Analysis of the interfacial heat transfer process in a pebble fuel

In this paper the steady state and transient behavior of the heat transfer process in a fuel element of a Pebble Bed Modular Reactor (PBMR) is presented. The pebble fuel is a heterogeneous system which consists of carbon microspheres, which contain the fuel elements. These are constituted by a UO2 kernel, surrounded by some layers of pyrolytic carbon and silicon carbide. The pebble model considers three representative materials: TRISO particles, graphite and a coating that is also composed of graphite, which is cooled by helium gas in this type of reactor. The averaging volume method (AVM) was used for the homogenization of the pebble fuel and two approximations were derived: one-equation and two-equation models. The AVM captures the significant physical phenomena as well as the interfacial heat transfer between TRISO particles and graphite, which are important for non-equilibrium thermodynamic effects. The closure relationships for the pebble fuel are proposed in this work for different numbers of TRISO particles. The numerical experiments for the temperature analysis distribution in the pebble fuel during steady state and transient behavior are presented. The results show that the non-equilibrium thermodynamic effects are relatively important, and that the two-equation model presents a greatest heat transfer resistance compared to the one-equation model.

► Steady state and transient behavior of the heat transfer in a fuel element of a PBMR.
► The averaging volume method (AVM) was used for the homogenization of the pebble fuel.
► AVM captures the significant physical phenomena of the interfacial heat transfer.
► The closure relationships for pebble fuel are proposed to obtain the temperature.
► The results show that there is non-equilibrium thermodynamics effects.