Burnable poisons: Materials that absorb excess neutrons so that criticality of the reactor is more easily maintained. Over time, these poisons are “burned” (i.e., after neutron capture, they are turned into other isotopes that absorb fewer neutrons). Ideally, as the fuel approaches its end of life, the burnable poison is fully depleted.
Conversion: The process of changing the fuel used to power a nuclear reactor. In the context of this report, the new fuel is expected to be less enriched in uranium-235 (235U) than the current fuel. The fuel is preferably enriched to less than 20 percent (low enriched uranium [LEU]). This involves establishing that a reactor can operate safely on the new fuel and may also involve modifications to the reactor design and operating parameters in order to accommodate the new fuel.
Critical mass: The smallest mass of a fissionable material (e.g., 235U) that will sustain a nuclear chain reaction at a constant level. http://www.thefreedictionary.com/Critical+mass+%28nuclear%29.
Dispersion fuel: Fuel for research reactors whose uranium is contained in particles dispersed in a metallic matrix, which is then enclosed within metal cladding to contain the radioactive decay products and prevent chemical reactions. Research reactor fuels used today are all of this type.
Downblending: In the context of this report, a process involving diluting HEU with LEU or other materials to give a material that is less usable (or unusable) in nuclear weapons but can still be used in research reactors.
End-of-life analysis: In the context of this report, analysis of when a research reactor will no longer be technically or economically feasible to operate. Factors influencing when the end of life occurs include the integrity of various reactor components that cannot be replaced, such as the pressure vessel, as well as the ability of the reactor to continue to be certified as safe to operate and to be licensed by the relevant regulatory bodies.
Fissile material: An atom whose nucleus is capable of undergoing fission, or splitting into two smaller nuclei, after capturing low-energy neutrons. http://www.nrc.gov/reading-rm/basic-ref/glossary/fissile-material.html.
Fuel element: A rod, tube, plate, or other geometrical form into which nuclear fuel is fabricated for use in a reactor.
NOTE: As a reactor is converted from HEU to LEU, it is important to keep the overall size and geometry of the fuel elements constant. http://encyclopedia2.thefreedictionary.com/Fuel+element.
For U.S. reactors, the term fuel element refers to a collection of fuel plates that are grouped together to form a single unit. For Russian reactors, the term fuel element refers to the fuel plates or tubes. To avoid confusion, the text uses the terms fuel assembly to describe a collection of fuel plates.
High-density LEU fuel: Reactor fuel that contains LEU that has sufficient uranium density to enable operation of high performance research reactors without significant degradation in performance. In practice, that means fuel with a uranium density exceeding about 8 gU/cm3.
Highly enriched uranium (HEU): Uranium enriched to 20 percent or above in 235U. http://www.nrc.gov/reading-rm/basic-ref/glossary/highly-orhigh-enriched-uranium.html.
High performance research reactor (HPRR): Research reactors that are characterized by high neutron flux, operating power above 10 MW, and compact cores.
Hot isostatic pressing: A manufacturing process used to reduce the porosity of metals and increase the density of many ceramic materials. The process subjects a component to both elevated temperature and isostatic gas pressure in a high-pressure containment vessel. An inert gas is used to
avoid chemical reactions. Pressure is applied to the material from all directions (hence, the term isostatic). http://dictionary.sensagent.com/hot%20isostatic%20pressing/en-en/.
Low enriched uranium (LEU): Uranium enriched to less than 20 percent 235U.
Monolithic fuel: Fuel for research reactors whose uranium is contained in a dense, uniform alloy enclosed within metal cladding to contain the radioactive decay products and prevent chemical reactions. It is possible to get much higher uranium density in the fuel using monolithic fuel rather than the more common dispersion fuel.
Neutron flux: The number of neutrons per area delivered in a given time. In this report, neutron flux values are given in neutrons per square centimeter per second.
Post-irradiation examination: The study of irradiated materials such as nuclear fuel to ascertain the effect of the radiation on the material structure and integrity. Such examination is important in the qualification of new types of reactor fuel in that it helps establish the limits of the fuel and can lead to understanding of failure modes. Such evaluation must be carried out in a hot cell to contain the radioactivity of the samples.
Reactor cycle length: The length of time a reactor can operate without refueling. In some reactors, fuel elements are moved from one position in the reactor to another between cycles, while other fuel elements are replaced with fresh ones. In other reactors, all fuel elements are replaced at the end of each cycle.
- Subcritical assembly: A subcritical mass of fissile material that does not have the ability to sustain a fission chain reaction. A population of neutrons introduced to a subcritical assembly will exponentially decrease.
- Critical assembly: An assembly of fissile material brought together in such a way that each fission event causes, on average, exactly one additional such event in a continual chain.
- Steady state: A reactor that can operate stably at a given power level for a long period of time by maintaining a balance between the quantity of neutrons available to induce fission events (through the presence of neutron absorbers) and the amount of fissile material.
Research reactor—from 10 CFR § 170.3: Research reactor means a nuclear reactor licensed by the Nuclear Regulatory Commission under the authority of subsection 104c of the [Atomic Energy] Act and pursuant to the provisions of § 50.21(c) of this chapter for operation at a thermal power level of 10 MW or less, and which is not a testing facility as defined in this section.
Separative work unit (SWU): The standard measure of the effort required to separate isotopes of uranium (235U and 238U) during an enrichment process in nuclear facilities; 1 SWU is equivalent to 1 kg of separative work.
Spallation: High-energy nuclear reaction in which a target nucleus struck by an incident particle of energy greater than about 50 million electron volts ejects numerous lighter particles such as neutrons.
Supercritical: Pertaining to a mass of radioactive material in which the rate of a chain reaction increases with time. http://dictionary.reference.com/browse/supercritical.
Test reactor—from 10 CFR 50.2: Testing facility means a nuclear reactor that is of a type described in § 50.21(c) of this part and for which an application has been filed for a license authorizing operation at:
- A thermal power level in excess of 10 megawatts; or
- A thermal power level in excess of 1 megawatt, if the reactor is to contain:
- A circulating loop through the core in which the applicant proposes to conduct fuel experiments; or
- A liquid fuel loading; or
- An experimental facility in the core in excess of 16 square inches in cross-section.
Transmutation: A phenomenon that occurs when a neutron bombarding an atomic nucleus is absorbed, changing it into a different isotope of the same element or causing the nucleus to split into multiple (usually two) different elements.