ML20058M929
| ML20058M929 | |
| Person / Time | |
|---|---|
| Site: | Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png |
| Issue date: | 09/29/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20058M925 | List: |
| References | |
| TASK-03-02, TASK-03-04.A, TASK-3-2, TASK-3-4.A, TASK-RR NUDOCS 9310070138 | |
| Download: ML20058M929 (3) | |
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Enclosure f *%,%
UNITED STATES O-[kr1j NUCLEAR REGULATORY COMMISSION I
f WASHINGTON. D C. 20 % 5-0001 s, m j SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO.
TO FACILITY OPERATING LICENSE NO. DPR-61
_(DNNECTICUT YANKEE ATOMIC POWER COMPANY HADDAM NECK PLANT DOCKET NO. 50-213
1.0 INTRODUCTION
As part of the NRC Systematic Evaluation Program (SEP), Topic III-2, Wind and Tornado loadings, and Topic III-4.A, Tornado Missiles, and the NRC Integrated Safety Assessment Program (ISAP) Topic 1.06, Wind and Tornado Loadings and Tornado Missiles, the staff has earlier reviewed the Haddam Neck Plant design with regard to its ability to withstand the effects of wind and tornado loadings including tornado missiles. The staff rated the implementation priority for ISAP Topic 1.06 as high i
.stegrated Safety Assessment Report, NUREG-1185, July 1987.
The Connecticut Yankee Atomic Power Company, licensee of the Haddam Neck l
Plant, has made several design changes and upgrades to the plant systems since 1983. These changer, and upgrades include implementation of a new switchgear building, separation of loads from Motor Control Center 5, and placing a new condensate storage tank in service. By letters dated Jur.6 30 and August 23, 1993, the licensee proposed to add an air-cooled diesel generator to be permanently connected to the on-site power system during the Cycle 19 refueling outage, approximately in the fall of 1996.
The licensee proposed to close out SF' icpic 'II-2, SEP Topic III-4.A, and ISAP Topic 1.06 with these modificat is.
.ie justification for closing out these SEP and ISAP topics is based o
> nnoabilistic risk assessment.
2.0 LICENSEE'S ANALYSIS In the probabilistic risk assessment, the licensee constructed an event tree for plant system failures, with the initiating event being a tornado striking the plant proper. The event tree sequences include the following plant safety systems, operation, or consequences: service water system, reactor trip system, small break loss of coolant, auxiliary feedwater system, high pressure safety injection system, cognitive operator action for feed and bleed operation, feed and bleed operation, long term cooling, and containment heat removal system.
Each sequence in the event tree is modelled as being mutually exclusive. On this basis, the occurrence frequency of a particular plant damage state can be obtained by suming the occurrence frequency of each The total s,equence that terminates in that particular plant damage state.
931007013e 930929 gDR ADOCK0500g3
. reactor core melt frequency is the sum of the occurrence frequencies of each plant damage state. Transients, early reactor core melt (less than two hours after the initiating event), and late reactor core melt (longer than two hours after the initiating event) were considered.
Fault trees for each of the plant safety systems in the event tree were developed to include random failures, tornado-induced failures, and operator errors. The TORMIS code was used to estimate the probabilities of tornado wind and missile damage to the plant structures, equipment, and piping systems that are needed for safe shutdown of the Haddam Neck plant.
The source of basic data on tornados comes from the National Severe Storms Center. The data include those reported for the period of 1950 to 1982, and have been screened to eliminate errors and oetliers. A set of plant-specific tornado data was developed for the Haddam Neck plant from these basic data.
The licensee first performed the probabilistic risk assessment of the Haddam Neck Plant without the plant modifications described above, and obtained a reactor core damage frequency of 7.6 E-5 per year due to a tornado initiating event. Then the licensee estimated the core damage frequency with the plant modifications since 1989, but with^ut the proposed air-cooled diesel generator.
The estimated reactor core damage frequency is 5.7 E-5 per year.
The results indicate that the dominant sequence is failure of the service water system due to tornado damaging of structures or piping. Service water is required for cooling the emergency diesel generators.
Following a tornado strike event, a loss of service water implies a total loss of AC power, leading to a reactor core melt with containment heat removal unavailable.
By incorporating the air-cooled diesel generator, which does not need service water to cool, into the plant systems, the licensee re-calculates the core melt frequency from a tornado strike event to be 3.4 E-5 per year as the lower bound (with 95 percent confidence level) and 3.9 E-5 per year as the best estimate.
The licensee indicates that the overall uncertainty in the estimated core damage frequencies is about a factor of three. The majority of this uncertainty is attributed to the relatively large uncertainty in the tornado hazard data.
3.0 STAFF EVALUATION The staff has previously reviewed the TORMIS code and issued a safety evaluation on October 21, 1992.
In the safety evaluation, the staff concluded that the use of the TORMIS code for tornado wind and missile risk analysis of the Haddam Neck plant is acceptable.
In the present evaluation, the staff has determined that the methodology used in the licensee's analysis is appropriate, the supporting data are relevant, and the results of the analysis are reasonable.
i The staff has further determined that, even with a relatively large uncertainty in the tornado data that were used in the licensee's analysis, the proposed air-cooled diesel generator is effective in reducing the reactor core damage frequency at the Haddam Neck Plant due to a tornado initiating event.
This determination is based on the fact that the proposed air-cooled diesel generator will provide an alternate success path for maintaining AC power given a loss of all service water.
4.0 CONCLUSION
On the basis of the above evaluation, the staff concludes that, with the_ plant modifications since 1989 and the proposed air-cooled diesel generator at the Haddam Neck Plant, the licensee's estimated reactor core damage frequency, due to wind and tornado loadings and tornado missiles, has been reduced significantly such that the likelihood of core damage is reasonably low. This conclusion is based solely on the staff's evaluation of the licensee's probabilistic risk assessment.
Principal Contributor:
J. Wing Date:
September 29, 1993 i
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