Study on Monte Carlo simulation method for three-dimensional continuous-energy stochastic neutron transport equation

[Background] Currently, all pressurized water reactor (PWR) power plants in operation and under construction in China are designed to start up with an external neutron source, except for the PWRs designed in Russia, which start up without one. Under the current international situation, obtaining primary neutron sources has become increasingly difficult, and start-up without an external neutron source has attracted increasing research attention. [Purpose] This study aims to develop an accurate simulation of the reactor core physical field during start-up without an external neutron source to support safe reactor operation. [Methods] During the start-up process, the stochastic neutron transport equation should be used instead of the Boltzmann transport equation to describe the neutron fluctuations in the early stage of the neutron field. A Monte Carlo (MC) simulation method for the three-dimensional (3D) continuous-energy stochastic neutron transport equation was developed based on the NECP-MCX code. [Results] Benchmarks, including bare plutonium spheres and bare uranium spheres, were calculated. For steady-state systems, the relative deviations of the calculated probability of initiation (POI), prompt neutron decay constant (PNDC), and burst waiting time from reference solutions were within 10%. For dynamic systems, the calculated probability of survival (POS) exhibited the same trend as that obtained from other simulation codes. [Conclusion] These results demonstrate the validity of the newly implemented solver for the three-dimensional continuous-energy stochastic neutron transport equation in the NECP-MCX code.