The waste produced in nuclear reactors − called spent nuclear fuel − is orders of magnitude more radioactive than fresh uranium fuel. This is because irradiation of uranium produces many types of radioactive particles.
It takes about 200,000 years for the radioactivity of spent fuel to decline to that of the original uranium ore body because of the time required for decay of transuranics and long lived fission products in the fuel. Transuranics or actinides are elements such as plutonium, which are heavier than uranium. Fission products are uranium break-up products; they are elements lighter than uranium.
For the high level waste stream from reprocessing (from which plutonium and uranium have been removed), it takes about 10,000 years for the radioactivity to decline to that of the uranium ore body. (Switkowski Report, 2006.)
A typical power reactor produces about 30 tonnes (10 m3) of spent nuclear fuel annually, as well as 300 m3 of low and intermediate level waste. (Switkowski Report, 2006.)
Annually, nuclear power plants around the world produce 12,000 to 14,000 tonnes of spent fuel, and about 200,000 m3 of low and intermediate level waste.
About 340,000 tonnes of spent fuel have been produced in power reactors around the world. About one third of that amount has been reprocessed and the remainder is stored.
These are small amounts of waste compared to the mass or volume of wastes generated by coal-fired electricity plants. However, there are very large waste streams generated across the nuclear fuel cycle, not least many millions of tonnes of uranium mine tailings wastes. Moreover it is not the volume or mass of spent fuel that is of concern but its extreme toxicity, longevity, heat generation, and the fact that it contains plutonium which can be extracted for use in nuclear weapons.
Storage of spent fuel in reactor cooling ponds for several years after its removal from the reactor is necessary to allow residual heat to decline to levels that facilitate handling. This is usually followed by longer term storage away from the reactor, pending reprocessing or eventual disposal.