A study of flow mixing in a PWR vessel in asymmetric cooldown faults using the FLOW3D code

The Harwell computational fluid dynamics code, FLOW3D has been used to simulate a flow mixing test in the Oconee-1 reactor. The object was to test the ability of FLOW3D to describe thermal mixing in a PWR pressure vessel for the conditions of an over-cooling fault. The code produced reasonable estimates of the thermal diffusion observed in the Oconee test, with a tendency to underpredict mixing. However, the test exhibited gross swirl and asymmetric mixing which was not predicted by FLOW3D. Sensitivity studies to investigate the effects of downcomer ovality, inlet flow vorticity and flow imbalance between loops, have not revealed the source of the observed asymmetries