Axial Block Shuffling in a Block-Type VHTR for Improved Core Performances

Fuel reloading should be done periodically in blocktype VHTRs (Very High Temperature Reactors) to maintain a required cycle length. In each reloading, part of fuels in the core is replaced by fresh fuels. Either a 2- batch or 3-batch fuel management is often adopted. A typical fuel reloading scheme is radial shuffling of fuel columns, in which positions of fuel columns are changed. However, a radial fuel shuffling has several drawbacks such as difficulty in power distribution control and limited shuffling due to control rod positions. As an alternative fuel management, the axial-only fuel shuffling schemes were introduced. A simple 2-batch axial scheme was used, while 3- and 4-batch axial fuel management strategies were adopted in Ref. 3. When the batch size is 3 or 4, there are lots of possible axial shuffling strategies. We tried to find an optimal axial shuffling strategy of a VHTR in terms of the fuel temperature and cycle length (or discharge burnup)