Conceptual Neutronic Design of Inverted Core for Lead-Bismuth Cooled Small Modular Reactor

Abstract

In this thesis, conceptual design of inverted core is studied in comparison with the reference design of a lead-bismuth cooled small modular reactor(SMR), URANUS-40MWe. Inverted core designs with hexagonal assemblies which contain liquid metal coolant pipes have been studied assuming that their fabricability can be established by emerging 3D printing technology to improve both economy and structural reliability of SMRs. An inverted core design with 78 hexagonal fuel assemblies made of UO2 fuel and ferritic-martensitic stainless cooling pipes has shown to have most favorable characteristics among cases examined. Both power density and core criticality period are increased by 42.34% and 25.00%, respectively, while core size, power peaking factor at BOC and heavy metal mass loaded is decreased by 16.2%, 20.9%, 27.6%, respectively, by the inverted design for the given power rating of the reference design of URANUS-40MWe. By the inverted core design, various neutronic design parameters are also improved

the absolute value of Doppler coefficient is increased by 8.14%, the void effect becomes more negative to -0.744%dk/k and -4.367%dk/k for the central and the hottest assembly, respectively. The control rod worth is found to be $16.74 that is adequate to assure shutdown margins. The lifetime and conversion ratio of the reactor have been extended from 20 years and 0.7286 to 25 years and 0.7651, respectively. The improvement of neutronic characteristics of inverted core can be understood by higher fuel volume fraction compare to normal core. Fuel to coolant volume ratio increased from 0.816 to 0.900. The increased fuel volume ratio improves the utilization of uranium, allowing the longer life. Peak fuel temperatures are calculated to be 967.7℃ and 812.6℃ for normal reference core and inverted core, respectively. The peak temperature of the inverted core is found to be as much as 155.1℃ lower than that of the normal core. Lower fuel temperature can reduce fission gas release and improve margin for peaking factors. Structural reliability is expected to be enhanced by removing grid and wire wraps without undermining fuel cladding integrity against fretting and corrosion. For LFR, therefore, the inverted core design can improve the neutronic characteristics, proliferation resistance, transportability, as well as thermal margins. In conclusion, inverted core design can be a desirable future option if fuel fabrication can be feasible. Favorable characteristics of inverted core designs warrant studies on challenging engineering issues, including fuel fabrication and fission gas pressure control By block-type inverted core concept, as suggested at the conclusion of this thesis, the core size can be significantly reduced while operation lifetime can be extended because the volume fraction of structural materials is reduced by uniting fuel assemblies to fuel blocks.