Study of the void coefficients of reactivity in a typical pool type research reactor

The spatial dependence of the void coefficient of reactivity in a low-enriched-uranium (LEU) material test reactor (MTR) core with central flux trap configuration has been determined experimentally. The voids were simulated by using Teflon and aluminium. In these measurements, the void coefficient of reactivity was found to be positive for the central flux trap region and negative for the rest of the core. The overall void coefficient of reactivity for the entire core is negative. Detailed theoretical calculations have been performed using diffusion-theory three-dimensional modelling of the core. The results of the calculations (C) show agreement with the corresponding experimental values (E) in regions where the core has small flux gradients, while deviations in the ClE ratio from unity are found elsewhere. In the central flux trap region, the induced reactivity increases when the void volume is increased from 117 to 468 cm3