ML14055A410
| ML14055A410 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 02/04/2014 |
| From: | Office of Information Services |
| To: | |
| Shared Package | |
| ML14055A421 | List:
|
| References | |
| FOIA/PA-2012-0325 | |
| Download: ML14055A410 (341) | |
Text
Oconee Nuclear Station Standby Shutdown Facility (SSF)
Flood Barrier Breach Violation informal reassessment USNRC UtDTATESNtCLEAI R Pn~tecting People and iMLtAmRI the Environment
Why isthe SSF wall isneeded?
@Two flood studies show the Oconee site vulnerable to flooding from a Jocassee dam failure
- 1982 flood study showed the need to protect from flooding up to 4.71 feet on the SSF
- 1992 FERC study showed that flooding could occur at 12 to 16 feet.
@Oconee decided to protect from "the most likely flood scenarios" by building a 5-foot wall in1988
- wall will protect against 80% of floods based on engineering judgment - no technical basis
@The probability that aflood will occur due to a failure of the Jocassee Dam contains large uncertainty.
I UNITED STATES PGULmTORY N'MCLEAR COMMIS*ION ProtectingPeople and the Environment
I .
SSF Wall Relative Heights of Flood Barriers and Penetration SSF FLOOD ELEVATION REFERENCE DRAWING EL 801' 9 (5.75 ft)
WE.BOf (W.5 A)I EL 801T (5.06')
EL 801' (5.0')
~TOP ssFFOToW RU W RliV. W'-. f"* A.
8OTTO.M (7$.CF ¶ MOTO oE E.. 8L .. .0.0... )
EL 8001Y.5 (4.71')
~SSF R=OO. ELY Th!ý4!_Q Fl.. ** * * .4 , .4 '.4.4 6**, 4 4 a6 .0a4 1* .4 0 a a 0 YAAD (I~~l IELFVýW7WfgO 11
. . ... .. . m.m.
- m .. ..
4 A' 1' A J A1 6 d 4 A a r i-~iI-*i't-ii ___ i-Il ___ ___ ___ ___ il-Il I-"tiI~-1Tl-~iii 3
U.SNRC UNITESTATES MXUtAR REMtATURY ProtecingPeople COMUSSION and tke Enuironmen
Nature of Violation/Performance Deficiency and SDP
- Open penetration access cover inthe SSF exterior wall for approximately 2years.
- No 50.65(a)(4) evaluation done by licensee.
- Opening below 5-ft max. flood height identified by licensee in IPEEE.
Susceptible to site flooding from rupture of Jocassee Dam 11-miles upriver.
Region IlSRA performed analysis based on information from the licensee and apportioned flood height based on split fractions to arrive at aquantitative WHITE finding.
Jocassee Dam Failure Mode Increase inCDF (per Reactor.Year)
Random 1.84 x106 Seismic 1.44 x10-6 Total 3.28 x10-6 4
UNTIED STATES NU.CLEAR REGULA70RYCOWMISSION ProteclngPeopleanmdthie Environrnext
Summary Timeline
. August 17, 2006 -SERP meeting assessed as preliminary WHITE based primarily on qualitative aspects (pre-MC 0609 App M)
- August 31, 2006- Choice letter sent to licensee.
- October 5,2006 - OCO provides written response choice letter to waived regulatory conference.
- Nov. 22, 2006 -FSD issued. WHITE based primarily on qualitative erosion of defense-in-depth,but includes quantitative CDF.
- December 20, 2006 -OCO appeals the FSD. Requests NRC to accept incomplete, un-docketed, Jocassee fragility study
- January 9,2007 - Appeal panel convened
- February 5,2007 - OCO sends completed seismic fragility analysis of Jocassee to NRC.
- March 1,2007 - Appeal panel upholds White finding
- May 3,2007 - OCO requests "reassessment of FSD"
- June 28, 2007 -Follow up telecom with OCO after seismic fragility analysis was evaluated.
- July 17, 2007- OCO response to analysis questions by email.
- June 22, 2007 - Reassessment of FSD assigned to RII
- August 27, 2007 - OCO 95002 inspection - Emergency AC White MSPI, U3 sump debris, SSF flood barrer breech white finding
- September 20, 2007 - RII reassessment results due to Oconee (90 days from June 22)
C 2US.NRC UNrr STATES RECILATORY NUCLEAR C04MMISSON ProtehuegPeopfe ad the Eaviroomeni
Reassessment Team Evaluation
- Seismic fragility review did not endorse licensee's conclusions completely
- NRC agreed that the seismic frequencies were sufficiently "low" to be insignificant. Therefore, only random dam failures were considered.
- Epistemic uncertainty inJocassee rupture frequency
- Licensee and NRC cannot resolve resultant flood height fractions resulting from below opening to 5-ft.
t Assume that mean dam rupture frequency producing a 4.71-ft flood identical to a mean frequency producing a 5-ft flood;
- in other words there isno probability distribution for floods of various heights, floods at any height on the 5.foot wall are equally likely
- Using licensee's minimal IPEEE SSF failure cutsets with OCO revised flooding frequency results inACDF of 8.22 x10.6 per year.
6 yUSNRC UNfEDSTATES NICL*ARRLGULTORYCOMMISSION Protecting People and *thEnzironmet
Jocassee Dam Random Failure
- Licensee computed random failure frequency based on operating industry history of failures for rockfill dams of over 50-ft inheight
@Discrepancy exists on counting the denominator of total operating dam-years.
- Licensee included operating years of other dam types indemoninator, but did not count their corresponding failures inthe numerator.
s NRC performed independent random dam failure analysis for rockfill dams >5 yrs old and >50 feet inheight
- Random frequency could be higher which might offset any uncertainty inthe resultant flood height difference between the bottom of the opening and the 5-ft height of the wall.
Resultant ACDF could range from WHITE to RED inseverity, SU.S.NRC PeColeR RKEI.RY COMMISSION ProEDcSTATEU
Risk-informed Regulation and Defense inDepth
, Used as away to account for uncertainties in equipment and human performance.
- The independence of barriers have been degraded.
, Licensee has an exclusive reliance upon the SSF to prevent core damage without redundancy or diversity of mitigation.
- Any functional degradation of the SSF flood barrier from these initiating events directly increased the failure probability of the SSF and therefore, increased the likelihood of core damage.
Using a blended qualitative and quantitative perspective, the NRC's final Significance Determination remains low to moderate (White)".
UNITEDSTATESN.1CUARlEGUIATORYCOMMISSION Protecting People 4nd the Environment
Judgment needed
, Does a 3.5.inch change inflood wall height that last for longer than a year result inan increase incore damage frequency greater than 1E-6?
, Should the NRC conduct additionalinspection to determine why alicensee did not assess the maintenance risk of asmall breach inthe only barrierbetween flooding and core damage and allow itto exist for two years?
%2jU.S.NRC NIJCILER uirrEDSTATES COW~SSION REGULATORY frtecringPeopl* and the Environmmnt