Spent nuclear fuel
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor. It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and, depending on its point along the nuclear fuel cycle, it will have different isotopic constituents than when it started.
Nuclear fuel rods become progressively more radioactive due to neutron activation as they are fissioned, or "burnt", in the reactor. A fresh rod of low-enriched uranium pellets will become a highly lethal gamma emitter after 1–2 years of core irradiation, unsafe to approach unless under many feet of water shielding. This makes their invariable accumulation and safe temporary storage in spent fuel pools a prime source of high-level radioactive waste and a major ongoing issue for future permanent disposal.
Nature of spent fuel
Nanomaterial properties
In the oxide fuel, intense temperature gradients exist that cause fission products to migrate. The zirconium tends to move to the centre of the fuel pellet where the temperature is highest, while the lower-boiling fission products move to the edge of the pellet. The pellet is likely to contain many small bubble-like pores that form during use; the fission product xenon migrates to these voids. Some of this xenon will then decay to form caesium, hence many of these bubbles contain a large concentration of.In the case of mixed oxide fuel, the xenon tends to diffuse out of the plutonium-rich areas of the fuel, and it is then trapped in the surrounding uranium dioxide. The neodymium tends to not be mobile.
Also metallic particles of an alloy of Mo-Tc-Ru-Pd tend to form in the fuel. Other solids form at the boundary between the uranium dioxide grains, but the majority of the fission products remain in the uranium dioxide as solid solutions. A paper describing a method of making a non-radioactive "uranium active" simulation of spent oxide fuel exists.
Fission products
Spent nuclear fuel contains 3% by mass of fission products of 235U and 239Pu ; these are considered radioactive waste or may be separated further for various industrial and medical uses. The fission products include every element from zinc through to the lanthanides; much of the fission yield is concentrated in two peaks, one in the second transition row and the other later in the periodic table. Many of the fission products are either non-radioactive or only short-lived radioisotopes, but a considerable number are medium to long-lived radioisotopes such as 90Sr, 137Cs, 99Tc and 129I. Research has been conducted by several different countries into segregating the rare isotopes in fission waste including the "fission platinoids" and silver as a way of offsetting the cost of reprocessing; this is not currently being done commercially.The fission products can modify the thermal properties of the uranium dioxide; the lanthanide oxides tend to lower the thermal conductivity of the fuel, while the metallic nanoparticles slightly increase the thermal conductivity of the fuel.
Table of chemical data
| Element | Gas | Metal | Oxide | Solid solution |
| Br Kr | Yes | - | - | - |
| Rb | Yes | - | Yes | - |
| Sr | - | - | Yes | Yes |
| Y | - | - | - | Yes |
| Zr | - | - | Yes | Yes |
| Nb | - | - | Yes | - |
| Mo | - | Yes | Yes | - |
| Tc Ru Rh Pd Ag Cd In Sb | - | Yes | - | - |
| Te | Yes | Yes | Yes | Yes |
| I Xe | Yes | - | - | - |
| Cs | Yes | - | Yes | - |
| Ba | - | - | Yes | Yes |
| La Ce Pr Nd Pm Sm Eu | - | - | - | Yes |
Plutonium
About 1% of the mass is 239Pu and 240Pu resulting from conversion of 238U, which may be considered either as a useful byproduct, or as dangerous and inconvenient waste. One of the main concerns regarding nuclear proliferation is to prevent this plutonium from being used by states, other than those already established as nuclear weapons states, to produce nuclear weapons. If the reactor has been used normally, the plutonium is reactor-grade, not weapons-grade: it contains more than 19% 240Pu and less than 80% 239Pu, which makes it not ideal for making bombs. If the irradiation period has been short then the plutonium is weapons-grade.Uranium
96% of the mass is the remaining uranium: most of the original 238U and a little 235U. Usually 235U would be less than 0.8% of the mass along with 0.4% 236U.Reprocessed uranium will contain 236U, which is not found in nature; this is one isotope that can be used as a fingerprint for spent reactor fuel.
If using a thorium fuel to produce fissile 233U, the SNF will have 233U, with a half-life of 159,200 years. The presence of 233U will affect the long-term radioactive decay of the spent fuel. If compared with MOX fuel, the activity around one million years in the cycles with thorium will be higher due to the presence of the not fully decayed 233U.
For natural uranium fuel, fissile component starts at 0.7% 235U concentration in natural uranium. At discharge, total fissile component is still 0.5%. Fuel is discharged not because fissile material is fully used-up, but because the neutron-absorbing fission products have built up and the fuel becomes significantly less able to sustain a nuclear reaction.
Some natural uranium fuels use chemically active cladding, such as Magnox, and need to be reprocessed because long-term storage and disposal is difficult.
Minor actinides
Spent reactor fuel contains traces of the minor actinides. These are actinides other than uranium and plutonium and include neptunium, americium and curium. The amount formed depends greatly upon the nature of the fuel used and the conditions under which it was used. For instance, the use of MOX fuel is likely to lead to the production of more 241Am and heavier nuclides than a uranium/thorium based fuel.For highly enriched fuels used in marine reactors and research reactors, the isotope inventory will vary based on in-core fuel management and reactor operating conditions.
Spent fuel decay heat
When a nuclear reactor has been shut down and the nuclear fission chain reaction has ceased, a significant amount of heat will still be produced in the fuel due to the beta decay of fission products. For this reason, at the moment of reactor shutdown, decay heat will be about 7% of the previous core power if the reactor has had a long and steady power history. About 1 hour after shutdown, the decay heat will be about 1.5% of the previous core power. After a day, the decay heat falls to 0.4%, and after a week it will be 0.2%. The decay heat production rate will continue to slowly decrease over time.Spent fuel that has been removed from a reactor is ordinarily stored in a water-filled spent fuel pool for a year or more in order to cool it and provide shielding from its radioactivity. Practical spent fuel pool designs generally do not rely on passive cooling but rather require that the water be actively pumped through heat exchangers. If there is a prolonged interruption of active cooling due to emergency situations, the water in the spent fuel pools may therefore boil off, possibly resulting in radioactive elements being released into the atmosphere.
Fuel composition and long term radioactivity
The use of different fuels in nuclear reactors results in different SNF composition, with varying activity curves.Long-lived radioactive waste from the back end of the fuel cycle is especially relevant when designing a complete waste management plan for SNF. When looking at long-term radioactive decay, the actinides in the SNF have a significant influence due to their characteristically long half-lives. Depending on what a nuclear reactor is fueled with, the actinide composition in the SNF will be different.
An example of this effect is the use of nuclear fuels with thorium. Th-232 is a fertile material that can undergo a neutron capture reaction and two beta minus decays, resulting in the production of fissile U-233. Its radioactive decay will strongly influence the long-term activity curve of the SNF around a million years. A comparison of the activity associated to U-233 for three different SNF types can be seen in the figure on the top right. The burnt fuels are Thorium with Reactor-Grade Plutonium, Thorium with Weapons-Grade Plutonium and Mixed Oxide fuel. For RGPu and WGPu, the initial amount of U-233 and its decay around a million years can be seen. This has an effect in the total activity curve of the three fuel types. The initial absence of U-233 and its daughter products in the MOX fuel results in a lower activity in region 3 of the figure on the bottom right, whereas for RGPu and WGPu the curve is maintained higher due to the presence of U-233 that has not fully decayed. Nuclear reprocessing can remove the actinides from the spent fuel so they can be used or destroyed.