Thermal-hydraulic effects of inserts in a fast reactor fuel bundle

The efficacy of inserts in the peripheral sub channels of a model fast reactor pin bundle with helical spacer wire has been investigated by 3 dimensional computational fluid dynamics (CFD) simulations as well as water experiments. Three different types of inserts, viz., triangular, semicircular and circular inserts have been considered along with a bundle without any insert. Sub channel sodium temperature, axial and circumferential temperature distributions of clad among the pins, Nusselt number and friction factor are the parameters targeted for the investigation. The CFD model has been validated by comparing the predicted pressure drop against measured pressure drop for water flow, for the cases of bundle with circular inserts and bundle without inserts. Thermal part of the CFD model has been validated against published Nusselt number for bundles without any insert. The inserts are found to reduce flow bypass in the peripheral sub channels and promote increase in central sub channel flow. As a consequence of this, the clad temperature with inserts was found to be lower than the bundle without any inserts. However, inserts were found to increase the friction factor. The increase in Nusselt number and friction factor for circular inserts are 86% and 17% more than that of a bundle without inserts. To find the optimum insert type, a Performance Enhancement Criterion (PEC) factor has been defined. This factor is found to be the maximum for circular inserts.