الفهرس 
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Abstract
Using 232Th, fuel pebbles loading pattern and online refueling method are designed for the MIT Modular Pebble Bed HTGR (MPBR) Reactor. To achieve the maximum use of 232Th with relatively minimal reactivity insertion in the event of a water ingress accident (WIA), various compositions of Thorium Low Enriched Uranium (ThLEU) have been examined. These compositions have varying Heavy Metals loading and 235U enrichment. Using an MCNP5 code odel for the MPBR reactor, these compositions are investigated and the optimum composition is selected. Different loading patterns of pebbles are examined using a mixture of pure LEU and ThLEU fuel pebbles with the chosen fuel composition. To attain criticality while mitigating the negative effects of fuel and reflector temperature coefficients of reactivity, as well as the poisoning effect of xenon build-up during reactor startup, one loading strategy is chosen. With the fewest possible batches of LEU fuel pebbles, this loading pattern also is designed to reduce the axial power peaking of the MPBR core central hot channel and flatten its radial power profile. To simulate the multicycle refueling mechanism and examine the effects of a suggested online refueling strategy for the MPBR reactor, a calculation procedure combining full core calculations and fuel pebble depletion is created. For each LEU and ThLEU fuel pebble, the nuclide concentrations have been identified as a function of the pebble burnup using the SERPENT code. The number of cycles needed for LEU and ThLEU fuel pebbles to achieve their maximum burnup is estimated based on a suggested multicycle online refueling strategy. Using the single pebble model in COMSOL software, the maximum fuel temperature is determined for the hottest stone at the maximum power density.