ML19093B014
| ML19093B014 | |
| Person / Time | |
|---|---|
| Site: | Surry  |
| Issue date: | 08/09/1977 |
| From: | Stallings C Virginia Electric & Power Co (VEPCO) |
| To: | Case E, Reid R Office of Nuclear Reactor Regulation |
| References | |
| Download: ML19093B014 (2) | |
Text
..
VIRGINIA ELECTRIC AND POWER COMPANY R10HMOND, VIHGIN"l.A. 23261 August 9, 1977
/Re*~-~"'
,.,__ -~Jt.ii Mr. Edson G. Case, Acting Director Nuclear Reactor Regulation U.. S. Nuclear Regulatory Connnission Washi_ngton, D.. C.
20555 Attention:
Mr. Robert W. Reid, Chief Operating Reactors Branch 4
Dear Mr. Case:
DPR-37 The purpose of this letter is to provide additional information as request-ed by the NRC staff r.egarding our.request f0r extended operation of Surry Unit No. 2.
In our previous letter serial no. 325 we presented information showing that a large number of tube failures would be.required.before 10CFRlOO dose limits would be exceeded during a steam line break (SLB) transient.
Although these high primary to secondary leak rates can be tolerated from the stand.point of site boundary dose, other affects such as DNBR may. dictate a more limiting primary to secondary leak rate.
Ten gallons per minute. (10. GPM) will be used as a limit to evaluate continued operation of Surry Unit No. 2 since this is the maximum value of total primary leakage permitted by technical specifications.
We have previously established that during a SLB, an existing.3 GPM leak will increase.to a maximum of.78 GPM.
We have*also established.that only tubes with 85%-100% thro_ugh wall flaws will crack through during the SLB.
These tubes would then leak at.05 GPM each during the remainder of the transient.
Based on these facts, approximately 184 tubes would have to crack through and leak during a SLB before the total primary to secondary leak rate would exceed 10 GPM.
In our letter serial no. 353B/113076 dated March _21, 1977, we presented in-formation showing that the growth of strain contours in the range of.08 in/in to.* 12 in/in (the range i:ri which over 90% of the leaking.tubes have occurred) is about~ tube row per month in the region of the strain boundary.
This means that the tubes adjacent to the present plugging.pattern in Surry Unit No. 2 will be at least partially subject to strains o*f. 08 in/in to.12 in/in during the requested period of additional operation.
Our inspection shows that 2A steam generator (worst case) has approximately 140 tubes adjacent to the present plugging pattern.
If we assume that half of these tubes sustain partial through wall flaws in the 85%-100% range during the period of extended operation, and that a. 3 GPM leak already exists; then the expected total primary to secondary leak rate during a SLB would be.4.28 GPM.
This provides sufficient margin to allow an additi0nal
_.114 tubes to crack and leak before a 10 GPM primary to secondary leak rate is achieved.
If a 10 GPM primary to secondary leak*rate were experienced during a SLB, the loss of coolant inventory would be trivial at the time the core thermal transient is terminated.
Previous analyses show that the SLB core transient is terminated in less than one minute by the safety injection system.
The 10 gallon additional.inventory loss imposed by primary to secondary leakage is intuitively 7 7~';;17000(
~---
e e
VrR1JINIA ELECTRIC AND POWER COMPANY TO Mr. Edson.G. Case Page No. 2 ins_ignificant since this represents less than. 02% of total reactor coolant volume.
Therefore, we do not believe that DNBR is affected by 10 GPM primary to secondary.leakage during SLB.
Since the expected leak rate during*SLB is acceptable from the standpoint of dose considerations and core performance, we believe that. the.health and safety of the general public will not be affected by continued operation of Surry Unit No. 2 thro_ugh September 15, 1977.. :
cc:
Mr. James P. O'Reilly Very truly yours,
~.me ~~ell--~
C. _M. Stallings Vice President-Power Supply and Production Operations
,