URGAP, a gap conductance model for transient conditions

A gap conductance model, URGAP, has been developed with contributions from solid, fluid and radiation heat transfer components. Model parameters are easily available, independent of different combinations of material surfaces. The model parameters were fitted to 388 data points under reactor conditions. For model verification, another 274 data points of steel-steel and aluminium-aluminium interfaces were used. For minor surface roughnesses normally prevailing in reactor fuel elements the model asymptotically yields Ross and Stoute's model for the open gap, which is thus confirmed. Materials data were carefully checked over a wide range of temperatures. Special attention was paid to the contact term for high temperatures. Thus, the model can be applied to transients. The URGAP model is currently being used successfully in several codes (e.g. URANUS, SSYST). (author)