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 Office of Nuclear Reactor Regulation
To:
References
IN-93-061
Download: ML031480397 (3)

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Home > Electronic Reading Room > Document Collections > General Communications > Information 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