Spent Nuclear Fuel

Spent nuclear fuel is literally just fuel that has been burnt or irradiated at a plant. According to Wikipedia, "a fresh rod of low enriched UraniumUraniumNatural Uranium VS \#stub 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."

Properties of Spent Fuel

During the operation of a nuclear core, temperature gradients cause fission products to move around the fuel rod. The zirconium moves toward the center where temperature is highest, and lower boiling products move to the edge. The pellet experiences pores or bubbles that form during the use - xenon moves to these voids. (some xenon decay to form caesium, specifically 135Cs)

MOX fuel (mixed oxide) has it where xenon diffuses out of plutonium rich areas, trapped in the uranium dioxide, whereas neodymium tends to not be mobile.

Fission Products

Spent fuel contains 3% by mass of fission products of 235U and 239Pu, considered radioactive waste and can be separated further for various industrial and medicinal uses. These products include every element from zinc to lanthanides - many products are either non-radioactive or short lived radioisotopes. Some countries have done research into segregating the rare isotopes into different elements such as Silver to offset the cost of reprocessing, but it is not being done commercially at this time.

Plutonium

1% by mass is 239&240 Pu, from the conversion of U-238. It is either classified as useful byproduct or dangerous waste.

Uranium

96% by mass is uranium, most u238 with some 235 (.8%) and little 236 (.4%). Reprocessed uranium will contain u236, not found in nature, and can be used as a fingerprint for spent nuclear fuel. If using Thorium fuel to product fissile u233, the SNF will have the same uranium. The presence of u233 will affect the long term radioactive decay, and compared with MOX fuel, the activity around one million years in throium will be higher due to the undecayed u233

In natural uranium, fuel is usually not discharged because the fissile material is fully used up, but because Neutron-Absorbing Fission Products have built up and the fuel is unable to successfully sustain a nuclear reaction. Some natural uranium fuels use chemically active claddingCladdingCladding is the thin walled metal tube that composes the outside of a fuel rod. It's purpose is to prevent corrosion of the fuel by the coolant & release of fission contents into the coolant. Although Zirconium alloy is common, aluminum and stainless steel is also used. Cladding Types Zirconium alloy has been used for so long due to it's properties being very good for nuclear reactors. * New research suggests that there is an alternative - SiGA cladding. This cladding is made from silicon car, such as Magnox, which need to be reprocessed as long-term storage and disposal is difficult.

Minor actinides

spent fuel contains traces of minor actinides, other then uranium and plutonium. the amount formed depends greatly upon nature of fuel and conditions

Storage

The main storage of spent fuel is in Spent Fuel PoolAir Traffic Control\#emptys, located at reactors directly in most cases. #WIP