Determination of the hot- and cold-temperature coefficient of reactivity in the AVR core

The measurements that were made in recent years to determine the hot and the cold coefficient of reactivity and the corresponding model calculations carried out by both Interatom and Hochtemperaturreaktorbau are described. The effects of the partial load of low-enriched uranium (LEU) fuel were of major interest. The cold coefficient is determined in shutdown periods by measuring the subcriticality at different core temperatures. The hot coefficient is determined under operation in connection with a change in coolant outlet temperature at constant power using a calibrated rod curve. In the case of the cold coefficient, experiment and model calculations are in perfect agreement. Calculated values for the hot coefficient are -- 20% lower than the experimental ones. However, neither experiment nor model calculation show any significant change of the hot coefficient when about one-fourth of the core content was being replaced by LEU fuel