FUSION ACTIVATION OF FERROUS-ALLOYS - DEPENDENCE ON FLUX, IRRADIATIONTIME AND FLUENCE

The induced activity, contact dose-rate and decay heat of a number of ferrous alloys have been calculated using the European Activation SYst em EASY for the first wall position in a number of different conceptua l 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 p ermit direct investigation of the influence of the different spectra o n the predicted activation. The activation levels of ferrous alloys pr edicted for the various reactor designs at a standardised flux are fou nd to vary by orders of magnitude at cooling times relevant to reactor decommissioning. The ways in which the predicted activation propertie s 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 i s governed by a relatively limited number of specific radionuclides an d can best be understood by following the production pathways for thes e nuclides and by examining their dependence on the flux and irradiati on time. These pathways can be strongly modified by changes in the irr adiation conditions and the amount of a particular radionuclide genera ted cannot be simply scaled if multiple step reactions are involved in its production. The cases of selected ferrous alloys are considered i n some detail; the production pathways for dominant radionuclides are identified and the way in which the pathways evolve as irradiation pro ceeds is described. The results demonstrate that an appreciation of th e 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.