United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-82: Concerns Regarding Essential Chiller Reliability During Periods of Low Cooling Water Temperature

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                          WASHINGTON, D.C. 20555-0001

                               December 5, 1994

                                   RELIABILITY DURING PERIODS OF LOW COOLING
                                   WATER TEMPERATURE 


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to concerns about the reliability of essential
chillers during periods of low cooling water temperature.  It is expected that
recipients will review this information notice 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.


Essential chillers operating in a simple refrigeration cycle (see Figure 1)
are frequently used at nuclear power plants to transfer heat from safety-
related components to an ultimate heat sink.  These essential chillers are
necessary to transfer heat in locations where the ultimate heat sink
temperature is frequently near or above the temperature necessary to maintain
safety-related components at an acceptable temperature by direct cooling. 
However, the chiller design may depend on special controls for continued
operation when the cooling water temperature is abnormally low.

Description of Circumstances
South Texas Project Electric Generating Station

In the NRC diagnostic evaluation team report for the South Texas Project
Electric Generating Station (STP), which was forwarded to Houston Lighting and
Power Company (the licensee for STP) on June 10, 1993, the team noted that the
licensee had not completely evaluated the potential for under-loading
essential chillers following accident signal actuation during cold weather
despite internal assessments identifying this concern.

Each plant at STP has three trains of essential chilled water, and each of
these trains is cooled by one 150-ton chiller and one 300-ton chiller.  The

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                                                      December 5, 1994
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chillers originally had valve actuators that automatically controlled the
refrigerant conditions in the condenser by varying the flow of cooling water. 

However, in 1989, the licensee replaced the valve actuators with manual valve
operators under a temporary modification because of continuing problems with
the valve actuators.  The licensee had evaluated essential chiller operation
with manual control of cooling water flow for a range of cooling water
temperatures at the design maximum heat removal rate, but had not evaluated
potentially more limiting operation at lower heat removal rates.

After the NRC diagnostic evaluation, the licensee evaluated essential chiller
operation at low cooling water temperatures, considering both maximum and
minimum heat removal rates.  On the basis of its evaluation, the licensee
modified procedures and hardware to ensure stable operation of the essential
chillers over the selected range of cooling water temperatures with minimal
operator actions.  The modifications included the installation of an
instrumented bypass line with a throttle valve that allows precise control of
cooling water flow to the chiller condenser; a procedural change that directs
operators to block an automatic start of the 150-ton chillers at low cooling
water temperatures, thereby increasing the load on the 300-ton chillers; and a
change to the chilled water temperature controller that reduces the peak load
on the 300-ton chillers.

Perry Nuclear Power Plant

Centerior Energy, the licensee for the Perry Nuclear Power Plant, reported
both trains of control room emergency recirculation to be inoperable for 10
hours and 47 minutes on January 28 and January 29, 1994, because of the low
temperature of emergency closed cooling water (Licensee Event Report 94-005,
March 11, 1994).  The control complex chillers are designed for an inlet
temperature of condensing water greater than 13 �C [55 �F] from the emergency
closed cooling system, which supplies cooling water when the control room
heating, ventilation, and air conditioning system is in the emergency
recirculation mode.  At a supply temperature of emergency closed cooling water
below 13 �C [55 �F], an emergency start and load of the control complex
chillers could be prevented by a low refrigerant temperature trip.  

The licensee also discovered that a similar condition had been noted at Perry
in 1986.  The corrective action taken by the licensee at that time was to
install a small-diameter essential service water bypass line to restrict the
flow of essential service water to the emergency closed cooling heat exchanger
when the temperature of essential service water was below 13 �C [55 �F]. 
However, the calculations and procedural modifications supporting this design
change only considered peak design heat loads for the emergency closed cooling
system, not reduced heat loads on the system.  The failure to consider reduced
heat loads in the calculations and procedural modifications directly
contributed to the failure to satisfy the design criterion for emergency
closed cooling water temperature on January 28 and January 29, 1994.

To correct these problems, the licensee has committed to install a temperature
control valve in the essential service water bypass line around the emergency
closed cooling heat exchanger.  The valve will be designed to automatically
maintain emergency closed cooling water temperature above its design limit. 
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                                                      December 5, 1994
                                                      Page 3 of 3

The licensee has also improved procedural guidance to maintain the temperature
for emergency closed cooling above 13 �C [55 �F] until the design change is


Over-cooling the condenser refrigerant for essential chilled water systems
when condenser cooling water supply temperature is abnormally low may cause
unstable chiller operation or actuate a self-protection feature that removes
the chiller from service.  Because cold weather increases the rate of heat
loss from both the source of cooling water and structures served by the
chilled water loop, low cooling water temperatures and low chiller heat loads
tend to occur concurrently during periods of cold weather.  The potential for
loss of a chiller at low cooling water temperatures due to over-cooling of the
condenser refrigerant is increased at low heat loads.  Because of a focus on
peak heat loading during the design process, the potential for the loss of an
essential chiller during periods of low cooling water temperature may be

Although heat transfer through structures to the environment during periods of
cold weather reduces the heat load on the chilled water system, some heat
removal through an essential chiller may be necessary to prevent equipment
temperatures from exceeding the maximum analyzed value.  Safety-related
equipment may fail if the equipment temperature exceeds its maximum analyzed
value following a loss of essential chiller function caused by an abnormally
low cooling water supply temperature.
This information notice requires no specific action or written response.  
If you have any questions regarding the information in this notice, please
contact the technical contact listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.

                                   /S/'D BY BKGRIMES

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

Technical contact:  Steven R. Jones, NRR
                    (301) 504-2833

1.  Figure 1, A simple Refrigeration Cycle      
2.  List of Recently Issued NRC Information Notices

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