ML19340C330

Forwards S&W Summary Rept on Turbine Missile Damage Probability Analysis Prepared for VEPCO.Long-term Measures Re Turbine Disk Cracking Will Be Submitted When Prepared

ML19340C330
Person / Time
Site:	North Anna
Issue date:	11/06/1980
From:	Christman J HUNTON & WILLIAMS
To:	Buck J, Rosenthal A NRC ATOMIC SAFETY & LICENSING APPEAL PANEL (ASLAP)
Shared Package
ML19340C331	List: ML19340C330 ML19340C341
References
ISSUANCES-OL, NUDOCS 8011140522
Download: ML19340C330 (4)

Text

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N Dr. John H. Buck Atomic Safety and Licensing Appeal Board U.S. Nuclear Regulatory Commission Washington, D.C. 20555 1

1 In the Matter of Virginia Electric and Power Company l (North Anna Nuclear Power Station, Units 1 and 2) l Docke t Nos . 50-328 OL and 50-339 OL  !

l Turbine Missiles Gentlemen: l l

On the subject of the turbine missile analyses for North Anna 1 and 2, I should like to inform y ou that the Stone &

Webster calculations of P 4, the probability of turbine mis-sile damage, have been completed for Unit 1. These results are summarized in a report called Summary Report on the mur-bine Missile Damage Probability Ana}yses for North Anna Units 1 and 2, a copy of which is enclosed for your information.

Since the probabilities are less consa.':vative than the origi-nal analysis in the FSAR, verco has filed a Licensee Event Report. Under the new Westinghouse analysis for P , because the

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,' HUNTON Oc W I L LI A M S Alan S. Rosenthal, Esq.

Dr. John H. Buck November 6, 1980 Page Two turbine disk cracking is a time-dependent phenomenon, the probability of missile generation increases with time. Ac-cordingly Vepco is considering a surveillance program to en-sure safety for the long term.

You will see, however, that the probability P4 after two years of continuous operation (17,520 hours0.00602 days <br />0.144 hours <br />8.597884e-4 weeks <br />1.9786e-4 months <br />) is only 9.528 x 10-7 for low-trajectory missiles and 1.207 x 10-7 for high-trajectory (Table IV), provided one considers perforatien but not " scabbing" and makes two other assumptions listed on page

.1 under " Criterion B." Stone & Webster advise that scabbing would be greatly attenuated for the structure that creates the greatest part of the risk from low-trajectory missiles (the main steam valve house or MSVH). The piping in the MSVH I is typically large-bore, heavy-wall carbon steel, and the valves are designed to fail in a safe position if loss of electrical power or air occurs, ilso, the MSVH contains major quantities of large-member structural steel that supporte pipe break restraints. Accordingly, serious damage is un-  !

likely to result from scabbing, which produces concrete mis- l siles that typically have only about 10 per cent of the velocity of the original turbine missile. I should add that the probabilities in Table IV do not take credit for most of the conservatisms listed on page 8 of the report.

Stone & Webster have also done a second case that is more conservative still. The results of this " Criterion A" analysis for two operating years is shown ".n Table III. This

HUNTON & WILLI A MS Alan S. Rosenthal, Esq.

Dr. John H. Buck November 6, 1980 Page Three analysis simply assumes that scabbing will occur, and that any scabbing will cause damage, if certain of the structures (including the nain steam valve house) are struck by a tur-bine missile. As a result the calculated probabilities are higher than in Table IV.

Vepco has changed the planned starting date for its next outage for Unit 1 by one month , until January 1, 1981, at which time the unit will have operated 17,740 hours0.00856 days <br />0.206 hours <br />0.00122 weeks <br />2.8157e-4 months <br />. Dur-ing the outage, as you know, the Unit 1 turbine will be ultrason-ically inspected for disk cracking. The basis on which Vepco justified operation until the outage was given in affidavits provided by Mr. Schmerling of Wes ti nghouse last spring. Be-cause the Westinghouse information on critical crack size and crack growth rate has been revised since then, Westinghouse has updated the tables of crack sizes that were a part of Mr.

Schmerling 's af fidavit. The results, labeled "Rev. 1/5/80,"

are attacLed; I ask that you give daem the same protection that other Westinghouse proprietary information has receive' in this proceeding. The ratios of estimated crack depth to critical crack size in these tables are still below 1.0 as of January 1, 1981, and Westinghouse's revised P ana lys is is con-1 sistent with the conclusion that th er e is reasonable assurance tha t the Unit 1 turbine can be operated without a rupture of the disk through that date.

Westinghouse has not yet completed the P y analysis for l North Anna Unit 2 but will do so in due course. You will see probabilities for Unit 2 in the Stone & ' ebs ter report ,

but Stone & Webster calculated these bv a s s u:..i ng that the

HUNTON Oc WILLI A M S Alan S. Rosenthal, Esq.

Dr. John H. Buck November 6, 1980 Page Four Un'; J turbine is identical to the Unit 1 turbine. The analy-sis for Unit 2 is regarded as a less pressing matter inasmuch as the Unit 2 turbine was ultrasonically inspected before it went into service and had operated only 1,278 hours0.00322 days <br />0.0772 hours <br />4.596561e-4 weeks <br />1.05779e-4 months <br /> as of mi dnigh t this past November 3.

What I should like to do next is s ubmit to you a more complete package of information, including Wes tinghouse 's P 1 reports, af ter the Unit 2 analyses are completed. West-inghouse es timates it can finish the Unit 2P1 analysis by about December 1, and Stone & Webster will require about two more weeks to complete the P 4 calculations for both Criterion A and B. I will then need a few weeks to assemble or pre-pare suf ficient documen ts to explain the methodology used, answer questions you have raised in the past, and try to an-ticipate other questions yc a may have. I will also report on what long-term measures Vepco plans for dealing with turbine disk cracking. l l

l Yours very truly, l M . C James N. Christman 126/586 Enclosures cc: Daniel T. Swanson, Esq.

Anthony J. Gambardella, Esq. (w/o Westinghouse attachments)

Chief, Docketing and Service Section (w/o Westinghouse attachments)