Simulation of gamma radiation-induced water radiolysis in Cs and Sr-bearing waste forms

This report presents the simulation of water radiolysis and H2 gas generation in Cs and Sr-bearing waste forms under laboratory irradiation conditions. The H2 gas generation was estimated by CHEMSIMUL software package. Since Cs and Sr-bearing waste forms were exposed to gamma irradiation immediately after casting, a period in which they still contained a significant amount water, their irradiation is almost similar to that of water. In this view, the concentration of water (55.5 mole/L) is used in the model to evaluate the extent of water radiolysis. The simulations were performed at pH=13 using cumulative doses of 48 kGy at dose rates of 250, 303 and 6000 Gy/h and 4032 kGy at a dose rate of 6000 Gy/h. The effect of simulated decay temperature was also evaluated. Based on the simulations, it was observed that at equivalent dose, that the dose rate has no effect on radiolytic H2 generation, which is consistent with literature reports. In addition, only a small fraction of initial H2O underwent radiolysis, which suggests that there may be no effect of water content on H2 production as long as there is sufficient water in the system. The results also show that the elevated temperature during irradiation does not affect the extent of H2 generation. It is therefore suggested that the decay heat may have no significant influence on water radiolysis in Cs and Sr-bearing waste forms. The simulations only considered gamma radiations. Since in real scenarios of Cs and Sr immobilization, the waste form will be exposed to both beta and gamma radiation, numerical modelling tools that account for both beta and gamma radiation could be useful in assessing the real extent of water radiolysis in Cs and Sr-bearing waste forms.