Flattening of UCFR Power Distribution with Fuel Zoning

The Ultra-long Cycle Fast Reactor (UCFR) utilizes a breed-and-burn strategy to achieve such a long cycle by using low enrichment uranium (LEU) as an igniter and natural uranium (NU) as a blanket material. Only the feasibility of the core from the neutronics point of view had been reported, so the optimization of UCFR was performed to consider thermal hydraulic feedback analysis and mitigate the power peaking issue. Also the study on using spent fuel for the blanket material has been performed. It was noticed that the high reactor power rating combined with high peaking could cause a very large amount of local power and consequently violate the limits of temperature or fast neutron fluence in the fuel and clad regions. Thorium fuel had been utilized as a blanket material to flatten the radial power distribution of UCFR-1000 and the result shows the radial peaking factor at the center decreases at the middle of cycle (MOC), which confirmed that thorium loading has an effect on the power flattening. In this paper, several designs of UCFR-1000 have been developed to flatten the power distribution not only at MOC but also throughout the whole cycle. They have an inner fuel region whose radius is 35% of the whole fuel region in contrast to the previous UCFR-1000 with an inner fuel region of only 1/4 the radius of the whole fuel region. It has been confirmed that fuel zoning can achieve radial power flattening, which decreases the maximum power peak. The Case 1 core has a center peak at BOC like the reference core, but the peaking factor has decreased due to the thorium fuel in the inner fuel region, which is shown by the fact that at year 20 it has decreased from 1.57 to 1.14. In contrast to the reference and Case 1 cores, the peak of the Case 2 and Case 3-1 cores appears in the peripheral region at BOC, which causes a power shape more flattened at MOC and EOC than that of Case 1. At EOC, Case 3-1 has a maximum peak of 1.15 while the reference case has a maximum peak of 1.34