Calculation Aspects of Detailed Radiation Heat Transfer

Radiation heat transfer is a major mode of heat transfer in High Temperature Gas-cooled Reactors because of the high operating temperatures. It is however a difficult phenomenon to calculate in full detail due to its geometrical complexity. One has to use either a numerical method or complex analytical view factor formulae. Except the difficulty of view factor calculation, a vast number of calculation elements are required to consider all interacting surfaces around a cavity. A common approximation in systems simulation codes is to connect only directly opposing surfaces with a view factor of one. The accuracy of this approximation was investigated with a finite volume, two-dimensional axial-symmetric reactor model implemented in the systems simulation code Flownex. A detailed radiation model using analytical formulae [1] for view factor calculation was also implemented with the HTR-10 as well as the 268MW version of the PBMR used as case studies. Loss-of-Flow events without SCRAM were simulated and the time to recriticality used as indicator of heat removal effectiveness. With the HTR-10, other nonlinear phenomena in the reactor core constrained the solution process, so that the number of radiation elements had no effect on solution time, while with the 268MW PBMR DLOFC, the use of a detailed radiation model increased solution time with 30 percent. (author)