ML20059N329

Forwards Response to Comments Contained in Util Re Installation of Hardened Wetwell Vent

ML20059N329
Person / Time
Site:	Millstone
Issue date:	10/02/1990
From:	Stolz J Office of Nuclear Reactor Regulation
To:	Mroczka E CONNECTICUT YANKEE ATOMIC POWER CO., NORTHEAST NUCLEAR ENERGY CO.
References
TAC-74872, NUDOCS 9010110265
Download: ML20059N329 (5)

Text

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October 2,1990 -

Docket'No. 50-245' DISTRIBUTION Docket File ACRS (10)!

NRC & Local.PDRs ABuslic PDI-2 Reading JKudri k -

Mr. Edward J. Mroczka-SVarga PDI-4 File

-Senior Vice President.

BBoger AThadani Nuclear Engineering and Operations SNorris' EWenzinger, RI Connecticut Yankee Atomic Power Company

.MBoyle Northeast Nuclear Energy Company OGC P.O. Box 270 EJordan Hartford, Connecticut 06141-0270 MThadani

SUBJECT:

INSTALLATION OF HARDENED WETWELL VENT - MILLSTONE NUCLEAR POWER STATION, UNIT NO. 1 (TAC NO. 74B72)

By letter dated September' 6,1990, the NRC staff provided you a response' to l

your letter dated September 4,1990, in which you committed to insta11'a hardened wetwell vent during the 1993 refueling outage of Millstone Unit No. 1.

In that letter we stated that a copy of our response,to comments contained:in your August 8,1990 letter was enclosed;. but we. inadvertently omitted to include the stated enclosure. _Therefore', we herewith enclose a copy of our response to-your comments.

We regret any inconvenience caused you by the delay in providing you the enclosed response.

Sincerely, Original signed by John F. Stolz, Director Project Directorate I-4 Division of Reactor Project - I/II' Office of Nuclear Reactor Regulation L

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Enclosure:

NRC Staff Response i

cc w/ enclosure:

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Mr. Edward J. Mroczka Millstone Nuclear Power Station Northeast Nuclear Energy Company Unit No. 1 CC:

Gerald Garfield, Esquire R. M. Kacich, Manager Day, Berry and Howard Generatinn Facilities Licensing-Counselors at Law Northeast Utilities Service Company.

City Place Post Office Box 270 1

Hartford, Connecticut 06103-3499 Hartford, Connecticut 06141-0270 l

W. D. Romberg, Vice President D. O. Nordquist Nuclear Operations Director of Quality _ Services Northeast Utilities Service Company Northeast Utilities Service Company' Post Office Box 270 Post Office Box 270 Hartford, Connecticut 06141-0270 Hartford, Connecticut 06141-0270 Kevin McCarthy, Director Regional Administrator Radiation Control Unit Region I Departnent of Environmental Protection U. S. Nuclear Regulatory Connission -

State 0ffice Building 475 Allendale Road Hartford, Connecticut 06106 King of Prussia, Pennsylvania 19406, Bradford S. Chase, Under Secretary First Selectmen Energy Division Town of Waterford-Office of Policy and Management Hall of Records 80 Washington Street 200 Boston Post Road Hartford, Connecticut 06106

~Waterford, Connecticut 06385 S. E. Scace, Nuclear Station Director W. J. Raymond, Resident'inspt:ctor -

Millstone Nuclear Power Station Millstone Nuclear Power Station Northeast Nuclear Energy Company c/o V. S. Nuclear Regulatory Connission.

Post Office Box 128 Post Office Box 811 Waterford, Connecticut 06385 Niantic, Connecticut 06357 J. P. Stetz, Nuclear Unit Director Millstone Unit No. 1 Northeast Nuclear Energy Company Post Office Box'128 Waterford, Connecticut M385 1

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RESPONSE TO NORTHEAST l-(

' UTILITIES LETTER DATED AUGUST 8, 1990 r

1. 0 INTRODUCTION L

1 On August 8, 1990, Northeast Utilities provided a response to the NRc staff's backfit analysis supporting the installation of a hardened wetwell vent at Millstone, Unit No.-1.

In that letter the lir.ensee provided several. comments on the staff's backfit' analyses and indicated that it has demonstrated that there are significant differenta between the staff's analyses and the licensee's s

analyses.

Specifically, the. licensee stated that the staff erred in calculating the frequency of.small break loss-of-coolant-accidents (LOCA), assumed 21 years of remaining plant life instead of 19 years, did not credit the benefits of existing venting capability,fand did not consider plant-specific features (e.g., tornado missile protection of the isolation condenser).

Additionally, the licensee stated that.the basis for the order now, rather than waiting for-four months for.the completion'of the individual plant examination (IPE) program, is not apparent.

The staff has reviewed the licensee's comments and notes the following in response.

2.0 RESPONSE Comment:

The staff's multiplication by 3 the freqt.ency of small break LOCA derived in the Millstor.e Unit i PRA to reflect the value used in i

NUREG-4550 is incorrect.

It appears that the Millstone Unit 1 PRA

.small break LOCA should be compared with intermediate break LOCA of NUREG-4550.

Response: The Millstone Unit 1 probabilistic risk assessment-(PRA): includes all of the.S1 category of the Peach. Bottom Study.and for steam breaks a good fraction of the S2 category.

Therefore, the staff's assignment of the 3 E-3 per year estimate-to small break appears valid.

Further support for the staff's value comes from the Shoreham PRA which used a frequency of 3 E-3 per year for 51 (medium LOCA) sequences 1

which were defined to include liquid breaks between 0.004 ar:d 0.1 sq. ft and steam breaks between 0.016 and 0.08 sq. ft.

The steam break range is wholly contained within the small break range for l-Millstone Unit 1, and the bulk of the liquid break range is also contained in the break range for Millstone Unit 1.

Comment:

The staff has~ assumed that the benefits of venting would accrue over the 21 years of remaining Millstone Unit i life.

However, since the vent would not be installed until 1993, the benefits would accrue for 18 years only.

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Response: The use of 18 years as the benefits accruit., period'would not signi-l ficantly change the staff's conclusions.

Ace '+.ionally, the staff expects that most licensees, including Northee ' Utilities, would

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opt for 20 year extension of plant life.

The 7 5elieves that it is prudent to consider accrued benefits for a p.

of additional 20 years of Millstone Unit 1 operations.

The use'of 38 years of accrued benefit period will clearly add further support to the staff's backfit decision.

Comment:

The staff did not consider the impact of the existing vent at Millstone Unit 1.

Response: The staff did not iny ae a credit for the existing, vent path at Millstone Unit 1, because the expected pressures at which; venting would be initiated could reach the primary containment $ pressure limit (PCPL).

j At that pressure, contrary to the licensee's' claim, the standby gas i

treatment system will fail and could cause failure'ofLsafety equip-ment located in.the Main Reactor Building.

Failure of the: safety equipment following a severe accident is expected to result in core damage.

It is important to note that.the sequences of importance in isolation condenser plants are not the transients followed by luss of decay heat removal capability, but sequences, involving stuck open safety relief valves and a spectrum of small break.LOCAs.

For'such sequences the isolation condenser is.not effective and>the radiation level in the drywell may be excessive.

Therefore, venting'from the drywell through the Main Reactor Building exhaust stack may not be appropriate because it would not have the benefit of suppression pool scrubbing.

Use of the hardened.wetwell vent,-however, provides the greatest reduction in core damage frequency and mitigation of radio-logical consequences.

l-l Comment:

The isolation condenser and its make up system significantly reduce the contribution of TW sequences to core damage frequency and' i

essentially accomplish the safety benefit which the staff desires from the installation of a hardened wetwell vent.

Response: The discussio' of the isolation condenser omits a-very important fact:

the isciation condenser is not effective in removing the decay heat from the containment due to transients followed by a stuck open safety relief valve or inadvertent' opening of a relief. valve. ;The o

NRC staff considered the presence of the isolation. condenser but determined that for the sequences of concern it would not be effective in removing the heat from the containment.

Additionally,the; isola-1 t

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tion condenser and its make up systems do.not accomplish-the safety l

benefit which the NRC staff expects from the installation of the-hardenet wetwell vent for sequences involving breaks in the primary i

system pressure boundary.

For such sequences a hardened vent path l

can remove the containment heat directly, while the isolation:

condenser will be ineffective for the purpose of removing containment heat.

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1 3.0 = CONCLUSION i

The above responses to specific comments contained in the Northeast Utilities August 3, 1990 letter demonstrate that, in spite of the presence of a reliable isolation condenser and existence of procedures to vent the containment, the contribution of accident sequences to core damage frequency can be significantly I

reduced and mitigation of consequences of potential core damage accidents I

significantly enhanced by the installation of a hardened wetwell vent path.

Therefore, the staff's conclusions contained in the oackfit analysis <emain unchangeu.

i Principal Contributors:. Arthur Busiik and Mohan Thadani i

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