Monitoring the distribution of energy release in BN-600 by γ scanning fuel assemblies

Abstract

A complex of computational and experimental measures for monitoring the distribution of energy release in the core has been perfected over the 25-year history of BN-600 operation in the Beloyarskaya nuclear power plant. Continual monitoring is conducted together with calculations based on three-dimensional multigroup calculations and periodic γ scanning of regular BN-600 fuel assemblies with upgrading of the core of this reactor. At the present time, in the course of switching BN-600 to a new core with four refuelings and maximum fuel burnup increased to 11.1% h.a., the experimental procedure has been upgraded and optimized taking account of the experience gained, three series of such measurements have been completed, and new experimental data on the character of the radial and axial neutron-field distribution have been obtained.One advantage of fast nuclear reactors is the stability of energy release. This made it possible to do without a special in-reactor system for monitoring the power of fuel assemblies in real-time in BN-600 in the Beloyarskaya nuclear power plant. Instead, a complex of measures for monitoring the distribution of the energy release in the core has been developed over the 25-year period of operation of the reactor. The crux of these measures reduces to the following. The fuel-assembly irradiation parameters in a regime with established refueling are continually monitored by computer simulation of the neutron field in a three-dimensional hexagonal geometry using programs for performing neutron-physical calculations, which solve the transfer equation in the multigroup diffusion approximation. During physical startup and upgrading of the core, special experiments are performed which make it possible to follow the actual state and changes of the neutron field in connection with the transition to a new modification. Starting with the physical startup of BN-600 in 1980, the distribution of the reaction rates and energy release in a fuel assembly were measured by means of γ scanning of fuel assemblies and needle activation detectors [1,2]. The former was simpler and adapted for monitoring the power of a fuel assembly under the conditions of a commercial fast reactor in a nuclear power plant and is now standard. The present article presents a description of the method and the application and results of the latest measurements undertaken in connection with the transfer of BN-600 to a new core 01M2 with enhanced burnup.