Spectral Effects in the Extrapolation of Pressure Vessel Surveillance Capsule Measurements

An evaluation of the effects of the neutron spectral shift between the surveillance capsule and vessel on the prediction of radiation-induced pressure vessel (PV) embrittlement has been made. A spectral lead factor, L_s, that accounts for this spectral shift and its effect on predicting change in vessel reference nil ductility temperature (ΔRT_NDT) is defined. Using multigroup neutron spectra calculated for the Three Mile Island-2 core/vessel configuration and the damage cross sections developed by Serpan, L_s has been determined and found to result in significant underpredictions of ΔRT_NDT. For a standard surveillance capsule located near the PV inner wall, ΔRT_NDT is underpredicted by ~10 and ~50% at the T/4 and 3T/4 vessel locations, respectively.