The influence of thorium, uranium and plutonium fuel mixture composition on the reactivity coefficients of pressurised water reactors

Abstract

Temperature reactivity coefficients of a PWR (Pressurized Water Reactor) containing three fuel mixture compositions of uranium, thorium and plutonium in the form of UO2 with Low Enriched Uranium,Th-UO2 with Low Enriched Uranium and Pu-MOX with Low Enriched Uranium were analysed. The LEU UO2 fuel is of 4.5% U-235 & 95.5% U-238 (Fuel model A). The LEU Th-UO2 is of 5% U-235, 93% U-238 & 2% Th-232 (Fuel model B). And the LEU Pu-MOX is of 0.8% Pu(PWR), 4.5276% U-235 & 94.6724% U-238 (Fuel model C). Each fuel mixture composition was simulated in an infinite unit cell fuel pin model with reflective boundaries (a cylindrical fuel rod surrounded by a cuboid moderator) in Monte Carlo N Particle code (MCNP 6.2). A 17x17 lattice PWR core with a three-batch refuelling scheme of fresh, once burnt and twice burnt fuel is assumed and it was mimicked using the infinite unit cell fuel pin model. The reactivity coefficients were analysed at the BOC (beginning of cycle) and at criticality (keff =1). To achieve criticality, burnable absorbers gadolinia and soluble boron are added in the fuel models. The reactivity coefficients of the three fuel mixture compositions were sufficiently negative as required by regulatory bodies and were all within the set ranges in terms of magnitude as published in the source Safety margins of operating reactors by the IAEA,2003. The DC ( Doppler Coefficient) and MTC(moderator temperature coefficient) behave quite differently with increasing temperature. The DC becomes less negative with increasing fuel temperatures and the MTC grows strongly negative with increasing moderator temperatures. The largest effect on both the Doppler coefficient and Moderator temperature coefficient of the fuel model compositions is owing to the thermal and resonance absorption in the fuel isotopes. The total capture-fission ratio of all fuel models substantially increases with increasing fuel temperatures from CZP (293.6K) to 1200k. A decrease in thermal fissions and an increase in resonance absorptions is directly linked to a decrease in moderator density and it was found that the density is highly influential on the MTC. The moderator density substantially decreases with increasing moderator temperatures from 1.003g/cc at CZP(293.6K) to 0.6613g/cc at HFP (600k), then to 0.098g/cc at boiling (620k) even under a pressure of 15.5MPa(155bar). Boiling that is significant rarely occurs in a PWR and it is seen from the results that by the time it occurs, the reactor long reached sub-criticality (keff <1). Overall ,the LEU Pu-MOX and the LEU Th-UO2 fuels produced more favourable temperature reactivity coefficients than the conventional LEU UO2 fuel.