United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 97-14: Assessment of Spent Fuel Pool Cooling

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

                                March 28, 1997


NRC INFORMATION NOTICE 97-14:    ASSESSMENT OF SPENT FUEL POOL COOLING


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 results of an assessment performed by the Office
for Analysis and Evaluation of Operational Data (AEOD) on operating experience
involving spent fuel pool (SFP) cooling and inventory control.  The AEOD
assessment is detailed in NUREG-1275, Volume 12, "Assessment of Spent Fuel
Cooling," February 1997.  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

NRC's Executive Director for Operations directed AEOD to independently assess
the likelihood and consequences of an extended loss of SFP cooling or
inventory.  The assessment focused on the collection and analysis of operating
experience involving SFP cooling and inventory control and comprised of six
major tasks:  

(1)   Develop generic configurations delineating SFP cooling and inventory
      control equipment for a boiling-water reactor (BWR) and a pressurized-
      water reactor (PWR).

(2)   Review and assess more than 12 years of operating experience for
      domestic and foreign reactors.  

(3)   Visit six sites to gain an understanding of the physical configurations,
      practices, and operating procedures for the licensees' SFPs.

(4)   Review applicable SFP regulations and the NRC Standard Review Plan
      (NUREG-0800) for the acceptance criteria and the applicable regulatory
      guides.

(5)   Perform independent assessments of electrical systems, instrumentation,
      heat loads, and radiation levels associated with the SFP.

(6)   Estimate near-boiling frequencies utilizing probabilistic techniques. 


9703270253.                                                            IN 97-14
                                                            March 28, 1997
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The findings and conclusions of the AEOD assessment are summarized below.

.     The loss of SFP coolant inventory in excess of 1 foot has occurred 10
      times over the last 12 years at a rate of about once every 100 reactor-
      years.  Loss of SFP cooling with a temperature increase in excess of 11
      �C [20 �F] has occurred 4 times in the last 12 years at a rate of
      approximately 3 times every 1000 reactor-years.  The consequences of
      these actual events have not been severe; however, some events resulted
      in loss of several feet of SFP coolant level and have continued for more
      than 24 hours.  The primary cause of these events has been human error,
      most preva- lently, valve mispositionings.  These errors are discussed
      in greater detail in NUREG-1275, Volume 12.

.     The relative risk due to loss of spent fuel pool cooling is low in
      comparison with the risk of events not involving SFPs.  The review
      determined that the likelihood and consequences of SFP loss-of-cooling
      events are highly dependent on human performance (e.g., valve
      manipulations, crane manipulations, spent fuel handling equipment
      manipulations) and individual plant design features.

.     The efforts by utilities to reduce outage durations have resulted in
      full core off-loads early in outages.  This increased fuel pool heat
      load reduces the time available to recover from a loss-of-SFP-cooling
      should such an event occur early in the outage.

On a plant specific basis the following observations are provided:

.     Failures of reactor cavity seals or gate seals or ineffective antisiphon
      devices could potentially cause loss of SFP coolant inventory, which
      could potentially result in uncovering the fuel or endangering makeup
      capability.

.     Ineffective configuration control was the most frequent cause of SFP
      loss-of-inventory events and loss-of-cooling events.

.     At multi-unit sites with common pools, there is the potential for SFP
      boiling conditions to affect safe-shutdown equipment for the operating
      unit, particularly during full core off-loads.

.     Improved procedures and training which recognize the time frames over
      which SFP events can proceed, the heat load and possibility of loss of
      inventory, could enhance control room operator response to loss-of-
      inventory and loss-of-cooling SFP events.

.     Insufficient control room instrumentation and non-safety-related power
      supplies to SFP instrumentation may hinder operator response to SFP
      events.
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                                                            March 28, 1997
                                                            Page 3 of 3

Discussion

The AEOD assessment found large variations in the designs and capabilities of
SFPs and related systems at individual nuclear plants.  The AEOD assessment
identified that the frequency of loss-of-inventory events was relatively high
compared to loss-of-cooling events, and prompt off-loads will lead to reduced
time to boil if cooling is lost.  A loss of SFP inventory in combination with
higher heat loads reduces the time to respond to SFP events.  Thus, the
emphasis of the findings were on instrumentation to quickly alert the
operators and effective procedures and training to facilitate prompt operator
response.  The operating experience review determined that loss-of-SFP-cooling
events are infrequent and the consequences of actual events have been small. 
The risk assessment indicates that the SFP events are not a dominant
contributor to overall plant risk.  However, human error initiators and
operator actions required to mitigate an error are subject to large
uncertainties, and there are large variations in design vulnerabilities among
plants.  In summary the following areas need emphasis:

(1)   Procedures and training to detect and respond to SFP loss-of-inventory
      and loss-of-cooling events, including those caused by loss of offsite
      power are needed and should address configuration controls that can
      prevent and/or mitigate such events.  They should be consistent with the
      time frames over which SFP events can proceed at the specific plant,
      recognizing the plant-specific heat load and the possibility of loss of
      inventory because of cavity seal or gate failures.

(2)   Reliable instrumentation is necessary to monitor SFP temperature and
      level and SFP area radiation, including periods following a loss of
      offsite power, in order to detect SFP loss-of-inventory events and loss-
      of-cooling events in a timely manner.

(3)   Testing, maintenance, and configuration control of plant features such
      as reactor cavity seals, gate seals, or antisiphon devices need to be
      examined for those plants where failures could potentially cause loss of
      SFP coolant inventory sufficient to endanger makeup capability or result
      in fuel uncovery.

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

                                    Thomas T. Martin, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Jose G. Ibarra, AEOD       Stephen Koenick, NRR
                     (301) 415-6345             (301) 415-2841
                     E-mail:  jgi@nrc.gov       E-mail:  ssk2@nrc.gov 
Page Last Reviewed/Updated Tuesday, December 03, 2013