hydrogen fire produced by radiolysis

A control room operator at nuclear power unit observed pressure spikes on the pressuriser level instruments. At the same time, a loud rumbling sound was heard in the containment. The containment vent radiation alarm activated, and as a precaution, the containment was evacuated.

At the time of the event, the pressuriser was drained and vented to both the pressuriser 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 INITATING CAUSE was the ignition and explosion of hydrogen which had formed by radiolysis in the cooling water of the reactor.
Welding activities associated with the pressurizer loop seal modification had ignited the hydrogen gas coming out of solution in the water and accumulated inside the pressurizer and associated piping.

The ROOT CAUSE is not provided by the NRC report. Nevertheless, the presence of hydrogen in solution in the cooling water of a pressurised water reactor is a well-known phenomenon. The depressurisation of the cooling water circuit created the conditions for hydrogen to leave the solution, forming a flammable mixture in the gas phase, and provisions should have been taken to eliminate the possibility of its presence or its ignition.

nuclear reactor pressuriser,

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 licensee imposed additional controls for welding on the primary system. These require that samples be taken and analysed before initiating an arc to determine if explosive gasses are present. The licensee also evaluated pressurizer stresses associated with the rapid hydrogen burn and completed a piping support inspection and internal inspection of the pressurizer.