Coupled MC–TH Transient Simulation of a Lead-Cooled Fast Reactor

Sammanfattning

Accurate transient analysis of lead-cooled fast reactors requires solving neutronics and thermal-hydraulics together, since temperature and density changes in the core affect the neutron flux and the power distribution simultaneously. The objective of this work is to develop a coupled Monte Carlo andthermal-hydraulic solver for the SEALER reactor, a small modular reactor concept developed by Blykalla. The SUNRISE-LFR v2 design is used as the reference design throughout the work. The neutronics solver is Serpent 2, operating in criticality mode and linked to a one-dimensional thermal-hydraulic solver through the multiphysics interface. The Improved Quasi-Static method is used to separate the fast amplitude evolution, integrated by the point kinetics equations, from the slower flux shape updates provided by Monte Carlo. A stochastic relaxation filter is applied to suppress statistical noise during the steady-state initialisation.The steady-state results show that the coupled solver converges to the expected axial power and temperature distributions, and the criticality search finds the critical control rod position in few iterations. Three transient scenarios are analysed: a control rod withdrawal, a rod drop anda coolant inlet temperature step. In all cases, the reactor response is consistent with the expected reactivity feedback from the Doppler effect and the coolant density change. The main limitations of the current implementation are the single-channel thermal-hydraulic model and the absence of structural expansion feedbacks.