United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 97-60: Incorrect Unreviewed Safety Question Determination Related to Emergency Core Cooling System Swapover from the Injection Mode to the Recirculation Mode

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

                                August 1, 1997


NRC INFORMATION NOTICE 97-60:  INCORRECT UNREVIEWED SAFETY QUESTION            
                               DETERMINATION RELATED TO EMERGENCY CORE COOLING
                               SYSTEM SWAPOVER FROM THE INJECTION MODE TO THE  
                               RECIRCULATION MODE


Addressees

All holders of operating licenses or construction permits for pressurized-
water reactors. 

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to an instance in which a licensee has incorrectly
determined that changes to its emergency core cooling system (ECCS) swapover
procedures that resulted in interruption of flow and a consequent core heatup
in the long-term cooling phase of a potential loss-of-coolant accident (LOCA)
did not involve an unreviewed safety question (USQ).  It is expected that
recipients will review this 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 May 1, 1989, a license amendment was issued approving a semi-automatic ECCS
swap- over procedure for Salem Nuclear Unit 2.  This procedure provided the
plant with the capability to switch the ECCS suction from the refueling water
storage tank (RWST) to the containment sump (i.e., from the injection mode of
operation to the recirculation mode of operation) without interruption of ECCS
flow to the core.  It further provided continuous suction to the high-head
safety injection (HHSI) and intermediate-head safety injection (IHSI) pumps. 
The semi-automatic swapover design provided this capability by automatically
completing the following steps:  (1) opening the suction line valves between
the residual heat removal (RHR) pump and the sump; (2) starting component
cooling water to the RHR heat exchangers; (3) opening the suction crosstie
valves between the HHSI and IHSI pumps; and (4) when the sump line valves are
fully open, closing the suction line to the RWST.  Manual


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operator action was still required to align the suction of the HHSI and IHSI
pumps to the discharge of the RHR pumps (i.e., the piggy-back mode of
operation) and to trip the con- tainment spray pumps.  Subsequently, both the
licensee's individual plant examination and the NRC technical evaluation
report on Salem human reliability analysis issued in August 1995 identified
ECCS swapover as one of the most risk-significant operator actions for Salem.

For Salem Unit 2, ECCS swapover is initiated when the level in the RWST
reaches the low-level alarm setpoint.  In addition, to prevent vortexing in
the RWST and to protect the ECCS pumps, emergency operating procedures (EOPs)
direct operators to trip all ECCS pumps taking suction from the RWST if the
RWST low-low level alarm setpoint is reached.  There- fore, the maximum time
available for successful completion of ECCS swapover is the time that it would
take the RWST to drain from the low-level alarm setpoint to the low-low level
alarm setpoint.  In 1995 and 1996 the licensee performed evaluations that
considered:  (1) uncertainties in specific operator action times, (2) the
introduction of "three-point" communications, (3) higher than previously
assumed containment spray pump flow, and (4) the identification of more
limiting single-failure scenarios.  The licensee concluded that  under certain
accident conditions, the operators may not be able to complete the swapover
before reaching the low-low level alarm setpoint.  For such scenarios, the
operators would  be directed to trip the ECCS pumps taking suction from the
RWST, thereby interrupting ECCS flow to the core.  

The licensee's analyses for the small break loss-of-coolant accident (SBLOCA)
and accumulator line break accidents allotted the operators 11.8 minutes and
12.9 minutes, respectively, for completing the ECCS swapover procedure from
the point that the RWST level reaches the low-level alarm setpoint.  Licensee
calculations showed that the low-low level alarm setpoint would be reached at
10 minutes for the SBLOCA and 7.9 minutes for the accumulator line break.  As
discussed above, this is the point at which the operators are directed to stop
all ECCS pumps taking suction from the RWST, thereby interrupting ECCS flow to
the core.  Therefore, for the balance of the time allotted for completing the
swapover procedure (i.e., 1.8 minutes for the SBLOCA and 5 minutes for the
accumulator line break), core cooling would have to be provided without ECCS
flow.  The licensee evaluated these changes to the swapover procedure,
including the introduction of interruption of ECCS flow, pursuant to 10 CFR
50.59 and determined that the changes did not involve a USQ.  Accordingly, the
licensee implemented the changes without NRC staff approval.

The NRC became aware of the above-described changes to Salem Unit 2's semi-
automatic ECCS swapover procedure during a special NRC inspection conducted
between March 24 and April 17, 1997.  The inspectors were concerned with the
introduction of interruption of flow into the ECCS swapover procedure as the
NRC staff had not reviewed this approach for Salem Unit 2.  Additionally, the
inspectors were concerned about the potential for such scenarios to violate
the requirement contained in 10 CFR 50.46(b)(5), "Long term cooling." 

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Discussion

Pursuant to 10 CFR 50.59, a licensee may make changes in its facility and
procedures as described in the safety analysis report (SAR) without prior
Commission approval, unless  the proposed change involves a USQ.  In part, 10
CFR 50.59 states that changes shall be deemed to involve a USQ if the
probability of occurrence of a malfunction of equipment important to safety
previously evaluated in the SAR may be increased, or if a possibility 
for a malfunction of a different type than any evaluated previously in the SAR
may be created.  

The changes to the swapover procedure in the Salem case introduced reliance on
manual operator actions, where such actions were not previously required to
stop and restart  ECCS pumps, and reliance on ECCS pumps to restart shortly
after being stopped during an accident.  

This was contrary to the intent of the 1989 design change, which was to
increase the reliability of ECCS swapover by automating certain features, so
that the RHR pumps would not have to be stopped and restarted, and reduce
susceptibility to human error in ECCS swapover.  As a result of the inspection
findings, the staff evaluated these changes and found them to have increased
the probability of a malfunction of equipment important to safety in that the
changes provide additional opportunities for operator error, especially
considering the conditions and the time constraints under which the operators
would be working.  In addition, restarting ECCS pumps shortly after they have
been stopped involves uncertainties as to whether the pumps will restart and
is not recommended by pump vendors.  Therefore, an increase in the probability
of malfunction of the ECCS pumps was also introduced.  On the basis of the
above discussion, the staff concluded in Inspection Report 50-311/97-11 that
these changes did involve a USQ.

In addition, the interruption of flow during ECCS swapover could potentially
result in another fuel uncovery following the initial blowdown and reflood
phase of a LOCA.  Consequently, the fuel could reheat during that phase of the
accident.  Therefore, such an interruption of ECCS flow could violate the
long-term cooling requirement of 10 CFR 50.46 (b)(5).  Additional cycling of
the fuel (i.e., introduction of heatups in the long-term cooling phase of the
accident) also increases the probability of cladding failure and therefore
involves a USQ as defined in 10 CFR 50.59.  For the Salem case, the licensee
evaluated the effects of the interruption of flow on the core.  The licensee
wanted to maximize the time available to the operators to complete the
swapover.  The allowable times for flow interruptions as noted above were
calculated on the basis of maintaining the core under water to avoid core
heating.   The licensee has since implemented EOP changes to allow operators
to complete the swapover without interruption of flow.  In addition, the
licensee verified through simulator exercises  that all operating crews could
perform the revised EOPs well within the time frame required to prevent
interruption of flow.  The NRC found the licensee's actions to resolve these
issues to be acceptable.
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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
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.   

                                                signed by S.H. Weiss for

                                    Marylee M. Slosson, Acting Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Mohammed A. Shuaibi, NRR         Thomas Koshy, NRR 
                     301-415-2859                     301-415-1176
                     E-Mail:  mas4@nrc.gov            E-mail:  txk@nrc.gov
      

                     Frank R. Orr, NRR                Doug Dempsey, RI
                     301-415-1815                     (610) 337-5347
                     E-Mail:  fro@nrc.gov             E-mail:  dad1@nrc.gov
Page Last Reviewed/Updated Tuesday, December 03, 2013