Effect of nuclear transmutation elements in low activation structural materials for nuclear fusion reactors

On low activation ferritic steels of primary structural material candidates used for the first walls and blankets of nuclear fusion reactor after its prototype reactors, helium embrittlement resistance at high temperature and for a long time caused by nuclear transmutation helium formed into materials by neutron irradiation were investigated. Post-irradiation creep were carried out by using specimens simulated on helium formation at nuclear fusion reactor environment by implanted helium at high temperature through alpha-ray irradiation into various low activation ferritic steels. As a result, all of low activation ferritic steels used for tests showed better helium embrittlement resistance, at all of helium amount levels containing helium amounts (1,000 appm) considering as maximum accumulation in prototype reactors, at every supposed maximum using temperature of 873 K in upper limit, and no meaningful embrittlement of creep feature by helium and no induction of grain boundary fracture were found. And, as a result of investigation on mechanism of the helium embrittlement, it was quantitatively clarified that it brought control of the helium embrittlement to avoid movement to the grain boundary by effectively remaining helium into the grains, and that the helium embrittlement was caused by unstable growth of helium voids present at the boundary and unstable energetically. (G.K.)