SNF results from the once-through fuel cycle (i. e., no further processing conducted). It contains greater than 99% of the radioactivity and has unique characteristics compared to wastes from fossil plants. Because only about 5% of the energy value has been consumed in the reactor, it can also rep­resent a future energy resource. The energy release from nuclear fission per ton of fuel is about a million times greater than the energy release from the burning of fossil fuels. The waste volume generated is about a million times less. The quantity of SNF is small per unit of energy produced. The small quantity (-20 tons per reactor per year) makes multiple waste management options economically feasible: multiple direct disposal options and multiple options to process the SNF chemically for recovery of selected materials for recycle and/or conversion into different waste forms.

Reactors discharge SNF that contains fissile materials (fuel) and fission products (waste). The radioactivity and decay heat of SNF decreases rapidly with time; thus, to reduce handling risks and costs, SNF is stored before transport, disposal, or recycling. SNF storage is a required step in all open and closed fuel cycles. This is a consequence of the nuclear characteristics of SNF. The radioactivity decreases rapidly with time, resulting in radioac­tive decay heat and gamma radiation decreasing rapidly with time. There are large safety and economic incentives to allow the radioactivity of SNF to decrease before transport, processing, or disposal.

Upon reactor shutdown, SNF is intensely radioactive and generates large quantities of decay heat — equal to about 6% of the power output of the reactor. However, the radioactive decay heat decreases very rapidly reach­ing 0.5% in one week. The refueling strategy in light water reactors (LWRs) is to transfer the SNF from the reactor core to the SNF storage pool where the water provides cooling and radiation shielding. After about ten years, the radioactivity will decrease by another factor of 100.

If SNF is to be disposed of in a repository, it will likely be stored for approximately 40-60 years prior to disposal. Peak temperatures in a geo­logical repository are limited to ensure long-term repository performance. If the temperatures are too high, the performance of the waste form, waste package, and geology may be impaired. Peak repository temperatures would be controlled by limiting the allowable decay heat per waste package. If the SNF is stored for several decades, several advantages would result: the decay heat per ton of SNF decreases; more SNF can be placed in each waste package; the waste packages can be spaced closer to each other underground; the size (footprint) of the repository is reduced; and the cost of the repository is reduced. Like SNF, the HLW will be cooled for 40-60 years before ultimate disposal to reduce the decay heat.