Perturbation algorithm for refueling reactivity coefficient in the pebble-bed high temperature reactor

The fuel management mode of the pebble-bed high temperature reactor (PB-HTR) is on-line continuous refueling, that is, the reactor is refueling at the same time of operation, and the reactivity is continuously supplemented by continuous refueling. In this paper, the perturbation algorithm for refueling reactivity coefficient is proposed, and the internal mechanism of reactivity introduced by refueling in the PB-HTR is revealed, that is, refueling reactivity depends on the dot product of the pebble flow velocity and the differential of atomic density in space. Based on the neutron diffusion equation, the difference and differential formulas of refueling reactivity coefficient are derived. The application of differential formula, which can be directly calculated according to the current parameters of the reactor core before the refueling, is simpler than the difference formula. Then, the equilibrium core and running-in phase of HTR-PM are used to verify the effect of the perturbation algorithm and the calculation results agree well with the reference value from the VSOP code. The distribution of refueling reactivity in different spatial regions can also be analyzed based on the difference and differential formulas.