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Steady State and Accident Transient Analysis Burning Weapons-Grade Plutonium in Thorium and Uranium with Silicon Carbide Cladding

Nathan Andrews, Koroush Shirvan, Edward E. Pilat, Mujid S. Kazimi

Nuclear Technology / Volume 194 / Number 2 / May 2016 / Pages 204-216

Technical Paper /

First Online Publication:April 21, 2016
Updated:May 3, 2016

A comparison of burning weapons-grade plutonium in a standard pressurized water reactor (PWR) using thoria or urania as a fuel matrix has been performed. Two cladding options were considered: a silicon carbide (SiC) matrix of 0.76-mm thickness and Zircaloy of 0.57-mm thickness. As expected, in terms of percentage and total plutonium mass burned, there was a large benefit in using thoria as a matrix compared to urania. Additionally, a smaller amount of plutonium is required in a core when SiC is the cladding because of lower neutron absorption in SiC. The thorium system was also better from a plutonium-burning viewpoint. It resulted in less weapons-useable U and Pu at discharge and more burned over an assembly’s lifetime. At discharge, the fuel was shown to have lower multiples of minimum amounts needed for weapons, even when 233U breeding was taken into account. Thoria-plutonia fuel has different kinetic characteristics from urania-plutonia or enriched urania fuel, so a limited safety comparison of such fuels was made for two reactivity insertion accidents: (1) the highest worth rod ejection and (2) main-steam-line break (MSLB). The accident analyses were performed at both beginning and end of cycle. While the control rod worths are higher in the simulated thoria-plutonia and urania-plutonia cores than in conventional urania-loaded cores, the enthalpy added during the accident was lower than current safety limits for conventional cores. During the MSLB accident, all cases showed acceptable behavior, indicating that the less negative moderator temperature coefficients of thoria-plutonia and urania-plutonia fuel were not limiting.

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