Neutron importance based lattice-to-core projection technique and its application for HTGR design using Monte Carlo method

In general, a nuclear core design process is divided into two subsequent phases, namely the fuel lattice design phase, and followed by the full core design phase such as for the light water reactor (LWR). The optimal design parameters obtained from the fuel design phase can be used in a straightforward manner in the full core design phase since the neutron mean free path is much smaller than the fuel assembly dimensions. Unfortunately, the above-mentioned favorable situation may not be applicable for a high-temperature gas-cooled reactor (HTGR) design process. In this paper, for the HTGR design process, we propose a new technique based on the neutron importance concept to make a projection of the optimal design parameters obtained from the fuel design phase to the full core design phase so that the optimization works required in the full core design phase can be significantly reduced. The proposed technique is implemented in the Japanese High-Temperature Engineering Test Reactor to find the optimal fuel composition that can achieve an average core discharge burnup of 80 GWd/tU using single-batch and multi-batch refueling schemes. Important core neutronics parameters, such as reactivity coefficient and power profiles, are also discussed. Moreover, the impact of the axial shuffling scheme on the fuel compact fuel temperature is also analyzed.