The density change of simulated radioactive waste glasses irradiated by the10B(n,α)7Li reaction was determined by a sink-float method as a function of irradiation exposure. Simulated waste glasses P0500, P0798 and GP98/12 swelled, while P0504 shrinked. The magnitude of the density change was less than 0.6% up to a fluence of 6.6 × 1025 reactions/m3, which corresponds to the cumulative irradiation during a few tens of thousand years after disposal of the waste glass from the spent fuel irradiated up to 33000 MWD/MTU. The processes which play an important role on the density change have not been clarified, but it is likely that one of the processes is helium bubble formation which was clarified by a carbon replica technique, in association with transmission electron microscopy.
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