ML20155C835

Forwards Proposed Methodology to Resolve SEP Topics III-2 & III-4.A,per 880407 Meeting W/Nrc.Util Has Determined That Proposed Methodology Utilizing Probabilistic Techniques Is Sound & Reasonable as Means of Resolution of Topics

ML20155C835
Person / Time
Site:	Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png
Issue date:	06/03/1988
From:	Mroczka E CONNECTICUT YANKEE ATOMIC POWER CO., NORTHEAST UTILITIES
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
TASK-03-02, TASK-03-04.A, TASK-3-2, TASK-3-4.A, TASK-RR A06968, A6968, B12935, TAC-51938, NUDOCS 8806140350
Download: ML20155C835 (13)

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OE General Offices e Selden Street, Berkn Connecticut I r '55'N"" P.O. BOX 270 HARTFORD. CONNECTICUT 06141-0270 k k J * * *SENN.cc

"'*""'"",~. (203) 665-5000 June 3, 1988 Docket No. 50-213 A06968 B12935 Re: SEP Topic III-2 and III-4.A ISAP Topic 1.06 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D.C. 20555 Gentlamen:

Haddam Neck Plant Propcsed Method to Resolve SEP Topics III-2 and III-4.A (ISAP Topic 1.06)

Wind and Tornado loadinas and Tornado Missiles (TAC #51938)

On April 7,1988, Connecticut Yankee Atomic Power Company (CYAPC0) met with the NRC Staff in Maryland to discuss the Wind and Tornad Missile topics with respect to the Haddam Neck Plant.goadings At thatand meeting, Torntdo CYAPC0 presented its approach to resolve these SEP topics. The Staff requested that we submit a report of our propcsed methodology. Attachment I contains that report.

CYAPC0 has determined that the proposed methodology, which utilizes

> probabilistic techniques, is both sound and reasonable as a means of resolution of SEP Topics III-2 and III-4.A. CYAPC0 respectfully requests that the Staff review the methodology described in Attachment 1 and provide concurrer.ce with this approach.

These SEP topics are combined into one Integrated Safety Assessment Program (ISAP) Topic-1.06. The current Integrated Schedule (IIS) indicates a mid-1989 completion of this study.gplementation (1) The April 7, 1988 meeting was requested by the Staff in their December 15, 1987 letter. See the A. B. Wang letter to E. J. Mroczka, "Haddam Neck Plant-SEP Topics III-2, III-4 and III-6," dated December 15, 1987.

(2) See the E. J. Hroczka letter to the U.S. Nuclear Regulatory Commission, "Integrated Safety Assessment Program, Haddam Neck Plant," dated November 13, 1987. j f

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U.S. Nuclear Regulatory Commission A06968/812935/Page 2 June 3, 1988 If you have any questions, please call us.

Very truly yours, CONNECTICUT YANKEE ATOMIC POWER COMPANY (26 4 SenfofVic~e PrgYident E. J. Mroczka cc: W. T. Russell, Region I Administrator A. B. Wang, NRC Project Manager, Haddam Neck Plant J. T. Shedlosky, Senior Resident Inspector, Haddam Neck Plant i

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Attachment 1 Proposed Method to Resolve SEP Topics III-2 and III-4.A Wind and Tornado Loadings and Tornado Missiles i

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RESOLUTION OF SEP TOPICS III-2 AND AND III-4.A UNDER ISAP TOPIC 1.06 AT THE HADOAM NECK PLANT General Northeast Utilities (NU) will perform an analysis of the plant shutdown systems at Haddam Neck to resolve SEP Topics III-2 and III-4.A under ISAP 1.06.

Consistent with the USNRC reconnendations outlined in References 1 and 2, this analysis is intended to demonstrate an adequate resistance for relatively smaller tornado loadings and that the probability of larger tornado loadings is acceptable. With regard to tornado missiles, this analysis is intended to demonstrate the plant shutdown capability using adequately protected equipment.

It will provide information on the f ailure probabilities of individual structures and components, subjected to wind, missile, and pressure drop dfects. The analysis will also identify the most vulnerable conponents and develop the failure probability for each safe shutdown system. The TORMIS methodology, a tornado computer risk analysis program developed through EPRI, will be used to detennine component and plant failure probabilities. It has been previously reviewed and adopted by the USNRC. Base case failure probabilities for the existing plant shutdown systems and the redections in risk as a result of physical component protection and/or other upgrade options will be determined. The construction costs will be estimated for each upgrade project. A cost-benefit analysis will be perfonned and presented to the USNRC.

The approach to the resolution of ISAP 1.06 will consist of five basic tasks:

(1) System Failure Review (2) Site-Specific Tornado Analysis (3) Component failure Modes (4) TORMIS Analysis (5) Cost-Benefit Analysis The steps in these tasks are reviewed in the following paragraphs.

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System Failure Review NV will perform a plant wide systems failure review of the feed and bleed and auxiliary feedwater safe shutdown systems. This review will document: (1) all system components required to ensure the plant could be brought to hot shutdown; (2) the required instrumentation and control for each component; (3) system failure logic (i.e., redandancy among components will be recognized); and (4) the required function or each component within the system. The results of this review will be utilized to confirm the system and component inputs to the TORMIS analysis. A preliminary system failure review has been perfonned to identify the primary components and structure required for these systems.

Site-Specific Tornado /1alysis A site-specific analysis has been perfonned to generate a tornado data set for the TORMIS analysis. The National Severe Storms Forecast Center (NSSFC) database (Reference 3) was used to identify a homogeneous subregion around the Haddam Neck Plant. Tornadoes were mapped for a large region and statistical tests were performed to identify a suitable subregion. The subregion tornado occurrence rates and intensities were compared to the regional data to ensure that the tornado risks are appropriately quantified. The results of this analysis have been compared to published regional data and previous tornado site characterization for Haddam Neck.1 Results of this site-specific analysis have been completed and are shown in Table 1. This table includes the results for a point target (i.e., a single small structure or component) and for an area target that encloses the safety-related components at Haddam Neck.

Table 1. Tornado Windspeed Execedance Probabilities at Haddam Neck P(V)V*) V* (mph)

(peryr) Point Plant 1 x 10-4 94 115 1 x 10-5 165 185 1 x 10-6 222 245 1 x 10-7 275 290 1 For example, the NRC has, previously published 168 mph as the tornado windspeed associated with an annual exceedance probability of 1 x 10-5 per year for Haddam Neck. (See Reference 4). f 2

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m ein ,s. n w Figure 1. Tornado Windspeed Exceedance Curve The difference between the point and plant windspeeds is due to the area of the plant and the increased risks of a tornado strike on an area target as opposed to a geometrical point. The analysis for each shutdown system will be based on the plant-wide tornado risk. Figure 1 sunmarizes the developed tornado hazard curve for Haddam Neck. The TORMIS analysis will use the data and results of this site-specific analysis in the overall tornado analysis at the Haddam Neck site.

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Component Failure Modes Failure modes will be finalized for each component in each shutdown system.

These failure modes will take into account the tornado effects of missile impact, dynamic pressure, and atmospheric pressure change. Table 2 sumarizes the wind effects and component / structure failure modes that will be considered.

Damage criteria will be developed for each component and these will be used in the analysis. As an example, spalling of a reinforced concrete or blockwall due to missile impact, as well as a predetemined wind velocity or pressure drop fragility, will define the criteria for failure of that wall. In an area such as the existing switchgear room, the failure of any such wall defines the failure of all switchgear contained in that room.

TORMIS Analysis The TORMIS methodology was developed to estimate the probability of damage to nuclear power plant structures and components from extreme winds (References 5,6). Figure 2 sumarizes the key elements of the methodology. The NRC issued a Safety Evaluation Report on 26 October 1983 (Reference 7), which concluded that the methodology "... can be utilized when assessing the need for positive tornado protection for specific safety-related plant features." NU will apply the TORMIS methodology consistent with the specific ca.ments in this review.

A detailed model of the plant has been developed. This model includes all components and structures which: (1) would affect the safe shutdown of the plant if they were to fail; (2) add significantly to the nissile population of the site; or (3) shield these structures and/or components required for the shutdown of the plant structures, such as the diesel generator, switchgear, and primary auxiliary buildings. Several of the structures included in the TORMIS model are outlined in Figure 3. Components included in the analysis includes tanks, piping, valves, and electrical switchgear. Missile sources have been detemined through a plant walkdown to specifically quantify missile populations and locatiuns. All relevant failure modes will be considered for each component / structure, as outlined above. The system failure logic for each shutdown system will reflect the results of the system failure review. The TORMIS analysis will provide probabilities of damage to each individual 4

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component and the probability of f ailure of each shutdown system due to a tornado strike on the plant. All of the inputs and results will be documented in a detailed final report for USNRC review.

At the Haddam Neck plant, a new switchgear building is currently under construction. This new switchgear building wili house rauch of the vital instrumentation and control required for the safe shutdown of the plant.

Although this new switchgear building is being constructed primarily to resolve 10CFR50 - Appendix R concerns, its design end construction does recognize the need for additional tornado wind and missile protection at the Haddam Neck site.

Tornado wind and missile protection is being provided through 2 feet thick building walls and roof slab. Cabling will be provided throughout the plant through buried reinforced concrete ductbanks. These modifications, along with ,

other ongoing projects which also recognize the need for added tornado ,

protection at the Haddam Neck site, will be reflected in the TORMIS analysis.

Cost-Benefit Analysis The TORMIS analysis will provide the probability of shutdown system failure for the existing feed and bleed and the auxiliary feedwater systems. These results will be referred to as the base case results. They will indicate the probabilities of damage to each component and will thus in(icate the most vulnerable components and structures. The base case results will thus provide a risk level to which the plant is currently protected.

The final step toward the resolution of high wind and tornado issues will be the assessment of hardening and upgrade options for the plant. The TORMIS methodology will be used to analyze component and system probabilities for hardening and protection / relocation of specific comonents. For exagle, if certain components are vulnerable to relatively weaker winds and tornado effects, the effects of protecting the components can be assessed. In this r.ianner, the reduction in probability of failure of each shutdown system can be developed and the effective windspeed resistance of the system ir. creased. Cost estimates will be developed for each protection / hardening upgrade construction project. By combining these results, a cost vs. probability of failure relation 8

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X'l 10 to X SYSTEM DMIAGE PROBABlUTY FOLLONING A TORNADO D'R'I ) AS DEVELOPED BY TORul5 Figure 4. Cost Benefit Analysis can be developed, as shown in Figure 4. This information will allow NU to assess cost versus risk and develop an effective plan for resolving the tornado and high wind issues at Haddam Neck (i.e. ISAP 1.06). The target overall system failure probability with regard to ISAP 1.% is anticipated to be consistent with the overall core melt frequency safety goal from all potential anticipated plant design basis events.

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References

1. U.S. Nuclear Regulatory Commission, "Integrated Plant Safety Assessment

-Systematic Evaluation Program - Haddam Neck Plant," Final Report,

} NUREG-0826, Docket No. 50-213, June 1%3.

2. U.S. Nuclear Regulatory Connission, "Integrated Safety Assessment Report -

Integrated Safety Assessment Program - Haddam Neck Plant," Draft Report, NUREG-1185, Docket No. 50-213, July 1%7.

3. Kelly, D. L. , Schaefer, J.T. , McNulty, R.P. , Doswell, C. A. , III, arid Abbey,

.R.F. , Jr. , "An Augmented Tornado Climatology," Monthly Weather Review, Vol.

1%, August 1978, pp.1172-1183.

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4. Markee, E., Presentation to ACRS on Tornado Risk, 4-5 April 1983, Washington, D.C.

5. Twisdale, LA., et al., "Tor.' ado Missile Risk Analysis," NP-768 and NP-769, Electric Power Research Institute, Palo Alto, California, May 1978.

6. Twisdale, L. A. , and Dunn, W.L. , "Tornado Missile Simulation and Design E Methodology," EPRI NP-2005, Electric Power Research Institute, Palo Alto,

- California, August 1981.

7. U.S. Nuclear Regulatory Comnission Safety Evaluation Report, "EPRI Topical Reports Concerning Tornado Missile Probabilistic Risk Assessment Methodology," Transmittal Memo dated October 26, 1983 from L.S. Rubenstein, Assistant Director for Core and Plant Systems, Division of Systems inte-gration, to F.J. Miraglia, Assistant Director for Safety Assessment, Division of Systems Integration.

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