A Brief Sensitivity Analysis for the GIRM and Other Related Technique using a One- Group Cross Section Library for Graphite-Moderated Reactors

The Graphite Isotope Ratio Method (GIRM) is a well-known technique developed to estimate cumulative energy production and plutonium production from graphite-moderated reactors. This method is based on the measurement of trace isotopes in the reactor’s graphite matrix to determine the change in their isotopic ratios due to burnup. These measurements are then coupled with reactor calculations to determine the total plutonium and energy production of the reactor. To facilitate sensitivity analysis of these methods, a one-group cross section and fission product yield library for the fuel and graphite activation products has been developed for MAGNOX-style reactors. This library is intended for use in the ORIGEN computer code. Using this library, a brief sensitivity analysis was conducted to assess various input parameters including 235U and 238U cross section values, aluminum alloy concentration in the fuel, and initial concentrations of trace elements in the graphite moderator. The results of the analysis yield insight into the GIRM method and the isotopic ratios the method uses as well as the level of uncertainty that may be found in the system results.