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The rate of accumulation of helium at grain boundaries is one of the important parameters determining the integrity and lifetime of the structural components of a fusion reactor. A diffusion calculation is made of the flux of helium to a grain boundary. The flux is found to depend on the gas production rate, the width of the cavity denuded zone, and the cavity sink strength in the grain interior. The calculated accumulation of helium is in good agreement with the measured gas content of grain boundaries in aluminum, PE 16, and 316 stainless steel. The flux of helium to grain boundaries increases with helium generation rate, but the increase is less than proportional to the generation rate. The loss of helium to grain boundaries during the nucleation of the bubbles within the grains has been estimated; no great loss is expected to occur. However, the loss would be considerably enhanced if any delay in bubble nucleation were to occur because of incubation effects. The role of material variables is found to be difficult to predict at present.
helium, generation rate, diffusion, radiation enhanced, aluminum, 600-MeV protons, 800-MeV protons, bubbles, cavities, dislocation, cold-work, alloying, flux, concentration, grain boundary, denuded zone, sink strength, incubation, nucleation
Senior research scientist, Metallurgy Department, Risø National Laboratory, Roskilde,
Principal Scientific Officer, Materials Development Division, A.E.R.E. Harwell, Oxfordshire