Fusion activation of ferrous alloys - dependence on flux, irradiation time and fluence

Abstract

The induced activity, contact dose-rate and decay heat of a number of ferrous alloys have been calculated using the European Activation SYstem EASY for the first wall position in a number of different conceptual reactor designs, namely CCTR, EEF, DEMO, STARFIRE, ITER, and NET. In the first instance all neutron fluxes were normalised to 1 MW m −2 to permit direct investigation of the influence of the different spectra on the predicted activation. The activation levels of ferrous alloys predicted for the various reactor designs at a standardised flux are found to vary by orders of magnitude at cooling times relevant to reactor decommissioning. The ways in which the predicted activation properties of a material scale with the duration of the irradiation, the first wall neutron loading and the corresponding fluence are systematically examined. The activation behaviour of candidate structural materials is governed by a relatively limited number of specific radionuclides and can best be understood by following the production pathways for these nuclides and by examining their dependence on the flux and irradiation time. These pathways can be strongly modified by changes in the irradiation conditions and the amount of a particular radionuclide generated cannot be simply scaled if multiple step reactions are involved in its production. The cases of selected ferrous alloys are considered in some detail; the production pathways for dominant radionuclides are identified and the way in which the pathways evolve as irradiation proceeds is described. The results demonstrate that an appreciation of the main generation routes for radionuclides produced by multiple-stage reaction chains is essential to a proper application of the flux-time scaling relationships in safety and environmental studies.