Information Notice No. 93-59: Unexpected Opening of Both Doors in an Airlock

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C. 20555

                                July 26, 1993


NRC INFORMATION NOTICE 93-59:  UNEXPECTED OPENING OF BOTH DOORS IN AN AIRLOCK


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.  

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a potential problem that may result when
interlocks which prevent both airlock doors being open at the same time are
not in use or not operable.  It is expected that recipients will review the
information for applicability to their facilities and consider actions, as
appropriate, to avoid similar problems.  However, suggestions contained in
this information notice are not NRC requirements; therefore, no specific
action or written response is required.  

Description of Circumstances

On February 25, 1993, when the plant was operating at 100-percent power,
personnel at River Bend Station inadvertently had both airlock doors open at
the same time.  The airlock at River Bend is a pneumatic airlock that was
designed for either automatic operation actuated by a pushbutton or manual
operation using handwheels located on the doors.  The original intent was to
operate the airlock in the automatic mode, which incorporated both electrical
and mechanical interlocks.  However, operation in the automatic mode was not
reliable and, since 1985, the airlock was operated in the manual mode.  

The manual mode incorporates only the mechanical actions associated with
turning the handwheel.  The electrical interlocks, which had been identified
as being for "personnel safety," were not being used, and the licensee did not
believe that they were necessary.  An operable mechanical interlock allows
only one handwheel at a time to be in the open position.  Operation of the
handwheel (1) positions two pins into keepers, (2) repositions two 3-way ball
valves used to inflate/deflate the pneumatic seal, (3) operates a valve that
equalizes pressure across the door, and (4) actuates the mechanical interlock.
  
On February 25, 1993, a licensee employee entering containment closed the
outer door (not completely) and turned the handwheel to the closed position. 
By placing the outer door handwheel in the closed position, the mechanical
interlock was satisfied, thereby permitting the handwheel on the inner door to

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be subsequently turned to the open position.  This subsequent action deflated
the door seal and air rushed from the containment past the employee in the
airlock.  A supervisor outside the airlock sensed the air flow out of the
airlock and alerted the employee in the airlock, who promptly re-inflated the
seals on the inner door and then properly (fully) closed the outer door.  A
second instance on the same day that involved different people nearly resulted
in a repeat opening of both doors.  An engineer leaving the containment nearly
opened the inner door while the outer door was not fully closed.  A technician
in the airlock had previously turned the handwheel on the outer door to the
closed position.  In both instances, the lack of an operable door position
interlock permitted the handwheel to be turned to the open position.  

The licensee has modified the interlock system on the airlocks to incorporate
the door closure switch into the circuit to prohibit manual operation of the
handwheel when the associated door is not fully closed.    

Discussion

There have been a number of other instances at other facilities when both
doors of a containment airlock were opened; however, only a small fraction
occurred during power operation.  Nevertheless, each instance represents a
potential for a large leak path.  Most of the known at-power occurrences were
attributed to failure to properly seat the door (due to rebound) or to
component failure.  In most instances, the time both doors were open was very
short.  Thus, the actual safety significance was low.  

The effects of changing the operating mode of an airlock from automatic to
manual may not be limited to a change from the pushbutton operating the
handwheel to manually operating the handwheel.  Operation in the manual mode
may also eliminate some of the design interlock features, such as those that
would prevent having both doors open at the same time and assuring that there
is no significant pressure differential across an airlock door before it is
opened.

Of particular concern in the River Bend event is that part of the interlock
system was removed from service, apparently without a fundamental
understanding of the function of the electrical interlocks in assuring the
integrity of the containment boundary.  The safety significance of positive
verification of door closure via functioning door position switches was not
recognized.  
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This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.  


                                       ORIGINAL SIGNED BY


                                    Brian K. Grimes, Director
                                    Division of Operating Reactors Support
                                    Office of Nuclear Reactor Regulation

Technical contact:  J. Carter, NRR
                    (301) 504-1153

Attachment:  
List of recently Issued NRC Information Notice 
.