Tracer gas testing within the Palo Verde Nuclear Generating Station unit 3 auxiliary building

High concentrations of radioactive noble gases (primarily Xe133, Xe135, and Kr85) have been observed on several occasions in the Auxiliary Buildings of both Units 1 and 2 at Palo Verde Nuclear Generating Station. This airborne activity has been detected on various levels of the Auxiliary Buildings, and often has not been identifiable as to source area or initiating activity. Multiple tracer gas testing, using low concentrations of selected halocarbon gases, was proposed as a method of determining the existence and magnitude of non-ducted airflow. These gases can be detected at very low concentrations and have been used previously in industrial facilities to unambiguously trace diverse air flows. Three distinct suites of tests were performed. Multiple tracer gas testing during the first two test suites demonstrated migration both along floors and in some cases between floors indicating that ventilation within the source rooms was insufficient to contain potential noble gas release. In addition, floor-to-floor migration was shown to occur through the building drain system, as well as through unsealed penetrations and pipe chases. The third suite provided an explicit demonstration of the isolation ability of the ventilation system while operating in the SIAS mode. The testing demonstrated the utility and applicability of multiple tracer gas techniques for diagnosing actual operating behavior of nuclear air and gas treatment systems, as well as for disclosing the existence of non-design leakage through unsealed penetrations, ducts and pipe chases. To our knowledge these techniques have never been used in the nuclear industry to verify that air cleaning and ventilation systems perform as designed