Information Notice No. 83-62: Failure of Redundant Toxic Gas Detectors Positioned at Control Room Ventilation Air Intakes

                                                           SSINS No.:  6835 
                                                           IN 83-62        

                                UNITED STATES
                           WASHINGTON, D.C. 20555
                             September 26, 1983 

                                   POSITIONED AT CONTROL ROOM VENTILATION 
                                   AIR INTAKES 


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


This information notice is provided as notification of events that involve 
the degradation and subsequent common-cause failure of toxic gas detectors 
for control room ventilation systems. It is expected that recipients will 
review the information herein for applicability to their facilities. No 
specific action or response is required at this time.  

Description of Circumstances: 

During the five-year period, 1977 through 1982, approximately 64 licensee 
event reports have involved the failure of one or more chlorine and ammonia 
detectors positioned at the air intakes of control room ventilation systems.
To date, for the present year, there have been four such failures. The 
frequency of events corresponds to a significant failure rate, since not all
nuclear power plants have toxic gas detectors. 

There have been several instances of releases of toxic gases at nuclear 
power plant sites. Although these events apparently were not complicated by 
concurrent failure of toxic gas detectors, this certainly is a possibility. 

During 1977 to 1982 actual chlorine gas releases occurred at Millstone 
(March 1978) and Browns Ferry (June 1979). While neither release involved 
more than a gallon of liquid chlorine, a total of 20 persons, including one 
control room operator, were hospitalized because of chlorine gas inhalation 
during both incidents. IE Circular No. 80-03 titled "Protection From Toxic 
Gas Hazards" provided detailed information pertaining to the Millstone and 
Browns Ferry releases, potential source's of toxic gas, and applicable 
regulations for toxic gas hazards. Information Notice No. 82-43 also 
dealt with deficiencies in LWR air filtration and ventilation systems. 


                                                         IN 83-62          
                                                         September 26, 1983 
                                                         Page 2 of 2       

More recently, an incident involving a chlorine gas release occurred on June
6, 1983 at the Oyster Creek nuclear plant. A plastic pipe failed on the 
outlet side of an evaporator, expelling the toxic gas.  The reactor and the 
turbine buildings were evacuated. Of the 25 people transported to the 
hospital, all but one were permitted to leave the same day. This occurrence 
and the other two previously noted, typify the significant and potentially 
fatal repercussions associated with chlorine or ammonia gas releases, and 
the need to have operable toxic gas detectors. 

The toxic gas detector most susceptible to failure is the type which 
utilizes a dripping electrolyte. This detector can fail because of a clogged
electrolyte wick or orifice, excess electrolyte consumption between 
scheduled replenishments, and absorption of the electrolyte by dust 
accumulation. Other factors contributing to failures are dust raised by 
nearby construction, periods of high wind or low humidity, and seasonal high 
pollen counts. 

Redundant toxic gas detectors are exposed to the same intake airflow for 
ventilation systems, and may therefore fail from a common cause, i.e., 
exposure to the same source of dirty air. The personnel of some facilities 
using such toxic gas detectors are considering either increased surveillance
or replacement with a more reliable type of detector to preclude continuing 

If you have any questions regarding this matter, please contact the Regional
Administrator of the appropriate NRC Regional Office, or this office. 

                              Edward L. Jordan, Director 
                              Division of Emergency Preparedness 
                                and Engineering Response 
                              Office of Inspection and Enforcement 

Technical Contact:  R. M Young, IE 
                    (301) 492-7275 

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