Unit 1 Plant power was stable at 90% following a plant startup. The Auxiliary Operator (AO) performed a pre-job brief with shift management before adding hydrogen to the main generator. While performing the addition, the AO attempted to verify open a half-inch hydrogen addition valve. The AO was unable to move the valve by hand and mistakenly assumed the valve was stuck on its closed seat. The valve is a normally open valve and the procedure step was to verify the valve was, in fact, open. The AO obtained a pipe wrench to assist in freeing the valve off of its "closed" seat. Using the pipe wrench, the valve handwheel was turned in the open direction. The AO attempted to open the valve by hand again. Unable to move the valve by hand, the pipe wrench was used to further open the valve approximately 6 turns (two turns of the handwheel for this particular valve results in full stem travel). The bonnet, handwheel, stem and valve internals were eventually ejected from the body of the valve by hydrogen system pressure. The AO promptly evacuated the area due to the hydrogen release and the hydrogen ignited within seconds. The AO informed the Control Room via radio of the leak and fire at the hydrogen add station. The AO and other operators proceeded immediately to the hydrogen bulk storage facility to isolate the hydrogen supply to the turbine building.

The control room supervisor directed a manual reactor trip. The plant Fire Plan and Abnormal Operating Procedure were entered and the fire brigade was dispatched. The hydrogen fire was extinguished following isolation of the hydrogen supply. However, there continued to be re-flash, arcing, and a small fire in the cable tray located in the overhead above the hydrogen add station. Fire extinguishers and eventually water were used to extinguish the fire in the cables.

A Notification of Unusual Event was declared for a fire not extinguished within 10 minutes. No injuries occurred as a result of this event. The damage in the cable tray required the replacement of 29 cables (480 volt). This and other damage to local components required the Unit to be shutdown for approximately five days to complete repairs.