An in-tank explosion occurred in a gaseous waste decay tank while the plant was in cold shutdown. The gaseous hydrogen present in the tank formed a flammable mixture and ignited due to air contamination of the inert nitrogen system used to control the hydrogen-oxygen concentrations in the tank. This resulted in a release of about 8.8 curies of noble gases and minor tank damage.Generally, the gas in pressurized water reactor waste gas systems is hydrogen rich and the oxygen concentration must be controlled to prevent flammable gas mixtures. Flammable concentration of gas mixtures can be prevented by limiting either the hydrogen or the oxygen concentration to less than 3 percent.The source of air was identified as instrument air leaking through check valves at the cross connections between instrument air and nitrogen lines. Under normal operating conditions, the pressure in the instrument air system is higher than that of the nitrogen system. These cross connections had been installed in response to a safety improvement request. The nitrogen system provided a backup gas supply to the air-operated steam supply valve for the steam-driven auxiliary feed water pump. This backup was installed to provide a "safety grade" auxiliary feed water system that satisfies the single-failure criteria. Other cross connections, which apparently did not leak air into the nitrogen system, had been previously installed to provide a redundant gas supply to the air-operated pressurizer relief valves and the associated block valves.