Thermal group contribution to the fuel-temperature coefficient revisited

A project at the Paul Scherrer Institute (PSI) involves the development of simulation models for the transient analysis of the reactors in Switzerland (STARS) (Ref. 1). This project, funded in part by the Swiss Federal Nuclear Safety Inspectorate, also involves the calculation and evaluation of certain transients for Swiss light water reactors. To generate the nuclear data for these analyses, we have employed various codes, including the cell/assembly codes BOXER (Refs. 2 and 3) and CASMO-3 (Ref. 4). For certain reactivity insertion accidents, the fuel-temperature coefficient (FTC) determines the major component of the feedback reactivity, which limits the peak fuel enthalpy during the transient. The FTC has a component due to the temperature dependence of the oxygen scattering in the fuel that causes changes in the thermal group cross sections. However, this and other sources of a thermal group contribution to the FTC (TFTC) are sometimes ignored. In this paper, we present some results of investigations of contributions to the TFTC