Radioactivity aspects of fusion reactors

Abstract

Abstract Activation characteristics, including radioactivity, decay heating rate, and integrated decay energy at times after shutdown of a D—T fusion power reactor were investigated for all potential reactor materials using a recently published comprehensive activation cross-section library and decay data handbook. It was found that among the potential structural elements, the shutdown activity could vary by four orders of magnitude, with C, O, and Si producing the least radioactivity and Mo giving the highest activity within a few days after shutdown, a period of importance to the reactor operation. Vanadium, Ti and Fe are among the lower activation elements with the activity levels higher than Si by about one (for V) to two (for Ti and Fe) orders of magnitude. As far as alloying elements are concerned, Cr and Si are best for minimizing the activity level; Mn, Ni, Ta and W are among the elements giving higher radioactivity and decay heat values. These higher activity elements are furthermore subject to the neutron spectral effect resulting in an increase of activation levels in a soft spectrum with higher neutron population at lower energies. The important elements, that need to be limited in fusion reactor materials in order to meet the 10CFR61 Class C shallow-land burial disposal goal, are Al, Si, Ni, Zr and Ta as alloying elements, and Nb, Mo, Ag, Gd, Tb, and Ho as impurities. The concentration limits of some of these elements such as Nb will also become more restrictive in a soft neutron spectrum, that is typical for the present fusion experimental facilities under investigation.