Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
October 20, 1993
NRC INFORMATION NOTICE 93-84: DETERMINATION OF WESTINGHOUSE REACTOR COOLANT
PUMP SEAL FAILURE
Addressees
All holders of operating licenses or construction permits for pressurized
water reactors (PWRs).
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a potential problem that could result from the
use of Westinghouse-designed reactor coolant pump (RCP) shutdown procedures
for pumps whose No. 1 seals have failed. 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.
Background
Westinghouse supplied the Braidwood Station RCPs, incorporating a three-stage
seal series arrangement to limit coolant flow up the pump shaft. The No. 1
seal, the main seal of the pump, is a controlled-leakage, film riding face
seal. The No. 2 and No. 3 seals are rubbing face seals. During normal
operation, an injection flow of nominally 30.3 liters [8 gallons] per minute
enters the pump below the No. 1 seal (see Attachment 1). Here the flow
splits. A portion (approximately 18.9 liters [5 gallons] per minute) flows
down through the thermal barrier heat exchanger and enters the reactor coolant
system. In this manner, the primary coolant is prevented from entering the
radial bearing and seal section of the pump unit. The remainder (11.4 liters
[3 gallons] per minute) (controlled leakage) passes through the pump radial
bearing and enters the No. 1 seal. Above the No. 1 seal, most of the flow
leaves the pump through the No. 1 seal leakoff line and returns to the
chemical and volume control system. Minor flow passes through the No. 2 seal
and its leakoff line to the reactor coolant drain tank. A back flush
injection of 0.8 liters [0.21 gallons] per hour from a head tank flows into
the No. 3 seal between its "double dam" seal area. At this point, half of the
flow passes through one side of the seal and out the No. 2 seal leakoff line
while the remaining flow passes through the other side and out the No. 3 seal
leakoff line. This arrangement ensures essentially zero leakage of reactor
coolant from the pump.
9310140192.
IN 93-84
October 20, 1993
Page 2 of 3
During normal operation, the No. 1 seal is subject to full reactor coolant
system pressure. When the injection flow passes through the No. 1 seal, it
produces a pressure drop of approximately 15.41 MPa [2235 psi]. The No. 2
seal has the capability to withstand full operating pressure, and its major
function is to act as a backup in case the No. 1 seal fails. If the No. 1
seal fails, leakage through the No. 2 seal increases. The No. 2 seal high
leak flow alarm actuates at 3.8 liters [1 gallon] per minute.
Description of Circumstances
In the middle of 1992, while developing shutdown procedures for the RCPs at
the Braidwood Station, Commonwealth Edison Company (the licensee) found that
its abnormal operating procedure contained inadequate instructions for
responding to a failure of the RCP No. 1 seal. On a high RCP No. 1 seal
leakoff condition, the abnormal operating procedure directed the operating
personnel to check the condition of the No. 2 seal. If the No. 2 seal was
acting as the primary pressure boundary on the basis of leakage of at least
3.8 liters [1 gallon] per minute from the No. 2 seal, the procedure required
that the RCP be shut down. If the condition of the No. 2 seal was normal, the
procedure required no further action and gave no further guidance.
Discussions with Westinghouse indicated that the condition of the No. 2 seal
might not be a sufficient indication of the operability of the No. 1 seal.
Discussion
The inadequacies in the operating procedure stem from the limitations of the
instrumentation used to monitor the leakage. The relevant No. 1 seal
instrumentation limits maximum readings to 22.7 liters [6 gallons] per minute.
Although the No. 2 seal high leak flow alarm was set at 3.8 liters [1 gallon]
per minute, very high No. 1 seal leak rates, more than 113.6 liters
[30 gallons] per minute, are needed for the No. 2 seal to reach this flow
rate. Thus, the No. 1 seal could have substantial leakage (e.g., 22.7-113.6
liters [6-30 gallons] per minute) even though the No. 2 seal high leak flow
alarm was not actuated. In the mean time, the RCP seal system might be
severely damaged if loss of injection were to occur or the No. 1 seal leak
rates were more than 30.3 liters [8 gallons] per minute. Therefore,
monitoring for high No. 2 seal leakage is not a reliable method of determining
whether the No. 1 seal has failed.
After finding that the condition of the No. 2 seal might not indicate the
operability of the No. 1 seal, Westinghouse provided guidance for the
Braidwood Station that significantly enhances operator ability to determine if
immediate shutdown of an RCP is required or if a more orderly shutdown can be
conducted. Westinghouse completed a safety evaluation and issued Technical
Bulletin, NSD-TB-93-01-R0, "Revised Procedures for RCP Shutdown with No. 1
Seal Leakage Outside Operating Limits," dated March 30, 1993, to other PWRs
that might be affected. An excerpt from this document is attached to this
information notice (see Attachment 2).
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IN 93-84
October 14, 1993
Page 3 of 3
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.
/s/'d by BKGrimes
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: Steve DuPont, RIII
(815) 458-2852
Martin Farber, RIII
(708) 790-5774
Chu-Yu Liang, NRR
(301) 504-2878
Attachments:
1. Simplified Braidwood Seal Water Injection
and Leakoff Flow Diagram
2. Excerpt from Westinghouse Technical
Bulletin, NSD-TB-93-01-R0, "Revised
Procedures for RCP Shutdown with No. 1
Seal Leakage Outside Operating Limits"
3. List of Recently Issued NRC Information Notices
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