Predicting the capability of decay-heat removal for a pebble bed of the high-temperature gas-cooled reactor is a cutting-edge issue in the research of generation IV advanced reactors. In this research, numerical simulations of heat transfer in the pebble bed reactor have been conducted by developing two kinds of models, a particle-scale model and a porous media model. The aim is to estimate the effect thermal conductivity of the pebble bed and predict the internal temperature field. The former model relies on the pebble–pebble conduction and radiation at a scale as fine as every fuel pebble. The latter one is an overall model at the scale mainly on the whole feature and capability of the pebble bed reactor. These two models have generated consistent results in an intermediate case considering the effective thermal conductivity for radiation and conduction of the pebbles. Based on this consistency, the pebble bed’s overall features of the internal field of the temperature field in pressurized helium have been well predicted. The simulation results validate the pretty good capability of the pebble bed reactor on decay-heat removal. The maximum temperature of the reactor core is limited to about 350 °C at the 7 MPa pressurized helium. This means the safety of the HTR-PM reactor could be reasonably guaranteed. •A new SPH kernel by combining a polynomial and a reciprocal term is proposed.•Validated by dam-break, the kernel is used for natural convection in reactor cores.•Comparing to existing kernels shows the differences and (dis)-advantages of them.•Various temperature differences, channel numbers/lengths are simulated.•A mapping method is used to transfer particles’ velocity/temperature to Eulerian.