Hydrodynamics of ECC Water Bypass and Refill of Lower Plenum at PWR-LOCA

摘要

In order to develop the hydraulic model of the ECC water bypass and the refill period at a PWR-LOCA, flashing transient and the downcomer Counter Current Flow Limit (CCFL) experiments were conducted with the large scale Cylindrical Core Test Facility (CCTF). The ECC water was bypassed by the two phase mixture swelled from the lower plenum to the downcomer at the flashing transient experiments. This swelling behavior was predicted well by using the void fraction correlation proposed by Okabe & Murao. The CCFL correlation based on Battelle experiments predicted well the bypass of the ECC water and its penetration into the lower plenum at the CCFL experiment.These swelling and CCFL models were combined to form the best estimate analytical model which was applied to a large break LOCA of a commercial PWR. Calculated results showed that (1) the proposed model predicted the existence of water at the initiation of the refill, (2) the present licensing calculation was conservative because it assumed no water at this time, and (3) this conservativeness mainly came from the neglect of the water in the downcomer at the ECC water bypass period.