ML20041B624
| ML20041B624 | |
| Person / Time | |
|---|---|
| Site: | Pilgrim |
| Issue date: | 02/05/1982 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20041B622 | List: |
| References | |
| NUDOCS 8202240373 | |
| Download: ML20041B624 (4) | |
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UNITED STATES
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTING A!!END!iENT N0. 53 TO FACILITY LICENSE !!0. DPR-35 BOSTON EDISON C011PANY PILGRIl4 NUCLEAR POWER STATION DOCKET NO. 50-293_
Principal Author:
Kenneth T. Eccleston 1.0 Introduction By letter dated January 15, 1982, Boston Edison Company (the lfcensee) requested an amendment of Facility Operating License No. DPR-35 for the Pilgrim Nuclear Power Station (the facility). The proposed amendment would reduce the maximum downcomer submergence to 3.25 feet and would reduce the minimum drywell-torus differential pressure from 1.5 psid co 1.17 psid. The licensee is shortening the length of the downcomers as part of the Mark I Containment Long Term Program (LTP) and has determined that a change in the Technical Specification requirements for drywell-torus differential pressure is necessary.
2.0 Evaluation The purpose of the Mark I Containment Long Term Program is to perform a complete reassessment of the suppression chamber (torus) design to include suppression pool hydrodynamic loads, which were neglected in the original design, and to restore the originally intended design safety margins of the structure.
NUREG-0661 describes the generic techniques for the definition of suppression pool hydrodynamic loads in a Mark I system and the related structural acceptance criteria.
One method of suppression pool hydrodynamic load mitigation that tne Mark I Owners Group has adopted for the LTP is reducing the initial submergence of the downcomer in the suppression pool to a minimum of at least three feet.
By shortening the length of the downcomer, the pool volume (i.e., thermal capacity) of the original design would be maintained. This approach,-
however, raises concern regarding the increased potential for uncovering the downcomers and steam condensation capability, both of which could lead to torus overpressurization.
2.1 Seismic Slosh The potential for downconer uncovery is addressed in the assessment of seismic slosh.
This assessment was performed at the most extreme con-g ditions that could potentially lead to uncovering of the downcomers and was predicted on a minimum three-foot downcomer submergence.
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2 Seismic motion induces suppression pool waves which can (1) impart an oscillatory pressure loading on the torus shell, and (2) potentially lead to uncovering the ends of the downcomers, which could result in steam bypass of the suppression pool and potential overpressurization of the torus, should the seismic event occur in conjuction with a Loss of Coolant Accident (LOCA). To assess these effects, the Mark I Owners Group undertook the development of an analytical model which would pro-vide plant-specific seismic wave amplitudes and torus wall pressures.
This model was based on 1/30-scale " shake test" data for a Mark I torus geometry.
Based on the results of plant-specific analyses, using the analytical model, the Mark I Owners Group concluded that (1) the seismic wave pressure loads on any Mark I torus are insignificant in comparison with the other suppression pool dynamic loads, and (2) the seismic wave ampli-tudes will not lead to uncovering the downcomers for any Mark I plant.
This conclusion was based on the maximum calculated pressure loads and the minimum wave trough. depth relative to the downcomer exit.
We have reviewed comparisons of the analytical predictions with scaled-up test data, the small-scale test program, and the seismic spectrum envelope used in the plant-specific analyses.
Based on this review, we conclude that the seismic slosh analytical p'redictions will provide reasonably conservative estimates of both the wall pressure loading and the wave amplitude, for the range of Mark I plant conditions.
Since the maximum local wall pressures were found to.be less than 0.8 psi at a 95% upper confidence limit, the Mark I Own rs Group has proposed that the seismic slosh loads may be neglected in the. structural analysis.
We agree that the seismic slosh loads are insignificant in comparison with the other suppression pool dynamic loads.
On this basis, we con-clude that neglecting seismic slosh loads for the plant-unique analyses is acceptable.
The results of the slosh wave amplitu.ie predictions indicate that, within the local area of maximum amplitude and with maximum suppression pool drawdown (resulting from ECCS system flows), the slosh waves will not cause uncovering of the downtomers.
We have reviewed the assumptions used in these analyses and conclude that they are sufficiently conserva-tive.
Based on the above discussion, we find the proposed change acceptable.
3 2.2 Condensation Capability Condensation capability of the suppression pool is a function of the local pool temperature in the vicinity of the downcomer exit.
Full Scale Test Facility (FSTF) test results and foreign test data have shown that thermal stratification occurs, and becomes more severe as the downcomer submerg6ce is reduced. The most severe thermal stratification has been observed in low flow tests with a quiescent pool.. However, in actual plant conditions, the Residual Heat Removal (RHR) system and Safety Relief Valve (SRV) discharge provide sufficient long-term pool mixing to minimize thermal stratification.
Even with vertical thermal stratification, we have determined that the high energy reposition ss accompanied by an increased flow and mixing, which prevent overpressurization of the torus.
In addition, the analytical predictions of the torus pressure and bulk temperature response have been found to be conservative when compared with FSTF test data for plant simulated initial conditions.
The local temperature variation in the pool which has been observed in the test data is not significant to the structure, and therefore, need not be considered in the structural analysis.
Based on this assessment, we conclude that a minimum initial downcomer submergence of three feet is acceptable, and there is suffic!ent conservatism in the containment response analysis techniques to accommodate the effects of thermal stratification.
2.3 Differential Pressure The introduction of a positive pressure differential between the drywell and the suppression chamber air volume reduces the height of the water leg insi @ the downcomer.
The reduced water leg permits the downcomers to clear earlier in the LOCA transient with the drywell consequently at a lower pressure.
This effect reduces both the downward and upward pressure loads on the containment. Mark I Containment LTP Quarter Scale Test Facility testing at a downcomer submergence of 3.25 feet and a 1.17, psi drywell to wetwell pre;sure differential shows a significant suppression chamber load reduction.
The length of the water leg inside the downcomer is limited by the downcomer submergence.
Due to the shortening of the downcomers the drywell to torus differential pressure will be reduced by an amount equivalent to the reduction (9") in the water leg inside the downcomer. Long Term Program analysis and modifications are based on a downcomer submergence of 3.25 feet and a drywell to torus differential pressure of 1.17 psi.
The licensee will submit for post-implementation review the Plant Unique Analysis for Pilgrim utilizing this pressure differential and downcomer submergence value to the NRC.
Since this modification will reduce the suppression pool hydrodynamic loads and act to restore the orignally intended margins of safety, the proposed
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modification is acceptable.
Therefore, we find the proposed Technical Specification changes acceptable.
4 3.0 Environmental Considerations We have determined that the amendment does not involve a change in effluent types or total amounts nor an increase in power level and will not result in any significant environmental impact.
Having made this determination, we have further concluded that the amendment involves an action which is insignificant from the standpoint of environmental impact and pursuant to 10 CFR Section 51.5(d)(4) that an environmental impact statement or negative declaration and environmental impact appraisal need not be prepared in connection with the issuance of the amendment.
4.0 Conclusions We have concluded, based on the considerations discussed above, that:
(1) because the amendment does not involve a significant increase in the probability or consequences of accidents previously considered ar.d does not involve a significant decrease in a safety margin, the amendment does not involve a significant hazards consideration, (2) there is reasonable assurance that the health and safety of the pcblic will not be endangered by operation in the proposed manner, and (3) such activities will be conducted in compliance with the Commission's regulations and the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Dated:
February 5,1982
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