Nuclear reactors burn nuclear fuel to generate electricity. Most reactors burn uranium oxide in the form of ceramic pellets in long tubes. The tubes comprise the core of the reactor where the fission reaction takes place. The zirconium cladding of the fuel rod tends to migrate into the center of the pellets while the lower boiling point fission products move to the edge of the pellet. Small bubbles form in the pellet which fill with cesium-137 from decaying xenon.
Three percent of the mass of spent nuclear fuel consists of the fission products of U-235 and Pu-239. Fission products include every element in the periodic table from zinc up to the lanthanide series. Some of the fission products are either non-radioactive or have short half-lives. Other fission products are long-lived radioisotopes including strontium-90, cesium-137, technetium-99, and iodine-129.
One percent of the mass of spent nuclear fuel is Pu-239 and Pu-240 which results from conversion of U-238. Pu-239 is a concern because it might be used in the creation of nuclear weapons. If the reactor is run for a long time, the amount of Pu-240 is greater than twenty percent and the proportion of Pu-239 is much less useful for weapons development. If the fuel is only irradiated for a short time, the proportion of Pu-239 is suitable for weapons production.
Ninety six percent of the mass of spent nuclear fuel is uranium, with just under ninety five percent being the original U-338. About eight tenths of a percent is unburned U-235 and four tenths of a percent is U-236.
Minor actinides including neptunium americium and curium will be present in minute amounts in spent nuclear fuel. The exact proportions will depend on the exact type of nuclear fuel being burned in the reactor. MOX or Uranium/Thorium fuels will produce slightly different rations of the actinides.
When natural uranium nuclear fuel is burned in a reactor, it starts out with about seven tenths of a percent of U-235. When it is considered spent and removed from the reactor, it still have about one quarter of one percent of U-235 and one quarter of one percent of Pu-239. It is spent not because there is no fissile material left in the fuel but because the buildup of fission products has begun to absorb so many neutrons that the ability of the fuel to sustain a chain reaction has dropped to low to be useful.
When spent fuel is removed from the reactor, it continues to generate fading amounts of heat due to beta emission of the fission products. Spent nuclear fuel that has been removed from the reactor core is stored in spent fuel pools of water. Most such pools circulate water around the fuel and then through a heat exchanger in order to dissipate the heat. The water in the pool also provides protection against the radiation emitted by the spent fuel. Current estimates are that the exiting spent fuel pools will be full within five years at current use of nuclear fuel.
Spent nuclear fuel pool – picture from radioactivechat.blogspot.com:
