Immobilisation in Ceramic
Although borosilicate glass remains the waste form of choice for many countries with nuclear waste, there has been a substantial effort to develop alternative ceramic nuclear waste forms. The alternative waste forms may be single or polyphase ceramics with several barriers against the release of radionuclides. Principal ceramic nuclear waste forms include: Synroc; tailored ceramics; TiO2-matrix ceramics; glass ceramics; monazite; and FUETAP concrete. In addition, there are a number of ‘novel’ ceramic waste forms which have been developed to only the most preliminary stages, and there are several multi-barrier strategies which encapsulate one ceramic waste form in another.
Immobilisation with a High Level Waste (HLW) Barrier
The presently stored HLW at Trombay is characterized by significant concentrations of uranium, sodium and sulphate in addition to fission products, corrosion products and small amount of other actinides. Sulphate in the waste is derived from ferrous sulphamate used as a reducing agent for conversion of Pu+4 to Pu+3 during partitioning stage of reprocessing and is one of the troublesome constituents with respect to vitrification.
Sulphate in the form of sodium sulphate can be accommodated in borosilicate matrix only to a limited extent i.e. 1 wt% maximum. At higher sulphate concentrations, a separate phase of alkali sulphate known as ìgallî is formed. Its presence in the glass is not desirable as this phase is enriched with Cs and has high solubility in water. It was also experienced that the presence of this soluble separated yellow phase adversely affects homogeneous distribution of radionuclides and also causes problems during pouring of vitreous products into storage canisters.
Barium-based borosilicate glass matrix (SB-44,) has been developed to accommodate sulphate homogeneously in the glass matrix. Lead Borosilicate showed phase separation on plant scale and was replaced by barium borosilicate.Barium borosilicate glass matrix is able to contain sulphate upto 3.5% wt without impairing the properties of the conditioned product. Vitrified Waste Product (VWP) made from simulated waste based on this matrix was evaluated for chemical durability, homogeneity, phase separation, thermal conductivity, viscosity, glass transition temperature, thermal stability etc. Experimental investigations indicated that conditioned product is homogeneous and has adequate leach resistance, meeting product acceptance criterion as per international norms.
Based on the satisfactory performance, the newly developed matrix has been adopted in the plant for vitrification of HLW, after passing through critical review by safety / regulatory processes applicable to BARC facilities. More than 100 canisters (9090 Kg) have been made so far immobilizing about 1,45,000 Curies of radioactivity safely and successfully.