Graduate Thesis Or Dissertation

Feasibility Study of Using HANARO Fuel Rods in WWR-SM Reactor

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  • Interest in increased fuel supply stability has driven an investigation into possible alternate fuel for use in the WWR-SM research reactor at the Institute of Nuclear Physics in Uzbekistan. The WWR-SM is a high-power, pool-type research reactor currently utilizing IRT-4M fuel made by a single Russian supplier. A candidate for new fuel is the Korean-made High-flux Advanced Neutron Application Reactor (HANARO) fuel rod, which maintains the low 19.75% enrichment of the current fuel but has a different configuration. To examine the safety and operational parameters of the HANARO fuel rods in the WWR-SM core, the Monte Carlo N-Particle transport code is used to model the neutronics and the PLTEMP code from Argonne National Lab is used to compute thermal-hydraulic parameters. While the core structure will remain unchanged, the plate-type IRT-3M assembly models will be replaced with models of assemblies using HANARO fuel rods that preserve the outer dimensions and central flux trap for irradiation of samples. Three candidate assemblies are assessed for neutronic viability. Two contain the standard HANARO fuel rods with different pitches between 16 fuel rods arranged within the space between basic structures of the IRT assemblies. The other assembly contains a modified HANARO fuel rod with a reduced fuel meat radius; this smaller rod allows for more fuel rods in the assembly, thus increasing the heat transfer area. Neutronic results indicate viability of one of the standard HANARO fuel rod assemblies. The modified HANARO fuel rod assemblies do not maintain sufficient excess reactivity when placed in the core configuration to operate at a critical state, making standard WWR-SM operations impossible. Whearas the undermoderated standard HANARO fuel rod assemblies have a significantly positive temperature coefficient of reactivity making them unsafe in positive temperature transient conditions. When 24 optimal pitch assemblies are in the WWR-SM core model, the core has sufficient excess reactivity to maintain a critical state and produces flux levels within the target range of 10¹² to 3 x 10¹⁴ n cm⁻² s⁻¹. Power distributions and depletion analysis suggest that the cycle length for this core would be at least comparable to the 24 day cycle for IRT-3M fuel assemblies, if not better. However, this core configuration has too much excess reactivity and does not meet the operational shutdown margin requirement. Minor modifications could be made to the core configuration to meet the shutdown margin but are not explored here. Thermal-hydraulic analysis of the optimal pitch HANARO fuel rod shows that low flow rates and the existing pressure drop across the WWR-SM core would result in nucleate boiling along the length of the fuel rod, though flow remains stable. In addition, the maximum temperature in the fuel in some simulations reaches temperatures within 20% of the operational design maximum of 350 °C. Temperature results from the PLTEMP code agree well with hand calculations with an average relative difference of 7%.
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