Simulation on fluctuations in molten salt reactor with 1D coupled neutronics/thermal-hydraulics model

WANG Jiangmeng, and CAO Xinrong^*

College of Nuclear Science and Technology, Harbin Engineering University, Harbin, 150001, P.R. China (wangjdmeng@163.com, caoxinrong@hrbeu.edu.cn)

Abstract: In this study, the more realistic behaviors of the fluctuations in neutron fluxes, fuel temperature and velocity induced by a propagating perturbation in the molten salt reactor (MSR) are simulated by coupling the neutronic and thermal-hydraulic models. The neutron kinetic model is established based on neutron diffusion theory, in which the neutron is classified into two groups and the delayed neutron precursors into an averaged family. The thermal feedback is considered by introducing a heat transfer model, in which the temperature-dependent group constants are produced with the HELIOS code. The equations for the fluctuations are deduced based on linear perturbation theory, assuming the perturbation is small enough. All the equations are discretized and numerically solved by developing a code. In the static case, the increasing fuel velocity is found to have significant influence on the precursors, whereas the fast and thermal neutron fluxes nearly stay the same due to the small delayed neutron fraction of the fuel. In the dynamic case, the main efforts are devoted to the effects of several factors on the fluctuations. The applicability of linear perturbation theory is also quantitatively evaluated. The results show that linear perturbation theory works well in the scope of 5K temperature change. In addition, the difference between the amplitudes of the temperature noises in the coupled and pure neutronic models increases oscillatorily along the flow direction, thereby indicating the much stronger spatial oscillation of the neutron noises in the realistic system. Moreover, the fluctuations in various kinds of group constants have significant effects on the neutron noises and their contributions should be accounted for.

Keyword: MSR; neutron noise; temperature noise; coupled neutronics/thermal-hydraulics; propagating perturbation.