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Bringing Fusion to the U.S. Grid (2021)

Chapter: 4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals

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Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
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Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
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Page 42
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 43
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 44
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 45
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 46
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 47
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 48
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 49
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 50
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 51
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 52
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 53
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 54
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 55
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 56
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 57
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
×
Page 58
Suggested Citation:"4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals." National Academies of Sciences, Engineering, and Medicine. 2021. Bringing Fusion to the U.S. Grid. Washington, DC: The National Academies Press. doi: 10.17226/25991.
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Page 59

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4 Innovations and Research Needed to Address Key Fusion Pilot Plant Goals It is clear that significant enthusiasm exists in the fusion community towards the proposed U.S. strategy of developing, designing and building a compact fusion pilot plant, which would carve out a unique niche in the worldwide pursuit of fusion energy involving smaller physical size, lower power output and lower capital cost. This enthusiasm is manifest in the recent fusion community planning process 1 and the numerous private startup companies that have garnered significant investment and are working to further the development of numerous fusion confinement and potential power plant concepts. However, research aimed at developing a fusion-based power plant has, to date, focused mainly on the plasma physics and confinement itself, including the plasma, the divertor and first wall, as well as the magnets and heating systems. It is obvious that substantial innovations will be required prior to completing a detailed engineering design and site selection leading to a decision to begin construction. This is necessary since private and public sector funding for fusion materials and technology, and corresponding research activities, have been substantially lower than for fusion confinement concepts. The desire to put fusion power on the grid in a pilot plant requires a significant investment into the research and development of materials and fusion nuclear technology to increase the technical readiness to a level that enables a pilot plant. The attractiveness of a fusion system, in terms of economics, safety and environmental considerations, is mainly determined by the materials and design of systems that will extract the fusion power and convert it to electricity and sustainably close the fuel cycle, which in the case of a deuterium-tritium (D-T)-based reactor design includes the generation, or breeding, of tritium. At present, these systems for the divertor and first wall armor PFCs, and integrated blanket are at a very low technology readiness level (TRL), and thus require substantial research and development. However, as noted in the Fusion Energy Sciences Advisory Committee report on Transformative Enabling Capability for Efficient Advance Toward Fusion Energy 2 there are numerous recent advances in advanced materials and manufacturing, high temperature and/or high field magnets, and tritium processing that offer the potential to significantly increase the TRL to enable construction and mitigate risks towards the initial operation of a compact pilot plant. The divertor, first wall and blanket systems for operation in a fusion power reactor represent a significant materials development challenge resulting from the neutron-induced degradation, thermal mechanical loading and corrosive environment. The operating environment envisioned for the materials- structures for fusion energy far exceed those of current technology, including light water reactors that are impacted by many materials degradation problems and expected in ITER as shown in Figure 4.1. 3,4,5,6 Figure 4.1 shows a schematic view of ITER in addition to calculations of the lifetime neutron fluence expected for ITER components compared to the annual neutron fluence anticipated in the European DEMO reactor, 7 and demonstrates that a demonstration reactor or pilot plant will experience neutron fluences and degradation significantly greater than ITER. To date, numerous designs have been proposed for fusion blankets to breed tritium, however, all such concepts are at a low TRL. An integrated strategy is needed to develop and test the integrated first wall and breeding blanket concepts in time for readiness for deployment of a compact, fusion pilot plant. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-1

FIGURE 4.1 Overview of neutronics calculation of (a) neutron fluence to components in ITER over the operation lifetime and in 1 year of operation of a European DEMO reactor, (b) showing the position dependent neutron flux. SOURCE: Republished with permission of Annual Reviews, Inc., from S.J. Zinkle and L.L. Snead, “Designing Radiation Resistance in Materials for Fusion Energy,” Annual Review of Materials Research 44: 241-267, 2014; permission conveyed through Copyright Clearance Center, Inc. There are two inter-related motivations for innovations and research. The first is to ensure that the key goals identified in Chapter 3 are met. The second is to use innovations to reduce the cost of the pilot plant and improve the economics of a first-of-a-kind (FOAK) power plant. The required innovations involve scientific and technical advancements, described below in the section “Scientific and Technical Innovations and Research Advances,” in order to demonstrate the required performance of the fusion confinement concept, extract the fusion power and to sustain the fusion fuel cycle. The sections “Participants in Developing a Pilot Plant” and “Models for Public-Private Partnerships” discuss the opportunities for public–private partnerships, given the success of such technology development partnerships including the NASA COTS program, and options for defining the private sector and federal government development priorities. Recommendation: To meet the challenge of having a viable design by 2028 and initial pilot plant operation in 2035-2040, innovations in fusion confinement concepts and technology to extract fusion power and close the fusion fuel cycle should be developed in parallel. This will enable the engineering design of a pilot plant and the construction decisions to be accelerated by a combination of government and private funding. SCIENTIFIC AND TECHNICAL INNOVATIONS AND RESEARCH ADVANCES Virtually every major component of a future nuclear fusion energy reactor, except for structural materials made of reduced activation ferritic-martensitic alloys, will require materials development in order to provide confidence in the ability to withstand significant limits of essential material properties PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-2

including: neutron damage, creep resistance, fracture toughness, surface erosion/re-deposition, corrosion, chemistry, thermal conductivity and many others. Further, a particular challenge is the need to safely and efficiently close the fuel cycle, which for deuterium-tritium fusion designs involves the development of blankets to breed and extract tritium, as well as the fueling, exhausting, confining, extracting, and separating tritium in significant quantities. 8 Although this is often put off for the future, the goal of economical fusion energy within the next several decades as a U.S. strategic interest 9 drives the need to rapidly increase the research and development of enabling materials, components and fusion nuclear technologies. Some of the capabilities needed for development and testing are straightforward and could be prepared in the short term, but a full research program will require test facilities producing environments increasingly similar to a fusion power plant to assess reactor-relevant power exhaust handling in the fusion neutron environment. In the 2019 report Final Report of the Committee on a Strategic Plan for U.S. Burning Plasma Research 10 (hereafter the “Burning Plasma report”), the higher magnetic field made possible by the development of demountable high temperature superconducting (HTS) magnets was identified as a key enabling technology that provides a potential path, when combined with advanced operating scenarios, to a compact fusion pilot plant with high fusion power density, and high poloidal beta enabling high bootstrap current fraction. Input to the committee stated that there are numerous other fusion confinement concepts that may provide a pathway to electricity generation. Integrated simulation has long been an important part of fusion energy research, with many recent examples of increasing physical fidelity in the Burning Plasma report. Advancement of the conceptual fusion pilot plant design(s) towards a construction decision will benefit from modeling and simulation incorporating multiple physics and multiscale phenomena with increasing fidelity into simulations to evaluate and refine design options. High fidelity simulation capability, validated by experiments, will continue to benefit from the emergence of exascale computing platforms, and can be employed both directly, and to facilitate development of reduced models, including via artificial intelligence. Physics, system and process models can be combined into comprehensive full device models which will likely contribute to evaluating the operations and maintenance of the pilot plant. Likewise, the incorporation of best practices and lessons learned from large scale project construction and system integration, including from ITER and recent nuclear fission power plants, in order to perform engineering computer aided design, structural analysis and process and control modeling will provide an important opportunity to optimize the design and integration of the fusion pilot plant. Such computational modeling and analysis will increase confidence in the operation and reliability of complex pilot plant components, including the divertor and plasma facing components, structural materials for the vacuum vessel, shielding and blanket modules, as well as functional and diagnostic materials that operate under severe thermal-mechanical, corrosion and radiation environments. It is also important to note that to meet the aggressive development timeline to meet the 2050 timeframe of putting a FOAK fusion plant on the electrical grid, it will be required to rapidly develop new programs and facilities to accelerate the scientific and technical innovation needed to finalize the engineering design of the fusion pilot plant. Fusion Performance and Plasma Confinement Producing significant net electric power from fusion requires achieving temperatures and pressures sufficient for high fusion power density, along with energy confinement times necessary to sustain those conditions with minimal auxiliary power. A high duty factor must be maintained for long periods of time, up to several months in the latter stages of pilot plant operation to meet the availability goals of a pilot plant. To achieve practical applications to replace present electrical generating processes with fusion will require us to make substantial progress in producing, maintaining, and heating of a burning plasma, while keeping it confined without damaging the engineered systems surrounding the plasma. Fusion PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-3

performance can be characterized by the fusion energy gain, or the closely related triple product of ion density, ion temperature, and energy confinement time. For D-T devices operating near the optimal temperature of 8-20 keV, a triple product of roughly 4–10 (in units of 1021 keV s m-3) is required to achieve the high fusion gain needed for net electricity production. Previous experiments on the JET, JT- 60U, TFTR and DIII-D tokamaks have achieved peak triple products of roughly 1, and sustained values of approximately 0.2 for several seconds, as shown in Figure 4.2. Stellarator experiments on W7-X and LHD have achieved peak values of roughly 0.1, and LHD has sustained lower performing plasmas for more than 1,000 seconds. Other magnetic fusion concepts, such as the field reversed configuration, have made rapid progress but remain orders of magnitude behind the triple product values achieved in tokamaks and stellarators. A combination of significant innovations enabling concept improvement, and incorporating technological advancements, such as HTS magnets, is needed to achieve conditions for high gain and long-duration or high repetition rate for pulsed systems at a scale that is potentially relevant for cost-effective fusion power production. FIGURE 4.2 Diagram of fusion plasma performance versus duration of plasma operation shows the fusion triple product versus duration, with the product ni(0)τETi(0) proportional to fusion energy gain. The highest performance is achieved for short duration. The longest plasma duration is achieved with superconducting magnets with sustained injection of external plasma heating power.TRIAM-1M (Japan) has the world record in plasma sustainment of 5 h and 16 min. Tore Supra (France) was sustained for more than 5 min, and the plasma was sustained in the superconducting Large Helical Device (LHD) in Japan for 48 min. SOURCE: Republished with permission of Springer, modified from M. Kikuchi and M. Azumi, 2015, Frontiers in Fusion Research II: Introduction to Modern Tokamak Physics, Springer International Publishing Switzerland; permission conveyed through Copyright Clearance Center, Inc. Finding: Significant progress is needed in fusion plasma confinement to demonstrate uninterrupted operation of a burning plasma in a high-performance confinement concept. Conclusion: Before proceeding to the final pilot plant design phase, a deuterium-tritium (D-T) fusion concept should simultaneously demonstrate temperatures of at least 100 million degrees C and a triple product greater than 2 (in units of 1021 keV s m-3), corresponding to a D-T equivalent plasma energy gain >1. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-4

Conclusion: For alternate fuels, equivalent parameters needed for net plasma energy gain must be demonstrated. Conclusion: For long pulse and sustained concepts, the values of plasma energy gain should be sustained for at least several characteristic plasma times including energy confinement times. The specific innovations required to advance a concept toward readiness for a pilot plant vary significantly with the characteristics of the concept. For brevity and concreteness, the remainder of this subsection focuses on the tokamak, as it is closest to readiness in terms of triple product, and was identified as the leading magnetic fusion concept in the 2019 Burning Plasma report. The following considerations would have to be modified for other fusion concepts and fusion fuels. In a tokamak, the toroidal component of the magnetic field is produced by external coils, which will need to be superconducting in a pilot plant, to avoid the resistive losses associated with copper coils. The other component of the confining magnetic field, the poloidal field, is produced by current which flows in the plasma itself. In a pulsed tokamak, this current can be driven via an external solenoid. However, if the tokamak is to operate continuously, the current must be driven by other means. A number of technologies can be applied, but will require substantial additional development, as discussed in below on innovations needed for plasma heating and actuators. If the tokamak can be operated at a high value of the poloidal beta (the ratio of plasma pressure to the pressure associated with the poloidal field), it can produce a substantial fraction of the needed current via the self-driven “bootstrap” current. The larger the fraction of the current which is self-driven, the less need there is for external current drive, and its associated recirculating power. Fusion performance of a sustained tokamak can then be characterized by three key performance metrics: (a) the fusion triple product, or fusion gain, which must be large enough for the plasma to produce net electricity and be predominantly self heated by fusion products (i.e., “burning plasma”), (b) the pressure, which determines fusion power density, and (c) the bootstrap current fraction, which must be high in order to avoid large recirculating power, which reduces net electric power, and enhances the heat load on material surfaces. Simultaneous optimization of (a), (b), and (c) requires both advanced technology, such as HTS magnets for high magnetic field, and advanced physics, to simultaneously reach high normalized performance via optimization of design parameters such as plasma shape and aspect ratio, as well as fueling and control actuators. In addition, fast particles must be well confined, and transients such as disruptions and edge localized modes must be avoided or strongly mitigated. Finding: Projecting regimes with confidence requires the development of theory and modeling tools, carefully validated against experiments, that are capable of predicting all the important aspects of plasma behavior, using both reduced models and integrated simulations spanning alpha particle physics, transport and confinement, stability, boundary layer physics, and plasma material interactions. Finding: All fusion concepts require significant innovations to address issues going beyond simple fusion performance metrics. Key integration issues include confinement of charged fusion products, exhaust of heat and helium ash, avoidance or strong mitigation of transient events, and integration of a hot fusion core with a boundary region compatible with exhaust requirements and long term survival of plasma facing materials. Recommendation: As fusion concepts approach performance metrics required for a pilot plant, the Department of Energy should support innovative facilities capable of solving key integration physics issues including core-edge integration of a high fusion performance core with a boundary consistent with long-term survival of plasma facing materials and exhaust of heat and helium ash. Support should incorporate extensive model and diagnostic development, and model validation. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-5

High Heat Flux Challenge for Plasma Facing Components and First Wall Components Power exhaust in high power density, compact fusion systems has two key challenges. One is the experimentally observed narrow steady-state e-folding length of power flow in the scrape-off-layer (SOL). Since the peak heat flux at the divertor plate (qdiv) is inversely proportional to the power e-folding length (𝜆𝜆 𝑞𝑞 ), narrow power e-folding length gives rise to an excessive heat flux at the divertor plate. Experimental observation shows 𝜆𝜆 𝑞𝑞 ~1/Bp 11,12 where 𝜆𝜆 𝑞𝑞 and Bp are power e-folding length and the poloidal field at the plasma surface. Since Bp~Ip/ap, power e-folding length in a compact fusion device tends to be smaller. However, the high operating density of a compact fusion device is likely beneficial for enhancing radiative cooling and to achieve detached plasma state. Another challenge is taming the transient heat flux including those due to ELMs (Edge Localized Mode). ELMs are an edge relaxation phenomena driven by the peeling/ballooning mode, whose onset is reasonably well understood and characterized. 13 ELM Suppression by methods such as application of 3D magnetic perturbations in DIII-D 14 reveals the promise for minimizing the impact of transient heat fluxes on the first wall, but much more research is required, especially for managing the heat flux challenge of a compact, high power density fusion reactor. Finding: High plasma core power density presents a significant heat exhaust challenge for the plasma facing components, armor, and first wall in fusion systems. Intensive experimental and theoretical research within the past several years has focused on the analysis and testing of advanced divertor configurations such as the Snowflake (SF) divertor, 15 super X (SX) divertor 16 and Small-Angle Slot (SAS) divertor. 17 Further, as discussed by Menard and co- workers, 18 the recent low aspect ratio HTS FNSF/Pilot plant design successfully showed that the long-leg /super X divertor can be implemented for the outboard divertor leg in a compact fusion system. As well, the SAS divertor tested in DIII-D has successfully demonstrated stable diverter detachment with a compact divertor geometry and indicates that a slot with a V-shaped corner is promising for a reactor. 19 Finding: Solutions are required to enable operation of controllable, dissipative boundary plasma conditions in magnetic toroidal devices, in addition to developing predictive understanding of divertor heat loads and methods to accommodate or avoid transient heat loading and transport and material migration in the plasma boundary/scrape off layer. Further, such solutions are necessary to develop the requirements for in-vessel components such as plasma heating systems, plasma- facing components, and first wall armor. Recommendation: The Department of Energy should support studies of the compatibility of innovative divertor designs in toroidal confinement concepts with divertor plasma detachment, which can significantly relax the radiated power requirement, and including the possibility of liquid metal PFCs. Recommendation: The Department of Energy should support a research program and new facilities, including linear devices for testing plasma facing components and non-plasma heat flux testing platforms, to identify, evaluate, and finalize a high-confidence, robust design for PFC and first wall armor materials, including both solid and liquid metal options, that are compatible with managing steady state and transient power loading. High-Temperature Superconducting Magnets It has been known for many decades that access to high magnetic fields is an important requirement for the achievement of magnetic fusion energy. Furthermore, for virtually all magnetic fusion PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-6

concepts, the fields must be generated by superconducting magnets. In general, the highest magnetic fields achievable in practical large scale superconducting magnets have been limited by the properties of the superconducting materials themselves. The two main well-established options are niobium-titanium (NbTi) and niobium-tin (Nb3Sn) with corresponding maximum fields approximately 7 T and 12 T respectively. In recent years, industry has developed two new classes of superconducting materials for large scale applications, with far superior properties 20,21,22: namely, rare-earth barium copper-oxides (REBCO) and bismuth-strontium copper-oxides (BSCCO). The fields and current densities are both much higher than for NbTi and Nb3Sn. REBCO is a recently developed superconductor applicable to large scale practical applications, with YBCO (yttrium-barium copper-oxide) a leading contender for fusion.20 YBCO superconductors typically have the form of a tape consisting of multiple deposition layers. YBCO tapes have superior electrical and mechanical properties compared to Nb3Sn, leading to significant interest and private investment within the fusion community. The main disadvantages of YBCO superconducting tapes are their high cost and the fact that it has yet to be demonstrated that they can actually be wound into large scale coils, although substantial efforts utilizing both private and public funding are under way to demonstrate the performance of such HTC magnets, as well as effort on insulators and stabilizers. However, it is important to build upon this recent investment to continue the development of HTC magnets, as well as to better understand the response of HTS magnets to 14 MeV neutron irradiation. Finding: The demonstration of sustained performance of HTS magnets at high field strength, including the avoidance or mitigation of magnet quench with required structural strength/load response, is required to contribute technological advances to multiple fusion confinement concepts. Recommendation: The Department of Energy should support a research program, including a facility for cryogenic neutron testing, to define neutron degradation limits on HTS magnets and/or optimize neutron shielding of magnets and to demonstrate mechanical design, magnet performance, and degradation limits of HTS magnets that impact multiple fusion concepts. Structural and Function Materials for First Wall and Vacuum Vessel Components The scientific understanding of the neutron-induced degradation of reduced activation ferritic martensitic (RAFM) alloys provides confidence up to a dose of 50 dpa/500 appm He (~5 MW-year m-2) within the temperature range from approximately 400 to 550 °C, as shown in Figure 4.3. 23,24 Figure 4.3a demonstrates the substantially lower volumetric swelling of RAFM, while Figure 4.3b shows the evolution of yield strength with temperature and dose, indicating that the yield strength and hence work hardening is not changing above 450 C. 25 Figure 4.4 demonstrates that the radiation embrittlement is generally manageable up to a helium concentration above 500 atomic parts per million (appm), 26 that corresponds to a neutron wall loading of ~5 MW-year m−2. This provides confidence in RAFM structural materials for use in a fusion pilot plant, although the degradation service limit above this fluence is not yet established. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-7

FIGURE 4.3 (a) Comparison of neutron radiation induced swelling of an austenitic stainless steel (annealed type 304L) and a 9-12% ferritic/martensitic steel in the temperature range from 400 to 550°C. (b) Yield strength of ferritic martensitic steels, including the reduced activation ferritic martensitic alloys F82H and 9Cr2WTaV as a function of temperature and neutron dose. Note the irradiation temperature and tensile testing temperature were approximately the same. SOURCE: (a) Republished with permission of Annual Reviews, Inc., from S.J. Zinkle and L.L. Snead, “Designing Radiation Resistance in Materials for Fusion Energy,” Annual Review of Materials Research 44: 241-267, 2014; permission conveyed through Copyright Clearance Center, Inc. (b) Reprinted from R.J. Kurtz and G.R. Odette, “Overview of Reactor Systems and Operating Environments for Structural Materials in Fusion Reactors,” pp. 51-102 in Structural Alloys for Nuclear Energy Systems, Elsevier, copyright 2019, with permission from Elsevier, http://www.elsevier.com. 𝛥𝛥𝑇𝑇𝑜𝑜 , of reduced activation ferritic martensitic alloys, which is a measure of the increase in the ductile to FIGURE 4.4 Predicted (lines) and measured (points) shift in the Master curve transition temperature brittle transition temperature. SOURCE: Reprinted from Y. Dai, G.R. Odette and T. Yamamoto, “The Effects of Helium in Irradiated Structural Alloys,” pp. 141-193 in Comprehensive Nuclear Materials (R.J.M. Konings, ed.), Elsevier, 2012, copyright 2012, with permission from Elsevier, http://www.elsevier.com. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-8

However, RAFM materials have not been fully demonstrated in the complex environmental loading conditions of a fusion pilot plant, which include multiple combined degradation modes including neutron degradation, He and H gas generation from nuclear transmutation, injected ions and permeating tritium, significant and potentially time varying heat flux, complex mechanical loading, magnetic fields and corrosive coolants, including the effects of radiolysis. Materials development efforts must focus on meeting all the requirements of a recognized code standard. Experience gained with licensing the structural materials of the cryostat, first wall armor and vacuum vessel of ITER can be applied to the fusion pilot plant. Although it is important to note that the pilot plant will not likely use the same materials, nor involve water cooling, and thus is expected to operate at higher temperatures for which no established code basis exists for materials licensing. The structural design criteria developed for ITER, and already being used for other fusion concept designs, can provide a starting point for the design of a pilot plant. Thus, robust mechanical property, corrosion, fabrication, and irradiation effects databases will need to be established that meet the requirements of appropriate regulatory authorities, including those consisting of high temperature and time varying stress state. This necessitates significant materials research and development to enable the design and function of all in-vessel and ex-vessel structural and functional materials in the fusion pilot plant environment. Finding: Confidence exists in the ability of low-activation ferritic martensitic alloys to survive D-T neutron-induced degradation up to a dose of 50 dpa/500 appm He (~5 MW-year m-2) at temperatures between 400 and 550°C; however, partially integrated testing is required to provide confidence in the performance of reduced activation ferritic martensitic components to the cyclic loading and environmental degradation required for Phase 1 and 2 operation of the pilot plant. Finding: Due to the anticipated higher operating temperature of a fusion pilot plant, the design criteria and licensing will be significantly different than for light water fission reactors or ITER, and will require development to address unique components, higher operating temperature and time varying stress state, corrosive coolants and stress/temperature gradients. Recommendation: The materials engineering community, supported by the fusion community and the Department of Energy, should develop high temperature structural design criteria that incorporate creep, fatigue, and corrosion behavior of in-vessel and ex- vessel structural and functional components to enable the engineering design and licensing of a fusion pilot plant as part of the conceptual design activities. Advanced manufacturing and complex material component design have transformative potential, 27 yet research is required to move beyond the early stage of developing these alloys and composites. This includes investigating neutron radiation effects, chemical compatibility and corrosion, response to plasma material interactions and tritium permeation, and component performance and degradation in the complex neutron, plasma material, and thermal-mechanical loading conditions. Studies will need to proceed from relatively simple single variable experiments to very complex, fully integrated, multiple-variable tests prior to Phase 1 pilot plant operation. For aneutronic fusion fuel cycles, the neutron flux and corresponding neutron-induced degradation and neutron activation concerns are reduced by one to two orders of magnitude. However, new concerns are introduced by the much larger heat and ion particle implantation into the near surface region of the first wall armor components. As well, the majority of the ion flux will consist of insoluble helium, which can degrade near surface thermal and mechanical properties. The implantation of helium into materials can cause blistering and spallation that can produce material loss. The maturity of materials for applications in high heat flux and ion implantation applications in advanced fuel cycles is at a low PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-9

TRL, that necessitates further research and development which should be defined as part of a more detailed technology roadmap for such fusion concepts. Recommendation: The Department of Energy should support a research program, including facilities to provide a limited volume prototypic neutron source for testing of advanced structural and functional materials and to assess neutron degradation limits of RAFM alloys beyond 5 MW-year m-2. Recommendation: The Department of Energy should support facilities for integrated blanket testing in representative time varying heat flux, mechanical loading, and corrosive environments, to identify, evaluate, and finalize a high-confidence, robust design for structural and functional materials. Innovations and Research Needed for Plasma Heating Systems and Actuators A broad range of heating and current drive technologies have been developed and employed in existing fusion experiments. These include gyrotrons and waveguides for electron cyclotron heating and current drive, antennas for ion cyclotron and lower hybrid wave heating and current drive, and neutral beams, including high energy negative ion beams. Examples of areas requiring further research and development include high frequency gyrotrons to enable electron cyclotron heating and current drive at high magnetic field, antenna structures compatible with high neutron and heat flux, and beam injection systems compatible with tritium breeding blankets. Solutions for the plasma heating systems and actuators for a fusion pilot plant are not in hand. In particular the normalized costs (dollars per watt) could be a significant factor in the overnight cost of the fusion pilot plant, while the electrical efficiency of the drivers/actuators clearly play a role in meeting its net electrical power goal. In addition, the durability of these heating/actuator engineering components exposed to the harsh fusion environment will be important to the operation times and costs of the fusion pilot plant. Finding: While ongoing advances in heating and current drive actuators have increased confidence in their capabilities to effectively drive and control fusion systems, significant progress is needed to demonstrate cost-effectiveness and compatibility with the needs of a pilot plant. Finding: A sustained pilot plant will require very high efficiency, months-long duration operation for actuators used in the stationary phase. Innovations in physics, such as very high bootstrap fraction for tokamaks, and in technology, such as radio frequency and beam technology that is both cost-effective and compatible with the neutron and heat flux environment, are needed. Actuators must be robust, reliable and efficient for various phases of the operation in the pilot plant’s fusion environment. Finding: A pulsed pilot plant will require very high efficiency, cost effective, durable and in some cases high repetition rate, drivers. Large reductions in cost per joule over existing drivers are required. Recommendation: The Department of Energy should invest in the development of actuator technology, and supporting theory, that is compatible with the requirements of leading pilot plant concepts. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-10

Innovations Needed for Closing the Fuel Cycle A D-T fusion reactor cannot function without a closed tritium fuel cycle, and this represents a fundamental feasibility issue for D-T fusion power production. Tritium provides a difficult species for control, accounting, and safety, yet it is critical to the fusion fuel cycle. In order for fusion to realize its maximum potential for safe operation and benign environmental impacts, high fidelity understanding of all processes involving tritium is required. The tritium fuel cycle has a very broad footprint on any fusion facility, which involves tritium burn fraction in the plasma, tritium processing time from plasma exhaust to fueling, the breeding of tritium in the blanket surrounding the plasma, extraction efficiencies from the breeder and coolant streams, tritium losses from and inventories in the fusion core, near-core and ex-core subsystems, and many more constitute a complex and interacting system. This is an essential capability for a D-T fusion pilot plant, and advances in these areas are required to meet the ambitious goals of a fusion pilot plant in the 2035-2040 timeframe. There are presently many blanket concepts being considered to meet the simultaneous demands of fusion power removal, neutron shielding and tritium breeding. A number of different blanket concepts have been proposed, and include a variety of solid and liquid blankets configurations to achieve these goals, however all concepts are at low technology readiness. This low readiness has to be addressed, since the blanket has significant design implications regarding the tritium breeding ratio, power conversion and the maintenance scheme for the pilot plant. The choice of coolant and the range of operating temperature of the blanket is important for overall efficiency of the pilot plant and this has far reaching consequences on the design performance of the blanket. The blanket also serves as a primary component in shielding sensitive components, such as magnets, from the deleterious effects of neutrons and high-energy photons, and as such, the radial thickness of the blanket is a key consideration in setting the overall size of the fusion pilot plant. Finding: Advancing the technology readiness of blanket technology is required in order to select and optimize the blanket concept for a fusion pilot plant. Virtually all of the technologies related to the tritium fuel cycle are at low technical readiness, with widely uncertainty in parameters that describe tritium migration through materials, across interfaces, and its retention in bulk solids and liquids, and retention and behavior in plasma facing materials. Extraction of tritium from either liquid or solid breeder materials is still highly uncertain, and the development of tritium barriers has been largely unsuccessful. ITER will provide a strong step in tritium processing and the fueling/exhaust tritium loop, with higher amounts of tritium required in the future (relative to ITER). Breeder material behavior and interactions are still at a low level of understanding. The breeding and recovery of tritium as it is processed raises a number of safety concerns to protect workers, the public and the environment. Tritium is highly mobile, and can readily permeate through metallic components, especially at elevated temperatures. ITER has a 4 kg maximum tritium inventory (~1.5 x 1018 Bq). As noted in Chapter 3, it is desirable to minimize the tritium inventory to ~1 kg. Also due to the sustained operation of a pilot plant, continuous processing technologies will be required. Expected tritium release limits are extremely low, and tritium needs to be accurately tracked to assure safety and proliferation. The grand challenges of tritium require improved scientific understanding of many interconnected phenomena including permeation, radiolytic chemistry, surface science and kinetics, liquid metal magnetohydrodynamics, and mass transfer. Systems and processes must be developed that can efficiently and safely continuously process tritium at flow rates and quantities far beyond current practice. Some of the technical issues associated with tritium control and recovery may be addressed by advanced fission reactors using lithium-based molten salts, and progress in these fission energy efforts may be applicable also to fusion systems. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-11

Finding: Maintaining low tritium inventory in a fusion plant is important for public safety, licensing, and many other considerations. Technologies that can enable decreasing tritium inventory in the fusion machine and the tritium processing plant are needed. Direct internal recycling of tritium is one example of a technology with the potential to reduce tritium processing rates and innovations that can enable direct internal recycling or similar technologies will be important. One challenge with direct internal recycling is that many of the impurities damage or poison known chemical processing methods and technologies for separation. Catalysis and removal of these corrosive species from the hot plasma is very challenging, but there are some potential avenues to be explored. Another important part of controlling overall tritium inventory is minimizing the captive inventory within the blanket material. The tritium must be rapidly recovered and kept out of collateral materials in order to minimize tritium inventory within the breeding loop. Direct internal recycling technologies, blanket tritium inventory reduction, or other similar methods to reduce the tritium inventory will need development before an FPP. Finding: Improvements in tritium accountability methods that can be applied in continuously operating fusion plants are needed and should be demonstrated in a fusion pilot plant. Recommendation: The Department of Energy should support research projects with the simultaneous objectives of reducing tritium inventory while increasing tritium processing rates that incorporate concepts such as process intensification. Technologies should be demonstrated prior to the construction of the tritium plant of a fusion pilot plant. Demonstration of tritium breeding and extraction technologies in the fusion blanket will be critical to the function of a fusion pilot plant and have not been demonstrated to date, although new advanced fission reactors using lithium-bearing fluoride salts may address some key questions relevant to a fusion pilot plant. The specific tritium breeding and extraction concept that will be implemented in a fusion pilot plant should be demonstrated during the design phase to provide reasonable confidence that it will work within a fusion pilot plant. A demonstration should include the ability to mitigate materials degradation of system materials due to exposure to blanket materials such as Pb-Li, FLiBe, or other relevant breeding blanket materials. If tritium extraction methods utilize halides, a method will need to be developed to ensure no halides or other potentially reactive impurities migrate to the tritium systems (e.g., a fluorine or impurity removal trap). Recommendation: The Department of Energy should support research projects and a demonstration to increase technology readiness and provide confidence that tritium breeding and extraction technologies can achieve fuel sustainability. Technologies should be demonstrated prior to the construction of the tritium plant of a D-T fusion pilot plant. Finding: Tritium emissions from a fusion energy pilot plant need to be controlled to meet applicable NRC guidelines. A fusion pilot plant should demonstrate implementation of tritium emission control measures and demonstrate the ability to maintain emissions within NRC guidelines for a FOAK or a pathway to achieve the required emissions reduction. PARTICIPANTS IN DEVELOPING A PILOT PLANT The design, construction and operation of a fusion pilot plant will require addressing scientific, engineering and regulatory issues in an integrated fashion and requires a broad spectrum of skills. While the United States is a leader in fusion research, the program has not built a facility of the scale of a pilot plant. Thus, new additional skills beyond that which currently exist will be needed, while continuing to PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-12

leverage strengths of the program. The pilot plant teams would be composed of fusion developers, component manufacturers, EPC (engineering, procurement and construction) contractors, universities, national laboratories and potentially Investor Owned Utilities. Large projects such as a pilot plant require strong teams of engineers, including systems engineering, project managers and individuals with licensing experience. Some of these skills currently exist within the program but will have to be augmented. Creating new teams is an opportunity to add members with the requisite skills. Embracing diversity, equity, and inclusion is key to building successful teams to solve the challenges that fusion faces. This is multi-faceted, and at the highest level, means a team of people that approach these challenges from a multitude of viewpoints. Different viewpoints arise from many different sources, including, but not limited to technical expertise, a person’s life experiences, gender, ethnic background, age, and many others. It is important to note that diverse teams are more effective at solving problems than those that are not diverse, however those teams must work together, otherwise they are less effective than non-diverse teams. 28 The recently released fusion community plan 29 recognizes this, identifying the cross-cutting opportunity to “Embrace diversity, equity, and inclusion, and develop the multidisciplinary workforce required to solve the challenges in fusion and plasma science.” The need to embrace diversity, equity, and inclusion is not new, nor newly discovered. There are many studies, including very recent ones, that support this conclusion, and provide evidence that while there has been progress, we have a long way to go. Studies such as the National Academies studies “Promising Practices for Addressing the Underrepresentation of Women in Science, Engineering, and Medicine: Opening Doors” 30 and “Expanding Underrepresented Minority Participation,” 31 provide actionable recommendations to drive positive systemic change. Change cannot happen without a visible and obvious emphasis. Finding: The need to embrace diversity, equity, and inclusion is not new, nor newly discovered. Change cannot happen without a visible and obvious emphasis. Recommendation: The participants in the development of the pilot plant should execute the recommendation of the Community Planning Process to “Embrace diversity, equity, and inclusion, and develop the multidisciplinary workforce required to solve the challenges in fusion and plasma science.” A continued tight coupling between the on-going research teams in the fusion program and the pilot plant teams will be needed. Universities have had a significant role in fusion research, have built and operated experiments, and have been key in workforce development. This role needs to continue, with increased effort on technology development, which had been previously reduced, and will now have to be strengthened. Some of the universities have also played a major role in spinning-off privately funded fusion developers. Discussions with component manufacturers surfaced a need for funding stability as an important requirement for their partnership in this effort. Companies are interested in working on fusion and developing technology and tooling needed to construct components; however, past experience reveals that industry requires a stable revenue stream to attract, develop and retain the expertise that is needed. Without such stability, the expertise that is developed in industry will go away. This has occurred previously in the fusion program and is another motivation for an accelerated program to ensure that the appropriate skills are developed and retained in industry. National laboratories have played a large role in the fusion program ranging from scientific research and technology development, to operation of large magnetic fusion facilities. They operate tritium facilities, including previously a D-T fusion facility, and have capabilities for siting nuclear facilities. They also perform a broad range of research from basic to applied. Similarly, the NNSA has operated facilities at national laboratories and at the University of Rochester to advance the scientific understanding of inertial confinement fusion. Experience gained at these facilities in addressing issues PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-13

related to tritium handling and neutron production can benefit the fusion pilot plant effort and add diversity in perspective. DOE Fusion Energy Sciences has operated national facilities at national labs, industry and universities. PPPL hosted TFTR (D-T tokamak) and now the NSTX-U facility. A private company, General Atomics, has built and operates the DIII-D National Fusion Facility on behalf of the Department of Energy. Similarly, a university, MIT, previously built and operated Alcator C-Mod. These DOE funded facilities have contributed to numerous scientific advances, and have achieved high values of the Lawson parameter, approaching conditions required for fusion breakeven. In the United States, there is far more private investment in fusion technologies today compared to 10 years ago. This private investment is a positive development because it enables parallel developmental paths, which is key to realizing success in this challenging technology. Private investors expect a return on their investments. Milestones need to be achieved that demonstrate progress to goal using a timeline that maps to market needs. Most of the privately-funded fusion technology companies have tapped into the expertise at the national laboratories and universities by directly funding work, by hiring staff from the national laboratories or through new funding programs such as INFUSE and ARPA-E BETHE including diagnostic teams and GAMOW, which is jointly funded by FES and ARPA-E. It was noted that currently INFUSE only supports national laboratories. Finding: Teams made up of private industries, national labs, and universities bring together important strengths: industry brings the focus on deploying a usable product on a timeframe that will meet market needs, and national labs and universities bring innovation and deep technical expertise. Recommendation: The Department of Energy should further encourage access of private industry to the broad range of technical experts resident at the national laboratories and universities. Recommendation: The Department of Energy (DOE) should use and expand the new programs to partner with industry in support of the pilot plant design, and DOE should include all FES-funded researchers, including those at universities and private companies, in the INFUSE program. MODELS FOR PUBLIC-PRIVATE PARTNERSHIPS Over the last several decades, fusion energy research has involved government sponsored programs, and for larger research efforts, international collaboration with the ITER program being the largest and most sustained example. However, over the last two decades multiple private sector efforts have been initiated to develop fusion energy concepts. The committee heard from the industry consortium that represents many of these companies, the Fusion Industry Association, along with representatives from several companies developing fusion technology and concepts. As of today, over $1 billion of private sector investment has already been made in fusion systems. 32 As noted in the “Economic Considerations” section in Chapter 3, a number of different federal government cost sharing approaches have been utilized to stimulate new energy production technology, including direct cost share, tax incentives, loan guarantees, grants to specific companies, ARPA-E funding of public-private projects, prizes and payment for milestones. Although there are significant differences in terms of technology maturity, the NASA Commercial Orbital Transportation Services (COTS) program has realized substantial cost reduction through an innovative cost and risk sharing approach that involved a fixed-price payment-for-milestones contracting structure to private sector companies working to develop crew and cargo space transportation capabilities. In parallel, NASA PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-14

pursued its conventional Artemis program to develop deep space exploration capabilities which could also satisfy similar capabilities to low Earth orbit if necessary. One of the first COTS awards was in 2006 to the start-up company SpaceX, which had been founded in 2002. By 2008 SpaceX had conducted its first successful launch of the single-engine Falcon 1, by 2012 it made its first cargo delivery to the International Space Station using the Falcon 9, by 2018 SpaceX had captured over half of the worldwide market for commercial space launch services, and by 2020 became the first commercial company to launch humans to space. Successful technology development efforts funded by private investment involve strong and positive public incentives to systematically address areas of technology and market risks with the ultimate objective of purchasing a commercially viable, cost-effective product or service. Public- private partnerships using payment-for milestones contracting, versus traditional cost, plus fee contracting, reinforce built-in incentives to be efficient, timely, and entirely success oriented. This is a principal reason that the NASA COTS model has emerged as a recommended possible strategy for advanced fission energy 33 as well as for fusion 34 although other models have also been used for developing advanced fission energy including loan guarantees and direct cost share agreements as being pursued in the Advanced Reactor Development Program. 35 Furthermore, the recently enacted bill HR-133 Sec. 2008 has defined a milestone-based fusion energy development program. Public cost-share support of private-sector efforts to develop fusion energy are well justified by the long-term societal benefits of successful commercial deployment of dispatchable, sustainable clean energy technology. However, it is important to note that SpaceX would likely not have been successful in such a short timeframe without access to NASA’s technical expertise and key infrastructure including launch range facilities. In the same way, it remains essential that DOE sustain a strong base program in fusion energy science and technology, including supporting infrastructure and research at national laboratories and universities, and pursuing development of a FPP as recommended in this report. As discussed above the DOE Innovation Network for Fusion Energy (INFUSE) program and the BETHE and GAMOW programs are designed to enable private sector companies to access these capabilities. Since the fusion program has not used a milestone based public-private partnership previously, gaining experience with such a program would be valuable. The following two examples of research that are needed is meant to illustrate how such a program could be applied. The response of materials to neutrons in the fusion environment is of universal interest because they are present in all fusion concepts, even though the relative fraction of fusion energy carried by neutrons and their energy varies. A particular challenge is faced in D-T fusion because the 14 MeV neutrons are at a much higher energy than from fission and therefore the relevance of materials exposed in existing reactors is limited. Furthermore, it is important that the exposure environment (thermal, chemical) is sufficiently representative of the fusion concept. Therefore, one must consider the development of dedicated capabilities and facilities that can determine the response of materials in a fusion neutron environment. This can lend itself to a public-private partnership because there are both scientific frontiers to explore and practical questions to answer for private fusion developers. If the facility can serve a broad set of developers or concepts, the combined financial and technical resources from the government and private could be leveraged. Furthermore, the cost-effective design, construction and operation of such a facility would be a target for a milestone payment program since the scientific requirements of the neutron exposure are already well understood. This program could include cost shares on the near-term maturation of neutron source technology presently being developed in the private sector. A related and synergistic topic is the response of superconductor magnets to the fusion environment. Many fusion concepts rely on strong magnetic fields produced by electromagnets. These are almost always superconductor magnets because they consume minimal electricity to operate. However, the fusion neutrons can affect the superconductors and therefore impact the overall performance and lifetime of the superconductor coils. Simultaneously, there are substantial and multiple efforts on the development of advanced HTS superconductors in the private sector. A milestone based public-private partnership that focuses on the magnet performance evolution under neutron bombardment in a shared facility would be a high leverage opportunity. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-15

Finding: The NASA COTS program achieved remarkable success in developing new commercially competitive space transportation capabilities at significantly less cost to the government and with an accelerated schedule using a payment-for-milestones public-private partnership. Finding: While the NASA COTS model holds promise, in general the TRL for space transportation systems is substantially higher than the TRL for major fusion energy systems. Due to the strategic importance of crew and cargo space transportation capabilities to the International Space Station and beyond low Earth orbit, NASA pursued a conventional technology development program (now called Artemis) in parallel with its COTS program to mitigate the commercial investment risk and to serve longer term deep space exploration goals. Recommendation: The Department of Energy should evaluate and identify the best model for public-private partnerships to accelerate development and reduce government cost for a fusion pilot plant. Note that the different phases of the development, including conceptual design and technology roadmap, detailed engineering design, construction and operation, may involve different or incremental public private partnership models, including fixed- price payment for milestones. ITER CONTRIBUTIONS TO A PILOT PLANT As an ITER member, the United States receives access to all ITER intellectual property (IP); the Department of Energy is responsible for determining how that IP will be available. ITER-related technologies are accessible to the U.S. fusion industry through partnerships with DOE national laboratories and universities. U.S. fusion and related industries benefit from the technologies, know-how, and experience that results from U.S. engagement in ITER. Leveraging the experience gained from the ITER project will be important to meeting the aggressive time scale for a pilot plant. The United States is responsible for hardware design and manufacturing for 12 different ITER systems. Experience in developing specifications for ITER hardware procurements and overseeing the design, fabrication, and delivery of the hardware that meets regulatory requirements is an important foundational step that will benefit the design and deployment of a fusion pilot plant. While a pilot plant will differ considerably from ITER, and may not even be a tokamak configuration, much of the experience gained through the ITER process is relevant to a pilot plant regardless of its configuration. The ITER systems that the United States is responsible for that could fit into this category include instrumentation and control, magnet conductors, steady-state electrical networks, plasma diagnostics, isotope separation, and vacuum systems. If the U.S. fusion pilot plant is a tokamak, then there are several specialized systems from the U.S. ITER effort that could be relevant, including the disruption mitigation system, which limits damage to internal components from off-normal events, high-efficiency radio frequency power transmission lines for heating systems, and the central solenoid, which produces most of the plasma current needed in a tokamak. In addition, the United States is contributing to the design of both the tokamak blanket/shield and the edge localized modes which support vertical stabilization of the plasma. Examples of how ITER activities benefit the U.S. fusion industry broadly include: • Tools and strategies for plasma control and performance. ITER, in conjunction with the international community, has led to advances in modeling, prediction, avoidance and control of plasma transients and disruptions in tokamaks; supporting technologies and techniques have also been developed. Non-tokamak fusion devices can also benefit from the improved understanding of plasma behavior derived from ITER R&D. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-16

• Superconducting magnet technologies. The accumulated mechanical properties of structural materials at cryogenic operating temperatures is an important asset for all magnet designs. In addition, lessons learned on the complexity and scale of ITER magnet systems can inform solutions for feedthroughs and joints and production/testing strategies, even if new superconductors are likely to be used in the pilot plant. • Radiation transport analysis. Higher resolution and faster radiation computational analysis tools have been applied to understanding ITER shielding and safety requirements, including shutdown dose rates. These tools, combined with advanced modeling and simulation, will be highly relevant to the design qualification, and safe operation of future fusion devices and power plants • High-powered plasma heating. State-of-the-art heating and current drive technologies developed in the United States and by ITER partners for a nuclear facility could be broadly applied across many magnetic confinement configurations. • D-T fuel cycle technologies. The knowledge and application of the ITER tritium plant design directly supports the development of a fusion pilot plant in that the processing rate of hydrogen isotopes is much greater than current facilities use and scale-up as well as validation of many components will be completed as part of constructing the ITER fuel cycle. The ITER design includes ash and impurity removal; recapture of the hydrogen; separation of the hydrogen isotopes; and the recycling of the isotopes back into the fuel stream. Specific technologies to be developed by ITER include: palladium-silver permeators; tritium impurity catalysis and removal; cryo-pumping of tritium; tritium storage and delivery; and advanced tritium process modeling. In addition to the fuel process loop, the ITER design includes a detritiation system that minimizes tritium releases to the environment. Progress has been made on many fronts in development of these systems for ITER, but challenges remain in component scale-up and system integration. • Continuous plasma fueling. ITER has led the development of continuous pellet fueling systems for long pulse operations. Pellet fueling may be an effective strategy for efficiently and reliably delivering hydrogen fuels to plasmas in various device configurations. • Fusion materials. ITER-scale requirements drove a demonstration path for the development and selection of appropriate plasma-facing and divertor materials that can handle the neutrons, magnetic fields, and heat flux of the ITER fusion environment. Even though the environment of a pilot is expected to be harsher (high power density, neutron fluence, different coolants, and higher operating temperature) this is foundational work for next generation fusion materials and components. • Fusion power and particle handling. ITER power levels and pulse length will exceed that of any current fusion device. The power and particle handling demands of many fusion pilot plant designs will require full steady state operation solutions informed by ITER R&D. This includes actively cooled internal components and high-speed tritium compatible vacuum pumps. • Burning plasma science. ITER’s mission is to achieve plasma energy gain of 10 and access the burning plasma regime. This has two impacts on a fusion pilot plant. The first impact is through the development of the detailed science tools needed to predict ITER’s performance. These tools can be applied to a fusion pilot plant, with the impact being dependent on the pilot’s configuration. These tools can be effective immediately. The second impact would be anticipated by the experimental achievement of the burning plasma regime after 2035 when D-T operations are expected to start in ITER. The insights gained from these results could be important to the fusion pilot plant starting operations at a similar time. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-17

ITER represents a major step forward in fusion science and technology funded by the U.S. government through the ITER agreement. The knowledge gained by participating in the ITER project is one of the key motivations for the United States to continue its support. Recommendation: The Department of Energy should assure maximum possible access to ITER information for the members of the fusion pilot plant design teams. INTERNATIONAL COLLABORATIONS The United States has had major international collaborations in fusion research in addition to ITER since the fusion program was declassified in 1958. U.S. scientists participate in experiments on major facilities throughout the world and in particular on the largest cutting edge facilities including Asdex Upgrade and W7-X in Germany, EAST in China, JET and MAST-U in the United Kingdom, K- STAR in Korea, LHD and JT60-SA in Japan, and WEST in France and many other smaller facilities around the world. International scientists participate extensively in U.S. experiments. Collaborations span a broad range of topics including not only experiments but also theory, modeling and technology development. The United States provides neutron irradiation facilities for collaboration with Japanese and European fusion programs. Construction of a pilot plant in the United States would provide an opportunity to expand these collaborations. In some technology areas, international collaborators would be able to provide experience and access to test facilities, which are not available in the United States. This can be mutually beneficial. NOTES 1. “A Community Plan for Fusion Energy and Discovery Science” can be found at: https://drive.google.com/file/d/1w0TKL_Jn0tKUBgUc8RC1s5fIOViH5pRK/view. 2. Fusion Energy Sciences Advisory Committee Report on “Transformative Enabling Capabilities for Efficient Advance Toward Fusion Energy” https://science.osti.gov/- /media/fes/fesac/pdf/2018/TEC_Report_15Feb2018.pdf?la=en&hash=A748443D03AD591E340AD046D 53F86537B37FAB0. 3. S.J. Zinkle and J.T. Busby, “Structural materials for fission & fusion energy”, Materials Today 12 (2009) 12-19. 4. S.J. Zinkle and G.S. Was, “Materials Challenges in Nuclear Energy”, Acta Materialia 61 (2013) 735-758. 5. S.J. Zinkle and L.L. Snead, “Designing Radiation Resistance in Materials for Fusion Energy”, Annual Review of Materials Research 44 (2014) 241-267. 6. R.J. Kurtz and G.R. Odette, “Overview of Reactor Systems and Operating Environments for Structural Materials in Fusion Reactors”, Structural Alloys for Nuclear Energy Systems, Elsevier 2019, 51-102. 7. S.J. Zinkle and L.L. Snead, “Designing Radiation Resistance in Materials for Fusion Energy”, Annual Review of Materials Research 44 (2014) 241-267. 8. A. C. Klein, 2017 American Nuclear Society special report. 9. National Academies of Sciences, Engineering, and Medicine 2019. Final Report of the Committee on a Strategic Plan for U.S. Burning Plasma Research. Washington, DC: The National Academies Press. https://doi.org/10.17226/25331. 10. National Academies of Sciences, Engineering, and Medicine 2019. Final Report of the Committee on a Strategic Plan for U.S. Burning Plasma Research. Washington, DC: The National Academies Press. https://doi.org/10.17226/25331. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-18

11. R.J. Goldston, Heuristic drift-based model of the power scrape-off width in low-gas-puff H- mode tokamaks, Nucl. Fusion 52 (2012) 013009. 12. T. Eich, et al., Inter-ELM Power Decay Length for JET and ASDEX Upgrade: Measurement and Comparison with Heuristic Drift-Based Model, Phys. Rev. Lett. 107 (2011) 215001. 13. P.B. Snyder, et al., A first-principles predictive model of the pedestal height and width: development, testing and ITER optimization with the EPED model, Nucl. Fusion 51 (2011) 103016. 14. T.E. Evans, et al., Suppression of large edge localized modes with edge resonant magnetic fields in high confinement DIII-D plasmas, Nucl. Fusion 45 (2005) 595. 15. D.D. Ryutov, Geometrical properties of a “snowflake” divertor, Phys. Plasmas 14 (2007) 064502. 16. M. Kotschenreouther, et al., The super X divertor (SXD) and a compact fusion neutron source (CFNS), Nucl. Fusion 50(20101)035003. 17. H.Y. Guo, et al., Small angle slot divertor concept for long pulse advanced tokamak, Nucl. Fusion 57 (2017) 044001. 18. J.E. Menard, et al., Fusion nuclear science facilities and pilot plants based on the spherical tokamak, Nucl. Fusion 56 (2016)106023. 19. H.Y. Guo et al, First experimental tests of a new small angle slot divertor on DIII-D, Nucl. Fusion 59 (2019) 086054. 20. See DOE FESAC Report on Transformative Enabling Technologies (2018). 21. D. Larbalestier, Nature, vol. 414 (2001) 368 22. W.H. Feitz, Fusion Engineering & Design, vol. 88 (2013) 440. 23. S.J. Zinkle and L.L. Snead, “Designing Radiation Resistance in Materials for Fusion Energy”, Annual Review of Materials Research 44 (2014) 241-267. 24. Y. Dai, G.R. Odette and T. Yamamoto, “The Effects of Helium in Irradiated Structural Alloys”, in Comprehensive Nuclear Materials (2012), R.J.M. Konings, editor, Elsevier, 141-193. 25. R.J. Kurtz and G.R. Odette, “Overview of Reactor Systems and Operating Environments for Structural Materials in Fusion Reactors”, Structural Alloys for Nuclear Energy Systems, Elsevier 2019, 51-102. 26. Y. Dai, G.R. Odette and T. Yamamoto, “The Effects of Helium in Irradiated Structural Alloys”, in Comprehensive Nuclear Materials (2012), R.J.M. Konings, editor, Elsevier, 141-193. 27. Fusion Energy Sciences Advisory Committee Report on “Transformative Enabling Capabilities for Efficient Advance Toward Fusion Energy” https://science.osti.gov/- /media/fes/fesac/pdf/2018/TEC_Report_15Feb2018.pdf?la=en&hash=A748443D03AD591E340AD046D 53F86537B37FAB0. 28. Steve L. Robbins, What If, 11th Anniversary Edition, 2018. 29. “A Community Plan for Fusion Energy and Discovery Science” can be found at: https://drive.google.com/file/d/1w0TKL_Jn0tKUBgUc8RC1s5fIOViH5pRK/view. 30. Promising Practices for Addressing the Underrepresentation of Women in Science, Engineering, and Medicine: Opening Doors, edited by Rita Colwell, Ashley Bear, and Alex Helman, The National Academies Press, 2020. 31. Expanding Underrepresented Minority Participation, The National Academies Press, 2011. 32. “Investments in privately funded fusion ventures grow” Physics Today, October 13, 2020, DOI:10.1063/PT.6.2.20201013a. 33. “In Search of a SpaceX For Nuclear Energy,” Nuclear Innovation Alliance, May 2019. https://www.nuclearinnovationalliance.org/search-spacex-nuclear-energy. 34. Cost-Sharing Partnerships With the Private Sector in Fusion Energy, Federal Register, 4/20/2020. https://www.federalregister.gov/documents/2020/04/20/2020-08312/cost-sharing- partnerships-with-the-private-sector-in-fusion-energy. 35. https://www.energy.gov/ne/nuclear-reactor-technologies/advanced-reactor-demonstration- program. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4-19

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Fusion energy offers the prospect of addressing the nation's energy needs and contributing to the transition to a low-carbon emission electrical generation infrastructure. Technology and research results from U.S. investments in the major fusion burning plasma experiment known as ITER, coupled with a strong foundation of research funded by the Department of Energy (DOE), position the United States to begin planning for its first fusion pilot plant. Strong interest from the private sector is an additional motivating factor, as the process of decarbonizing and modernizing the nation's electric infrastructure accelerates and companies seek to lead the way.

At the request of DOE, Bringing Fusion to the U.S. Grid builds upon the work of the 2019 report Final Report of the Committee on a Strategic Plan for U.S. Burning Plasma Research to identify the key goals and innovations - independent of confinement concept - that are needed to support the development of a U.S. fusion pilot plant that can serve as a model for producing electricity at the lowest possible capital cost.

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