On February 3, 1994, a control room operator at Surry Power Station, Unit 1, observed pressure spikes on the pressurizer level instruments. At the same time, a loud rumbling sound was heard in the containment. The containment vent radiation alarm was received, and as a precaution, the containment was evacuated.At the time of the event, the pressurizer was drained and vented to both the pressurizer relief tank and the containment atmosphere through a filtered covering over the piping where the safety valves had been removed. The reactor vessel head had just been detensioned. There was fuel in the reactor vessel. A modification to eliminate the pressurizer loop seals was in progress.The pressure fluctuations inside the pressurizer caused the expulsion of radioactive gases from the pressurizer into the containment causing alarms on both the containment radiation monitor and the ventilation radiation monitor where the ongoing filtered containment purge was directed. Licensee personnel calculated the release rate through the monitored ventilation purge path to be approximately 30 percent of the technical specification limits. One worker received an estimated internal exposure of 70 Sv [7 mrems] and a total dose of 160 Sv [16 mrems].The licensee investigation showed that the screens for excluding foreign material that were taped over the pressurizer side of the piping where the three safety valves were removed were discoloured and appeared burned. The licensee concluded that a hydrogen gas burn had occurred inside the pressurizer. Welding activities associated with the pressurizer loop seal modification had ignited the hydrogen gas that had come out of solution and accumulated inside the pressurizer and associated piping. There were no personnel inquiries.