Void induced reactivity in a mixed MTR core

Experiments in research reactors often pose the necessity of void insertion in several core positions. Also, sub-cooled boiling may introduce fluctuating quantities of bubbles. The reactivity change and its effect on the safety are important matters that must be assessed. In the present work, the reactivity induced by void insertion in various locations of the pool type, mixed (high and low enrichment uranium fuel) core of the Greek Research Reactor, was measured. The induced reactivity due to the void introduction was determined by measuring the reactor period. The measured reactivity was found to be strongly space-dependent. Results obtained with the diffusion code CITATION were found in good agreement with the experiment. Conclusions on the best utilization of diffusion codes when calculating local void effects on reactivity, in regions of steep flux gradients, were drawn. The sensitivity of the local void reactivity to the core position, as well as to the fuel enrichment and burn-up was examined. An analytical point reactor model with two neutron energy groups was developed, to investigate qualitatively the effects of parameters related to void reactivity. The analytical results were found in good qualitative agreement with measurements and numerical simulations.