Hot wall experiments in a simulated multiloop PWR geometry

Results are presented of an investigation of the effects of heated vessel and core walls on downcomer flow behavior in the absence of core or cold leg steam flows. Energy stored in these walls could interact with emergency core coolant (ECC) water during a postulated loss-of-coolant accident (LOCA) in a pressurized water reactor (PWR) and delay the delivery of this water to the lower plenum. Several of the parameters which affect this process were explored experimentally in a 1/15-scale, planar model of a multiloop PWR downcomer. The variables which strongly affect the hot wall delay period were found to be: inlet water temperature, initial wall temperature, downcomer gap spacing, and downcomer length. For example, as the inlet water subcooling was decreased from 1600F to 00F the hot wall delay time increased by an order of magnitude. The influence on the delay time of wall and water temperatures, downcomer dimensions, number of cold legs, and water flow rate is presented. Because the hot wall effect is not an off/on phenomenon, a number of representative lower plenum filling rate experimental traces are presented along with a technique for reconstructing other traces from the tabular data presented. A theory for the hot wall effect is presented and compared with the delay time and lower plenum filling rate data. (U.S.)