Experimental study of void behavior in a suppression pool of a boiling water reactor during the blowdown period of a loss of coolant accident.

摘要

The possible failure of an Emergency Core Cooling System (ECCS) train due to a large amount of entrained gas in the ECCS pump suction piping in a Loss of Coolant Accident (LOCA) is one of the potential engineering problems faced in a Boiling Water Reactor (BWR) power plant. To analyze potential gas intrusion into the ECCS pump suction piping, the study of void behavior in the Suppression Pool (SP) during the LOCA is necessary. The void fraction distribution and void penetration are considered as the key parameters in the problem analysis. Two sets of experiments, namely, steady-state tests and transient tests were conducted using the Purdue University Multi-Dimensional Integral Test Assembly for ESBWR application (PUMA-E) to study void behavior in the SP during the blowdown. The design of the test apparatus used is based on the scaling analysis from a prototypical BWR containment (MARK-I) with consideration of the downcomer size, the SP water level, and the downcomer water submergence depth. Several instruments were installed to obtain the required experimental data, such as inlet gas volumetric flow, void fraction, pressure, and temperature.;For the steady-state tests, the air was injected through a downcomer pipe in the SP in order to simulate the physical phenomena in the SP during the initial blowdown of LOCA. Thirty tests were performed with two different downcomer sizes (0.076 and 0.102 m), various air volumetric flow rates or flux (0.003 to 0.153 m3/s or 0.5 to 24.7 m/s), initial downcomer void conditions (fully filled with water, partially void, and completely void) and air velocity ramp rates (one to two seconds). Two phases of the experiment were observed, namely, the initial phase and the quasi-steady phase. The initial phase produced the maximum void penetration depth; and the quasi-steady phase showed less void penetration with oscillation in the void penetration. The air volumetric flow rate was found to have a minor effect on the void fraction distribution and void penetration during the initial phase, which was in the range of high air volumetric flow rate conditions; however, it strongly impacted the void fraction distribution and void penetration during the quasi-steady phase for the entire ranges of air volumetric flow rate conditions. The initial downcomer void conditions were found to strongly affect the void fraction distribution and void penetration during the initial phase. The air velocity ramp rates were found to have a minor impact on void distribution and penetration in both phases. The downcomer cross-sectional areas did not significantly impact the average void penetration (both axial and radial) for those tests having comparable initial air volumetric flux.;For the transient tests, sequential flows of air, steam-air mixtures, and pure steam, with the various flow rate conditions, were injected from the Drywell (DW) through a downcomer pipe in the SP. Eight tests were conducted with two different downcomer sizes (0.076 and 0.102 m), various gas volumetric flux levels (17 to 84 m/s) at the downcomer, and two different initial air concentration conditions in the DW (80% and 100% of air concentration). Three phases of experiments, (i.e., initial phase, quasi-steady phase, and chugging phase) were observed. The void penetration depth was at the maximum level in the initial phase and at a reduced level in the quasi-steady phase. The chugging that occurred at the tail end of the experiment provided renewed void penetrations that were comparable to those in the initial phase. It was determined that the void fraction distribution and the void penetration in the SP were governed by the gas volumetric flux at the downcomer and by the air concentration in the downcomer. The downcomer cross-sectional areas did not significantly impact the average void penetration (both axial and radial) for those tests having comparable initial gas volumetric flux.