Bulletin 78-03: Potential Explosive Gas Mixture Accumulations Associated With BWR Offgas System Operations

                               UNITED STATES 
                           WASHINGTON, D.C. 20555 

                             February 8, 1978  

                                                     IE Bulletin No. 78-03 


Description of Circumstances: 

On December 13, 1977, two hydrogen explosions occurred at the Millstone 
Nuclear Power Station Unit No. 1, a boiling water reactor (BWR). 

The first explosion occurred at 9:30 a.m. and was mostly confined to the 
offgas system. Damage was relatively minor, including the breaking of glass 
faces on offgas system flow differential pressure gages, the blowout of a 
rupture disk and the evacuation of loop seals between the offgas system 
drain lines and stack base sump. 

The second explosion occurred at 1:00 p. m. outside the offgas system in the
two-level room at the base of the plant stack. This explosion blew the stack
base space door into a warehouse about 200 feet away, breached the 
reinforced concrete ceiling between the stack chimney and the stack base 
space, extensively damaged the ceiling beams, dislodged the 2-ton concrete 
plugs in the floor above the offgas system particulate filters, damaged the 
stack radiation monitor isokinetic probe supports, and produced cracks in 
the stack. These cracks were vertical, with a maximum separation at the 
surface of 1/16 inch; however, the overall structural capability of the 
stack was not impaired. 

One man was injured by the blast, receiving a concussion, skin abrasions, 
and contamination. He was hospitalized because of the concussion and was 
released four days after the event. 

The licensee's review of the event concluded that the action taken to 
restore offgas system drain line loop seals in the stack base space had not 
been successful. Without these seals, gases from the offgas 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. 

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IE Bulletin No. 78-03                                    February 8, 1978 

By way of background, it is recognized that accumulation of explosive 
hydrogen mixtures was considered in the design of the BWR offgas system. As 
a result, the design has prevented major releases of airborne radioactivity 
in the approximately 25 known hydrogen gas explosions that have occurred 
within the offgas systems of operating BWRs. Extensive mechanical damage to 
equipment and structures, however, in addition to uncontrolled release of 
radioactive material, have resulted from five explosions, including the 
Millstone event, where hydrogen gas accumulated outside of the offgas 

Action to be Taken by Licensees: 

For all BWR power reactor facilities with an operating license: 

1.   Review the operations and maintenance procedures related to the offgas 
     system to assure proper operation in accordance with all design 
     parameters. Include in this review measures you have taken or will take
     to prevent inadvertent actions (such as arc strikes) which might cause 
     ignition of the mixture of gases contained in the offgas piping. 

2.   Review the adequacy of the ventilation of spaces and areas through 
     which offgas system piping containing explosive mixtures of gases pass. 
     The review should consider ventilation losses and off-normal offgas 
     system operation, such as lack of dilution steam, lost loop seals, 
     blown rupture disks, bypassing recombiners, and leakage of offgas into 
     isolated portions of systems. 

3.   For those spaces and areas identified, describe what action you have 
     taken or plan to take to assure that explosive mixtures cannot 
     accumulate, that monitoring equipment will warn of such an 
     accumulation, and that disposal of such mixtures will be controlled 
     without resulting in a damaging explosion. 

4.   Loop seals are potential offgas leakage paths following a pressure 
     transient in the offgas 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 offgas system 

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IE Bulletin No. 78-03                                    February 8, 1978 

6.   Within 45 calendar days of the date of issue of this bulletin, report 
     in writing to the Director of the appropriate NRC Regional Office, the 
     results of your review and your plan of action with regard to Items 1 
     through 5. A copy of your report should be sent to the United States 
     Nuclear Regulatory Commission, Office of Inspection and Enforcement, 
     Division of Reactor Operations Inspection, Washington, D. C. 20555. 

For all BWR power reactor facilities with a construction permit, this 
Bulletin is for information only and no written response is required. 
Approved by GAO, B180225 (R0072); clearance expires 7/31/80. Approval was 
given under a blanket clearance specifically for identified generic 

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