Thermal hydraulic effect of fuel plate surface roughness

This study presents surface roughness measurements characteristic of the pre-film layer applied to a typical Advanced Test Reactor (ATR) fuel plate. This data is used to estimate the friction factor for thermal hydraulic flow calculations of a Gas Test Loop (GTL) system proposed for incorporation into ATR to provide a fast neutron flux environment for the testing of nuclear fuels and materials. To attain the required neutron flux, the design includes booster fuel plates clad with the same aluminum alloy as the ATR driver fuel and cooled with water supplied by the ATR primary coolant pumps. The objectives of this study are to (1) determine the surface roughness of the protective boehmite layer applied to the ATR driver fuel prior to reactor operations in order to specify the machining tolerances for the surface finish on simulated booster fuel plates in a GTL hydraulic flow test model and (2) assess the consequent thermal hydraulic impact due to surface roughness on the coolability of the booster fuel with a similar pre-film layer applied. While the maximum roughness of this coating is specified to be 1.6 μm (63 μin.), no precise data on the actual roughness were available. A representative sample coupon autoclaved with the ATR driver fuel to produce the pre-film coating was analyzed using optical profilometry. Measurements yielded a mean surface roughness of 0.53 μm (21 μin.). Results from a sensitivity study show that a ±15% deviation from the mean measured surface finish would have a minimal effect on coolant temperature, coolant flow rate, and fuel temperature. However, frictional losses from roughnesses greater than 1.5 μm (∼60 μin.) produce a marked decrease in flow rate, causing fuel and coolant temperatures to rise sharply.