ECC delivery study: experimental results and discussion

The results of a series of separate effects and combined effects tests in a 1/15-scale planar model of a Pressurized Water Reactor (PWR) downcomer region at or near atmospheric pressure are presented. These tests are part of a continuing effort to develop analytical tools and predictive techniques to permit the calculation of lower plenum filling rates, as a function of time, during a postulated Loss-of-Coolant Accident (LOCA) in PWRs. The effects of reversed core steam flow, cold leg steam flow, and superheated walls on the timing and rate of delivery to the lower plenum of simulated Emergency Core Coolant (ECC) were investigated separately and in various combinations along with variations in the downcomer gap spacing, ECC water temperature, ECC water flow rate, and other parameters. The tests with reversed core steam flow and neutral walls showed that the downcomer gap size does not correlate the results, and also that increased water flow rate or water subcooling resulted in delivery to the lower plenum at higher steam flow rates (and hence earlier in the postulated LOCA transient). Reversed core steam flow and superheated walls combined produced an effect on water delivery which was greater than either effect acting separately. In addition, plenum filling was found not to be an ''off-on'' process and thus the filling rate as a function of time is the important result from downcomer penetration studies