There are several sources of waste in the nuclear fuel cycle, from the initial mining to the treatment of spent fuel. For example, after naturally occurring uranium sources have been mined, crushed, and leached out of the ore, the remainder (the tailings) still contains radioactive uranium. There is also the spent fuel from the reactor: fuel rods are replaced on a regular basis, rotating out partially depleted fuel rods and replacing them with fresh fuel. Some of the U-235 undergoes neutron capture instead of fission and, ultimately, only about 75% of the U-235 is consumed in the fuel rod. Hence, the spent fuel contains uranium isotopes, Pu-239, other radioactive fission products, and minor actinides, as shown in Figure 9.5. The unfortunate fact that about 1% of the spent fuel consists of plutonium isotopes means that it can be a source of material for nuclear weapons. And although the minor actinides (neptunium-237, americium-241, and curium-244) make up only the tiniest amount of the waste, they are the most hazardous in terms of long-term radiotoxicity (see below) (Hudson et al. 2012).
Nuclear waste can be classified as low-level (LLW), intermediate-level (ILW), or high-level nuclear waste (HLW) depending upon the percent of radioactivity:
High-level waste is hot both thermally (in terms of radiated heat from ongoing nuclear decay processes) and with respect to radioactivity and is stored in cooling ponds for an indefinite period of time as it decays (see below). The typical output for a 1-GW nuclear power plant is 200-350 m3 of low – and intermediate-level waste per year and 10-20 m3 of high-level waste (for comparison, the capacities of nuclear generating plants in the United States range from about 0.5 to 1.3 GW (U. S. Energy Information Administration 2011)). High-level waste that is not reprocessed (Section 126.96.36.199) undergoes vitrification in which it is encased in borosilicate glass and then sealed in casks for storage in geologic structures for thousands of years (the half-life for Np-237 is 2.2 million years). These storage areas are deep rock structures that are back-filled with some sort of impermeable material like clay. However, since the radioactivity level of high-level waste decreases significantly in the first few decades, the spent fuel is often “temporarily” stored in cooling ponds for several years (International Energy Agency 2007).