HLW solidification

Table 11.8 Softening and melting temperatures of HLW solidification products and associated materials...

By contrast, HLW from LWR fuel reprocessing is stored ia cooled, well-agitated, stainless steel tanks as an acidic nitrate solution having relatively few sohds. Modem PUREX flow sheets minimise the addition of extraneous salts, and as a result the HLW is essentially a fission-product nitrate solution. Dissolver soHds are centrifuged from the feed stream and are stored separately. Thus the HLW has a low risk of compromising tank integrity and has a favorable composition for solidification and disposal (11). 

The solidification of waste takes place in two steps, a calcining step and an incorporation of the calcined material into borosilicate glass. Calcining can be done in various ways but primarily involves the removal of volatile products like water or NO3" and conversion of all species in the HLW to solid stable oxides. The oxides are then mixed with SiC>2, B2O3, and the like to make a borosilicate glass, which is then prepared for geologic storage (Fig. 16.13). 

The planned composition for the acidic HLW solutions includes 7.0M nitric acid with only trace quantities of sodium and sulfate. The most critical specification for liquid storage will be volume however, sodium and sulfate content will be the most limiting specification for solidification. The new acidic HLW facility will be designed to meet all the natural phenomena and EPA criteria as well as to include the same degree of confinement conservatism as the existing HLW systems. 

The americium and curium isotopes formed during irradiation of nuclear reactor fuels are diverted into the high-level waste (HLW) stream during fuel reprocessing. The HLW is thus the biggest potential source for these elements, and R+D activities to develop a process for the recovery of Am and Cm from HLW were started in 1967. A major condition was that the process to be developed must not essentially increase the waste amount to be processed further, must not use strongly corrosive reagents, and must be compatible with the final waste solidification procedure. 

HLW (high level waste). The radioactive liquid containing most of the fission products and actinides present in spent fuel — which forms the residue from the first solvent extraction cycle in reprocessing — and some of the associated waste streams this material following solidification spent fuel (if it is declared a waste) or any other waste with similar radiological characteristics. Typical characteristics of HLW are thermal powers of about 2 kW/m and long lived radionuclide concentrations exceeding the limits for short lived waste [2]. 

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