Information Notice No. 82-28: Hydrogen Explosion While Grinding in the Vicinity of Drained and Open Reactor Coolant System

                                                            SSINS No.: 6835 
                                                            IN 82-28  

                               UNITED STATES 
                           WASHINGTON, D.C. 20555 

                               July 23, 1982 

                                   VICINITY OF DRAINED AND OPEN REACTOR 
                                   COOLANT  SYSTEM 


All nuclear power reactor facilities holding an operating license (OL) or 
construction permit (CP). 


This information notice is provided as a notification of an event that may 
have safety significance. It is expected that recipients will review the 
information for applicability to their facilities. No specific action or 
response is required at this time. 

Description of Circumstances: 

On April 10, 1982, a hydrogen explosion occurred at Unit 1 of Arkansas 
Nuclear One while maintenance personnel were grinding a recently cut 
high-pressure injection (HPI) pipe, approximately 18 inches from the nozzle 
connecting the HPI pipe to the reactor coolant system (RCS) piping. At the 
time of the explosion, the RCS was partially drained and the water level in 
the reactor coolant piping was just below the HPI nozzle to permit 
radiography of the nozzle and subsequent repair. (Information Notice No. 
82-09 provides details concerning the cracking problem in HPI piping at 
Babcock & Wilcox plants.) The reactor coolant temperature was being 
maintained at approximately 100F by the decay heat removal system, and 
nitrogen cover gas was being maintained in the reactor coolant piping. These
conditions existed since the RCS was depressurized and partially drained on 
March 29, 1982. 

At approximately 1240 hours on April 10, 1982, the craftsmen, who were 
grinding on the HPI pipe in preparation for welding, observed a bright flash
at the outlet of the HPI line and heard a loud "bang". The craftsman 
actually performing the grinding was physically blown away from the HPI pipe 
a distance of about three feet. Personnel in other areas of the Unit 1 
containment building heard the explosion and felt the resulting concussion 
and mechanical vibration. Additionally, some personnel outside of the 
containment building, including operators in the Unit 1 control room 
reported that they heard the explosion and felt varying degrees of 
vibration. Although there were no physical injuries as a result of this 
event, it should be mentioned that the craftsman's life was endangered as he 
was working on a scaffold that was over 30 feet high. 

The most recent RCS measurement of dissolved gas in reactor coolant had been
taken on March 26, 1982, just before commencing the plant cooldown and 
shutdown. It indicated 39 standard cc of total gas/liter of coolant. The 


                                                           IN 82-28  
                                                           July 23, 1982  
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concentration was 43% of this total. Because the total gas concentration 
exceeded the maximum allowed by plant procedures (30 cc/liter) for RCS 
depressurization, the RCS was degassed for approximately 14 hours as 
cooldown progressed. The total gas concentration was not measured after 
degassing had been terminated because the RCS had been depressurized and the 
sampling method is effective only when the RCS pressure is greater than 
several hundred psig. Atmospheric samples had not been taken to measure 
hydrogen and oxygen concentrations in the vicinity of the open HPI pipe. 
(This had been done at other Babcock and Wilcox plants which were undergoing 
nozzle repair.) 

The reason for the presence of an explosive concentration of hydrogen is 
unknown. It could have been caused by (a) inadequate degassing, (b) failure 
to purge the HPI pipe with nitrogen, or (c) failure to temporarily plug the 
open HPI pipe. 

Subsequent inspection of the affected HPI line, the first upstream check 
valve in the HPI line, and the corresponding nozzle and safe-end on the RCS 
cold leg indicated no signs of damage as a result of the explosion. 

No written response to this information is required. If you need more 
information about this matter, please contact the Regional Administrator of 
the appropriate NRC Regional Office or this office. 

                              Edward L. Jordan, Director 
                              Division of Engineering and 
                                Quality Assurance 

Technical Contact:  W. Marinelli 

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