United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-01: Potential for High Post-Accident Closed-Cycle Cooling Water Temperatures to Disable Equipment Important to Safety

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

                                January 3, 1996


NRC INFORMATION NOTICE 96-01:  POTENTIAL FOR HIGH POST-ACCIDENT CLOSED-CYCLE
                               COOLING WATER TEMPERATURES TO DISABLE EQUIPMENT
                               IMPORTANT TO SAFETY


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 the potential for high post accident closed-
cycle cooling water system temperatures to disable equipment important to
safety.  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 November 14, 1994, the licensee for Fort Calhoun initiated a plant shutdown
because engineering analysis had shown that the control room air conditioners,
which operate in a vapor compression refrigeration cycle, could be disabled by
a large primary coolant system pipe rupture or a main steamline break inside
the containment.  Loss of the control room air conditioners could cause
certain engineered safety feature equipment in the control room to become
overheated.  The engineering analysis was initiated during preparations for a
service water system operational performance assessment.

At Fort Calhoun, the closed cooling water (CCW) system operates in a closed
cycle and transfers heat to the raw water system from various pieces of
equipment, including the containment coolers and the control room air
conditioning units.  The raw water system operates in an open cycle and
rejects the heat to the Missouri River.  A large primary coolant system pipe
rupture or main steamline break inside the containment could cause the CCW
temperature to rise rapidly because of the large heat input from the
containment coolers during these postulated accidents.  The licensee
calculated that under design-basis conditions, with the maximum available
containment cooling capacity and the minimum permissible heat rejection
capability of the CCW system, the system temperature could reach a maximum of 


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                                                            Page 2 of 3


86 �C [187 �F], which exceeds the maximum post accident CCW temperature
specified in the final safety analysis report (FSAR) and the temperature used
to calculate thermal stress in certain piping segments.  

The control room air conditioning units, which operate with the condenser
refrigerant in a two-phase equilibrium state, are equipped with rupture discs
that were designed to blow out at a CCW supply temperature of 54.5 �C      
[130 �F].  If the refrigerant was released, the air conditioning units could
not be recovered.  The licensee does not consider a release of the refrigerant
to be a personnel safety concern.  However, without any air conditioning, and
with the control room ventilation system operating in the emergency
pressurization mode, the control room temperature could increase to levels
that could hinder operator activities and cause the design temperatures of
safety-related equipment in the control cabinets to be exceeded.  

The licensee identified two root causes associated with the design deficiency
(Licensee Event Report 50-285/94-010 and supplements):

1.    The use of an inappropriate methodology to establish the maximum post
      accident CCW system temperature.

2.    The failure to include a maximum condenser temperature for post accident
      conditions in the procurement specifications for the air conditioning
      units.  The procurement specifications gave the normal CCW system
      temperatures.

The Fort Calhoun FSAR documented the loss of an emergency onsite power source
as the most limiting single failure for the CCW system.  However, such a
failure would reduce containment heat transfer to the CCW system, as well as
reduce the heat rejection capability of the CCW system.  During preparations
for the self-assessment, the licensee identified certain initial equipment
configurations that could result in a significant reduction in the heat
rejection capability of the CCW system without reducing the potential heat
transfer from containment.

The licensee justified continued operation until the March 1995 refueling
outage based on implementation of certain compensatory measures and the
existence of low river water and air temperatures at that time.  The licensee
subsequently completed modifications that installed air-cooled condenser units
for the control room air conditioning units, increased the minimum design CCW
heat rejection capability, and determined that the calculated piping thermal
stress was within acceptable limits for post accident CCW transient
temperatures.  The licensee has also implemented administrative controls to
ensure that post-accident CCW temperatures remain within acceptable limits
while a request for a Technical Specification amendment is pending.

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                                                            January 3, 1996
                                                            Page 3 of 3


Discussion

Closed cooling water systems are subject to significant transient temperature
variations because of the limited system heat capacity and the potential for
substantial changes in heat addition and heat rejection rates.  The complex
nature of some such systems may make difficult the correct identification of
the most limiting potential operating configuration of the system.  Certain
safety-related components served by CCW systems, such as air conditioning
units and emergency diesel generators may fail in a non-recoverable manner as
a result of temperature transients outside the system design basis.  Because
temperature transients initiated by an accident may affect redundant parts of
the closed cooling water system, safety-related components in redundant trains
necessary for mitigation of an accident may be affected.

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.


                                          original signed by
                                    
                                    Dennis M. Crutchfield, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Steven R. Jones, NRR
                     (301) 415-2833
                     Internet:srj@nrc.gov
                        
                     William F. Burton, NRR
                     (301) 415-2853
                     Internet:wfb@nrc.gov
Page Last Reviewed/Updated Thursday, November 21, 2013