Release of Hydrogen Produced by Radiolysis
On February, 20, 1987, the Vogtle nuclear power plant reported a hydrogen leak inside the auxiliary building. This plant was recently licensed, had never been critical, and was in cold shutdown at the time of the event. The discovery of this problem was as a result of an unrelated event involving the activation of a chlorine monitor in the control building. When additional samples indicated no chlorine gas, the shift supervisor ordered further investigation into other plant areas. Because there was no installed detection equipment, portable survey instruments were used to determine gaseous mixtures. Hydrogen was detected in the auxiliary building and 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 on-shift supervisor declared an unusual event (UE) with a subsequent report to the NRC via the emergency notification system (ENS).
Event Date
February 20, 1987
Record Quality Indicator
Region / Country
Event Initiating System
Classification of the Physical Effects
Nature of the Consequences
Cause Comments
When hydrogen was discovered in the auxiliary building, the licensee isolated the cryogenic hydrogen skid outside the turbine building and soon located the source of the leak as packing on a globe valve in a small line to the volume control tank (VCT). The licensee opened doors that quickly caused the hydrogen to dissipate. The globe valve was of a conventional design and had no special packing. The globe valve was located in a vertical pipe chase where little ventilation was present because of ongoing HVAC testing.
Facility Information
Application Type
Application
Specific Application Supply Chain Stage
Components Involved
Liquid hydrogen skid and an auxiliary building
Storage/Process Medium
Location Type
Location description
Industrial Area
Operational Condition
Pre-event Summary
This plant was recently licensed, had never been critical, and was in cold shutdown at the time of the event. The liquid hydrogen storage system ('skid') is located outside the turbine building. The use of liquid hydrogen is being used as a cover gas in the VCT, and also used in the plants waste gas system and to cool the generator. A cryogenic gas line connect the skid to the building.The discovery of this problem was as a result of an not associated event involving the activation of a chlorine monitor in the control building.
Currency
Lessons Learned
Lessons Learned
The lessons of this event fall into five categories: (1) proper in-plant communications during events, (2) proper valve application for use with hydrogen, (3) excess flow check valve set point, (4) heating ventilation and air conditioning (HVAC) maintenance and flow testing, (5) hydrogen line routing.The following measures were considered by the licensee:-ways to improve communications in the plant during events and the training of personnel in reading portable instruments. -the use of other types of valves, such as valves with a diaphragm or bellows rather than conventional stem packing, in lines containing hydrogen.-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.
Event Nature
Emergency Action
Hydrogen was detected in the auxiliary building and percent of the lower flammability limit (LFL) for hydrogen. A level of about 1.2 percent hydrogen by volume.When hydrogen was discovered in the auxiliary building, the licensee isolated the cryogenic hydrogen skid outside the turbine building and soon located the source of the leak as packing on a globe valve in a small line to the volume control tank (VCT). The licensee opened doors that quickly caused the hydrogen to dissipate.
Release Type
Release Substance
Hydrogen Release Concentration (%)
1.20
Detonation
No
Deflagration
No
High Pressure Explosion
No
High Voltage Explosion
No
Source Category
References
References
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND INFORCEMENT, WASHINGTON, D.C. 20555, April 20, 1987,
available (2019) at
https://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices/19…
US NRC Issue 195:
https://www.nrc.gov/sr0933/Section%203.%20New%20Generic%20Issues/195.ht…
(accessed Dec 2022)