Determination of the neutron flux and γ radiation in the core of operating and shut-down reactors using quartz glasses and element monitors

Abstract

The neutron fluxes and the intensity of γ radiation are measured in 26 channels of a VVR-SM reactor and its thermal column. The fast neutron fluxes in the channels are determined using Ni, Fe, Co, Au, and Mn element monitors with different threshold energies, together with a theoretical calculation using the MCNP-4C program. The energy distribution of the neutron flux inside the fuel assembly is obtained for selected channels around the core. The flux of neutrons with energies >1 MeV is in the range (0.5-43)·10 12 cm -2 sec -1 , depending on the location of the channel. A linear correlation is discovered between the induced optical absorption at the 215 nm line (E′ center) of SiO 2 -BaO glass and the fast neutron flux in the channels. The γ-ray intensity in the thermal channel is estimated for the reactor during operation (~38.4 Gy/sec) and 24 hours after it is shut down (~24.7 Gy/sec) using the E′ centers induced in pure quartz glasses. The observed difference in the efficiency with which oxygen defects are formed during dry and wet irradiation of glass owing to the radiolysis of water must be taken into account when developing radiation technology and during the burial of radioactive waste.Discrete determinations of the neutron flux and the accompanying γ radiation in nuclear reactors are needed in studies of the formation of radiation defects and in the development of radiation technologies. The dose rate for γ radiation is estimated customarily at 15 Gy/sec for a reactor power of 1 MW. The γ radiation is detected using ionization chambers, silicon detectors, and solid state dosimeters based on phosphate or quartz glass with germanium doping [1,2].A linear correlation between the increase in the optical density of glass in the 215 nm band (E′ center) and the fast neutron flux has been established by irradiating samples of specially pure quartz glass and nickel monitors in 10 vertical channels of a reactor operating at 10 MW [3]. Irradiation of lithium fluoride (LiF) crystals in a 60 Co device at 7.65 Gy/sec and parallel irradiation in a shut-down reactor showed that the γ dose rate was 26 Gy/sec [4].This paper reports an experimental determination, by means of quartz glasses and element monitors, of the fluxes of neutrons at various energies and γ-rays in all the test channels in the core and in the center of the fuel assemblies of the IRT-3M with 36% enriched 235 U for the operating and shut-down VVR-SM reactor.Experimental Data. The neutron flux and the γ-ray intensity were measured in six channels in the center of the fuel assembly and in 20 vertical channels in the first and second series surrounding the core (Fig. 1a) at various times after reloading the fuel assembly and with differing degrees of burnup.