Control of Radioactive Waste Glass Melters: II, Residence Time and Melt Rate Limitations

Abstract

Slurries of simulated high‐level radioactive waste (HLW) and glass formers have been isothermally reacted and analyzed to identify the sequence of the major chemical reactions in waste vitrification, their effect on glass production rate, and the development of leach resistance. Melting rates of waste batches have been increased by the addition of reducing agents (formic acid, sucrose) and nitrates. The rate increases are attributable in part to exothermic reactions which occur at critical stages in the vitrification process. Nitrates must be balanced by adequate reducing agents to avoid the formation of persistent foam, which would destabilize the melting process. The effect of foaming on waste glass production rates is analyzed, and melt rate limitations are defined for waste glass melters, based upon measurable thermophysical properties. Minimum melter residence times required to homogenize glass and assure glass quality are much smaller than those used in current practice. Thus, melter size can be reduced without adversely affecting glass quality. Physical chemistry and localized heat transfer of the waste glass melting process are examined, to refine the available models for predicting and assuring glass production rate. It is concluded that the size of replacement melters and future waste processing facilities can be significantly decreased if minimum heat transfer requirements for effective melting are met by mechanical agitation. A new class of waste glass melters has been designed, and proof of concept tests completed on simulated HLW slurry. Melt rates have exceeded 155 kg · m−2· h−1 with slurry feeds (32 lb · ft−2· h−1), and 229 kg · m−2· h−1 with dry feed (47 lb · ft−2· h−1). This is about 8 times the melt rate possible in conventional waste glass melters of the same size.