Impact of Ad Hoc Improvement of Decay and Cross-Section Data on the Prediction of Fusion-Neutron-Induced Radioactivity in Zirconium and Tungsten

Measured decay rates resulting from neutron irradiation of zirconium and tungsten samples in a typical fusion environment have been compared with the computed values, and the sources of errors in the data and the calculational method have been identified. Comparison of four codes showed large differences that arise mainly from differences in the data libraries provided with these codes. The following reactions were found to be most important in terms of their contribution to the decay photon emission rate: ⁹⁰Zr(n, 2n)-^89m+gZr) ⁹⁰Zr(n,p)^90mY, ⁹⁰Zr(n,α)^87mSr, ⁹¹Zr(n,p)^91mY, ¹⁸⁶W(n,y)¹⁸⁷W, ¹⁸⁶W(n,p)¹⁸⁶Ta, ¹⁸⁶W(n,np)(n,d)-¹⁸⁵Ta, ¹⁸⁴W(n,p)¹⁸⁴Ta, ¹⁸³W(n,p)¹⁸³Ta, ¹⁸²W(n,p)-¹⁸²Ta, and ¹⁸⁶W(n, α)¹⁸³Hf. However, decay data and cross sections for these reactions are not adequate in currently available libraries. An effort was made to improve the decay data by using the values from the most recent Table of Radioactive Isotopes and to improve the cross sections by using a simple curve-fitting procedure. Modified or improved decay data and cross sections were implemented in a representative code, and the computation was performed again. A great improvement in the computed results was observed for both sample cases. This work can easily be extended to other fusion-relevant materials by utilizing the methodology presented here. The improved decay and cross-section data were applied to an International Thermonuclear Experimental Reactor (ITER) blanket using tungsten as a first-wall coating material and Li₂ZrO₃ as a breeding material. The specific photon yield in each zone was computed, and as much as three orders of magnitude difference in the photon yield in the tungsten zone and ∼10 to 15% difference in the zirconium-containing breeding zone were observed between the results using the improved decay and cross-section data and those using the original data.