ML20072S935
| ML20072S935 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 08/18/1994 |
| From: | Chien S, Fenstermacher, Hook T SOUTHERN CALIFORNIA EDISON CO. |
| To: | |
| Shared Package | |
| ML20072S884 | List: |
| References | |
| PRA-2-3-94-013, PRA-2-3-94-13, NUDOCS 9409140308 | |
| Download: ML20072S935 (9) | |
Text
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BUCLEAR SAFETY GROUP PROBABluSTIC RISK ASSESSMERT REPORT PRA EVALUATION i
OF RISK IMPACT OF PROPOSED ONE-TIME EXEMPTION FROM THE REQUIREMENT OF 10CFR50, APPENDIX J FOR ILRT TESTING AT SAN ONOFRE NUCLEAR GENERATING STATION UNITS 2 AND 3 August 18, 1994 NSG/PRA REPORT PRA-2/3-94-013 Prepared by:
(T. G. Hook)
Prepared by:
6~T (S.
CIleni tr-- Q Reviewed by:
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1 (T. E Fen'te cher, PLG)
Reviewed by:
E (R. J. Lee) p Approved by:
O/Wh5'. O A /..
(C.
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p$[IkoockOb h1 P
L,. '.o NSG/PRA REPORT PRA-2/3 94 013 PURPOSE The purpose of this evaluation is to determine the risk impact'of a proposed one-time exemption from the requirement of 10CFR50,-
Appendix J,Section III.D.1.(a) for an integrated leak rate test (ILRT) of the San Onofre Nuclear Generating Station (SONGS) Units 2 and 3 Containment.
The exemption would allow a one-time test interval of 60 10 months such that the ILRTs currently scheduled for the Units 2 i
and 3 Cycle.8 refueling outages may be deferred until the Cycle 9 refueling outages.
BACKGROUND i
i SCE has requested a one-time exemption from the requirement of 10CFR50, Appendix J,-Section III.D.1.(a) which requires three ILRTs to be performed every 10 years (Ref. 1).
The exemption 1
would delay the'ILRT scheduled for the next refueling outage (Cycle 8) at each unit to the following Cycle 9 refueling outage.
The purpose of 10CFR50, Appendix J leak rate test requirements, as stated in the Introduction to 10CFR50 Appendix J, is to i
" assure that-(a) leakage through the primary reactor containment _
and systems and components penetrating primary containment shall not exceed allowable leakage rate values as specified in the technical specifications or associated bases-and (b) periodic surveillance of reactor containment penetrations and isolation valves is performed _so that_ proper maintenance and repairs are made during the service life of the containment, and systems and components penetrating primary containment."
Part (a) of the stated purpose of Appendix J applies to leakage through the primary reactor containment, which is the portion.
l relevant to_the, exemption request.
10CFR50 Appendix,J,Section III.D.1(a) requires three Type A tests at approximately equal
-intervals during each 10-year service period.
SONGS Units 2 and 3 Technical-Specification-4.6.1.2 requires three ILRTs to be performed at 40 1 10 month intervals during each 10-year service period.
According to this schedule, an.ILRT will need to be
. performed during the Units 2 and 3 Cycle 8 refueling outages.
.The proposed one-time exemption would allow.these ILRTs to be delayed until the Cycle 9. refueling outages.
RIf a proposed revision to Appendix J is approved by the NRC according to the current schedule, Edison would take credit for 1
the new rule and adjust the ILRT testing frequency to 1 test per 10 years.
The most recent ILRTs for Units 2 and 3-were performed in October 1 -
- i. e NSG/PRA REPORT PRA 2/3-94 013 1991 and March 1992, respectively.
A proposed exemption would result in a one-time ILRT test interval of 60 10 months.
As required by Appendix J, three ILRTs would still be performed during the current 10-year service period.
For Unit 2, the current 10-year service period is from February 16, 1992 to February 16, 2002.
For Unit 3, the current 10-year service period is from November 15, 1992 to November 15, 2002 (Ref. 1).
Data available for San Onofre Units 2 and 3 show that since 1988, there have been only 2 local leakage rate test failures for Type B and C penetrations out of approximately 830 total penetration tests.
Data previous to 1988 involves startup issues and is not representative of the current performance basis.
Both of the leakage rate failures were actually detected by LLRTs, not ILRTs.
No excessive local leaks have been detected by Type A testing at San Onofre Units 2 and 3 during this period (Ref. 1).
The major containment leakage paths include:
1)
Penetration Seal Leakage: Air lock door seals; doors with resilient seals or gaskets except for seal welded doors; penetrations whose design incorporates resilient seals, gaskets, or sealant compounds, piping penetrations fitted with expansion bellows, and electrical penetrations fitted with flexible metal seal assemblies are all possible leakage sources.
Type B tests address these types of potential leakage sources.
2)
Containment Isolation Valve Leakage:
These valves provide a direct connection between the inside and outside atmospheres of the primary containment under normal operation, are required to close automatically upon receipt of a containment isolation signal in response to controls intended to effect containment isolation, are required to operate intermittently under post accident conditions.
Leakage through these valves can be caused by leaking valve seals, isolation valve closure failure, or failure to return a penetration to its normally closed condition following maintenance.
For all of these events except post-maintenance /LLRT errors, this type of leakage is detectable by Type C local leak rate testing.
Following any maintenance or operation of a Containment Isolation valve, an LLRT is performed followed by an independent valve alignment verification to ensure that leakage remains within acceptable levels.
3)
Gross Containment Leakage:
This type of leakage is only detectable by a Type A ILRT test.
a, itSG/PRA REPORT PRA-2/3-94-013 10CFR50 Appendix J, Section II.K defines the acceptable leakage limit L, as, "the maximum allowable leakage rate at pressure P.
(calculated design basis accident peak containment pressure) as specified for preoperational tests in the technical specifications or associated bases, and as specified for periodic tests in the operating license."
According to Technical Specification 3.6.1.2, the acceptance criterion for an ILRT at SONGS Units 2 and 3 is 0.75 L., which is 162800 seem for Unit 2 and 162801 for Unit 3.
The three previous ILRTs for both Units 2 and 3 have all shown acceptable leakage rates as shown in Table 1 below.
Table 1 ILRT Test Results at San Onofre Units 2 and 3 Unit Date ILRT
- 0. 75 L.,
Measured Leak l
performed acceptance Rate, sccm limit, scem 2
February 1985 162800 93339 2
October 1987 162800 86827 2
October 1991 162800 98115 3
November 1985 162801 104248 3
July 1988 162801 133714 3
March 1992 162801 92688 Data available for San Onofre Units 2 and 3 show that since 1988 there have been only two local leakage rate test failures for Type B and C penetrations out of approximately 830 total penetrations tested.
Data previous to 1988 involves startup issues and is not representative of the current performance.
The NRC evaluated the impact of containment leak rate test changes in Draft NUREG-1493, " Performance-Based Containment Leak Test Program" (Ref. 6).
Several sensitivity studies were performed for changes in the ILRT testing frequency.
" Alternative 4" from this study examines relaxing the ILRT frequency from 3 in 10 years to 1 in 10 years.
Using best estimate data, the draft NUREG concludes that the increase in population exposure risk to those in the vicinity of the five reference plants ranged from 0.02 to 0.14%.
This very low impact on risk is attributed to 1) the effectiveness of Type B and C tests in identifying potential leak paths (about 97%), 2) a low likelihood of ILRT-identified leakages in excess of 2 times allowable, and 3) the insensitivity of risk to containment leak 3-
~.
NSC/PRA ItEPORT PRA 2/3-94-0U rate (e.g., no discernable increase in population dose risk with containment leak rates 100 times greater than currently allowed).
This led the authors of draft NUREG-1493 to conclude that even increasing the ILRT frequency to once in 20 years would " lead to an imperceptible increase in risk."
The exemption request for San Onofre Units 2 and 3 is concluded to be bounded by the analysis of draft NUREG-1493 since: 1) the exemption would result in a one-time test interval of five years, which is much shorter than the test intervals deemed to " lead to an imperceptible increase in risk", 2) the population density around the San Onofre plant is well below that of the reference plants studied in NUREG-1493, and 3) no ILRT has previously failed at San Onofre Units 2 and 3.
The following is an evaluation of the impact of the requested exemption on the increased likelihood of population exposure to radiation (i.e.,
increase in population dose risk) from San Onofre Units 2 and 3.
METHODOLOGY The increase in likelihood of population exposure to radiation due to the requested delay in performing the ILRT is determined by performing a sensitivity study of the San Onofre Units 2 and 3 Level.3 probabilistic risk assessment (PRA) (Ref. 5).
In the sensitivity study, the impact of the delay in performing the ILRT is assumed to create the potential for an undetected containment leakage path with a leak rate of twice the Technical Specification allowed leak rate L,.
This assumption is consistent with that used in draft NUREG-1493 based on historical ILRT data from industry tests.
According to draft NUREG-1493, "The small number of leaks detectable only by ILRTs were characterized by only marginal deviations from existing requirements (~2L ). "
The increased leak rate was factored into the San Onofre Units 2 and 3 Level 3 PRA by changing the containment leak size in the 15 Level 2 PRA source term MAAP runs to that matching two times the Technical T8pecification leak rate (i.e., 2L.).
The Level 3 PRA was then resolved using the modified source terms, resulting in a new population dose risk (man-rem /yr).
The proposed exemption request increases the likelihood that an undetected leak of the containment exceeding Technical i
Specification limits could occur.
Historical data indicates that ILRTs detect only 3 percent of leaks (i.e., LLRTs detect the remaining 97% of containment leaks) (Ref. 6).
The average time between tests with-the current 3 in 10 year test frequency is 36 1
-4 i
i
. w NSG/PRA REPORT PRA-2/3-94-013 months.
The average time between tests with the proposed ILRT exemption request would be 60 months.
Assuming that a pocential leak would go undetected one-half of the time between tests, the average time that a leak would go undetected would increase from 18 to 30 months with the requested change.
This increase represents a factor of 1.67 increase in risk of undetected leakage exceeding Technical Specification limits.
Since ILRTs detect only 3 percent of leaks, the overall leakage probability would increase by a factor of 5 percent due to the requested ILRT exemption.
The increase in population dose risk due the requested ILRT exemption is calculated by multiplying the population dose risk with an assumed containment leak rate of 2L.
by 5 percent.
This is the method used in draft NUREG-1493, Section 7.1, Alternative 2 to determine the risk increase from an ILRT frequency change from 3 in 10 years to 2 in 10 years (Ref.
6).
ASSUMPTIONS The delay of the scheduled ILRTs from the Cycle 8 refueling outage to the Cycle 9 refueling outage are assumed to lead to an increased likelihood of a containment leak prior to the Cycle 9 test which may exceed Technical Specification limits.
It is assumed that the maximum leak size expected during this period would not exceed two times the Technical Specification leak rate limit (i.e.,
2 L. ).
This assumption is based on a review of historical data documented in draft NUREG-1493, which found that ILRTs have detected only marginal increases in leakage above technical specification limits.
ANALYSIS The fifteen Level 2 source term MAAP analyses were rerun using a containment leak size of 8.5E-4 sq ft.
The source term information for each sequence (radionuclide release rates and sensible heats of plumes) was transferred to the Level 3 MACCS code inputs.
The fifteen source term sequences were analyzed using MACCS to determine the population dose consequences.
RESULTS S
The fifteen MACCS output files are located in Appendix A.
The increase in population dose risk was calculated by multiplying the frequency of each source term by its population dose consequence, summing the risks for all fifteen source term sequences, and multiplying the result by 5%.
Table 2 provides a summary of the calculation. _
a NSG/PRA REPORT PRA-2/3 94 013 Table 2 Summary of Results from ILRT Exemption Case Sequence Frequency (per Population Dose Risk yr)
Dose (man-rem)
(man-rem /yr)
PCS-4 9.0E-6 4.66E+2 4.19E-3 MLO-4 6.8E-6 1.65E+3 1.12E-2 LLO-4 4.3E-6 1.15E+4 4.94E-2 ATWS-26 2.8E-6 6.79E+2 1.90E-3 LOP-48 2.1E-6 2.68E+5 5.63E-1 PCS-35 2.7E-6 3.60E+4 9.72E-2 SLO-20 6.9E-7 4.40E+6 3.04E+0 LLO-13 6.2E-8 1.65E+6 1.02E-1 SBO-17 1.9E-8 3.68E+6 6.99E-2 LLO-32 1.4E-9 1.81E+6 2.53E-3 VSEQ-2 6.5E-7 1.75E+7 1.14E+1 SGTR-20 7.9E-7 4.06E+6 3.21E+0 SGTR-33 2.2E-7 1.83E+5 4.03E-2 SGTR2-48 1.5E-7 4.78E+6 7.17E-1 SGTR2-66 2.8E-7 4.54E+6 1.27E+0 TOTAL 3.0E-5 NA 2.05E+1 The increase in risk from the ILRT exemption is calculated by subtracting the SONGS Base Case Level 3 dose risk from that calculated above, and multiplying by St.
The SONGS Base Case Level 3 dose risk was calculated in NSG/PRA Report PRA-2/3-94-012 to be 1.8(B+1 man-rem /yr (Ref. 5).
7 Increase in dose risk - (ILRT Exemption - Base Case)
- 5%
(2.05E+1 - 1.86E+1)
- 5%
=
- 9.5E-2 man-rem /yr or 0.51% increase 6-
7~
n ;.
s NSG/MtA REPORT PRA 2/3-94-013 CONCLUSIONS The increase in population dose risk from the proposed ILRT exemption is estimated to be 9.5E-2 man-rem /yr or 0.51% from the current risk.
This increase in risk is considered acceptably small.
t REFERENCES 1.
Draft Proposed Change Number (PCN NPF-10/15-438, Change to Technical Specification 3/4/6/1/2, " Containment Leakage",
San Onofre Nuclear Generating Station Units 2 and 3, transmitted via memo from J. L. Rainsberry to D.
P. Breig, et al. on June 14, 1994.
1 l
2.
" Individual Plant Examination Report for San Onofre_ Nuclear Generating Station, Units 2 and 3, in Response to Generic Letter 88-20", Southern California Edison, April 1993.
f 3.
NUREG/CR-4691, "MELCOR Accident Analysis Consequence Code System," Sandia National Laboratories, Prepared for the Nuclear Regulatory Commission, February 1990.
4.
" SONGS Consequence Analysis Using MACCS", Halliburton NUS Environmental Corporation, August 1991.
5.
NSG/PRA Report PRA-2/3-94-012, " Population Dose Risk from Severe Accidents at San Onofre Nuclear Generating Station Units 2 and 3," August 1994.
6.
Draft NUREG-1493, " Performance-Based Containment Leak Test Program," Nuclear Regulatory Commission, Draft for Comment.
t i
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NSG/PRA REPORT PRA-2/3-94 013 I
l APPENDIX A l.
i MACCS Dutput File Listings (AVAILABLE UPON REQUEST)
I 1
l A-1
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