Storage and release of radiation energy in salt in radioactive waste repositories

The results showed that appreciable amounts of gamma-radiation energy can be stored under certain exposure conditions. The results also showed that thermally activated annealing takes place at elevated temperatures and that the rates of this annealing at temperatures above about 1500C are such that negligible amounts of energy will be stored in salt in a repository where the salt is at temperatures above about 1500C. We are not able to show that thermally activated annealing takes place in rock salt at temperatures below about 1500C, although it may do so. There may also be some radiation-induced annealing. The results of measurements of energy stored at irradiation temperatures between 30 and 1500C, together with results of theoretical considerations, showed that the maximum stored energy that would be formed in salt in a repository with no annealing whatsoever would be about 50 cal/g. We did not conceive a means by which the stored energy could be released abruptly, nor was any significant hazard believed to be possible from the release if it should occur abruptly by some unforeseen mechanism. In practice, salt temperatures below about 1500C will occur only with isolated or semi-isolated canisters in a repository, or with most canisters, after very long times during which the gamma-dose rate will have dropped off markedly. Available evidence from thermal annealing observations and from calorimetric and dissolution measurements indicated that there were no significant losses of either chlorine or sodium during or after irradiation. One effect of the stored energy which must be accounted for in a safety analysis is the generation of H2, which takes place upon aqueous dissolution of radiation-damaged salt; about 0.1 cm3 of H2 is generated per calorie of stored energy. However, we have not recognized any problems arising from this effect which cannot be counteracted by appropriate design and operation of the repository