ML20040F967
| ML20040F967 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 01/19/1982 |
| From: | Engle L, Jeffrey Mitchell Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20040F966 | List: |
| References | |
| NUDOCS 8202110007 | |
| Download: ML20040F967 (5) | |
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NUCLEAR REGULATORY COMMISSION
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9, SAFETY EVALUATION BY THE OFFICE ON NUCLEAR REACTOR REGULATION SUPPORTING AMENDMENT N05.36AND 16TO FACILITY OPERATING LICENSE N05. NPF-4 AND NPF-7 VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION, UNITS NO. 1 AND NO. 2 DOCKET N05. 50-338 AND 50-339
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Introduction:==
By letter dated March 6,1981 (Serial No.109) the Virginia Electric and Power Company (the licensee) requested a change to the Technical Specifications (TS) for the North Anna Power Station Units No. 1 and 2.
The licensee's request would change the NA-l&2 TS limits for enrichment of new and spent fuel. The licensee's March 6, 1981 letter was supplemented by letters dated March 26, 1981 (Serial No. 195) and August 18, 1981 (Serial No. 495).
The licensee initially proposed to raise the enrichment limit to 4.1 weight percent U-235 with a_burnup limit of 45,000 megawatt days per Metric-Ton-Uran-ium (mwd /MTU). However, the licensee later requested that we approve 4.1 per-cent U-235 at 38,000 mwd /MTU, which this Safety Evaluation Report (SER) addresses.
On April 29, 1981, we issued Amendments No. 27 and No. 8 to Facility Operating Licensee No. NPF-4 and NPF-7 for NA-l&2 respectively. The SE for these amend-ments evaluated the safety aspects.of storing 4.1 weight percent of U-235 in the new and spent fuel racks. However, the TS limit was set at 3.7 weight per-cent U-235.until the safety aspects of operating at 4.1 weight percent U-235 could be evaluated.
We have now evaluated the safety aspects of operating with 4.1 weight percent U-235 to 38,000 mwd /MTU with the exception of the effect of fuel failure rates.
Unti; we can complete our review of fuel failure rates the use of 4.1 weight percent U-235 fuel is based on a burnup to 37,000 mwd /NTU. Other parameters, as discussed below, are evaluated at 38,000 mwd /MTU.
Increases in fuel enrichment and burnup beyond the traditional range covered in the Regulatory Guides and Standard Review Plan could affect the radiological consequences of accidents by changes in the fuel failure rate, changes in the total inventory and mix of radiosotopes in the fuel, the fraction of isotopes accumulated in the fuel-clad gap, iodine spiking behavior, and the effect of fuel rod gas pressure on decontamination factors assumed for fuel handling i
accidents.
The parameters, as noted above, are discussed below.
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Discussion:
s CORE FISSION PRODUCT INVENTORY Changes in anrichment and burnup would affect the total inventory of. fission products in the fuel elements, as well as the relative abundance of various isotopes. Based on many years of experience with radiological consequence calculation, the staff's analyses codified in the Regulatory Guides and Standard Review Plan are based on the assumption that the iodine and noble gas isotopes present the radiologically limiting radionuclides. For enrich-l ment /burnup beyond the traditional limits it is necessary to verify that this assumption is valid.
Calculations confirm that the radiciodines and short lived noble gases remain quite constant, but also show an increased. core content of other raoiologically important nuclides such as Cs-137 or Sr-90. Two types of accidents must be 4
evaluated in light of the changing mix of nuclides:
the loss of coolant accident where the release is calculated for l'eakage through the containment (LOCA),
i and the accidents for which the leakage bypasses containment and leaks, for in--
stance, through the steam generator.
In the case of the LOCA, the most important mitigating feature is the containment, which would be equally effective for the retention of all fission products.
j The.Surry and North Anna plants have sub-atmospheric. containments which will-prevent leakage of fission products except for a short pressure spike above atmospheric in case of.a large break LOCA. The staff's Safety Evaluation Report (SER) con 3ervatively assumed that.the leakage through the containment would continue for one hour. The conservatism in.this value is sufficient to accommodate the increase shown in Cs-137 and Sr-90 in the preliminary calculations.
For the LOCA pathway which bypasses the containment, the recirculation leakage, these plants have. safety arade filters which will filter the effluent prior to release to the environment. The efficiency for-filtration of Gs or Sr are are than that assumed for iodine, by enough margin to account, again, for the chl ater culated core inventory increase.
Therefore, the LOCA can be. evaluated for this extension of burnup by traditional methods. The licensee provided results by letters dated March 26, 1981 and August 18, 1981 which showed the noble gas and radiciodine inventories of a core totally fueled with 4.1 weight percent U-235 and burned to the end of the cycle where the batch average discharge burnup would be 38,000 mwd /MTV.
The calculation was specific for the North Anna 17 x 17 fuel, but the 1 icensee
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stated that the minor changes in inventory were representative of both North i
Anna and Surry. The noble gas inventory was in no case larger than that assumed in the North Anna Final Safety Evaluation Report (FSAR) and therefore the whole '
body doses from the LOCA are still bounded by the FSAR calculations. However,
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due to minor differences in radiciodine yields among the fissile nuclides,.
the calculation showed between 9% and 3% increases.in the core content of specific radiciodine isotopes over the FSAR values. The staff's calculation i
l of the thyroid dose at the exclusion area. boundary (EAB) reported in the SER was 113 rem; allowing a 9% increase (the value appropriate to I-131) the cal-culated dose of 123 rem still indicates that the plant is adequately designed against the LOCA and that the dose mitigating features are adequate. The same conclusions apply to Surry; the staff's evaluation of the thyroid. dose from the LOCA, 220 rem at the EAB, would still be below the guideline value for the increased inventory.
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. For other accidents that bypass the containment and release, for instance through the steam generator, the gap content of radionuclides is important and is discussed as follows.
GAP INVENT 0RY In considering those accidents where the content and pressure of the rod are important, the staff evaluated whether the traditional gap fraction.of volatile radionuclides (10% except Kr-85 which is 30%) remains adequate.
The gap fractions of radioactive volatiles are determined mainly by three parameters: The half life, the linear heat generation rate (LHGR).and the 4
burnup.(other factors of less importance since the fuel designs have remained.
relatively unchanged are surface to volume ratio of the pellets and the effective density). The state-of-the-art calculational technique for gap release is embodied in the ANS proposed standard 5.4.
The licensee provided calculations using this model which showed that the propensity to release more volatiles into the gap due to burnup was more than compensated by the reduction in LHGR within the high burnup modules. This conclusion is dependent on the proposed fuel management scheme where the high burnup modules are in non-
' limiting locations, and the burnup limit of 38,000 mwd /MTU. The staff then evaluated whether the gap inventories of modules in limiting locations would exceed the usually assumed 10% value. Since the gap release model is a "best estimate," the peaking factors used to infer a. peak LHGR were plant specific technical speficifation maxima, thereby preserving a suitably " conservative" resulting inventory. For all the plants, the traditional release fractions (to the gap) for noble gases remains conservative for first and.second cycle t
fuel. For North Anna, the maximum LHGR is 11.4 KW/ft. and the ANS 5.4 model predicts that 10% release is not exceeded for all iodine isotopes. This assures that thyroid. doses calculated on the basis of 10% of each isotope of iodine are still conservative.
More than 10% of the Cs-137 is in the gap for rods of the North Anna peak LHGR no earlier than about 10,000 mwd /MTU. However, for the higher burnup fuel, where the Cs-137 rod content is increased, the LHGR is lower than this maximum.
For this species the lower volatility compared to elemental iodine will limit its release.
I0 DINE SPIKING The phenomenon of iodine spiking has been. considered by the licensee. No changes in the plants' technical specifications are requested for the mag-nitude of the equilibrium or the " spike" iodine concentration, the surveillance requirements, or the restriction on the total time a plant may operate above the equilibrium concentration. Combined, then, with the lower fraction of radioiodines in the gaps of higher burnup rods, these factors assure that the i
staff's modeling of the " spiking" in accident calculation remains conservative.
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s DECONTAMINATION FACTORS The total pressure of gas in the fuel rods is increased at high burnup. During a fuel handling accident, the higher pressure would cau m the bubbles con-taining the radionuclides to rise more quickly to the pool surface, thereby reducing the time available for diffusion of iodine into the water. The licensee has provided a reanalysis of a decontamination experiment performed by Westinghouse Electric Corporation in'1970 (proprietary). The reanalysis, r.
which was based on a best fit to the data, showed that, at the pressure.that would be obtained following high burnup, a decootamination factor of 600 would be appropriate. The staff has independently reviewed the data and has deter-mined that a value that can be supported by more than 90% of the data should be used, especially where extrapolation beyond the range of the experiment is necessary. The staff has concluded that a factor no higher than 300 is just-ified.by the data. However the traditional value of a decontamination factor of 100, will provide additional margin for uncertainties in the experiment.and is
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acceptable for a bubble rise distances in the pool to about 22 feet at 38,000 mwd /MTU batch average at discharge.
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i FUEL FAILURE RATE m
We have assessed information currently available from operation with similar' i
fuel at other Westinghouse plants and.we conclude that there is reasonable l
l assurance that an increase in batch average burnup to 37,000 mwd /MTU would impose no significant hazards considerations and would not endanger the public i
health and safety.
i Evaluation:
4 We conclude, therefore, that the use of 4.1 weight percent U-235 exposed to a burnup of 37,000 mwd /MTV batch average at discharge does not substanially alter the previously calculated consequences of accidents provided the LHGR remains at the value 1mplied by the present Technical Specification on peaking factors. The minor increase in the calculated LOCA dose, due to minor differ-ences in yield of iodines.among the fissioning species can be accommodated within the 10 CFR Part 100 guidelines.
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Therefore, based on the above, the NA 1&2 TS limits are hereby revised to allow an increase in enrichment for new and spent fuel to 4.1 weight percent U-235.
Environmental Consideration We have determined that the amendments do not authorize a change in effluent types or total amounts nor an increase in. power level and will not result in.
any significant environmental impact. Haviog made this determination, we have further concluded that the amendments involve an action which is insignificant from the standpoint of environmental impact and. pursuant to 10 CFR g51.5(d)(4),
that an environmental impact statement or negative declaration and environmental impact appraisal need not be prepared in connection with the issuance of these
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amendments.
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Conclusion We have concluded, based on the considerations discussed above, that: (1) be-cause the amendments do not involve a significant increase in the probability or consequences of accidents previously considered and do not involve a sig-nificant decrease in a safety margin, the amendments do not involve a significant hazards cor.. sideration, (2) there is reasonable assurance that the health and safety.of the public will-.m.t be endangered by operation in the proposed v
manner,'and (3)
"ch activities will be conducted in compliance with the ComL miss fun's regel and the issuance of these amendments will not be inimical to the common s and security or to the health and safety of the public.
,e Date:31an.uary 23, 1982
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" PrincipalContributors Jocelyn A Mitchell'
' Leon B. Engle
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