School for Mechanical Engineering, Northwest University, Private Bag X6001, Potchefstroom, 2520, South Africa; Postgraduate School for Nuclear Engineering, North-West University, South Africa

van Antwerpen, H.J., School for Mechanical Engineering, Northwest University, Private Bag X6001, Potchefstroom, 2520, South Africa; Greyvenstein, G.P., Postgraduate School for Nuclear Engineering, North-West University, South Africa

Radiation heat transfer is a major mode of heat transfer in high temperature gas-cooled reactors (HTRs) 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 was developed and also implemented in the Flownex reactor model. This paper also describes the analytical formulae for view factor calculation in this detailed radiation heat transfer model. The HTR-10 and the 268 MW version of the PBMR were used as case studies in which Loss-of-Flow events without SCRAM were simulated. In these simulations, the time to reach recriticality was used as an indicator of heat removal effectiveness. With the HTR-10, other non-linear 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 268 MW PBMR DLOFC, the use of a detailed radiation model increased solution time with 30%. With both the HTR-10 and the PBMR, the radiation model had negligible effect on the total heat resistance from the reactor, as indicated by the time elapsed until recriticality. For system simulation codes that focus on transient response of a plant, it is not considered worthwhile to use a detailed radiation model, as the gain in accuracy does not justify the increased solution time or the implementation and verification effort. © 2008 H.J. van Antwerpen.

Codes (symbols); Fuel tanks; Gas cooled reactors; Heat exchangers; Heat resistance; Heat transfer; Heating equipment; High temperature reactors; Numerical methods; Polynomial approximation; Radiation; Thermoanalysis; Two dimensional; A-Plant; Analytical formulas; Case studies; Finite volumes; Flownex; Geometrical complexity; Heat removal; High temperature; High-temperature gas-cooled reactors; Non-linear phenomenon; Operating temperatures; Radiation heat transfer; Radiation modeling; Reactor modelling; Reactor systems; Solution processing; Solution time; System simulations; Systems simulation; Transient responses; View factor; Modal analysis