ML20137R811

Forwards Final Accident Sequence Precursor Analysis of Potential Operational Condition at Licensee Facility Re 950309 LER 213/95-010

ML20137R811
Person / Time
Site:	Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png
Issue date:	04/09/1997
From:	Fairtile M NRC (Affiliation Not Assigned)
To:	Feigenbaum T, Mellor R CONNECTICUT YANKEE ATOMIC POWER CO.
References
NUDOCS 9704140255
Download: ML20137R811 (11)

Text

_

April 9, 1997 Mrc Ted C. Feigenbato Executive Vice, President--

c/o Mr. Russell'Mellor

Director of Site Operations Connecticut Yankee Atomic Power Co. ,

362 Injun Hollow Road i East Hampton, CT 06424-3099  !

u

SUBJECT:

-FINAL ACCIDENT SEQUENCE PRECURSOR ANALYSIS OF CONDITION AT HADDAM '

l NECK l

Dear Mr.~Feigenbaum.

i Enclosed for your information is a copy of the final Accident Sequence - l Precursor analysis of the potential operational condition at Haddam Neck ,

reported in Licensee Event Report.No.- 213/95-010. The date of discovery was ,

March 9,:1995, and the description of the event is: . Multiple safety i .jection  !

' valves are susceptible to pressure locking. With the plant in a permanently '

shutdown status, this potential is no longer pertinent.

This final analysis _ (Enclosure 1)'was prepared by our contractor at the Oak Ridge National Laboratory, based'on' review and evaluation of your comments on

'the preliminary analysis, comments received from the NRC staff and from our independent contractor, Sandia National Laboratories. Enclosure 2 contains ,

our responses to your specific comments. In our review of your comments we employed the criteria contained in'the material which accompanied the -

preliminary analysis; The results of, the final'. analysis indicate that this was an accident sequence, precursor.for-a-condition ^ existing in'1995.

Pleasecontactmeat(3'01)415-1442jif'yo'uhaN~anyquestionsregardingthe enclosures. Wef recognize and aprireciate. the. effort expended by you and your staff in reviewipg ard providing comments on the preliminary analysis.  ;

,, N s , ,. s

^ Sincerely,

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, Original signed by:

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p kk. hkh.kg hh, .Mortdn Non-Power B. Fairtile, Senior Reactors and Project Manager Decommissioning j ProjectsDirectorate Division of Reactor Program Management Office of Nuclear Reactor Regulation Docket No. 50-213

Enclosures:

As stated j cc: See next page ,

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% . . . . . p' WASHINGTON, D.C. 20555 4001 April 9, 1997 Mr. Ted C. Feigenbaum Executive Vice President c/o Mr. Russell Mellor Director of Site Operations Connecticut Yankee Atomic Power Co.

362 Injun Hollow Road East Hampton, CT 06424-3099

SUBJECT:

FINAL ACCIDENT SEQUENCE PRECURSOR ANALYSIS OF CONDITION AT HADDAM NECK

Dear Mr. Feigenbaum:

Enclosed for your information is a copy of the final Accident Sequence Precursor analysis of the potential operational condition at Haddam Neck reported in Licensee Event Report No. 213/95-010. The date of discovery was March 9, 1995, and the description of the event is: Multiple safety injection valves are susceptible to pressure locking. With the plant in a permanently shutdown status, this potential is no longer pertinent.

This final analysis (Enclosure 1) was prepared by our contractor at the Oak Ridge National Laboratory, based on review and evaluation of your comments on the preliminary analysis, comments received from the NRC staff and from our independent contractor, Sandia National Laboratories. Enclosure 2 contains our responses to your specific comments. In our review of your comments we employed the criteria contained in the material which accompanied the i preliminary analysis. The results of the final analysis indicate that this i was an accident sequence precursor for a condition existing in 1995. )

l Please contact me at (301) 415-1442, if you have any questions regarding the enclosures. We recognize and appreciate the effort expended by you and your '

staff in reviewing and providing comments on the preliminary analysis.

Sincerely, 1Mb b , b %k Morton 8. Fairtile, Senior Project Manager Non-Power Reactors and Decommissioning <

Project Directorate Division of Reactor Program Management Office of Nuclear Reactor Regulation

-Docket No. 50-213

Enclosures:

As stated cc: See next page

- Northeast Utilities Service Company Haddam Neck Plant Docket No. 50-213 cc:

Lillian M. Cuoco, Esq. Regional Administrator Senior. Nuclear Counsel Region I Northeast Utilitfes Service Company U.S. Nuclear Regulatory Commission

-P. O. Box 270 475 Allendale Road

- Hartford, CT 06141-0270 King of Prussia, PA 19406 Mr. Kevin T. A. McCarthy, Director Board of Selectmen Monitoring and Radiation Division Town Office Building Department of Environmental Haddam, CT 06438 Protection 79 Elm Street Resident Inspector

' Hartford, CT 06106-5127 Haddam Neck Plant c/o U.S. Nuclear Regulatory Commission Mr. Allan.Johanson 361 Injun Hollow Road Assistant Director East Hampton, CT 06424-3099 Office of Policy and Management Policy Developrs7t and Planning hr. James S. Robinson Division Manager, Nuclear Investments and 450 Capitol Avenue-MS#52ENR Administration P. O. Box 341441 New England Power Company Hartford, CT 06134-1441 25 Research Drive Westborough, MA 01582 I

[ Mr. F. C..Rothen 7 Vice President - Work Services Mr. G. P. van Noordennen

' Northeast Utilities Service Company Manager - Nuclear Licensing P. O. Box 128 Northeast Utilities Service Company Waterford, CT 06385 362 Injun Hollow Road East Hampton, CT 06424-3099 Mr. D. M. Goebal Vice President . Nuclear Oversight Ms. Deborah B. Katz, President Northeast Utilities Service Company Citizens Awareness Network P. O. Box 128 P. O. Box 83 Waterford, CT 06385 Shelburne Falls, MA 01370-0083 Mr. J. K. Thayer

, Recovery Officer, Nuclear Engineering and Support Northeast Utilities Service Company P. 0. Box 128 Waterford, CT 06385 Mr. T. C. Feigenbaum Executive Vice. President tc/o~Mr. Russell:Mellor Director of Site Operations Connecticut Yankee Atomic Power Co.

L 362 Injun Hollow Road-East Hampton, CT 06424-3099:

9

---

___-------------a

ENCLOSURE 1

p Appendix B LER No. 213/95-010 i

B.2 LER No. 213/95-010 Event

Description:

Multiple safety injection valves are susceptible to pressure locking Date of Event: March 9,1995 Plant: Haddam Neck B.2.1 Event Summary In preparation for the closcout of Generic Letter (GL) 89 10, personnel at Haddam Neck determined that the following motor-operated valves (MOVs) were potentially susceptible to pressure locking (Fig. B.2.1):

i

. Valves SI MOV-861 A,'-861B,-361C and 861D (the HPSI admission valves), j

- valves SI-MOV 871 A and -871B (the LPSI admission valves), and j Valve SI-MOV 873 (the common LPSI isolation valve).

This analysis assumes the susceptible valves could impact the plant response to a large-break LOCA ,

(LBLOCA). An increase in the core damage probability (CDP) during the time that the necessary conditions i 4

for pressure locking these valves exists is 4.7 x 10 The nominal CDP for the same period is 2.1 x 104 The -

uncertainty in the frequency of LBLOCAs and the uncertainty in the likelihood that the pressure locking  !

conditions will exist contribute to the uncertainty in this estimate.

B.2.2 Event Description On March 9,1995, personnel at Haddam Neck determined that several safety injection (SI) valves were susceptible to pressure locking, which could preclude them from performing required safety functions following a postulated LOCA (Ref. I and 2).

J-Pressure locking occurs when the fluid in the valve bonnet is at a higher pressure than the adjacent piping at the time of the valve opening. The two most likely scenarios for elevating the pressure in the valve bonnet relative to the pressure in the valve system are given below.

1. Thermal pressure locking (or bonnet heatup) can occur when an incompressible fluid is trapped in the valve bonnet (e.g., during valve closure), followed by heating-up the volume in the bonnet. The bonnet heatep scenarios include heating the valve bonnet by an increase in the temperature of the environment during an accident, heat up due to an increase in the temperature of the process fluid on either side of the valve, etc. (Normal ambient temperature variation is not considered because it occurs over a long time period and pressure changes tend to be alleviated through extremely small amounts ofIcakage. Further, operating experience shows that normal temperature variations are not a source of pressure locking events.)

B.2-1 NUREG/CR-4674, Vol. 23

9 LER No. 213/95-010 ' ,

Appendix B

2. Hydraulic pressure locking (or pressure-trapping) can occur when an incompressible Guid is trapped in the valve bonnet, followed by depressurization of the adjacent piping prior to valve opening. Examples of hydraulic pressure locking scenarios include back-leakage past check valves, and system operating pressures that are higher than the system pressure when the valve is required to open.

Pressure locking is of concern because the pressure in the space between the two discs of a gate valve can become pressurized above the pressure assumed when sizing the valve's motor operator. This prevents the valve operator from opening the valve when required.

Thermal binding is a phenomenon where temperature changes of the valve internal components causes the valve stem to expand after closure. This results in a higher required opening thrust that may be above the '

operung thrust assumed when sizing the valve motor operator.

In 1990, plant personnel reviewed the potential of flexible wedge gate valves becoming pressure locked and thermally bound in response to the Institute of Nuclear Pewer Operations' significant operating events report (SOER) number SOER 84-7. As a result of these reviews, personnel implemented remedial measures consisting of procedural changes (stroking valves during plant heatup), analytical treatment of pressure locking effects, and limited testing of valves to address the high priority valves found subject to pressure locking and thermal binding.

In order to upgrade the quality of the documentation on pressure loci,ing and thermal binding issues in preparation for the closcout of GL 8910, personnel detennined that several of the valves in the safety injection system were potentially subjected to pressure locking conditions that were more significant than previously concluded. According to plant personnel, the concem is the thermal pressure locking of the HPSI admission valves, the LPSI admission valves, and the common LPSI isolation valve (Ref. 3 and 4).

B.2.3 Additional Event-Related Information NRC Information Notice (IN) 95-18 (Ref. 5), which addresses the Haddam Neck event, elaborates on the mechanisms of pressure locking:

Pressure-locking may occur in flexible wedge and parallel disk gate valves when fluid entrapped in the bonnet becomes pressurized and the actuator is incapable ofovercoming the additional thrust requirements needed to overcome the increased friction resulting from the differential pressure on both valve disks from the pressurized fluid. IN 95-14 discusses several ways in which fluid may enter the valve bonnet . . These mechanisms represent potential common-cause failure modes that can render redundant trains of safety-related emergency core cooling systems incapable of performing their safety functions.

1 Accordmg to personnel at Hadhn Neck, the pressure locking condition of concern for the HPSI admission valves, the LPSI admission valves, and the common LPSI isolation valve is thermal pressure locking. Hence, these valves are susceptible to becoming pressure locked if (1) water (the incompressible fluid) becomes NUREG/CR-4674, Vol. 23 B.2-2

(

a Appendix B LER No. 213/95-010

. trapped in the bonnet during valve closure and (2) water in the valve bonnet becomes heated by an increase in the temperature of the environment or the process fluid on either side of these valves.

B.2.4 Modeling Assumptions Personnel at Haddam Neck indicated that the failure mode of concern for the high-pressure and low-pressure safety injection valves is believed to be thermally induced pressure locking, wherein water trapped in the valve bonnets may expand during plant heatup and prevent the valves from opening. This analysis assumes that valves SI MOV 861 A, -861B,' 861C -861D and valves SI-MOV-871 A, -871B could be unavailable ,

i because of pressure locking following a large-break LOCA, which would render LPSI and HPSI inoperable, The potential failure of valve SI MOV-873 was not considered because its failure is only significant if valves SI-MOV 871 A and -871B function correctly, which is assumed not to be the case.

The Haddam Neck Individual Plant Examination (IPE) (Ref. 6) indicates that LPSI will provide adequate makeup during a LBLOCA to prevent ccre damage. The simple event tree model used for this event (Fig.

B.2.2) consists of a postulated LBLOCA initiating event with the success or failure of the following two l J

modes of operation: LPSI and decay heat removal (DilR). Consistent with other ASP analyses, an annual j LBLOCA frequency of 2.7 x 10d/yr was assumed (Ref.1).

The significance of an unavailability such as this event is estimated in the Accident Sequence Precursor (ASP) l Program 'in terms of the increase in CDP over the unavailability period, which is also referred to as the importance. Because a nonrecoserable failure of the HPSI admission valves and the LPSI admission valves i

will fail both high- and low pressure injection, and injection is required following a large-break LOCA, the significance of the event can be estimated directly from the change in the probabihty ofinjection failure and l

the probability of a large-break LOCA in the unavailability period. The time interval during which the SI valves could have been inoperable is difTicult to determine. This an aysis assumes that the valves may have been unavailable for a total of I week during the prior year because once the pressure in the bonnet equalizes, pressure locking is no longer a concern. IIence,i.ne temperatures on both sides of the valve equalizing and

~

- normal valve leakage will remove the susceptibility to pressure locking. Figure B.2.3 explores the impact of difTerent assumptions regarding the duration of the time these valves are unavailable.

The CCDP associated with this event is estimated to be 2.7 x 10" 'CCDP for a LBLOCA I + 8.2 x 10 4 fCCDP from the IRRAS

$2 Iin a i-.t period ) 52 L bue cue for 1 wt period.

4

=- 6.8 x 10 otal CCDP

}

' a 1 wt period I.

The importance for this event (CCDP - CDP) is estimated to bc

6.8 x 10 4

Total CCDP

- 8.2 x 10 4 l CDP from the IRRAS I

in a 1 st period. 52 L bue cue for 1 wk periodi B.2-3 NUREG/CR-4674, Vol. 23

=o w

O f LER No. 213/95-010 Appendix B j i

4 i 2.5 a 10 lRepresentathe bue cue .

52 l LBLOCA CDP 1 .

4

= 4.7 = 10 (Importance).

B.2.5 Analysis Results ,

6 An increase in the core damage probability (CDP) during the time that the necessary conditions for pressure 4

locking these valves exists is 4.7 x 104. Tbc nominal CDP for the same period is 2.1 x 10 . The dominant -

core damage sequence for the event (sequence no. 3 on Fig. B.2.2) involves:

+ a postulated large break LOCA, and ,

• failure oflow-pressure injection. '  ;

This estimate is basd on estimated frequencies oflarge-break LOCAs. No irrge-break LOCAs have been observed to date, so there is substantial uncertainty associated with the frequency estimate. The CCDP estimate also is dependent on the assumption that the pressure locking phenomenon would prevent the

" injection valves from opening during large-break LOCAs. This assumption is consistent with those made in ,

the analysis reported in LER 213/95-010, but may be pessimistic.

B.2.6 References ,

I. LER 213/95-010, Rev. O," Pressure Locking of Safety Injection Valves," April 6,1995.

2. LER 213/95 010, Rey,1, " Pressure Locking of Safety Injection Valves," November 8,1995.

3. Conference call with personnel frcm Haddam Neck, the NRC's Olrice for Analysis and Evaluation of Operationas Data, and the Oak Ridge National Laboratory (ORNL), January 23,1997.

4. Personnel communication between P. D. O'Reilly, U.S. NRC, and M. D. Muhlheim, ORNL.

5. Infonnation Notice 95-18, " Potential Pressure-Locking of Safety-Related Power Operated Gate Valves," U.S. Nuclear Regulatory Commission, March 15,1995.

6. Haddam Neck Plant ,IndividualPlant Examination.

7. NUREG/CR-4674, Vol. 21, Precursors to PotentialSevere Core Damage Accidents: 1994, A Status

- Report, Appendix H, U.S. Nuclear Regulatory Commission, Decen'ber 1995.

8.- FinalSafety Analysis Report, Connecticut Yankee Atomic Power Company, Haddam Neck Plant.

NUREG/CR-4674., Vol. 23 B.2-4 7

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LER No. 213/95-010 Appendix B

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. FllCIDSURE 2 Appendix F LER No. 213/95-010 [

F.2 LER No. 213/95-010 Event Descnption: Mulople safety injection valves are susceptible to pressure  ;

los ing  :

Date of Event: March 9,1995 >

Plant: Haddam Neck F.2.1 Licensee Comments f

Reference:

Letter from T. C. Feigenbaum, Connecti:ut Yankee Atomic Power Company, to the U.S.

Nuclear Regulatory Commission, transmitting "Haddam Neck Plant Comments on Preliminary Accident Sequence Precursor Analysis", letter no 50-213-B15951, October 30, l l

1996.

, i Comment 1: Connecticut Yankee Atomic Power Company (CYAPCO) did not provide specific comments on the Accident Sequence Precursor analysis of event LER 213/95 010, but rather fonvarded an assessment of the event performed by Northeast Utilities Services Company (NUSCO), j l

which used different assumptions. Regarding the ASP analysis, Feigenbaum's letter states:

CYAPCO believes that the ASP report is too conservative in estimatmg the conditional core damage probability. The NUSCO quantification assumed a conditional probability of valve failure other than 1.0. The basis for this assumption was provided in the LER as to why the i valves would likely have functioned for a large break LOCA without loss-of-offsite power. 1 Response 1: Instead of commenting directly on the ASP analysis, the licensee for Haddam Neck submitted a repo t prepared by NUSCO: "An Analysis of the Risk Impact Due to Pressure Locking and Thermal Binding of CY ECCS MOVs" Because the ASP analysis and NUSCO's analysis could not be directly compared due to the different approaches taken to estimate the importance of pressure locking, a series of conference calls were held between  ;

personnel at ORNL, AEOD, and CYAPCO. Through these conference calls, sufficient information necessary to realistically estimate the likelihood that those valves susceptible ,

to pressure locking would fail, given the existence of the conditions expected to cause pressure locking, was obtained. Consequently, the ASP analysis no longer assumes the  ;

conditional probability of valve failure to be 1.0. Although NUSCO's analysis and the ASP analysis still cannot be directly cornpared due to the different approaches taken to estimate  ;

the importance of pressure locking, the results should be comparable. An increase in the core damage probability (CDP) (an importance measure) during the time that the necessary 4

conditions for pressure locking these valves exists is 4.7 x 10 This compares to a change F.2-1 NUREG/CR-4674, Vol. 23

l 4 -.  !

-4 LER No. 213/95-010 Appendix F of core damage frequency (CDF) calculated by NUSCO of 1.76 x 10~5 Based on the ASP l analysis, the nominal CDP for a 1 year period is 1.1 = 10". This compares favorably with d

,- NUSCO's estimate of 1.3 = 10 The uncertainty in the frequency of LBLOCAs and the uncertainty in the hkelihood that the pressure locking conditions will exist contribute to the ,

uncertainty in this estimate.

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