Computational-experimental studies of the neutronics of the SM reactor with different variants of the neutron-trap arrangement

Abstract

The results of experimental studies of the neutronics of the high-flux SM reactor with different arrangements of the neutron trap are presented. The MCU series of high-precision computer programs implementing the Monte Carlo method is used for computations. Experimental data on reactivity effects, the effectiveness of safety and control rods, and the coefficients of nonuniformity of energy release in the core have been obtained in experiments on a critical assembly ‐ a physical model of the SM reactor ‐ and directly in experiments in the reactor. The error is 4.2‐10% in determining the reactivity parameters and 5‐10% for the relative energy release in the fuel elements. Information on the neutron field formed in the volume of the neutron trap has been obtained for two arrangements of the beryllium and water moderators. The differential and integral energy spectra of the neutrons in the energy interval from 0.5 eV to 20 MeV are obtained for three points inside the trap (external, central series, center). The flux density of thermal, superthemal, and fast neutrons are determined. The SM high-flux research reactor was designed to produce radionuclides and test reactor materials, fuel compositions, and fuel elements [1]. The results of the studies performed with this reactor are used to substantiate the serviceability and safety of nuclear reactors; the radionuclides produced find applications in industry and medicine. The SM reactor has been operated with three different arrangements of the neutron trap. In 1961‐1990, an experimental channel which was connected to an autonomous secondary loop with light water as the coolant was located at the center of the reactor. The next phase of the construction of the reactor, the purpose of which was to increase operational safety, was completed in 1991‐1992 [2]. To increase 252 Cf production, the radionuclide in greatest demand at the time, the water neutron trap was replaced with a centralized beryllium block for holding transuranium targets. The coolant in the primary loop of the reactor started to remove heat from the central block. The cross section of the rebuilt SM reactor is shown in Fig. 1. In 2002, the central beryllium block in the central moderating cavity of the reactor was replaced with a separator structure consisting of 27 14-mm in diameter and 0.5-mm thick zirconium tubes with water in the space between the tubes. The purpose of the change in the construction was to increase production efficiency of 63 Ni, 75 Se, and other isotopes which were in demand at the time and to accumulate which a low-energy neutron spectrum with high flux density is required. The cross section of the central moderating cavity of the SM reactor with different arrangements of the neutron trap is shown in Fig. 2.