THORIUM-BASED FUEL UTILIZATION ANALYSIS IN FLUORIDE SALT-COOLED HIGH-TEMPERATURE REACTOR

The Th-U fuel cycle, benefiting from its excellent breeding capability in both thermal and fast reactors, lesser long-lived minor actinide (MA), and the abundant reserves of Thorium, has become increasingly prominent and new Thorium reactors needs to be developed. The Fluoride-cooling High-temperature Reactor (FHR) [1] , also known as Solid-fueled Molten Salt Reactor (MSR-SF) is a candidate reactor for Th-U fuel cycle. It can achieve excellent performance on safety and economy with a high temperature output. In the past, thorium-based fuels have been successfully utilized in HTGRs [2,3] in Germany, USA, Japan and The Russian Federation. The graphite coated fuel particles of the MSR-SF present some attractive advantages as high resistance to irradiation damages and high melting points, enabling to reach a high fuel burnup. The different layers of TRISO particle constitute excellent fission product and radionuclide barriers, which make TRISO particles a robust and attractive waste container. This guarantees the Thorium fuel based MSR-SF (TMSR-SF) can be operated in a “high burnup open fuel cycle”. The irradiated thorium pebble fuel would be discharged and temporarily stored out of the core to convert the 233U isotope, then refilled in batch mode or continuously to achieve a very high burnup, which is essential for thorium utilization. In the present work, the neutronic properties, thorium utilization and radio-toxicity of three kind thorium-based fuel (enriched with 233U, 235U and 239Pu) in a wholecore model of TMSR-SF is compared. Recirculating pebble-bed TMSR-SF is distinguished from all other reactor types by the upward movement through and reinsertion of fuel into the core during operation. Core simulators must account for this movement and mixing in order to capture the physics of the equilibrium cycle core. A simple scheme is developed to simulate the construction of the equilibrium cycle in a TMSR-SF. As shown in Fig. 1, the core is divided into sub-zones in axial and radial directions; the pebbles in the top sub-zones will be discharged and mixed, then stored out of the core temporarily to convert the 233U. The reusable pebbles will be refilled into the core, while the spent pebbles which achieved the maximum burnup will be discarded. The spatial step of the axial and radial sub-zone as well as the depletion time step of the sub-zone in the core will be simulated and optimized to acquire the equilibrium cycle scheme that satisfied the burnup and safety features. Fig. 1 Sketch of the pebble fuel recirculating The achievable burnup of the TMSR-SF at equilibrium state that characterizes the core neutronic properties is essential for the analysis of the equilibrium composition, the thorium utilization and the radio-toxicity. However, the depletion analysis is particularly complex in the pebble bed reactor because the fuel elements are small, numerous, and continuously recirculated through the system. A simplified methodology based on the infinite uniform bed method [5] was used to estimate the achievable burnup and determine the equilibrium composition and neutronic properties of the core as well [6] . SCALE [4] version 6.1 is used for describing the thorium fuel based TMSR-SF. A 238-group ENDF/B-VII library is selected for time-dependent cross-section processing which reveals fuel composition variation during irradiation. The depletion is performed at constant power and 388 nuclides are tracked in trace quantities. The neutron spectra of the three fuel types at beginning of life(BOL) are shown inFig. 1. With similar graphite-to-heavy metal ratio (C/HM), the spectra of different fuels reveal different inherent characteristics. However, the typical inelastic scattering resonance of FLiBe, particularly 19F and 6Li, is presented at the resonance dips around 0.1 MeV energy range for each