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92

Information Notice No. 93­92: Plant Improvements to Mitigate

Common Dependencies in Component Cooling Water Systems

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

December 7, 1993

NRC INFORMATION NOTICE 93‐92: PLANT IMPROVEMENTS TO MITIGATE COMMON

DEPENDENCIES IN COMPONENT COOLING WATER SYSTEMS

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 potential problems resulting from common

dependencies in component cooling water (CCW) systems. Such dependencies on

loss of the CCW system were found to be contributors to the core damage

frequency at several nuclear power plants. 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

On November 23, 1988, the NRC issued Generic Letter 88‐20, "Individual Plant

Examination for Severe Accident Vulnerabilities," requesting that each nuclear

power plant perform a systematic examination to identify any plant‐specific

vulnerabilities to severe accidents and report the results to the Commission.

As stated in the letter, licensees are expected to expeditiously correct any

identified vulnerabilities. This notice provides insights gained from the

individual plant examination (IPE) process regarding improvements in plant

design or operational practices and procedures to mitigate potential problems

resulting from common dependencies in CCW systems.

Description of Circumstances

CCW systems provide cooling for reactor coolant pump (RCP) seals, and in many

cases also supply cooling water to emergency core cooling system (ECCS)

components. Consequently, failure of the CCW system could cause a loss of RCP

seal cooling leading to an RCP seal loss of coolant accident (LOCA) and could

also disable the necessary accident mitigation systems. Several licensees

have identified these common dependencies as contributors to the total plant
core damage frequency in their IPEs. The following cases illustrate the

measures taken by several utilities to mitigate the consequences of such

accidents.

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IN 93‐92

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Turkey Point Units 3 and 4

The Turkey Point IPE documented a potential accident sequence caused by an

unisolable rupture of the CCW system that could occur while the B charging

pump is out of service. In this scenario, cooling to the RCP seals would be

lost, as would cooling to the A and C charging pumps. If the charging pumps

are run without CCW, they are predicted to fail. This scenario would,

according to the IPE, cause an RCP seal LOCA because all means of providing

seal cooling would be lost from either the normal CCW system supply or the

alternate seal injection supply from the charging pumps. Additionally,

because the CCW system supplies cooling to the low and high head safety

injection pumps, these pumps also would fail. Thus, an unmitigated RCP seal

LOCA would occur.

This finding led the licensee to modify the A and C charging pumps in 1991 so

that the service water system can be aligned as an alternate cooling water

supply to any of the charging pumps. (The B charging pump had been similarly

modified in 1976.) Thus, a loss of the CCW system alone would not disable all

of the ECCS components required for LOCA mitigation. This modification

reduced the contribution of this sequence to the total core damage frequency.

However, during an NRC Operational Readiness Assessment Team inspection at the

facility, procedural deficiencies were identified that could have prevented

this modification from performing its intended function. The licensee

corrected the deficiencies.

H.B. Robinson Steam Electric Plant, Unit 2

During development of the Robinson IPE, the licensee identified an accident

sequence that could be initiated by a total loss of the CCW system. The

configuration at Robinson is similar to that at

Turkey Point in that the CCW system supplies cooling to both the RCP seals and

the charging pumps. Thus, loss of the CCW system could lead to an RCP seal

LOCA coincident with the inability to provide high pressure makeup.

This discovery led the licensee to implement procedural revisions that allow

the alignment of an alternate cooling supply to the charging pumps by

connecting the firewater system to existing fittings on the charging pump

couplings. This procedural revision reduced the estimated core damage

frequency contribution for the sequence.

Watts Bar Nuclear Plant, Unit 1

Before performing the IPE, the licensee modified a procedure to align a backup

cooling water supply to the 1A‐A centrifugal charging pump (CCP) via the

emergency raw cooling water (ERCW) system. The CCPs depend on the CCW system

for normal cooling to the pump lube oil and gear oil coolers. The CCW system

also supplies cooling to the residual heat removal pumps, safety injection

pumps, containment spray pumps, and the

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IN 93‐92

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RCP thermal barrier coolers. In the IPE, the licensee credited operator

actions associated with aligning the ERCW backup during a total loss of CCW

system scenario. The IPE indicated that alignment of the ERCW system to the

CCP reduced the incremental core damage frequency.

A recent NRC Emergency Operationg Procedure team inspection at the facility

identified procedural deficiencies that could have prevented this alignment

from performing its intended function. The licensee is developing measures to

correct these deficiencies.

Diablo Canyon Nuclear Power Plant, Units 1 and 2

At Diablo Canyon Units 1 and 2, the CCW system provides cooling to the

charging pumps, which supply cooling water to the RCP seals. While developing

the probabilistic risk assessment, the licensee discovered that loss of the

CCW system could result in loss of the charging pumps and the RCP seals and

could lead to an RCP seal LOCA. To mitigate this consequence, the licensee

provided an alternate means of cooling the charging pumps from the fire water

system on loss of the CCW system. Hose connections and dedicated hoses were

provided and operational procedures were changed to facilitate this alternate

means of cooling. As a result, the licensee indicated that the core damage

frequency associated with loss of the CCW system was reduced.

Donald C. Cook Nuclear Plant, Units 1 and 2

The CCW system for the Cook Nuclear Plant provides cooling to the RCP seals

and various primary plant components. Loss of the CCW system leading to an

RCP seal LOCA was identified as a dominant contributor to the core damage

frequency. The licensee is currently investigating changes to operational

procedures to instruct the operator to open the cross‐tie valve of the

chemical and volume control system of one unit to the opposite unit early in

the accident response to provide RCP seal cooling and to prevent seal damage.

Discussion

As indicated above, several licensees have identified, through the IPE

process, that a common dependency in the CCW system to provide cooling for RCP

seals and ECCS components could result in a significant contribution to the

core damage frequency. Licensees found that changes in plant operational

practices and procedures or cost‐effective improvements in design could reduce

the calculated core damage frequency.

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IN 93‐92

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This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contacts listed below or the appropriate Office of Nuclear

Reactor Regulation project manager.

/s/'d by BKGrimes

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contacts: J. L. Shackelford, RII
(404) 331‐0338

M. T. Drouin, RES

(301) 492‐3917

Attachment:

List of Recently Issued NRC Information Notices

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