Nuclear power plant off-gas explosion

During the startup of the reactor with a new off-gas system, a spark from a closing valve initiated an explosion, caused by the presence of a flammable oxygen-hydrogen mixture.

The INITIATING CAUSE was the ignition of a flammable hydrogen-oxygen mixture in the off-gas system.
The use of a non-explosion proof valve provided the energy required for the ignition. However, the most effective preventive measure to avoid this type of mishaps would be to avoid the presence of both radiolitic gases in concentration exceeding the lower or higher flammability limits of the mixture.
Therefore, the ROOT CAUSE may be probably related to a shortcoming of the design. Considering that the affected system was a new off-gas system, inadequate procedures in the execution some operative step cannot be excluded.