Validation of the Monte Carlo Model Designed to Simulate the Neutronic Characteristics of Advanced Boiling Water Reactor Assembly

In the last few years, interest in burnup calculations using Monte Carlo methods has increased. Previous burnup codes have used diffusion theory for the neutronic portion of the codes. Diffusion theory works well for most reactors. However, diffusion theory does not produce accurate results in burnup problems that include strong absorbers or large voids. MCNPX code based on Mont Carlo Method, is used to design a three dimensional model for a BWR fuel assembly in a typical operating temperature and pressure conditions. A test case was compared with a benchmark problem and good agreement was found. The model is used to calculate the distribution of pin by pin power and flux inside the assembly. The effect of axial variation of water （coolant） density, and of control rods motion on the neutron flux and power distribution is analyzed. The effect of addition of Gd2O3 to natural uranium （0.711%） on both the thermal neutron flux and normalized power are analyzed. The concentration of U^235, U^238, Pu^239, and its isotopes is also calculated at burn-up 50 GWD/T.

Improvement of solidification model and analysis of 3D channel blockage with MPS method

In a severe accident of a nuclear power reactor,coolant channel blockage by solidified molten core debris may significantly influence the core degradations that follow.The moving particle semi-implicit(MPS)method is one of the Lagrangian-based particle methods for analyzing incompressible flows.In the study described in this paper,a novel solidification model for analyzing melt flowing channel blockage with the MPS method has been developed,which is suitable to attain a sufficient numerical accuracy with a reasonable calculation cost.The prompt velocity diffusion by viscosity is prioritized over the prompt velocity correction by the pressure term(for assuring incompressibility)within each time step over the“mushy zone”(between the solidus and liquidus temperature)for accurate modeling of solidification before fixing the coordinates of the completely solidified particles.To sustain the numerical accuracy and stability,the corrective matrix and particle shifting techniques have been applied to correct the discretization errors from irregular particle arrangements and to recover the regular particle arrangements,respectively.To validate the newly developed algorithm,2-D benchmark analyses are conducted for steady-state freezing of the water in a laminar flow between two parallel plates.Furthermore,3-D channel blockage analyses of a boiling water reactor(BWR)fuel support piece have been performed.The results show that a partial channel blockage develops from the vicinity of the speed limiter,which does not fully develop into a complete channel blockage,but still diverts the incoming melt flow that follows to the orifice region.