This appendix discusses the variety of options available for generating energy using either nuclear fission or nuclear fusion. The focus of this appendix is primarily on second-generation nuclear fission reactors, as they are more likely than fusion power plants to be introduced in the near term. Descriptions of some of the proposed new technologies are provided below. These descriptions are not intended to be a comprehensive and critical analysis of the technological options for future development of nuclear power nor an endorsement of particular technologies. Such an analysis will be provided in a forthcoming National Research Council Energy Engineering Board report.
Current advanced nuclear fission reactor development projects worldwide are summarized in Table G.1. Any reactor development project aims to improve performance in the categories of both safety and economics; however, their individual improvement differs substantiallyreflecting differing evaluations of which types of improvement are needed most. Reactor development projects can be categorized according to whether they give primary emphasis to economics or safety. The class of goals chosen for emphasis usually guides formulation of the basic reactor concept.
The optimized reactor design is usually very similar to current nuclear reactors, but reflects an approximate attempt to maximize either economics or safety, while attempting to improve performance in the other category of goals to at least the minimal extent required by the safety authorities or the economics of competing technologies.
Reactors emphasizing economic performance attempt to reduce the costs of generating electricityusually focusing on the cost components of capital and plant operational availability. Reactors emphasizing improved safety performance attempt to gain public acceptance by decreasing the risk of core damage or radioactive release. Reactor safety is improved by increasing
TABLE G.1 Worldwide Programs of Nuclear Power Technology Development as adapted from Golay (1990)
NOTE: Abbreviations are as follows:
the mechanical reliability of the post-shutdown reactor cooling and post-fuel damage maintenance of containment integrity functions. However,corresponding reductions in nonmechanical safety function unreliability,subtlecommon-mode failures, and severe external events (e.g., strong but rare earthquakes) appear to be more difficult to achieve. Thus, from the safety-oriented concepts substantial performance improvements can be expected, but hopes that perfect or idiot-proof technologies will result may be poorly founded.
The term often used for reactor concepts emphasizing safety is passively safe. In a nuclear power plant, passive safety features are those that perform a safety function using only sources of motive force found in nature (e.g., gravity for cooling water instead of pumps). There are developments on concepts that use passive safety features only. In other cases, particularly in light water reactors, a blend of passive and active safety features is being pursued. These concepts could be termed semi-passively safe.
In this appendix, the state of the art for the following three fission concepts are reviewed briefly:
• Light Water Reactors (LWR)
• High-Temperature Modular Gas-Cooled Reactors (HTGR)
• Integral Fast Reactors (IFR)
For the LWR, both passively safe and semi-passively safe developments are included. For HTGR and IFR, only the concepts using passively safe features exclusively are described. As noted earlier, this is not intended to be a comprehensive or critical assessment of all the nuclear technologies available or an endorsement of particular technologies, but a description of a few options to demonstrate some of the nuclear technologies currently being discussed. For example, not all varieties of light water reactors are discussed, nor are heavy water reactors, which are currently not licensed in the United States.
Light Water Reactors
In most countries the focus of nuclear fission development activities remains on light water reactorsemphasizing economic-oriented designs. Among the more important elements are plant operational availability and shortening plant construction time. Even for those developments, there is always a serious concern for safety. In the advanced pressurized water reactor (PWR) and boiling water reactor (BWR) designs, automatic mechanical and electronic devices have supplemented human operators in the performance of many emergency dutiesmaking safety systems more redundant.
Light water reactors are divided into two types: large evolutionary light water reactors and mid-sized light water reactors. Large evolutionary light water reactors include the advanced boiling water reactor, advanced pressurized water reactor, and the system 80+ standard design pressurized water reactor. Mid-sized light water reactors with passive safety features include the advanced passive pressurized water reactor and the simplified boiling water reactor. The large evolutionary light water reactors are the most likely to be implemented in the time frame in the Mitigation Panel's analysis. The cost of these reactors is the basis for the calculation described in Appendix J.
One way to reduce risk is to build smaller reactors that contain less radioactive material and, in principle, can be cooled more easily in case of malfunction. The Electric Power Research Institute (EPRI) has encouraged two efforts to develop smaller advanced (LWRs)the ASBWR and AP-600. These reactors utilize semi-passive safety systems. They are designed to utilize a small number of reliable ''active" components to initiate safety operations, but do not require large AC power sources to run the emergency equipment. All water required for heat removal in the primary system drains by gravity to the reactor core. Large volumes of water are stored above the reactor core. Typically, there is sufficient water to flood the reactor containment and reactor system. By so doing, some of the economic penalties of purely passive design may be avoided.
The LWR concept that utilizes exclusively passive safety features is the process inherent ultimately safe (PIUS) reactor conceived by ASEA-Atom
in 1979. Their criteria included safety from external events such as earthquakes and terrorist attack. The intrinsic protection can last a week or more without calling into action any active safety equipment or human actions. Currently, coordinated activities to develop PIUS are under way between ABB and companies in Italy, South Korea, China, and the United States.
High-Temperature Modular Gas-Cooled Reactors
The high-temperature gas-cooled reactor (HTGR) technology utilizing pebbles for fuel was developed first in West Germany. In the United States, the Fort St. Vrain reactor, which used General Atomic's HTGR technology, was less than successful. The current U.S. reference plant design developed by the Department of Energy's HTGR Program uses four steam generating reactor modules and two steam-turbine-generator sets to achieve a nominal plant rating of 550 MWe.
The modular gas-cooled reactor (MGR) is predicated on the possibility of building reactors that will not, under any circumstances that can be perceived at this time, release fission products to the environment. In a reactor with localized fuel, this is equivalent to the requirement that the fuel cladding retain its integrity even if all coolant (not just coolant flow) is lost and all control rods are fully withdrawn. This requirement is claimed to be met in the MGR by limiting the power density and reactor size in such fashion that the hottest location in the reactor core never reaches temperatures capable of damaging the fuel. To achieve this, it is essential both that the reactor possess a temperature coefficient of reactivity sufficiently negative for the chain reaction to be quenched before damaging temperatures are reached, and that the processes of conduction and radiation are sufficient to carry the after heat of the fission products to the environment without exceeding the design basis temperature. The latter requirement imposes important limitations on power density and size.
Although it is possible in principle to meet these requirements with a number of reactor types, the gas-cooled reactor with ceramic-coated fuel has the combination of cladding strength and temperature capability that holds promise in technical feasibility. The challenge of the MGR program is not the usual one of trying to make an economical reactor safe, but rather its inversethat of making a safe reactor economical. The issues are low power density with its obvious cost penalties and small unit size with its apparent cost in economies of scale. In addition, requiring a containment facility (not in current plans) would increase the cost. There are two different approaches to ameliorating the economic disadvantages of the gas-cooled reactor: the first is based on making the unit size as large as possible and combining the output of several steam-generating units to achieve economies of scale.
Another design concept, developed at the Massachusetts Institute of Technology
(MIT), utilizes a high-speed gas turbine in a direct-Brayton cycle and generates electricity at a high frequencyhigher than 60 hertzutilizing power electronics for frequency conversions. The aim is to reduce the size of the balance of plants and thus improve the economics.
The macroscopic properties of gas-cooled reactors are closely coupled to the small-scale properties of the fuel. As a result, incremental improvements in fuel quality can have substantial effects on existing reactors and, more significantly, on the design of commercially improved versions. This requires the most stringent quality control on the fuel manufacturing process. Recent improvements in fuel quality (a two-order-of-magnitude improvement in fission product retention and a method for coating individual pebbles with silicon carbide to render them fireproof) promise more evolutionary improvement.
All modern gas-cooled reactors are based on microencapsulated fuel in which submillimeter spheres of uranium oxide are surrounded by a series of nested spherical shells of pyrolytic carbon and silicon carbide. This fuel was developed by General Atomic and was brought to its current level of capability in the West German nuclear program, where the concept of the passively safe small-scale modular reactor was first developed. In the West German program, intensive series of tests of the fuel under a variety of temperature and radiation environments have been conducted, and several loss of coolant events were tested, and monitored and compared with computer models. Potentially important advances would follow from the development of higher-temperature coatings, which would facilitate the deployment of higher power density systems, with concomitant economic advantage, or the development of 1000°C systems for process heat applications.
Integral Fast Reactors
A new fission reactor concept developed at Argonne National Laboratory is the integral fast reactor (IFR). If the IFR, which uses a new formulation of fissile fuel material, performs as claimed, it will breed its own supply of fresh fuel on site and burn up the ultra-long-lived radioactive transuranic fission products (the actinides: elements 89 through 103, including plutonium (94)). This would leave a waste product that could potentially decay in radioactivity to original ore levels in several hundred years. The IFR features a very high fuel burnup and, in two subscale experiments, has demonstrated the ability to survive a total loss of cooling.
The IFR is a sodium-cooled breeder reactor with fuel in the form of a metallic alloy. Conventional reactors employ a ceramic oxide fuel form; the new fuel alloy is a far better conductor of heat, which is one of the factors enhancing the safety of the IFR design. The metallic fuel is reprocessed electrochemically rather than by solvent extraction. Reprocessing may be carried out on site, avoiding long-distance transport.
The utilization of IFR facilities to reduce the radioactivity of used fuel from conventional reactors has been suggested, but this use is still in the early exploratory stage and controversial. Nevertheless, despite all its benefits, one may still expect the IFR concept to display vulnerabilities of its own. For example, disposal of the radioactive inventory in the event of an early reactor shutdown would be no less difficult than for a conventional reactor. Also, loss of sodium coolant, as in an earthquake, could produce a major disaster. Further, the cost for a full-scale reactor is yet to be evaluated and may be quite high.
The safety of the IFR concept was demonstrated in two very interesting experiments, both carried out in the EBR-II Idaho reactor on April 3, 1986. Although small, with just 20 MW of electric generating capacity, the power density of the EBR-II is nevertheless typical of fast reactors. In the first experiment, while the EBR-II was operating at full capacity, power was shut off to the primary coolant pumps. Because of the reactivity feedback characteristics of the IFR, the reactor shut itself down without safety system or operator action. There was no damage to any part of the system. Later that day, the reactor was brought back to full power and a loss-of-heat sink without scram test was carried out. Again, no damage was observed.
In summary, the IFR concept may offer energy and power from nuclear fission while avoiding almost all of the major hazards associated with conventional fission reactors. The reactor breeds its own fresh fuel on site, features high fuel burnup, and burns up the ultra-long-lived radioactive transuranic fission products, leaving a waste product with lower radioactivity. Shipment of large amounts of fissile fuel to and from a reprocessing plant is obviated, as is the need to ship and store, for a geologic time period, a highly pernicious radioactive waste. The on-site fuel is unattractive for diversion to weapons use, and safe recovery in the face of a complete cooling failure has been observed in pilot experiments.
In sharp contrast to the existence of many operating fission reactors, the feasibility of economic controlled nuclear fusion has yet to be demonstrated. Research on this intensely difficult problem has been under way since the very early 1950s, and several orders of magnitude of progress has been made in temperature and confinement time. Temperatures of 300,000,000°C have now been achieved in fusion experiments, six times hotter than that necessary for ignition under ideal circumstances. At lower temperatures, confinement of such very hot gases has been extended to periods as long as a second. Densities of these hot gases are also close to those needed for fusion reactor operation.
Fusion experiments may achieve ignition of a deuterium-tritium plasma by the end of this decade and thus provide a definitive demonstration of
fusion reactor feasibility. The first third of the twenty-first century may then see a prototype fusion power plant in operation, depending on the energy cost and environmental situation at that time. The cost of nuclear fusion is expected to be very high in comparison with alternative nuclear reactor designs. On the other hand, nuclear fusion offers additional environmental protection compared to nuclear fission. By midcentury, some fraction of energy to the national electrical grids might possibly come from fusion reactors.
The advantages of fusion power with respect to safety and the environment follow:
• No danger from nuclear reactor runaway. The amount of nuclear fuel in the reaction chamber at any given time is minuscule, and a system failure of any sort can lead only to a cooling down of the reacting plasma.
• Enormously reduced amounts of nuclear waste. The nuclear ash from fusion is helium, a stable and totally benign gas. Almost all of the neutrons coming out of the reacting gas will be absorbed in a lithium 6 blanket, generating fresh tritium to replace that used up in the deuterium tritium reactions. Also, although the vacuum-chamber wall is expected to become radioactive due to bombardment by these transiting neutrons, the material of the vacuum chamber can be chosen to reduce the radioactivity level and character of this radioactivity and problems associated with storage or disposal. Further research is needed in this area.
• No production of gases deleterious to the environment such as oxides of carbon and nitrogen; however, there is some concern on the potential leakage of tritium into the water supply.
• No inherent production of fissile materials.
The acid test of fusion power feasibilityachieving nuclear ignition in a confined plasmais anticipated no earlier than the end of this decade, and a prototype fusion power plant should not be expected before the year 2020. Further progress is clearly needed in the science, technological development, and economics of nuclear fusion before it can actually be implemented. Nevertheless, in view of its minimal impact on atmospheric pollution and greenhouse warming, and the very much reduced level of nuclear hazard, controlled fusion still merits its reputation as a major option for the future generation of electric power.
Golay, M. W. 1990. Testimony before the U.S. House Committee on Interior and Insular Affairs. Washington, D.C., March 10, 1990.