The Impact and Status of Neutron-Activation Cross Sections for Short-Lived Reaction Products of Interest in Fusion Reactor Design

The impact and status of the cross sections for production of short-lived radioactivities in the intense high-energy neutron fields associated with deuterium-tritium fusion reactors is investigated. The main concern relative to these very radioactive species is that they may represent enhanced radiation sources not accounted for in typical transport calculations. These enhanced radiation sources may affect heat removal and shielding requirements. The status of nuclear data required to assess these issues is surveyed. Among the factors considered in defining the relevant reactions and setting priorities are quantities of the elemental materials in a fusion reactor, isotopic abundances within elemental categories, the decay properties of the induced radioactive by-products, the reaction cross sections, and the nature of the decay radiations. Attention has been focused on radioactive species with half-lives in the range from ~1 s to 15 min. Available cross-section and reaction-product decay information from the literature are compiled and examined. The evaluated data sets are collapsed using neutron spectra from three fusion reactor designs - ARIES I and II and the International Thermonuclear Experimental Reactor (ITER). The group-averaged cross-section sets are then used to produce neutron-spectrum-averaged, one-group cross sections, which are, in turn, used to produce decay heating reaction rates for each of the reactions. The decay heating rate is used as a measure of the radiation source strength associated with a given reaction. The decay heating reaction rates are compared against neutron heating reaction rates. Calculated decay heat to neutron heating ratios are required to be >10% in order for the reaction to be considered of importance for further study. The reactions of importance are identified as ²⁸Si(n,p)²⁸Al, with a ratio of ~10%, and ²⁰⁷Pb(n,n')^207mPb, with a ratio >50%. The ²⁸Si(n,p)²⁸Al reaction could affect heat removal requirements for reactors employing silicon carbide as a structural material. The ²⁰⁷Pb(n,n')^207mPb reaction could affect heat removal and shielding requirements for shield designs employing lead. Identified reactions of slightly less importance are ²⁷Al(n,p)²⁷Mg, ⁹Be(n,)⁶He, ⁵²Cr(n,p)⁵²V, ¹⁶O(n,p)¹⁶N, and ²⁰⁴Pb(n,2n)^203mPb - all of which have ratios between 1 and 4%.