We are devoted to performing multi-physics high-ﬁdelity ”virtual experiments” on nuclear reactor cores which are more cost-eﬃcient and versatile than physical experiments for testing the conceptual ideas and optimising the designs. The computational ﬂuid dynamics (CFD) approach is used for the thermal-hydraulic simulations and the Monte Carlo method accounts for the neutron transport simulations. These two branches proceed in parallel with the ﬁnal goal to being able to run accurate coupled simulations for the reactor cores. With our recent achievements, we are now focusing on two directions. For the CFD part, we are intending to construct mechanistic models for heat and mass transfer in two-phase annular flows with disturbance waves, as well as heat transfer in supercritical water, based on CFD data. For the Monte Carlo part, we are intending to improve the eﬃciency for large scale Monte Carlo simulations.