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Spent fuel storage
Spent fuel assemblies taken from the reactor core are highly radioactive and give off a lot of heat. They are therefore stored in special ponds which are usually located at the reactor site, to allow both their heat and radioactivity to decrease. The water in the ponds serves the dual purpose of acting as a barrier against radiation and dispersing the heat from the spent fuel.
Spent fuel can be stored safely in these ponds for long periods. It can be dry stored in engineered facilities. However, both kinds of storage are intended only as an interim step before the spent fuel is either reprocessed or sent to final disposal. The longer it is stored, the easier it is to handle, due to decay of radioactivity.
There are two alternatives for spent fuel:
Reprocessing to recover the usable portion of it
Long-term storage and final disposal without reprocessing.
LESSON # 3
URANIUM ENRICHMENT
The uranium enriched in uranium-235 is required in commercial light water reactors to produce a controlled nuclear reaction. Several different processes are used to enrich uranium.
Enriching Uranium
Gaseous Diffusion
Gas Centrifuge
Enriching uranium increases the amount of “middle-weight” and “light-weight” uranium atoms. Not all uranium atoms are the same. When uranium is mined, it consists of heavy-weight atoms (about 99.3% of the mass), middle-weight atoms (0.7%), and light-weight atoms (< 0.01%). These are the different isotopes of uranium, which means that while they all contain 92 protons in the atom’s center (which is what makes it uranium). The heavy-weight atoms contain 146 neutrons, the middle-weight contain 143 neutrons, and the light-weight have just 142 neutrons. To refer to these isotopes, scientists add the number of protons and neutrons and put the total after the name: ura- nium-234 or U-234, uranium-235 or U-235, and uranium-238 or U-238.
The fuel for nuclear reactors has to have a higher concentration of U-235 than exists in natural uranium ore. This is because U-235 is the key ingredient that starts a nuclear reaction and keeps it going. Normally, the amount of the U-235 isotope is enriched from
7% of the uranium mass to about 5%. Gaseous diffusion is the only process being used in the United States to commercially enrich uranium. Gas centrifuges can also be used to enrich uranium. Although this enrichment process is not used in the United States, the NRC is conducting licensing activities concerning two planned centrifuge facilities.
Gaseous Diffusion
Process: In the gaseous diffusion enrichment plant, the solid uranium hexafluoride (UF6) from the conversion process is heated in its container until it becomes a liquid. The container becomes pressurized as the solid melts and UF6 gas fills the top of the container. The UF6 gas is slowly fed into the plant’s pipelines where it is pumped through special filters called barriers or porous membranes. The holes in the barriers are so small that there is barely enough room for the UF6 gas molecules to pass through. The isotope enrichment occurs when the lighter UF6 gas molecules (with the U-234 and U-235 atoms) tend to diffuse faster through the barriers than the heavier UF6 gas molecules containing U-238. One barrier isn’t enough, though. It takes many hundreds
of barriers, one after the other, before the UF6 gas contains enough uranium-235 to be used in reactors. At the end of the process, the enriched UF6 gas is withdrawn from the pipelines and condensed back into a liquid that is poured into containers. The UF6 is then allowed to cool and solidify before it is transported to fuel fabrication facilities where it is turned into fuel assemblies for nuclear power reactors.
Hazards: The primary hazards in gaseous diffusion plants include the chemical and radiological hazard of a UF6 release and the potential for mishandling the enriched uranium, which could create a criticality accident (inadvertent nuclear chain reaction).
Gas Centrifuge
The gas centrifuge uranium enrichment process uses a large number of rotating cylinders in series and parallel formations. Centrifuge machines are interconnected to form trains and cascades. In this process, UF6 gas is placed in a cylinder and rotated at a high speed. This rotation creates a strong centrifugal force so that the heavier gas molecules (containing U-238) move toward the outside of the cylinder and the lighter gas molecules (containing U-235) collect closer to the center. The stream that is slightly enriched in U-235 is withdrawn and fed into the next higher stage, while the slightly depleted stream is recycled back into the next lower stage. Significantly more U-235 enrichment can be obtained from a single unit gas centrifuge than from a single unit
gaseous diffusion stage. No gas centrifuge commercial production plants are operating in the United States.
Fuel fabrication
Fuel fabrication facilities convert enriched UF6 into fuel for nuclear reactors. Fabrication also can involve mixed oxide (MOX) fuel, which is a combination of uranium and plutonium components. NRC regulates several different types of nuclear fuel fabrication operations.
Types of nuclear fuel fabrications are:
Light Water Reactor Low-Enriched Uranium Fuel
Reactor Mixed Oxide Fuel
Non-Power Reactor Fuel
Other Types of Fuel Fabrication
Light Water Reactor Low-Enriched Uranium Fuel
Fuel fabrication for light (regular) water power reactors (LWR) typically begins with receipt of low-enriched uranium (LEU) hexafluoride (UF6) from an enrichment plant. The UF6, in solid form in containers, is heated to gaseous form, and the UF6 gas is chemically processed to form LEU uranium dioxide (U02) powder. This powder is then pressed into pellets, sintered into ceramic form, loaded into Zircaloy tubes, and constructed into fuel assemblies. Depending on the type of light water reactor, a fuel assembly may contain up to 264 fuel rods and have dimensions of 5 to 9 inches square by about 12 feet long.
Light Water Reactor Mixed Oxide Fuel
MOX fuel differs from LEU fuel in that the dioxide powder from which the fuel pellets are pressed is a combination of U02 and plutonium oxide (Pu02). The NRC was directed by Congress to regulate the Department of Energy’s (DOE’s) fabrication of MOX fuel used for disposal of plutonium from international nuclear disarmament agreements.
Nonpower Reactor Fuel
Nonpower reactors are much smaller reactors that do not generate electrical power but are used for research, testing, and training. Nonpower reactors can include research reactors and reactors used to produce irradiated target materials. The fuel design varies with the reactor type and manufacturer. Plate-type fuel consists of several thin plates containing a uranium mixture clad with aluminum. Another fuel is in the shape of rods and consists of a uranium and zirconium/hydride mixture. There are also compact, self- contained, low-power (less than 5 watts) tank-type reactors. Although use of highly enriched uranium (HEU) fuel can reduce the size of a nonpower reactor, the NRC adopted a policy of discouraging use of HEU fuel.
Other Types of Fuel Fabrication Facilities
NRC also regulates some fuel fabrication facilities that have DOE contracts to down-blend HEU with other uranium to create LEU reactor fuel. The HEU being blended down to lower enrichment comes from Russian or U.S. weapons programs as part of an international arms control agreement.
Safety Concerns at Fabrication Plants
Chemical, radiological, and criticality hazards at fuel fabrication facilities are similar to hazards at enrichment plants. Most at risk from these hazards are the plant workers. These facilities generally pose a low risk to the public.
WORDLIST
Barrier - диафрагма
porous membrane - пористая мембрана
mishandling - несоблюдение правил эксплуатации
trains - цепочка,
cascade - каскад,ступень
low-enriched uranium (LEU) - низкообогащенный уран
mixed oxide (МОХ) fuel - смешанное оксидное яд. топливо
nonpower reactor - неэнергетический реактор
plate-type fuel - пластинчатое яд. топливо
tank-type reactor - корпусной ядерный реактор
hazards - опасные факторы
irradiated target material - облученный материал мишени
depleted uranium - обеднённый уран
EXERCISES: