A Successful Approach to Containment Integrated Leakage Rate Testing

Abstract

The integrated leakage rate test is one of the most important safety related tests performed at a nuclear-power plant in that it is the only one which demonstrates the ability of the containment to limit post LOCA releases to the outside environment. In theory the test is quite simple to perform. The containment is pressurized to design basis accident pressure and isolated. Leakage rate is then calculated as the rate of change of the absolute pressure/mean temperature ratio. Pressure is easily measured to the required levels of sensitivity and stability using commercially available ManoMeters. Unfortunately, all of the regulations and standards which govern such testing require that total pressure be reduced by mean vapor pressure to account for condensation and evaporation of water in the containment. The determination of mean vapor pressure can be an error prone process, often to the extent that the error exceeds the actual vapor pressure change (in extreme cases, the change in calculated leakaqe rate resulting from the error in vapor pressure correction may exceed the true leakage rate; and, the end result may not be conservative). As is the case with pressure, individual temperatures can be measured with the necessary sensitivity and stability. However, temperatures are often measured in such a way as to make calculation of a true mean temperature all but impossible. An all too common result of this is that the calculated P/1 ratio meanders in a rather erratic fashion, making leakage rate computations meaningless. Of course, after sufficient time (usually on the outage critical path) temperature change becomes negligible and leakage rate is effectively determined by pressure decay. A number of rather unique tests are used to determine whether or not containment atmospheric condition data are adequate for leakage rate determination. These tests, which set limits on data scatter and meander, are useful but are no substitute for proper attention to instrumentation. Examples in this paper illustrate how a well selected and carefully installed group of instruments can effectively ensure successful completion of an integrated leakaqe rate test in minimum time. Other examples illustrate some of the Major problems which arise when tests are conducted, as they all too often are, without due consideration for the importance of good measurements.