Special design issues. Induced radioactivity

In fission power plants energy producing reactions result in radioactive products. In fusion power plants the major concern is the neutron-induced radioactivity in the materials around the reaction chamber. Fusion offers the possibility of a significant reduction in the total radioactive inventory and shorter waste lifetime if an appropriate choice of the materials under neutron irradiation can be made. Unlike in MFE (magnetic fusion energy) plants, the neutrons in IFE (inertial fusion energy) plants are strongly moderated before they reach the reaction chamber wall, which modifies the neutron spectrum as seen by the reaction chamber materials. Therefore, objections can be raised against using MFE studies for the selection of low activation materials, as done in most of work to date. In fact, studies of representative IFE neutron spectra show that the long term activation response of some important elements is very different for hard (MFE) and soft (IFE) neutron spectra. In this document the selection of elements and optimization of the compositions of materials (steels, V alloys and ceramics) for IFE blankets and first structural walls is illustrated by considering the INPORT units in the HIBALL-II and LIBRA reactors. The concentration limits imposed by the two options for suitable waste management, namely: recycling and near surface burial, for all the stable elements is used in the analysis of several materials. From that analysis, critical radionuclides and the principal transmutation and decay pathways for their generation are identified. For greater completeness the Cascade reactor is analyzed as a representative design utilizing advanced materials and engineering solutions. Emerging concepts are also mentioned. Among the conclusions, it is stated that (i) designs using high Z materials should be avoided; (ii) Fe, Ni, Mo, and W should be restricted owing to their long-term hazards while they also dominate the gamma dose rate; (iii) activation steels taken from MFE do not perform better than conventional steels in an IFE reaction chamber; (iv) acceptable alloy constituents are Si, Ti, Cr, Y, and possibly Al. 37 refs, 8 figs, 5 tabs