Information Notice 1993-61, Excessive Reactor Coolant Leakage Following a Seal Failure in a Reactor Coolant Pump or Reactor Recirculation Pump

Excessive Reactor Coolant Leakage Following a Seal Failure in a Reactor Coolant Pump or Reactor Recirculation Pump

ML031480397
Person / Time
Issue date:	08/09/1993
From:	Grimes B K Office of Nuclear Reactor Regulation
To:
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IN-93-061
Download: ML031480397 (3)

Information

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Notices > 1993 > IN 9 UNITED STATES NUCLEAR REGULATORY

COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 August 9, 1993 NRC INFORMATION

NOTICE 93-61: EXCESSIVE

REACTOR COOLANT LEAKAGE FOLLOWING

A SEAL FAILURE IN A REACTOR COOLANT PUMP OR REACTOR RECIRCULATION

PUMP

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 the potential

for excessive

reactor coolant leakage following

a seal failure in a reactor coolant pump or reactor recirculation

pump. 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

Oconee Nuclear Station, Unit 1 On May 24, 1992, the licensee commenced

a reactor shutdown from 100 percent power because of excessive

leakage from the 1A2 Reactor Coolant Pump seal.The maximum leakage was approximately

23 liters per minute 6 gpm]. The seal failed because of the premature

degradation

of obsolete seal parts that had mistakenly

been installed.

Westinghouse

supplied the Unit 1 reactor coolant pumps, incorporating

a three-stage seal series arrangement

to limit coolant flow up the pump shaft.Although Westinghouse

had provided the necessary

information

on the design change of the seal, the information

was not properly communicated

to plant personnel.

As a result, the obsolete seal parts were not removed from the station stock and appropriate

maintenance

procedures

were not revised to reflect the change. These deficiencies

contributed

to the fact that maintenance

personnel

inadvertently

installed

the obsolete seal parts.Further details can be found in Licensee Event Report (LER) 50-269/92-09 and NRC Inspection

Report No. 50-269/92-13.

9308030200.

IN 93-61 August 9, 1993 Millstone

Unit 1 http://www.nrc.gov/reading-rmldoc-collections/gen-comm/info-notices/1

993/in9306

1 .html 03/13/2003 Information

Notice No. 93-61 On May 25, 1989, the licensee started up Unit 1 for Cycle 13 operation.

During plant heatup on May 27, 1989, operators

noted indications

of intermittent

seal failure for the A' Reactor Recirculation

Pump inner seal.The licensee continued

power escalation

and cycle startup testing while making plans to replace the seal. On May 29, 1989, while at full-power

operation, the drywell leakage exceeded the Technical

Specifications

limit and the licensee commenced

plant shutdown.

The leakage was approximately

34 liters per minute 9 gpm] at the start of the event and increased

to about 174 liters per minute 46 gpm] during the shutdown process. After reviewing

the failed seal and consulting

with the pump manufacturer, Byron Jackson, the licensee still did not identify the exact cause of the seal failure. However, the licensee did determine

that the pump seal had probably failed as a result of one or more of the following

causes: (1) improper seal handling prior to installation, (2) introduction

of debris and corrosion

products into the seal cavity, and (3) improper depressurization

following

hydrostatic

testing of the seal assembly.Further details can be found in LER 50-245/89-14, Revision 1, and in NRC Inspection

Report No. 50-245/89-12.

Clinton Unit 1 On May 21, 1989, the licensee took the reactor to critical for Cycle 2 operation.

On May 25, 1989, the pressure in the seal outer cavity decreased to approximately

414 kPa gauge [60 psig], indicating

failure of the upper seal stage. Approximately

10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> later, the seal appeared to reseal and operated properly.

On June 1, 1989, with the reactor at about 42-percent

power during power ascension, upon shifting the B- Reactor Recirculation

Pump speed from low to high, the operators

immediately

noted indications

that both the upper and lower seals in the pump had failed. The seal failures resulted in increased

flow from the drywell floor drain sump inlet; the leakage reached a maximum of 242 liters per minute [64 gpm]. The licensee then initiated

plant shutdown.

Although the exact cause of the seal failure was not determined, the licensee indicated

that the probable cause was improper assembly or improper installation.

Further details can be found in the licensee special report submitted

to NRC on June 30, 1989, and in NRC Inspection

Report No. 50-461/89-21.

Discussion

Both reactor coolant pumps and reactor recirculation

pumps use a series of primary and secondary

seals to limit the reactor coolant leakage to containment.

A loss-of-coolant

accident (LOCA) can occur if leakage through the seals of reactor coolant pump or reactor recirculation

pump exceeds the capacity of the normal makeup systems. Thus, the failure of these seals can represent

a significant

degradation

of the reactor coolant pressure boundary.IN 93-61 August 9, 1993 The NRC has, over a period of years, evaluated

the issue of reactor coolant pump and reactor recirculation

pump seal-related

problems (Generic Issue 23)and the need for additional

licensing

requirements

to reduce the potential core-melt

risk resulting

from the failure of these pump seals. An evaluation

program was initiated

to resolve the generic issue and address several reactor coolant pump seal leaks that occurred in the late 1970s and the early 1980s.Analysis performed

in conjunction

with the evaluation

indicated

that the overall probability

of core-melt

due to small-break

LOCAs could be dominated by reactor coolant pump seal failures.

The two conditions

under which seals have failed or could fail, normal operating

conditions

and off-normal

operating

conditions, are addressed

below: Seal performance

under normal operating

conditions

Based on the review of LERs and feedback from industry, some licensees

appear to have recently made major improvements

in reactor coolant pump and reactor recirculation

pump seal performance.

This improvement

is attributed

to a combination

of factors, including

the following:

enhanced seal quality http://www.nrc.gov/reading-rmldoc-collections/gen-commlinfo-notices/1993/in93061 .html 03/13/2003 Information

Notice No. 93-61 assurance

programs, modified seal design, improved maintenance

procedures

and training, closer attention

to detail, improved seal operating

procedures, more knowledgeable

personnel

involved in seal maintenance, reduction

in frequency of transients

that stress the seals, and seal handling and installation

equipment

designed with the appropriate

precision.

However, not all plants have implemented

such measures, and some seal failures have caused substantial

reactor coolant leakage (as described

above).Development

and implementation

of appropriate

procedures

and training can help assure correct operator action for operational

conditions

related to seal degradation

and can assist to identify seal degradation

in a timely manner.This can thereby prevent or mitigate cascade failure of multistage

seal assemblies.

Section III of the ASME Boiler and Pressure Vessel Code has included specific exclusions

for seal components

under NB-3411.2 and NB-2121(b)

relative to design requirements.

However, code exclusions

by themselves

do not relieve licensees

from other pertinent

regulatory

requirements

such as Appendix B to 10 CFR Part 50, as applicable.

For those items covered by Appendix B, as reflected

in plant-specific

licensing

bases, a quality assurance

program is required.

The staff is considering

additional

generic action to address whether all licensees

should treat certain seal components

as safety-related.

Seal performance

under off-normal

operating

conditions

With respect to off-normal

operating

conditions, particularly

loss of all seal cooling water which can be caused by station blackout, loss of component cooling water or loss of service water, the major concerns involve seal failures due to adverse temperature

effects on elastomer

seal materials

and performance

instabilities

at the primary seal face related to coolant flashing and two-phase

flow. The staff is proceeding

with rulemaking

to address issues of ensuring reactor coolant pump seal function or compensating

for seal failure during loss of seal cooling events, including

station blackout.IN 93-61 August 9, 1993 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 Brian K. Grimes, Director Division of Operating

Reactor Support Office of Nuclear Reactor Regulation

Technical

contacts:

Jai Raj N. Rajan, NRR (301) 504-2788 Peter C. Wen, NRR (301) 504-2832 Attachment:

List of Recently Issued NRC Information

Notices http://www.nrc.gov/reading-rmldoc-collections/gen-commlinfo-notices/l

993/in93061 .html 03/13/2003