Patent Number: 060884185
Section: summary

BACKGROUND OF THE INVENTION 1. Field of Invention The present invention relates generally to an apparatus and method for controlled de-pressurization of a nuclear reactor, and more particularly, to an improved gas sparging system for reducing loads acting on structures submerged in a suppression pool. 2. Discussion In the event of over-pressurization of a nuclear reactor, relief valves may vent steam or reactor coolant into a suppression pool--a tank filled with liquid coolant--to dissipate the energy of the vented steam. The relief valve's abrupt opening, and subsequent delivery of high-pressure steam to the suppression pool, results in dynamic loads on suppression pool walls and structures. These dynamic loads, if large enough and if not properly accounted for during plant design, can damage structures submerged in the suppression pool. Dynamic loads within the suppression tank are thought to occur through at least two different mechanisms. In a typical pressure relief system, a relief valve exhausts high pressure steam into a discharge line, which is connected to a group of gas spargers. The spargers generally consist of vertical pipes whose ends are submerged in the suppression pool. When the pressure relief valve vents high pressure steam into the exhaust line, the steam must first displace noncondensable gas and liquid coolant present in the sparger pipe. During this sparger line clearing process, the high pressure steam compresses the noncondensable gas because of the relatively large inertia and high flow resistance of the liquid coolant. As the compressed noncondensable gas emerges from the sparger nozzles, it expands rapidly and then contracts due to over expansion. The expansion and contraction of the noncondensable gas repeats during the line clearing process, resulting in oscillatory pressure waves that impact submerged structures within the suppression pool. At some point after the liquid coolant has cleared the sparger pipe, the sparger injects high pressure steam into the suppression pool, creating a vapor-phase injection zone adjacent to the sparger nozzles (in practice there appears to be no clear transition between non-condensable gas venting and steam venting). Because of time-dependent imbalances between the steam mass flux and condensation rate, the high pressure steam injection process results in pressure oscillations. Like the line clearing process, oscillatory pressure waves during steam injection give rise to dynamic pressure loads on submerged structures within the suppression pool. In many conventional pressure relief systems, the gas spargers simultaneously exhaust steam into the liquid coolant at different locations, which distributes pressure forces acting on submerged structures within the suppression pool. But, dynamic loads on submerged structures can still be large because pressure disturbances from different spargers can combine. For example, if pressure disturbances from two adjacent spargers have the same frequency and phase relationship, the amplitude of the two pressure disturbances will add, resulting in a combined pressure disturbance that is greater than the individual pressure disturbances. Thus, pressure relief systems that take into account the interaction of pressure disturbances from individual spargers in order to minimize dynamic loads on structures within the suppression pool would be desirable. SUMMARY OF THE INVENTION In accordance with one aspect of the present invention, there is provided a method of mitigating pressure disturbances resulting from venting gas through a series of spargers into a suppression pool. The method comprises the steps of obtaining fundamental frequencies of the pressure disturbances arising at each of the spargers, and adjusting the time delay between the start of gas venting of any two successive spargers in order to optimize at substantially the following relation: ##EQU1## where .tau. represents the time delay and f represents the fundamental frequency of the disturbance at the later venting sparger. In accordance with another aspect of the present invention, there is provided a second method of mitigating pressure disturbances resulting from venting gas through a series of N spargers submerged in a suppression pool. The method comprises the steps of obtaining fundamental frequencies of the pressure disturbances arising at each of the spargers, and adjusting phase angles of the disturbances at two successive spargers so that they optimize at substantially the relation ##EQU2## where .phi..sub.i and .phi..sub.i-1 represent the phase angles of the disturbances at two successive spargers, i is an integer greater than one and less than or equal to N and denotes the serial position of the sparger, and m is a positive integer greater than or equal to zero. The step of adjusting the phase angles to satisfy the phase angle relationship is repeated for every pair of successive spargers. In accordance with a further aspect of the present invention, there is provided an apparatus for mitigating pressure disturbances resulting from venting steam from a nuclear reactor into a suppression pool. The apparatus comprises a series of N spargers submerged in the suppression pool, and a header sequentially connecting each of the spargers. The spargers are configured in such a way that when steam is vented into the header from the nuclear reactor, pressure disturbances arising at any two successive spargers to optimize at substantially the relation ##EQU3## where .phi..sub.i and .phi..sub.i-1 represent phase angles of the disturbances at two successive spargers, i is an integer greater than one and less than or equal to N and denotes the serial position of the sparger, and m is a positive integer greater than or equal to zero.