The Effect of Glass Chemistry on Caesium Volatility

The UK currently has 99,000 m3 of intermediate level waste (ILW). The current preferred treatment route is cementation. However, thermal treatment could offer a better long term option due to higher waste loading and lower storage costs. This project will focus on volatile retention in the vitrification process to make thermal treatment a more viable option. A series of iron phosphate base glasses were prepared with the nominal molar composition xM_yO_z(40 - x)Fe₂O₃-P₂O₅ (M_yO_z = B₂O₃, MnO, ZnO with x = 5%). Samples are given names of the form IPG-1-X5 where X = B, Mn or Zn. For all batches, reagents were weighed and mechanically mixed to produce 150 g of glass. melted at 1150 deg. C for 3 h, Samples were immediately transferred to an annealing furnace and annealed at 450 deg. C for 1 h before being cooled to room temperature at 1 deg. C min^-1. 75 g batches was produced with a target of 2 wt.% caesium oxide. Each batch was placed into a mullite crucible and placed in the sealed system seen below. Each canister was placed into a top-loading resistance furnace and ramped to 950 deg. C at a rate of 3 deg. C min^-1 under a nitrogen atmosphere with a flowrate of ∼0.9 L min^-1. The samples were held at this temperature for 3 h before power to the furnace was removed and the system allowed to cool naturally. Once cool, the steel canister was opened and the entire apparatus (steel canister, mullite crucible and tubing) was washed through with ∼50 ml of 0.5 M HNO₃ to allow for a complete mass balance of the caesium species. A series of Iron Phosphate glasses have been produced and characterised to investigate the influence of glass chemistry on the volatility of caesium. All glasses were doped with ∼2 wt.% Cs₂O and re-melted at 950 deg. C for 3 hours in a specially designed furnace set-up. Results of these experiments showed that the addition of 5 mol. % ZnO greatly increased the retention of Cs₂O whilst B₂O₃ was detrimental to the Cs₂O retention. A series of iron phosphate based glasses have been produced with 5 mol.% additions of B₂O₃, MnO and ZnO, substituted in place of Fe₂O₃. All samples were analysed before and after being doped and re-melted with a 2 wt.% addition of Cs₂O. The effect of each additive on the base glass has been reported showing that both B₂O₃ and ZnO increase the glass transition temperature, whilst MnO additions decrease the thermal stability of the glass. The Q-speciation of the glasses was investigated, and the results showed that the additives did not significantly influence the Q-speciation in comparison to the base glass. It has been shown that the addition of ZnO dramatically increases the retention of Cs during re-melting by almost a factor of 10 in comparison to the base glass, whilst the addition of MnO addition increased the caesium retention but only by a factor of ∼3. The addition of B₂O₃ had a detrimental effect on the volatility in comparison to the 40Fe₂O₃-60P₂O₅ base glass