Breeding of fissile materials in thermal reactors

Abstract

It is predicted that by the end of the 21st century nuclear power plants with fast reactors will be able to meet no more than 30% of the electricity demand [1]. For thermal reactors with similar power, the proven reserves of cheap uranium will be consumed over approximately 50 years. Thermal reactors will be able to compete with fast reactors if they breed more fissile material. In this problem, preference is given to reactors that will burn fuel containing thorium. For example, a variant of an upgraded CANDU reactor where 233 U is produced completely is presented in [2].The breeding ratio for fissile materials in a thermal reactor can be made to approach 1 with a low concentration of fissile substances in the fuel, decreasing neutron losses in the structural materials and leakage. The following questions become important. Is it possible in this approach to reach high burnup? What will be the admissible neutron losses in materials and due to leakage? How realistic will the design be? What niche in power production will these reactors be able to fill?An acceptable result according to certain indicators can be achieved with the following solutions [3]:• the reactor operates according to a core superposition technology, where the excess neutrons present at the start of a run are used for breeding fissile materials at the end of the run, and the reactivity integrated over the campaign is close to zero; • the reactor operates in a closed fuel cycle with equilibrium chains of nuclides, whose parents are 232 Th and 238 U; • neutron losses are as low as possible, for which reactivity compensators are eliminated and structural materials enriched with isotopes with small neutron capture cross-sections are used; • an additional neutron source is present -the reaction n-2n. A variant of a reactor with the smallest possible dimensions is displayed in Fig. 1. Its core contains 475 technological channels. Figure 2 displays the geometry of a fuel assembly. The fuel elements cladding and the channel housing are made of zirconium-niobium alloy with up to 99% 99 Zr. This variant, according to the number of channels in the core and the core dimensions, is close to optimal for thermal power ranging from 600 to 1500 MW(t).The characteristics of the reactor were calculated in the following sequence. The construction of the reactor core and fuel assemblies was worked out. The MCNP program was used to calculate the neutron-physical characteristics and to determine, based on these characteristics, the neutron losses in the structural materials and due to leakage, and the cross-sections of nuclides for the single-group point model used for a reactor in the Dinamika program [4]. Next, the change in the composition of the fuel during a run and the neutron losses in fission products with prescribed run time were found. If the conditions for equilibrium of nuclides or zero reactivity integral were violated in the calculations, then a new series of calculations was conducted with new initial conditions. The reprocessing of the spent fuel from t...