Positive temperature reactivity coefficient of a TRIGA reactor at room temperature

A positive isothermal temperature reactivity coefficient has been measured at zero power conditions at the TRIGA research reactor of the J. Stefan Institute in Ljubljana. The coefficient is measured in a critical reactor for different core configurations at room temperature and at low power. The reactor is filled with standard, 20 % enriched, fuel elements containing 12 wt % uranium with 6 % average burn-up. The experiments are analysed by reactor calculations using WIMSD code to provide the reactor physics explanations of the results measured. The positive temperature reactivity effect can be explained by reduced absorption in water prevailing at low fuel temperature over negative temperature effects on the reactivity in the fuel elements. The positive effect is due to the thermal spectrum shift in water and is not the consequence of a change in water density. The positive effect decreases with increasing temperature. The negative reactivity effect in fuel prevails at fuel temperatures above 50°C. The positive isothermal temperature reactivity coefficient has no effect on normal reactor operation in steady state and pulse mode.