Part 21 Report - 1996-670

 
 
ACCESSION #:  9608120142

Westinghouse
Energy                   NUCLEAR SAFETY ADVISORY LETTER
Systems
Business
Unit

THIS IS A NOTIFICATION OF A RECENTLY IDENTIFIED POTENTIAL SAFETY ISSUE
PERTAINING TO BASIC COMPONENTS SUPPLIED BY WESTINGHOUSE.  THIS
INFORMATION IS BEING PROVIDED TO YOU SO THAT A REVIEW OF THIS ISSUE
CAN
BE CONDUCTED By YOU TO DETERMINE IF ANY ACTION IS REQUIRED.

                              P.O.  Box 355, Pittsburgh, PA 15230-0355

Subject: Containment Fan Cooler Operation         Number NSAL-96-003
During a Design Basis Accident

Basic Component: Containment Fan Coolers          Date: 6/20/96

Plants: Westinghouse and MHI NSSS Plants

Substantial Safety Hazard or Failure to Comply    Yes [ ]  No [ ]
  Pursuant to 10 CFR 21.21(a)
Transfer of Information Pursuant to 10            Yes [ ]
  CFR 21.21(b)
Advisory Information Pursuant to 10 CFR           Yes [x]
  21.21(c)(2)

Reference:

SUMMARY

The purpose of this letter is to provide Westinghouse NSSS Owners
information regarding the potential performance of some containment fan
cooler units (CFCUs) during accident conditions which may compromise the
containment cooling system capability.

This Westinghouse Nuclear Safety Advisory Letter deals with the potential
susceptibility for the cooling water of certain CFCUs to flash to steam
during a design basis Loss of Coolant Accident (LOCA) with either a
concurrent Loss of Offsite Power (LOOP) or a delayed sequencing of
safety-related equipment.  The applicability of this issue to a given
nuclear unit depends on plant specific aspects of the containment cooling
system design and its heat removal system.  The Nuclear Regulatory
Commission has been notified by an LER (Reference 1).

Additional information, if required, may be obtained from the originator.
Telephone 412-374-5750.

Originator(s)
          J.  T.  Crane            W.  R.  Rice, Interim Manager
          Regulatory & Licensing   Regulatory & Licensing Initiatives

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ISSUE DESCRIPTION

The potential for this condition was discovered during an investigation
into Component Cooling Water (CCW) system design issues by a Westinghouse
NSSS Owners The CCW System provides cooling water directly to the CFCUs
in that plant's design.  Depending on the specific design conditions, it
may be applicable to plants that utilize cooling water systems other than
the CCW for their CFCUs.

This issue concerns a type of containment cooling design that has a
single set of CFCUs for heat removal during both normal operating
conditions and accident conditions. In the Licensee's design the CFCUs
are equipped with a two-speed blower that operates at about 1200 rpm for
normal operation and about 600 rpm for accident conditions.  Cooling
water to the heat exchanger coils for both normal and accident operation
is supplied by the CCW system.  The Licensee's containment analysis
assumes the fan coolers operate during postulated accidents, removing
heat so as to maintain containment pressure within design basis limits.

During a postulated LOOP coincident with a design basis LOCA, power is
lost to both the CCW pumps and the CFCU blowers.  The CCW pumps are
calculated to coast down to 0 rpm within 1-2 seconds while the CFCU
blowers are calculated to coast down from a nominal speed of 1200 rpm to
600 rpm in approximately 30 seconds and then to 0 rpm an estimated 700
seconds later.  Accounting for diesel-generator start times and emergency
buss loading sequence, the CFCU blowers are re-energized approximately 20
seconds after the initiation of the LOOP.  This timing of events provides
for hot, steam-laden containment air to be drawn over the heat exchanger
coils at a relatively high velocity for 30 seconds before cooling liquid
flow is reestablished to the coils.

The high heat content of containment atmosphere under accident conditions
being drawn over the CFCU heat exchanger coil with no pumped liquid flow
ha been calculated to result in steaming of the stationary liquid
inventory of the coils.  When the CCW pumps an re-energized, the pumped
liquid flow acts to collapse the steam void in the CFCU piping.
Collapsing the stem void is predicted to result in a waterhammer with
sufficient energy and force that may impact the integrity of either the
heat exchanger coil or its associated piping, resulting in a loss of
integrity of the CCW system.

To prevent flashing of the CFCU cooling water, the Licensee has installed
a nitrogen pressurization system on the CCW system.

TECHNICAL EVALUATION

The following discussion provides several items to consider in evaluating
your CFCUs.

In general, a plant that has CFCUs with a 2 speed blower motor and a
diesel loading sequence that results in water flow through the cooler
coil law than 15 seconds after a design basis large break LOCA, may have
a similar situation.  However, there are other plant specific aspects
that can impact CFCU functioning.  The containment cooling design in this
issue has a single set of fan coolers that are used for containment
cooling during both normal and accident conditions.  Flashing and
subsequent void collapse may not apply if a plant has separate sets of
equipment where the accident CFCUs are not operating prior to a design
basis accident.  These accident related fan coolers would be at
containment ambient conditions with no forced corrective heat transfer at
the beginning of the

1317A:WFWIN                                                  Sheet 2 of 3

accident.  Since their fans would not be operating at the beginning of
the postulated accident (as compared to a single set of fan coolers which
would be coasting down) the heat release from a design basis accident
would be transferred to the coil cooling water at a much slower rate,

Flashing of the CFCU cooling water may not occur in a specific plant
design because of factors such as heat transfer capability, saturation
pressure of the cooling system, coastdown time of the fan blower, and
coastdown time of the cooling water pump(s).  Also, the timing of when
ECCS equipment will start in a design basis accident can determine if the
CFCUs do or do not have stagnant cooling water.

In summary, the issue is whether safety related CFCUs will function as
intended during a design basis event, if they are postulated to
experience flashing of the water in the cooling coils.  If flashing can
be postulated, then the issue of whether the CFCUs will maintain
structural integrity during subsequent void collapse should be addressed.

ASSESSMENT OF SAFETY SIGNIFICANCE

Westinghouse cannot determine the potential for flashing in the CFCUs on
a generic basis because of the various plant specific designs for the
containment cooling system and situation specific beat transfer
conditions.  Thus the most effective action that can be taken is a
notification to all Westinghouse plants.

REPORTABILITY

The NRC is aware of this issue via an LER (Reference 1).

RECOMMENDED ACTIONS

Licensees should review their containment cooling system to determine if
their safety related containment fan coolers are susceptible to cooling
water void formation and subsequent void collapse and waterhammer during
a design basis accident.

REFERENCE

1.   Pacific Gas & Electric letter:  PG&E DCL-96-097, April 26, 1996 to
     U. S.  Nuclear Regulatory Commission, "Licensee Event Report 1-96-
     005-00 (Voluntary) Potential for Flashing in Containment Fan Cooler
     Units"

1317A:WFWIN                                                  Sheet 3 of 3

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