Development of Hybrid Neutron Dynamics Algorithm Based on Transient Fission Matrix Combined Method

The deterministic method and the Monte Carlo method are used in the transient analysis of nuclear reaction systems, but high-fidelity methods are inefficient in computation, and the accuracy of the low order method is not sufficient. This paper proposed a transient patching fission matrix method based on the theories of transient fission matrix and patching fission matrix, offering a precise and rapid approach for transient analysis. The transient fission matrix combined (TFMC) method is a hybrid approach capable of transport calculations, but it requires extensive computation time to generate transient fission matrices. The patching fission matrix method efficiently processes the fission matrix database and quickly computes the whole core steady-state, yet it has not been applied in transient analysis. The TFMC theory includes the database generation technique, the transient fission matrix combined method, and the control rod correction ratio technique. All elements in the TFMC database were arranged according to material properties. The homogeneous model was simulated using a fixed-source calculation method. A database of various material properties was prepared, enabling this calculation to be performed only once. The TFMC method needs to consider the type of neutrons, so it is necessary to use prompt and delayed neutron spectra sources in the neutron source, and the prompt and delayed neutrons generated by fission in each cell also need to be separately counted. Based on the actual insertion of the control rods, select data concatenation in the database to combination real transient fission matrix. TFMC also employs a control rod correction ratio method, which solves the issue of needing to recalculate for control rods at different positions. This paper validated the transient patching fission matrix method through transient conditions in a single rod model and an assembly model. This work verified the transient fission matrix combined method through the single rod model and the assembly model. The results show that in the single-rod model, the steady-state process k_eff and the distribution of fission rate have a deviation of 4 pcm from the reference solution, and the power root mean square error is 0.14%. The trend of relative power change in the transient process is consistent with the reference solution. The TFMC, compared to the transient fission matrix, reduces the total computation time by four orders of magnitude without losing accuracy. In the component model rod insertion and extraction process, compared to the Monte Carlo method, the TFMC completes high-precision three-dimensional transient calculations in 61 CPU hours. The trend of relative power change is highly consistent with the Monte Carlo method, and the computation time is nearly 70 times shorter than that of the Monte Carlo method.