As LWR and PHWR (i. e. CANDU and its derivatives) reactors represent, by far, the majority in commercial use, spent fuel characteristics will be mainly discussed with respect to a UO2 matrix fuel with zircalloy cladding. Gas reactor fuels tend to be different: an AGR uses oxide fuel with stainless steel cladding and the few remaining Magnox reactors have metallic uranium in a magnesium alloy can. There is also some historic and AGR fuels with stainless steel cladding. There is very little information available about the management of fast reactor spent fuel.

In this chapter some key characteristics of spent fuel will be described. As these depend on the burn up of fuel, they will be discussed, wherever possible, in relation to burnup. During the fission process in nuclear reactors, the fuel undergoes a number of changes, such as: depletion of U-235, transformation of U-238 to Pu-239, build-up of fission products and decay products (i. e. helium gas), generation of neutron activation products, etc. The most important consequence, of course, is that the fuel becomes intensely radioactive. The main sources of this radioactivity are the fission products and the actinides, both minor (Np, Am, Cm, etc.), and major (U and Pu). It is important to remember that this changes with burnup. The contribution of the cladding to the overall radioactivity is usually very small.

The detailed composition of spent oxide fuel depends mostly on the fuel burnup at discharge (here simply referred to as ‘burnup’). For an LWR fuel assembly, this is expressed as the average power (GW) generated by the assembly, multiplied by the number of days at power (GWd) and divided by the amount of heavy metal (usually uranium) that the fuel assembly contains. While burnup varies from one reactor type to another, there has been, since the earliest days of nuclear energy, a consistent tendency for it to increase. Figure 15.3 shows how burnup increased in the period up to 2005. Such increases require higher enrichment fuel although fuel endurance, including its ability to survive unplanned events, will also impose limits.

For PWRs and BWRs the overall average burnup is approaching 50 GWd/tU with some power plants reaching 59 GWd/tU.2 This burnup tendency also

applies to Russian design reactors VWER-440 and VWER-100, which reach 48-50 and 45-55 GWd/tU respectively. The Russian fuel for the channel type rector (RBMK) achieves burnups of 30-35 GWd/tU. Typically, higher burnup is achieved with higher initial enrichments of nuclear fuel, which is nowadays approaching 5% U-235. For heavy water reactors that use natural uranium, burnups are lower, Around 7.5 GWd/tU, with the latest tendencies to use slightly enriched uranium fuel (SEU) with consequently higher burnups reaching about 9.5 GWd/tU.

It has to be pointed out that many characteristics of spent mixed oxide fuels are in general similar to uranium oxide fuel but more pronounced. The isotopic characteristics of the MOX fuel are initially different.