Measurements of the reactivity temperature coefficient during zero-power tests in the nuclear power plant Gundremmingen

According to the regulatory codes during the zero-power tests before the beginning of each reactor cycle it is mandatory to demonstrate by measurements that the reactivity temperature coefficient reaches negative values before reaching the operation temperature. In the past the reactivity temperature coefficient of the fuel loading composed of uranium fuel elements with final burnup smaller 40 MWd/kg was negative at about 20 C. For modern fuel loadings with higher enriched uranium fuel elements or mixed oxide fuel elements that reach final burnup of 60 MWd/kg the reactivity temperature coefficient is negative at relatively high temperatures. This fact increases the metrological expenses for the conventional measuring method. The authors describe a modified measuring technique that allows a significant time saving.