Computational and experimental study of RBMK load fragment with vertically shaped fuel assembly

Abstract

A series of experiments has been performed on the RBMK critical stand at the National Research Center ‐ Kurchatov Institute using assemblies which included single models of shaped fuel assemblies. The neutron fields computed with the MCNP and MCU-KNV codes over the height of a fuel assembly and reactivity effects after it is put into place are compared. It is shown that on the whole Monte-Carlo calculations secure high-quality matching to the measured neutron flux distribution. It is found that the neutron spectrum varies over the height of shaped fuel assemblies, which variation results in discrepancies of the thermal neutron flux distribution and 235 U fission rate and must be taken into account when interpreting the experimental data. When a uniform fuel assembly is replaced with a shaped assembly, the computed reactivity effect is close to the experimental effect. It was pointed out during the early stage of RBMK operation that vertical fuel-enrichment shaping in fuel assemblies can improve the physical characteristics of the reactor. The positive experience gained with vertically nonuniform loads in the operation of commercial uranium-graphite reactors, which served as a prototype for the development of RBMK, was taken into account. In particular, it was proposed that this method be used to decrease the steam coefficient of reactivity and increase the stability of the vertical power distribution in RBMK [1]. The possibility of vertical shaping of the enrichment, mainly to increase fuel burnup and decrease neutron leakage, is now being analyzed. At the same time, it has been proposed that the erbium content be shaped, which should give more uniform fuel burnup along the core height. Experimental batches of shaped fuel assemblies are already undergoing reactor tests. An experimental and computational study of the neutron-physical characteristics of model assemblies with separate models of shaped fuel assemblies was performed as part of work based on a critical bench at the National Research Center ‐ Kurchatov Institute. The bench is designed for neutron-physical experiments with RBMK load fragments within the dimensions of the