Release of hydrogen produced by radiolysis

A hydrogen leak was discovered inside an auxiliary building of a nuclear power plant recently licensed. The plant had never reached criticality and was in cold shutdown at the time of the event.

The hydrogen presence was discovered by chance. When a chlorine detector acivated in the control building, but no chlorine gas was found, the shift supervisor ordered further investigation in plant areas. Because there were no fixed detectors, portable survey instruments were used to identify gaseous mixtures. Hydrogen was detected in the auxiliary building at percent of the lower flammability limit (LFL) for hydrogen. A level of about 30% of LFL corresponds to about 1.2 percent hydrogen by volume. This reading was erroneously reported to the control room as 20 to 30 percent hydrogen by volume. The presence of hydrogen was due to a leaking valve on the hydrogen supply circuit used for the turbine cooling.
The on-shift supervisor declared an " unusual event" with a report to the NRC via the emergency notification system .

The INITIATING CAUSE was a small leak of hydrogen on a supply line of the hydrogen system of auxiliary services.

The small leak was in a auxiliary building, on a globe valve of a conventional design with no special packing. The globe valve was mounted in a vertical pipe chase where little ventilation was present because the ongoing HVAC testing had temporarily shut down the ventilation.

ROOT CAUSE was related to the needs of small improvement to the safety design of the hydrogen supply system.

globe valve

This plant was recently licensed, had never been critical, and was in cold shutdown at the time of the event.

HYDROGN USES in the POWER PLANT
The liquid hydrogen storage system ('skid') waa located outside the turbine building. A cryogenic gas line connect the skid to the building.
The liquid hydrogen was being used in three auxiliary systems :
(1) as a cover gas in the Volume Control Tank,
(2) in the plants waste gas system
(3) to cool the generator.

No consequence of any type, but modification by the licensee to the plants and its operation following lesson learned

The fact has been reported in a US.NRC information notice, which however did only issued suggestions, not requirements; no specific action or written response has been required.

When hydrogen was discovered in the auxiliary building (1.2% in volume), the operator isolated the cryogenic hydrogen skid outside the turbine building and located the source of the leak on the packing of a globe valve in a small line to the volume control tank . The operator opened doors allowing hydrogen to dissipate quickly.

This event is a near miss with a component of false alarm.
The false alarm was caused by the erroneous communication of the hydrogen concentration values by the investigation team to the control room. The team measured percentages of hydrogen related to the lower flammability limit, 4% but communicated them as absolute hydrogen concentrations in air. With 30% of hydrogen in air, the operator had to notify the authority of the case. With the real value of only 1.2%, this would not have been necessary. This misunderstanding is not uncommon in facilities dealing with hydrogen, because both values are provided as % by monitors. To ensure the correct understanding of is being measured the situation requires a deep understanding of gaseous hydrogen behaviour, its detection methods and know-how o what is measured and monitored.

The near miss was related to a small leak on a valve that was not fit for hydrogen use. Probably it could not have cumulated to level detectable by safety sensors, if the building ventilation would not have been switched off during the testing of the HVAC. More worrying was that the fact that apparently fixed detectors were not available, in an enclosed space where hydrogen was constantly present.

The lessons consisted in the following important elements:
(1) To ensure an effective in-plant communications during unplanned events.
(2) To use valves fit-for-use for (liquid) hydrogen.
(3) To calibrate correctly excess flow check valves.
(4) To maintain and test regularly heating, ventilation and air conditioning (HVAC).
(5) To optimise hydrogen line routing. Although the plant involved in this event re-examined the routing of hydrogen lines throughout.
the auxiliary building and found no problems, older plants may not have examined this question in detail.

The following measures were considered by the licensee:
(1) ways to improve communications in the plant during events and the training of personnel in reading portable instruments.
(2) the use of other types of valves, such as valves with a diaphragm or bellows rather than conventional stem packing, in lines containing hydrogen.
(3) the set point for the excess flow check valves the hydrogen lines. These check valves are designed to limit the flow of hydrogen in the event of a large leak so that when combined with proper ventilation in rooms with hydrogen lines, hydrogen levels would remain within specified limits throughout the plant.

Also because of this event, The NRC staff reviewed a previously published EPRI/BWROG report titled "Guidelines for Permanent BWR Hydrogen Water Chemistry Installation," . In the new document there will be guidelines for design, operation, maintenance, surveillance, and testing of hydrogen supply systems.