Verification of MOC Transient Solver in Neutron Transport Code STREAM

In the state of the art of computer technology, the transient analysis in the nuclear reactor has become on demand of the nuclear engineering field. Recently, the two-dimensional (2D) transient capability has been adopted to the neutron transport code STREAM developed in UNIST. This paper first presents the approach to solve the delayed neutron precursors equation based on a second-order approximation for the fission source. The Theta method with the well-known Crank-Nicholsen scheme providing a second-order accurate is then applied to tackle the time integration in the right-hand side of the time-dependent neutron transport. Eventually, the Methods Of Characteristic (MOC) solver in the steady state with excellent performance and accuracy in STREAM is modified with the delayed neutron term to solve the transient cases. Additionally, a multi-group CMFD accelerator to alleviate the computational burden of the simulation. The 2D C5G7-TD cases, namely TD0, TD1, TD2, TD3 in the C5G7-TD benchmark suite are then used for verification. The STREAM time-dependent MOC calculation results show good agreement with results of a deterministic transport analysis code, nTRACER with the maximum root mean square in the disparity for the power level change is 0.444 % for the TD3-4 case. With this high fidelity to replicate the solution in the time-dependent transport equation, the 2D transient analysis capability of STREAM code has been proved. This work plays as a foundation for the later 3D whole core transient solver accompanied with TH1D feedbacks for practical transient problems