Information Notice 1995-47, Unexpected Opening of a Safety/Relief Valve & Complications Involving Suppression Pool Cooling Strainer Blockage

Unexpected Opening of a Safety/Relief Valve & Complications Involving Suppression Pool Cooling Strainer Blockage

ML031060261
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	11/30/1995
From:	Crutchfield D Office of Nuclear Reactor Regulation
To:
References
IN-95-047, Rev. 1, NUDOCS 9511270084
Download: ML031060261 (10)

________ - . I

I-

UNITED STATES

NUCLEAR REGULATORY COMMISSION.

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001 November 30, 1995 NRC INFORMATION NOTICE 95-47, REVISION 1: UNEXPECTED OPENING OF A

SAFETY/RELIEF VALVE

AND COMPLICATIONS INVOLVING

SUPPRESSION POOL COOLING

STRAINER BLOCKAGE

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

PurDose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this revised

information notice to alert addressees to a recent failure of a safety/relief

valve (SRV) to remain closed during steady-state reactor operation and the

attendant complications involving suppression pool cooling including strainer

blockage. 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

Information Notice 95-47, "Unexpected Opening of a Safety/Relief Valve and

Complications Involving Suppression Pool Blockage," was issued on

October 4, 1995. The notice described a failure of a safety/relief valve to

remain closed, and licensee planned followup actions. This revision provides

clarifying details on the licensee action plan for monitoring leakage through

these valves (using tailpipe temperature monitoring), makes minor corrections

on event details, adds an additional related generic communication and notes

the NRC staff plans to further evaluate implications of SRV leakage.

DescriDtion of Circumstances

On September 11, 1995, the Limerick Unit-l plant was being operated at

100 percent power when control room personnel observed alarms and other

indications that one SRV ("i") was cpen. Emergency procedures were

implemented. Attempts to close the valve were unsuccessful and within

2 minutes a manual reactor scram was initiated. The main steam isolation

valves were closed to reduce the cooldown rate of the reactor vessel. The

maximum cooldown rate during the event was 69 *C/hr (156 OF/hr]. Before the

SRV opened, the licensee was running the 'Al loop of suppression pool cooling

to remove heat being released into the pool by leaking SRVs.

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IN 95-47, Rev. I

November 30, 1995 The licensee has 2-stage vertical discharge SRVs manufactured by Target Rock

Corporation. This particular valve design, which is oriented such that

condensate collects on the main stage valve seat, is believed to be the cause

of the continued problems with main stage leakage and is unique to the

Limerick units. Other licensees use Target Rock valves that have a similar

2-stage design, but that are configured to discharge horizontally without

condensate collecting on the valve seat. Even though the licensee had

modified the valve bodies to promote drainage of condensate buildup, these

leaking valves were assumed to still have the same leakage problem through the

main stage valve seat.

Shortly after the manual scram, and with the SRV still open, the *BO loop of

suppression pool cooling was started. Operators continued working to close

the SRV and slow the cooldown of the reactor vessel. Approximately 30 minutes

later, fluctuating motor current and flow were observed on the Unit 1 DAY

suppression pool cooling loop. Cavitation was believed to be the cause and

the loop was secured.

After checking out the pump, the "Al pump was restarted, but at a reduced

flowrate of 8 kL/m [2000 gpm]. No problems were observed so the flow rate was

gradually increased to 32 kL/m [8500 gpmj. No problems were observed so the

licensee continued to operate the pump at a constant flow. A pressure gauge

located on the pump suction was observed to have a gradually lower reading, which was believed to be indicative of an increased pressure drop across the

pump suction strainer located in the suppression pool. After about 30 minutes

of additional operation, the suction pressure remained constant.

The rest of the reactor shutdown was routine and there were no further

complications.

Discussion

Safety Relief Valve:

Shortly after the licensee started up following a refueling outage in March

1994, three SRVs ("F," IN," and IS") were leaking, as determined by tailpipe

temperatures which ranged from 79 *C [175 OF] to 104 *C [220 'F]. These

valves had been refurbished and reset before the restart and they were not

leaking when installed. However, SRVs 'Mm and ISO had been stroked during a

3550 kPa [500-psi] automatic depressurization system operability test.

Reactor operation continued from March 1994 until September 1995, except for

two short mini-outages. Prior to the recent opening of the OKI SRV, SRVs RD"

and *LO were also observed to be-leaking. Tailpipe temperatures of the five

leaking SRVs were reported as ranging between 102 *C [215 OF] to 141 'C

• [285 'F] and, because of prior experience, the leakage was believed to be past

the main stage valve seat. Tailpipes at Limerick are uninsulated.

IN 95-47, Rev. I

November 30, 1995 After the September 11, 1995, shutdown, the leaking SRVs were removed and the

and *SP SRVs sent offsite for inspection to determine the root cause for

M*'

the OM' SRV opening. The ON* SRV was found to have been leaking through the

pilot valve; the other four valves were leaking through the main valve.

Disassembly of the H*'SRV disclosed that the pilot valve disk was badly

eroded; the nose of the disk had been'steam cut 360 degrees around the disk

and had separated from the rest of the disk. The interior of the disk and the

push rod also showed evidence of erosion. The pilot valve seat was eroded, but to a lesser degree. The reason for the initial leakage is not known. The

pilot valve seat.and disk were fabricated of Stellite 6 and Stellite 6B,

respectively, and were not expected to erode so severely.

The licensee has replaced the five leaking SRVs and has resumed operation.

The licensee is investigating techniques for identifying whether the leakage

is through the pilot valve or main stage valve. Until such a technique is

found, the licensee will assume that all leakage is through the pilot valve.

As discussed in a letter dated October 6, 1995, the licensee has established a

tailpipe temperature action plan. When the tail pipe temperature exceeds an

"alert' level of 107 *C [225 OF], the licensee will log temperature more

frequently, project when temperature would be expected to reach 135 *C

[275 OF], and initiate preparations for an outage to replace the affected

valve(s). The projection will be based on historical trends and industry

experience. If the temperature reaches the 'action level" of 121 OC [250 OFJ,

a planned outage will be scheduled to replace the affected SRV before tail

pipe temperature is expected to reach 135 *C [275 OF].

Steam leakage through the pilot valve of about 450 kg/hr [1000 lb/hr] is

estimated to cause a tailpipe temperature of 121 'C [250 *F]. Testing has

demonstrated that this amount of leakage will not cause either the pilot valve

or the main stage valve to open. Actual experience at Limerick Unit 1 showed

that the AMP SRV operated for more than a year with a tailpipe temperature in

excess of 121 'C [250 OF] before it failed. The last recorded temperature of

the uninsulated tailpipe before the SRV opened was 141 *C [285 *F]; the

temperature was recorded as 146 *C [295 OF] a week earlier.

The NRC staff plans to further assess the safety implications of pilot valve

leakage, considering possible effects on SRV valve operability and leakage

detection capabilities. The staff also plans to examine the efficacy of the

Automatic Depressurization System (ADS) operability test and licensee

practices of routinely operating suppression pool cooling to cope with leaking

SRVs.

Suppression Pool:

Limerick Unit 1 has been in comnercial operation since 1986 without having had

the suppression pool cleaned; cleaning was scheduled for the 1996 refueling

outage. The pool of Unit 2 was cleaned during the 1995 refueling outage.

IN 95-47, Rev. 1 Novgember 30 1995 After a plant cooldown following the blowdown event, a diver was sent into the

Unit 1 suppression pool to observe the condition of the strainers and general

pool cleanliness. Each strainer is a 8TO arrangement with two truncated cones

fabricated from perforated plate; the entire cone surface is covered by a

12x12 316 L stainless steel wire mesh. The suction strainer in the *Al loop

of suppression pool cooling was found to be covered with a thin Kmat" of

material, consisting of fibers and sludge. The IBO strainer had a similar

covering, but to a lesser extent. These are the two loops that had been used

for suppression pool cooling necessitated by the leaking SRVs. The other

strainers in the pool were covered with a dusting of sludge. Debris was

subsequently brushed off the surface of the strainers, and the suppression

pool floor and water were cleaned by use of a temporary filtration system. It

is believed that, during operation of the suppression pool cooling system, the strainer filtered out fibers that were in the pool water. The resulting

6mat" of fibers improved the filtering action of the strainers thereby

collecting sludge and other material on the surface of the strainer. The

licensee believes that the SRV opening increased the rate of accumulation on

the strainer surfaces. The licensee removed about 635 kg [1400 lb] of debris

from the pool of Unit 1. A similar amount of material had previously been

removed from the Unit 2 pool.

Analysis showed that the sludge was primarily iron oxides and the fibers were

of a polymeric nature. The source of the fibers has not been positively

identified, but the licensee has determined that the fibers were not inherent

with the suppression pool. There was no trace of either fiberglass or

asbestos fibers.

Related Generic Communications

• NRC Information Notice 95-06: Potential Blockage of Safety-Related

Strainers by Material Brought Inside Containment"

• NRC Bulletin 95-02: Unexpected Clogging of a Residual Heat Removal

(RHR) Pump Strainer While Operating in Suppression Pool Cooling Model

• NRC Information Notice 93-34 and Supplement 1: *Potential for Loss of

Emergency Core Cooling Function due to a Combination of Operational and

Post-LOCA Debris in Containment'

• NRC Bulletin 93-02 and Supplement 1: 'Debris Plugging of Emergency Core

Cooling Suction Strainers"

• NRC Information Notice 92-85: "Potential Failures of Emergency Core

Cooling Systems caused by Foreign Material Blockage'

• NRC Information Notice 92-71: *Partial Plugging of Suppression Pool

Strainers at a Foreign BWRO

T4 95-47, Rev. 1 ember 30, 1995

- ge S of 5 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 project manager.

IeW rrec or

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contacts: Robert Elliott, NRR

(301) 415-1397 Jerry Carter, NRR

(301) 415-1153 Attachment:

List of Recently Issued NRC Information Notices

Attachment

i95-47, Rev. 1

">ember 28, 1995 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

94-13, Control and Oversight of 11/28/95 All holders of OLs or CPs

Supp. 2 Contractors during Re- for nuclear power reactors.

fueling Activities and

Clarification of Applica- bility of Section 50.120 of

Title 10 of The Code of

Federal Regulations to

Contractor Personnel

95-13, Potential for Data 11/22/95 All holders of OLs or CPs

Supp. 1 Collection Equipment to for nuclear power reactors.

Affect Protection System

Performance

91-29, Deficiencies Identified 11/22/95 All holders of OLs or CPs

Supp. 3 during Electrical for nuclear power reactors.

Distribution System

Functional Inspections

94-86, Legal Actions Against 11/15/95 All holders of OLs or CPs

Supp. 1 Thermal Science, Inc., for nuclear power reactors.

Manufacturer of Thermo-Lag

95-52 Fire Endurance Test Results 11/14/95 All holders of OLs or CPs

for Electrical Raceway Fire for nuclear power reactors.

Barrier Systems Constructed

from 3M Company Interam Fire

Barrier Materials

95-51 Recent Incidents Involving 10/27/95 All material and fuel

Potential Loss of Control cycle licensees.

of Licensed Material

95-50 Safety Defect in Gaumamed- 10/30/95 All High Dose Rate

12i Bronchial Catheter Afterloader (1IDR) Adapters.

Clapping Adapters

95-49 Seismic Adequacy of 10/27/95 All holders of OLs or CPs

Thermo-Lag Panel s for nuclear power reactors.

OL = Operating License

CP - Construction Permit

IN 95-47, Rev. 1 November 30, 1995 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 project manager.

Orig. signed by DN Crutchfield

Dennis M. Crutchfield, Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contacts: Robert Elliott, NRR

(301) 415-1397 Jerry Carter, NRR

(301) 415-1153 Attachment:

List of Recently Issued NRC Information Notices

Fk-ar

~ Ca %A-

• See previous concurrences.

DOCUMENT NAME: 9547REV1.IN

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IN 95-47 Rev. 1 November xx, 1995 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 project manager.

Dennis M. Crutchfield, Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contacts: Robert Elliott, NRR

(301) 415-1397 Jerry Carter, NRR

(301) 415-1153 Attachment:

List of Recently Issued NRC Information Notices

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OFFICIAL RECORD COPY

IN 95-47 Rev. 1 November xx, 1995 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 project manager.

Dennis M. Crutchfield, Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contacts: Robert Elliott, NRR

(301) 415-1397 Jerry Carter, NRR

(301) 415-1153 Attachment:

List of Recently Issued NRC Information Notices

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