Assessment of the safety impact caused by the variations of the critical position of control rods and by the presence of an in-core flux trap on the power peaking factors in an MTR-type research reactor

Abstract

The main objective of this work is to evaluate the safety impact caused by the variations of the critical positions of control rods and by the core configurations with flux trap on both the locations of the hot channels and on power peaking factors (PPFs) values in a research reactor core. Thanks to an appropriate computational methodology and to parallel computing capabilities of MCNP5, we conducted a demonstration study on two core configurations and twenty selected critical states of the NUR light water research reactor.The results obtained show that the locations of the hot channel in a given core vary continuously, in particular, around the central positions of the reactor cores as a function of the critical positions of the control rods. Moreover, it was found that the axial and radial power peaking factors depend mainly on the critical positions of the control rods whereas the local power peaking factors strongly depend on the presence of a flux trap in the core configuration. The maximum total power peaking factor can exceed, in some cases, the safety limit value (3.0) as found for the core configuration where a central flux trap was present. Indeed, it has been observed that the hot channels are mainly located at the central positions of the reactor cores of interest, that is where the fuel elements are the most consumed.Overall, due to the prohibitive values exceeding the safety limits found for the PPFs, in some critical states, it is of crucial importance to apply such methodological approach to accurately calculate them for all potential critical positions likely to be encountered during the useful life of a reactor.