Optimization of a seed and blanket thorium-uranium fuel cycle for pressurized water reactors

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2003.Includes bibliographical references (p. 189-194).A heterogeneous LWR core design, which employs a thorium/uranium once through fuel cycle, is optimized for good economics, wide safety margins, minimal waste burden and high proliferation resistance. The focus is on the Whole Assembly Seed and Blanket (WASB) concept, in which the individual seed and blanket regions each occupy one full-size PWR assembly in a checkerboard core configuration. A Westinghouse 4-loop 1150 MWe PWR was selected as the reference plant design. The optimized heterogeneous core, after several iterations, employs 84 seed assemblies and 109 blanket assemblies. Each assembly has the characteristic 17x17 rod array. The seed fuel is composed of 20 w/o enriched annular UO2 pellets. Erbium is used in the fresh seed to help regulate local power peaking and reduce soluble boron concentrations. Erbium was evenly distributed into all pin central holes except for the peripheral pins and four corner pins of each assembly where more erbium was used due to their higher power level. The blanket fuel is a mixture of 87% ThO2 - 13% UO2 by volume, where the uranium is enriched to 10 w/o. The blanket fuel pin diameter is larger than the seed fuel pin diameter. There are two separate fuel management flows: a standard three-batch scheme is adopted for the seed (18 month cycle length) and a single-batch for the blanket, which is to stay in the core for up to 9 seed cycles. The WASB core design was analyzed by well known tools in the nuclear industry. The neutronic analysis was performed using the Studsvik Core Management System (CMS), which consists of three codes: CASMO-4, TABLES-3 and SIMULATE-3. Thermal-hydraulic analysis was performed using EPRI's VIPRE-01.(cont.) Fuel performance was analyzed using FRAPCON. The radioactivity and decay heat from the spent seed and blanket fuel were studied using MIT's MCODE (which couples MCNP and ORIGEN) to do depletion calculations, and ORIGEN to analyze the spent fuel characteristics after discharge. The analyses show that the WASB core can satisfy the requirements of fuel cycle length and safety margins of conventional PWRs. The coefficients of reactivity are comparable to currently operating PWRs. However, the reduction in effective delayed neutron fraction (eff) requires careful review of the control systems because of its importance to short term power transients. Whole core analyses show that the total control rod worth of the WASB core is about 1/3 less than those of a typical PWR for a standard arrangement of Ag-In-Cd control rods in the core. The use of enriched boron in the control rods can effectively improve the control rod worth. The control rods have higher worth in the seed than in the blanket. Therefore, a new loading pattern has been designed so that almost all the control rods will be located in seed assemblies. However, the new pattern requires a redesign of the vessel head of the reactor, which is an added cost in case of retrofitting in existing PWRs. Though the WASB core has high power peaking factors, acceptable MDNBR in the core can be achieved under conservative assumptions by using grids with large local pressure loss coefficient in the blanket. However, the core pressure drop will increase by 70% ...by Dean Wang.Ph.D