Increasing the fuel cycle length of a PWR by means of a homogeneous uranium, thorium and plutonium fuel design

Abstract

In this study a new homogeneous fuel pellet was designed for a Pressurized Watercooled Reactor (PWR), aimed at increasing its fuel cycle length. The standard 4.5 wt. % Low Enriched Uranium (LEU) fuel pellet of the South African Koeberg Pressurized Water-cooled Reactor (PWR) was taken as reference. The aim was to alter the isotopic composition of a geometrically standard fuel pellet, in order to increase the fuel cycle length of the core and capacity factor, ultimately improve the profitability of the plant. The fuel cycle length is dependent of the burnup level of the fuel and is determined by the rate at which the infinite neutron multiplication factor ()k decreases. The fuel cycle ends when k has decreased to the point that the core becomes sub-critical, thereby terminating the sustainable fission chain reaction. The aim was to increase the fuel cycle length by increasing the enrichment of the fresh fuel and/or reducing the rate of decline of k with burn-up. The chosen constraints include that the fuel economy should be uncompromised by the aforementioned measures, all safety limitations for the fuel rods, such as the maximum power density and all anti-nuclear weapons proliferation limits on the isotopic composition of the fresh and spent fuel should be adhered to. A particular constraint was the assumption that k at the beginning of life (BOL) for the fresh fuel rod should not exceed that of the fresh reference fuel rod. The neutronic performance of each fuel design was simulated by creating a model for an infinite fuel pin in MCNP 6.1 Beta. This was done by surrounding a section of the fuel rod with the appropriate volume of water, which is again boxed in by reflective boundaries on all sides. The geometries of the fuel pin, fuel rod and surrounding blocks of water were kept unchanged, i.e. only the isotopic composition of the fuel pellets was altered. As a point of departure the enrichment of the LEU Koeberg reference was increased to the predetermined upper limit of 5 a/o 235U, this is the highest enrichment that is available on the international market. This of course increased k for the fresh fuel to above the maximum limit i.e. that of the reference 4.5 a/o 235U infinite fuel pin. k was then restored to the upper limit by diluting the LEU with thorium - 232 and/or adding natural boron, a well know neutron poison/absorber to the fresh fuel pellets. It was found that replacing all 238UO2 in the LEU with an equivalent amount of ThO2 substantially reduced the initial k for the fresh fuel. This is mainly due to the fact that 232Th has a much higher radiative, as well as total capture cross section in the thermal energy spectrum, compared to 238U. Further investigation also indicated that 232Th undergoes less fast fissions reactions than 238U, contributing to the higher initial infinite neutron multiplication factor ( k ). However, homogeneous mixing, e.g. equal volumes, of 232Th and 238U reduces k  even further, and resulted in a substantial increase in total neutron captures in both of the aforementioned. One possible explanation for this phenomena, is the reduced resonance escape probability in the epithermal energy spectrum, due to the summation of all the captures resonances peaks. Therefore only 2% 232Th was sufficient to reduce k for the 5 a/o LEU fuel composition to that of standard 4.5 a/o LEU fuel currently implemented in Koeberg. The logic behind the addition of natural boron was that the 10B will largely burn away within months, which means that the boron will not place a substantial drag on the neutron economy of the latter parts of the fuel cycle. An alternative approach was to determine the feasibility, of reactor grade plutonium, and MOX fuel, as a substitute, or as supplement for the standard UOX fuel composition. The changes in fuel performance, caused by to the modifications to the isotopic composition of the fresh fuel pellets, were analysed in terms of the neutron reaction rates of the predominant fissile and fertile isotopes. Preliminary burnup data suggest that some of these fuel designs are viable substitutes for currently implemented Low Enriched Uranium (LEU) fuel designs