In the UK, HLWs are defined as materials that are heat generating due to their radioactivity, and this heat generation needs to be considered during the design of storage and disposal facilities.2 HLW in the UK is dominated by materials derived from spent nuclear fuel reprocessing (see Figure 1).2 Prior to reproces­sing, spent fuel is stored for several years in a holding pond to allow short lived radionuclides to undergo radioactive decay and to dissipate the resultant heat. At the Sellafield site, spent fuel from a range of reactor types is either taken out of its cladding or chopped up, dissolved in nitric acid and is chemically processed in the PUREX process, to separate uranium, plutonium and waste fission products. During reprocessing, the fission products are ultimately separated into a solution phase which is known as highly active raffinate (HAR). This fission product laden nitric acid solution contains the bulk of the radioactivity that was associated with the spent fuel. It will continue to generate heat for decades and is dominated by radioisotopes such as 90Sr and 137Cs. The HAR is converted to a

more stable wasteform by glassifying or vitrifying it prior to storage and ulti­mately, disposal.3 As extracted uranium and plutonium may have economic value, they are not currently classified as waste and are instead stored.