Where uranium and plutonium are removed for recycling, immobilisation of fission product raffinate, the liquid HLW that results from reprocessing, proceeds by calcining — boiling to dryness and then heating in an oxidising environment. The fission product oxides are then dissolved in molten borosilicate glass to produce vitrified HLW. This is poured into stainless steel canisters and then stored for decades to allow the rate of heat production to subside before deep disposal. The absence of plutonium eliminates an important source of heat but, nevertheless vitrified waste has a higher heat output per cubic metre than spent fuel because one vitrified waste canister contains the fission products from several fuel assemblies. Over the course of a few hundred years, however, the virtual absence of plutonumn in vitrified HLW ensures that its heat output eventually falls below that of the same volume of spent fuel.11

Spent nuclear fuel (SNF)

SNF continues to generate heat long after it has been removed from the reactor and, unless destined for reprocessing, freshly discharged fuel is held in water- filled pools at the reactor site for 5-10 years (Chapter 15). Subsequently, the fuel may be sent for longer-term wet or dry storage before disposal. Because of the impact of heat generation on disposal (not least on its cost), the aim will be to allow the heat generation rate to reach an acceptably low value; usually this will be less than 1 kW/tHM. In broad terms the amount of heat produced by uranium SNF is proportional to its burnup. As discharge burnup has been steadily increased over the past 40 years, so too has the cooling time needed to allow decay heat to reach levels deemed low enough for disposal. MOX fuel, which because of the greater quantities of plutonium, produces even higher levels of decay heat, requires even longer cooling times.12 In some cases these times may be so long that reprocessing is deemed a necessity.