Two Hydrogen Explosions Occur at Boiling Water Reactor (1/2)
Two hydrogen explosions occurred at a boiling water reactor (BWR). The first explosion occurred at 9:30 a.m. and was mostly confined to the off-gas system. Damage was relatively minor.The second explosion occurred at 13:00 outside the off-gas system in the two-level room at the base of the plant stack. This explosion damaged considerable the stack however, the overall structural capability of the stack was not impaired. One man was injured.The review of the event concluded that the action taken to restore off-gas system drain line loop seals in the stack base space had not been successful. Without these seals, gases from the off-gas system accumulated in the space, resulting in an explosive mixture which was probably ignited by a spark from the level switch in the stack base sump. Inadequate ventilation of the stack base space and the lack of equipment installed to monitor explosive gas concentrations within the enclosed area were concluded to be contributing factors.
Event Date
December 13, 1977
Record Quality Indicator
Region / Country
Event Initiating System
Classification of the Physical Effects
Nature of the Consequences
Cause Comments
The IMMEDIATE CAUSE was defective seals in the off-gas system, which hallowed release/accumulation of hydrogen outside the system. CONTRIBUTING CAUSES were the inadequate ventilation and the absence of gas monitoring equipment.The ROOT CAUSE could be a combination shortcoming in management, design and possibly also installation/maintenance.
Facility Information
Application Type
Application
Specific Application Supply Chain Stage
Components Involved
Nuclear power plant
Storage/Process Medium
Location Type
Location description
Industrial Area
Operational Condition
Lessons Learned
Lessons Learned
LESSON LEARNEDIn 1978, following the investigation of this and previous similar cases, the NRC recognised that the BWR off-gas system design was considering the possibility of accumulation of explosive hydrogen mixtures. Thanks to this, the design has prevented major releases of airborne radioactivity in the approximately 25 known hydrogen gas explosions that had occurred so far within the off-gas systems of operating BWRs. However, cases of hydrogen gas accumulation outside the off-gas system occurred. In five cases, including this one, this resulted in five explosions, extensive mechanical damage to equipment and structures, and uncontrolled release of radioactive material.CORRECTIVE MEASURES REQUESTEDFor all BWR power reactor facilities with an operating license, NRC requireed the following: (1) Review the operations and maintenance procedures related to the off-gas system to assure proper operation in accordance with all design parameters. Include measures to prevent inadvertent ignition causes. (2) Review the adequacy of the ventilation of spaces and areas around the off-gas system piping, considering ventilation losses and off-normal off-gas operation (dilution steam, lost loop seals, blown rupture disks, bypassing recombiners) and the possibility of leakage. (3) Consequently take action to assure that explosive mixtures cannot accumulate, that monitoring equipment is in place to warn for accidental accumulation and that disposal of such mixtures can take place safely. (4) Loop seals are potential off-gas leakage paths following a pressure transient in the off-gas system piping. Describe your design features to minimize and detect the loss of liquid from loop seals and describe operating procedures which assure prompt detection and re-seal of the blown loop seals. (5) Review operating and emergency procedures to assure that your operating staff has adequate guidance to respond properly to off-gas system explosions.
Event Nature
Emergency Action
Unknown
Detonation
No
Deflagration
No
High Pressure Explosion
No
High Voltage Explosion
No
Source Category
References
References
NRC, Bulletin 78-03
https://www.nrc.gov/reading-rm/doc-collections/gen-comm/bulletins/1978/…
(accessed Aug 2023)
Wkipedia,
https://en.wikipedia.org/wiki/Millstone_Nuclear_Power_Plant#cite_note-18
(accessed Aug 2023)