Fuel Utilization Improvements in Heat Pipe Monoblock Microreactors

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She, Aaron Ziyuan
Petrovic, Bojan
Kotlyar, Dan
Biegalski, Steven R.
Sabharwall, Piyush
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Nuclear microreactors are an attractive technological concept that combine the advantages of lower capital costs and modularity to deliver reliable power generation for niche applications and remote communities that are otherwise not well served by conventional power utilities. Heat pipe microreactors use no moving parts and can operate at much higher temperatures than conventional light water reactors, which has advantages for remote operation and improved thermal efficiency. However, due to their physically small size, nuclear microreactors suffer from high neutron leakage, lowering their fuel utilization and increasing fuel cycle costs. This thesis investigates design tradeoffs to improve the fuel utilization and discharge burnup of a 15 MWth heat pipe monoblock microreactor fuel cycle, while retaining the advantages of microreactor concepts in economics and remote utility. Reactivity control is achieved using burnable absorbers, control sliders, and an emergency shutdown rod. It was found additional burnable absorber loading had a diminishing return on the cycle peak criticality while the penalty incurred to discharge burnup increased rapidly. Studies on the shutdown of the microreactor illuminated a positive reactivity coefficient due to the spectral shift of the neutron flux with increasing temperature – it is not clear if the single shutdown rod alone would be sufficient to shutdown the reactor and more studies are recommended. Heat pipe thermal operating limits were investigated. A sintered nickel wick heat pipe design is proposed that would enable the reactor to remain resilient to a failure of a heat pipe in the reactor hotspots with some margin. Finally, a simplified economic estimate suggests that the microreactor would be economically competitive and that implementing technology to improve the fuel utilization at the cost of a longer cycle is a worthwhile tradeoff. The proposed microreactor has a fuel cycle estimated to last 25 years without refueling, with a discharge burnup of about 60 MWd/kgU.
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