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Samples of polycrystalline rock salt were gamma-irradiated to doses up to 5.6 Grad, resulting in stored energies up to 28 J/g. They were subsequently annealed in the presence of added brine at temperatures between 70 and 120 C. Irradiation dose rates varied between 17 and 4 MRad/h., in a monthly cycle, and so did the irradiation temperatures (130 to 50 C, respectively). By increasing the temperature up to 120-130 C, during the whole irradiation period, the stored energy/total dose ratio increased. It has been shown in the annealing experiments that fluid-assisted recrystallization always takes place in the presence of added brine. Dry samples do not recrystallize. Recrystallization eliminates the damage-related stored energy: stored energy levels in recrystallized material are the same as in unirradiated material. Solid state back-reactions are not as effective as recrystallization in reducing stored energy levels: non-recrystallized material has the same stored energy level before and after the anneals. The results imply that if rock salt around HLW canisters contains about 0.1 % volume of brine on grain boundaries, in the temperature range 50-130 C, radiation damage levels will be significantly reduced by recrystallization.

Additional information

Authors: CELMA A G, Department of Geology, Faculty of Geology and Geophysics, University of Utrecht (NL)
Bibliographic Reference: EUR 11849 EN (1988)
Availability: (2)
ISBN: ISBN 92-825-9055-0;CD-NA-
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