We proposed the “Flexible Fuel Cycle Initiative” (FFCI), which has flexibility for the uncertainties like the introduction speed of FBRs. On the other hand, during the FBR introduction period, Pu from LWR spent fuel is used for startup of FBRs. But the FBR core being loaded with Pu from LWR spent fuel has larger burnup reactivity due to its larger isotopic fraction of Pu-241 than the core being loaded with Pu from the FBR multi-recycling core. The increased burnup reactivity may reduce the cycle length of the FBR. In this paper, an FBR transitional core concept to handle the issues of the FBR introductory period was investigated. Core specifications are based on the compact type sodium-cooled MOX-fueled core designed in the Japanese FBR cycle feasibility studies, because the lower Pu inventory should be better for the FBR introductory period in view of its flexibility for the required reprocessing amount of LWR spent fuel to start up the FBR. The reference specifications are selected as follows. Output is 1500MWe and the average discharge fuel burnup is about 150GWd/t. Minor Actinides (MAs) recovered from LWR spent fuels which provide Pu to startup FBR are loaded to the initial loading fuels and exchanged fuels during some cycles until equilibrium. We set a kind of MA fraction rate of the initial loading fuel with 4 as the number of the fuel exchange batches. The average of the MA fraction of the initial loading fuel assumed is 3%, and the MA fraction of the exchange fuel is set as 5%. This 5% maximum of the MA fraction is based on the irradiation results of the experimental fast reactor Joyo. The core performance including burnup characteristics and reactivity coefficient were evaluated, and we confirmed that the transitional core from the initial loading until equilibrium cycle loaded Pu from LWR spent fuels could keep the resemble performance with the FBR multi-recycling core.

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