Improvement of the numerical stability of a subchannel analysis code by the single-pass marching scheme

A subchannel analysis for low-pressure and low-flow conditions was performed by using a single-pass marching scheme. Most of the subchannel analysis codes for analyzing the thermal- hydraulic behavior of the nuclear reactor core use a multi-pass marching scheme, which uses an initial-value solution approach to a boundary-value problem. The multi-pass marching scheme uses the iteration of the axial sweep from the reactor core inlet to exit to satisfy the uniform-pressure boundary condition at the core exit, which results in the accumulation of numerical errors bringing about some numerical instabilities at low-pressure and low-flow conditions. As an alternative method to resolve this problem, the single-pass marching scheme has been implemented into the subchannel analysis code MATRA, which obtains a converged numerical solution through passing from the core inlet to exit only once by the iteration in an axial node until the lateral pressure difference between subchannels at each axial plane is sufficiently small, based on the assumption that the pressure distribution is uniform at all axial planes. The performance of the single-pass marching scheme was assessed for the low-pressure conditions used in the CHF experiments carried out at the Winfrith Establishment of the UKAEA. As the result, it was revealed that the single-pass marching scheme predicted the flow and enthalpy distributions obtained by the multi-pass marching scheme approximately well, and was numerically more stable