Thermoluminescence response of calcic bentonite subjected to conditions of high nuclear waste underground storage.

ABSTRACT

Bentonite is regarded as a backfilling material for underground storage facilities of highly radioactive nuclear waste built on granite formations. In these facilities, bentonite will be subjected to a gradient of temperature and dose rate, achieving a very high integrated dose and, therefore, changes in its structure and physical properties may take place. Two experiments to discriminate between the thermal and the irradiation effect were performed. In the first (named BIC 2A), samples were subjected to temperature while in the second (named BIC-2B) the combined effect of temperature and irradiation was studied. The experimental conditions were a thermal gradient between 130 degrees C and 90 degrees C, a maximum dose rate of 3.5 kGy.h(-1) and a gradient of the integrated dose between 1.75 MGy and 10 MGy. Both experiments lasted a total of 124 days. An irradiation source of 60Co with an activity close to 300,000 Ci, and bentonite samples of 200 mm in length and 50 mm in diameter were used. After the experiment, the samples were ground and two fractions were obtained a fine fraction (<2 microm) enriched in montmorillonite clay mineral and a coarse fraction (>80 microm). The results are described of thermoluminescence analyses on the two fractions obtained which showed that the coarse fraction can be 100 times more sensitive to radiation than the fine fraction. On the other hand, the heated and irradiated samples showed a thermoluminescence response around 50 times greater than the samples that were only heated. In addition to this, the temperature and dose rate conditions are relevant parameters in the generation and stabilisation of radiation induced defects. Finally, the response of samples heated and irradiated for two months was quite similar to that obtained on samples heated and irradiated for four months, indicating a saturation phenomenon.