United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-42: Unexpected Opening of Multiple Safety Relief Valves

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                         WASHINGTON, D.C.  20555-0001

                                August 5, 1996


NRC INFORMATION NOTICE 96-42:  UNEXPECTED OPENING OF MULTIPLE SAFETY RELIEF 
                               VALVES 


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 recent event in which indication was received
that multiple safety relief valves (SRVs) had opened during steady-state
reactor operation without any apparent initiating cause.  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 June 6, 1996, Grand Gulf Nuclear Station was being operated at about 100-
percent power when alarms sounded and the control room operators noticed that
the reactor vessel water level was decreasing, the suppression pool
temperature was increasing, and six SRVs were open.  No maintenance or test
activities related to SRV operation were being performed.  Because the
suppression pool temperature was increasing rapidly, operators scrammed the
plant to prevent exceeding the maximum temperature limit of 43 �C [110 �F].   

Neither the emergency core cooling system nor the reactor core isolation
cooling system was challenged by this event.  However, both the "A" and "B"
residual heat removal systems were placed in the suppression pool cooling mode
of operation to reduce the suppression pool temperature.  The main steam
isolation valves remained open, and steam continued to go to the main
condenser.  There was no appreciable cooldown of the reactor vessel during
this transient until after the reactor was scrammed.  

The six open SRVs automatically closed after the scram when reactor pressure
decreased.  The six SRVs were open for approximately 2.5 minutes.  Reactor
pressure was subsequently controlled with the main turbine bypass valves.  The
reactor water level was restored to the normal range through use of the
feedwater control system.  The rest of the reactor shutdown was routine, and
there were no further complications.  


9607300191.                                                            IN 96-42
                                                            August 5, 1996
                                                            Page 2 of 3


Discussion

Grand Gulf uses a BWR-6 reactor having four steam lines, each with either four
or six SRVs.  All SRVs have the same design and each has a relief capacity of
approximately 6 percent of the full steam flow.  The SRVs have both spring
safety operation and a power-operated (relief) mode.  SRV power actuation may
be accomplished either automatically at a predetermined relief set pressure or
manually at any desired pressure.  Actuation is achieved by positioning
solenoid-operated valves that either admit compressed air into an operating
cylinder or vent compressed air from the cylinder.  Two of the SRVs on each
steam line are also actuated by the automatic depressurization system logic.

Six of the SRVs are provided with a low-low set relief logic that minimizes
the number of valves that reopen following a reactor isolation event.  The
low-low set relief logic provides all six SRVs with lower closing setpoints
and two of these valves with lower opening setpoints that are below the normal
operating reactor pressure of 7.1 Megapascals [1025 psig].  These setpoints
override the normal set points following initial opening of the SRVs and act
to hold these valves open longer and prevent simultaneous reopening of
multiple SRVs.  This low-low set relief logic seals in when the SRVs receive
an opening signal.  The function of this logic is to prevent hydrodynamic
loads during subsequent SRV actuations from exceeding the containment design
basis.  

The control logic for the relief mode of operation consists of two divisional
trains (safety groups), with two logic channels per division.  Both logic
channels must be satisfied in order to operate the respective divisional
solenoid valves on the SRV air actuators.  Logic cards for both channels of
each respective division are located in the same card file.  The SRV logic
card files are one of four card files supplied from the 24-Vdc power supply.  

The licensee identified a transient in the 24-Vdc power supply circuit of the
Division II SRV logic as the root cause for the lifting of the SRVs.  All
20 SRVs can be actuated by either Division I or Division II logic, and the
20 SRVs received an unanticipated 200-millisecond open signal.  This open
signal was sufficient to actuate the "seal-in" for the six SRVs that have the
low-low set relief feature.  However, the signal duration was too short to
sustain opening of the other 14 SRVs.  The SRV logic card file is one of four
card files supplied from the Division II 24-Vdc power supply.  Each card file
contains several trip unit cards that have logic circuits for safety-related
components.  Licensee personnel found a blown fuse to one of the other circuit
card files and determined that a trip unit card in the card file associated
with the failed fuse had a failed capacitor.         
.                                                            IN 96-42
                                                            August 5, 1996
                                                            Page 3 of 3


The licensee determined that this event was bounded by the safety analysis,
which includes opening of all 20 SRVs.  The component loading effects were
determined to be within the piping design capabilities for the SRV transients,
as well as for the containment.  In addition, the design of the SRV and the
low-low set logic was verified as meeting the single-failure criterion for
initiation of a safety system and that inadvertent system actuations were
analyzed for acceptable safety consequences.  However, to prevent future
occurrence of a similar event, the licensee is pursuing the following
corrective actions:

1.    Evaluation of design modifications to minimize the vulnerability to a
      single failure actuating several SRVs.

2.    Evaluation of the consequences of failures in similar logic cards in
      other safety systems that are also subject to a similar initiating event
      (high-voltage spike) to verify that the design-basis accident analyses
      are bounding.

In addition, the licensee plans to revise of the final safety analysis report
description to clarify the application of the single-failure criterion.

This information notice requires no specific action or written response. If
you have any questions about information in this notice, please contact one of
the technical contacts listed below or the appropriate Office of Nuclear
Reactor Regulation project manager.




                                    Brian K. Grimes, Acting Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Jerry Carter, NRR
                     (301)415-1153
                     Internet:tjc@nrc.gov 

                     Jeff Tedrow, RIV
                     (601)437-4620
                     Internet:jet@nrc.gov 

                     S.V. Athavale, NRR 
                     (301)415-2974
                     Internet:sva1@nrc.gov 

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