Investigation of the characteristics of GIR-2 neutron fields

Abstract

The characteristics of the neutron fields (spectrum, fluence) at four points of the GIR-2 reactor are measured. It is shown experimentally that the energy spectrum of leakage neutrons from the reactor core through an opening in a spherical converter, which consists of light elements and inside which the core is placed, becomes harder. This effect is due to the shift of the region in the core where the number density of fissions is high toward the lateral surface of the core as a result of the presence the converter, consisting of light elements, outside the core. On the basis of the results of these investigations, GIR-2 neutron fields are certified as reference standards for measurements.The GIR-2 reactor is used together with the high-current LIU-10 linear accelerator to produce mixed-radiation fuels (neutron, electron, and γ radiation) for studying the effect of radiation on experimental objects [1].The present paper presents the results of an investigation of the characteristics of neutron fields at only four points of the GIR-2 reactor. These points are located on the same horizontal axis, passing through the center of the reactor core and the center of an opening in a spherical polypropylene [-CH 2 -CH(CH 3 )-] converter, in which the core is located. The first point is located directly on the lateral surface of the core inside a 300 mm in diameter cylindrical opening in the converter; the second point lies behind the converter opposite the opening in it. The third and fourth points are located on the same axis at a distance of 500 mm from the core center on the side of the opening in the converter and behind the converter, respectively. The dimensions of the experimental hall containing the core are 10 × 10 × 8 m (see Fig. 1). The center of the core lies 1.7 m above the floor of the hall. The spherical core of GIR-2 [1], with outer diameter 300 mm, is made of uranium-molybdenum alloy with molybdenum mass content 9%. The interior part of the core is made of spherical layers of uranium enriched to 90% 235 U. The outer spherical layer of the core, 16 mm thick, is made of uranium with enrichment 36%. The total mass of the uranium-molybdenum alloy is 178 kg. The converter in which the core is placed is pressed from mixtures of polypropylene and cadmium oxide powders. Its main purpose is partial conversion of neutron into γ radiation. The outer diameter of the spherical converter is 500 mm, and the wall thickness is 60 mm. The mass fraction of cadmium oxide in the converter material is 9%.The formation of the neutron spectra at the points is due to the presence of a converter consisting of light elements (hydrogen, carbon) outside the spherical metallic core. The formation of the neutron spectrum at each point differs substantially. Thus, the neutron spectrum at the first point is formed predominantly by leakage neutrons from the core and neutrons scattered in the converter material; the neutron spectrum at the second point is due predominantly to neutrons flowing out of the core and neutrons scattered in the co...