ML20205F038

Discusses Revs to Facility FSAR Description of Helium Leak Rate from Pcrv. Calculation of DBA-1 Pcrv Penetration Leak Rate & Updated FSAR Pages Encl

ML20205F038
Person / Time
Site:	Fort Saint Vrain
Issue date:	08/13/1986
From:	Brey H PUBLIC SERVICE CO. OF COLORADO
To:	Berkow H Office of Nuclear Reactor Regulation
Shared Package
ML20205F044	List: ML20205F038 ML20205F047
References
P-86497, TAC-56565, NUDOCS 8608190074
Download: ML20205F038 (3)

Text

hPublic Service ~ ='g a 2420 W. 26th Avenue, Suite 100D', Denver, Colorado 80211 August 13, 1986 Fort St. Vrain Unit No. 1 P-86497 Director of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, D.C. 20555 Attn: Mr. H. N. Berkow, Director Standardization and Special Projects Directorate Docketfjo. 50-267

SUBJECT:

PCRV Leak Rates

REFERENCES:

1) PSC letter, D. W. Warembourg to B. Grimes, dated 4/1/80 (P-80066)

2) PSC letter, R. O. Williams to H. N. Berkow, dated 7/22/86 (P-86466)

Dear Mr. Berkow:

This letter discusses revisions that Public Service Company of Colorado (PSC) has made to the Fort St. Vrain FSAR description of helium leak rates from the Prestressed Concrete Reactor Vessel (PCRV). These leak rates are used in the FSAR analysis for Design Basis Accident-1 (DBA-1, Permanent Loss of Forced Circulation) and were discussed with your staff in a telephone conference on May 30, 1986. While the overall primary coolant leak rate assumed in the FSAR analysis is not affected, the changes involve corrections in how the postulated overall leak rate is allocated to the two potential leak paths (i.e., PCRV concrete permeability and penetration closure leakage).

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, P-86497 Page 2 August 13, 1986 FSAR Section D.I.3.4.2, PCRV Leakage and Reactor Building Effects, in Revision 3 and prior revisions, concluded that a conservative assumption for overall PCRV leakage at 5 psig* is 0.2%/ day. This conclusion was based on consideration of the maximum leakage rate attributable to concrete permeability, which was assumed to be 0.2%/ day, and that attributable to the outer (secondary) penetration closures which was assumed to be less than 0.024%/ day.

In 1980, in connection with PSC's evaluation of Emergency Planning Zone Distances (Reference 1), PSC advised NRC that reexamination of the concrete permeability analysis revealed errors that caused the assumed leak rate to be overestimated by a factor of 200. The corrected PCRV concrete permeation leak rate at 5 psig* was calculated to be 0.001%/ day. However, for the purposes of the Emergency Planning evaluation, the historical 0.2%/ day was assumed to exist as an upper limit of all potential contaminated primary coolant leakage including permeability through the concrete.

PSC has also recently reexamined the basis for the assumption of 0.024%/ day leakage through the PCRV secondary closure seals. FSAR Revision 3, Section D.1.3.4.2 cited the maximum design basis leak rate through secondary closures as 100% of the PCRV volume per year at design pressure. This led to the conclusion that the corresponding accident leak rate at 5 psig* would be less than 0.024%/ day by volume. This extrapolation by volume was erroneous.

In fact, the maximum design leak rate from all components including the PCRV and the helium storage system is 100% of the total helium inventory (i.e., by mass) per year, as stated correctly in FSAR Section 5.2.3. The extrapolation by mass of the design leak rate at normal operating pressure from PCRV secondary closure seals to the leak rate at the 5 psid* assumed following the Permanent Loss of Forced Circulation accident (DBA-1) has been corrected in FSAR Section D.I.3.4.2 Revision 4 to 0.072%/ day.

The calculations that support the primary coolant leakage rate during DBA-1 attributable to concrete penneability are described in the attachment to Reference 1. The calculations that support the 0.072%/ day accident leak rate attributable to PCRV secondary closure leakage are described in Attachment 1 to this letter. These two calculations support the conservatism of tne historical FSAR assumption for overall PCRV leakage of 0.2%/ day.

• Gauge pressure (psig) is equal to differential pressure (psid) for these analyses of leakage to the atmosphere.

P-86497

. Pagg 3 August 13, 1986 Attachment 1 also includes an extrapolation of the PCRV penetration leak rate allowed by Technical Specification LC0 4.2.9, PCRV Closure Leakage to DBA-1 conditions. That analysis shows that the LC0 leak rate extrapolates by mass to a leak rate of 0.05%/ day at 5 psid*

which is less than the 0.2%/ day assumed in the FSAR analysis of DBA-1.

PSC considered it desirable to revise FSAR Section D.1.3.4.2 to correctly describe the relative contributions of concrete permeability and PCRV penetration closure leakage to the overall leak rate assumption. The fourth revision of the Updated FSAR reflects these changes to Section D.1.3.4.2 on pages D.1-48 and D.1-49. These Revision 4 replacement pages were submitted to NRC by Reference 2.

Both Revision 3 and Revision 4 pages are included as Attachment 2 to this letter, for convenient reference.

If you have any questions or coments concerning this matter, please call Mr. M. H. Holmes at (303) 480-6960.

Very truly yours, s'

94WW Q H. L. Brey, Manager Nuclear Licensing and Fuels HLB /JPL/TRM:jmt Attachments l

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