Heterogeneous effects in simulating a fast nuclear reactor on the BFS facility
The simulation of fast neutron reactors are modeled to compare experimental and calculated data on the neutron-physical characteristics on the zero power stands. This article discusses the BFS facility, which is in operation in Russia (Obninsk). The geometrical arrangement of materials in the actual design of reactors (fuel elements, fuel assemblies, coolant geometry) differs from the design on the BFS. This can cause differences in the experimental results at the BFS from theoretical calculations even in the case of careful observance of homogeneous concentrations of all materials of the reactor. Differences of neutron-physical parameters due to the geometry of the location of materials with the same homogeneous concentrations are called heterogeneous effect. Heterogeneous effects tend to increase with increasing reactor power and its size which is mainly due to changes in the neutron spectra.
The calculations of a number of functional values are carried out to assess the heterogeneous effects for different spatial arrangements of the reactor’ materials. The calculations were carried out for: a) heterogeneous distribution of materials in accordance with the design of a fast neutron reactor; b) heterogeneous arrangement of materials in accordance with the capabilities and design features of the BFS facility; c) a homogeneous presentation of core materials and reproduction zones.
The basic version of the calculations, in relation to which the effect called the heterogeneous shift of the functional value (HSF), was calculated and adopted by the layout of materials in accordance with the design data of the BN-1200 reactors type.
The effect of neutron leakage on the HSF as a result of calculations with different boundary conditions was estimated. All calculations were carried out at the same homogeneous concentrations of all materials for all three compositions. Calculations were also carried out when using metallic plutonium on the BFS.
The values of the following functionals were calculated for different arrangements of materials: the effective multiplication factor (reactivity), the sodium void reactivity effect, the average neutron energy causing fission, and the ratios of the radioactive capture to the fission cross sections for 239Pu.
The calculations were performed using the Monte Carlo software package for neutron physics modeling Serpent 2.1.30 (VTT, Finland) and the libraries of the evaluated nuclear data ENDF/B-VII.0 and JEFF-3.1.1. The effect of various methods of materials replacing on the values of keff was greatest when replacing the dioxide of fissile material with metal of the same materials (about 1.6%). The homogeneous composition reduces the keff by about 0.4%.
The average neutron energy causing fission significantly depends on the leakage of neutrons and the presence of sodium (the average energy of the neutrons increases when sodium reaches about 100 keV, that is, by about 11 – 13%). Replacing fissile metallic materials with their dioxide on the BFS facility (while maintaining homogeneous concentrations, including oxygen) reduces the average energy of the neutrons causing fission by about 60 keV.
The highest values of HSF, reaching 65%, are observed when calculating the sodium void reactivity effect with a homogeneous arrangement of materials, but when calculating the model of the reactor at BFS it is 1.5%. In the absence of neutron leakage (infinitely extended medium), the sodium void reactivity effect becomes positive and the HSF is 4 – 7%.
The heterogeneous effect of α for 239Pu significantly depends (6 – 8%) only on the replacement of metallic plutonium with its dioxide (naturally, while maintaining homogeneous concentrations).
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