ML20057F217
| ML20057F217 | |
| Person / Time | |
|---|---|
| Site: | Comanche Peak  |
| Issue date: | 10/05/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20057F214 | List: |
| References | |
| NUDOCS 9310140302 | |
| Download: ML20057F217 (6) | |
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NOS.19 AND 5 TO FACILITY OPERATING LICENSE NOS. NPF-87 AND NPF-89 TEXAS UTILITIES ELECTRIC COMPANY. ET AL.
COMANCHE PEAK STEAM ELECTRIC STATION. UNITS 1 AND 2 DOCKET N05. 50-445 AND 50-446
1.0 INTRODUCTION
By application dated October 19, 1993, Texas Utilities Electric Company (the licensee) requested changes to the Technical Specifications (Appendix A to facility Operating License No. NPF-87) for the Comanche Peak Steam Electric Station (CPSES), Unit No. 1.
By letter dated March 17, 1993, the licensee expanded the application to include CPSES Unit 2 (Facility Operating License No. NPF-89). By letters of April 1, and August 6,1993, additional information was provided in support of the application.
These additional letters were clarifying in nature and thus, within the scope of the initial notice and did not affect the NRC staff's proposed no significant hazards considerations determination.
The license amendment request proposed changes to the CPSES Technical Specifications (TS) which are listed below:
- TS 3/4.1.2.5 - increase the minimum baron content of fluid in the Refueling Water Storage Tank (RWST) from 2000 ppm to 2400 ppm in Modes S and 6.
- TS 3/4.1.2.6 and 3/4.5.4 - increase the boron content range of the RWST fluid from 2000-2200 ppm to 2400-2600 ppm in Modes 1, 2, 3 and 4.
- TS 3/4.5.1 - increase the boron content range of fluid in the cold leg injectaon accumulators from 1900-2200 ppm to 2300-2600 ppm in Modes 1, 2 and 3.
- TS 3/4.9.1 - increase the minimum boron content of fluid in the RWST from 2000 ppm to 2400 ppm in Mode 6.
This increase in boron concentration is necessary to accommodate shutdown margin and safety analysis requirements associated with an extension of the reload cycle to 18 months.
9310140302 931005 PDR ADOCK 05000445 P
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- The RWST's primary purpose is to provide a source of borated water to the spent fuel pool during refueling operations. The boration path to the spent fuel pool during refueling operations is accomplished by way of the reactor vessel and the refueling cavity. Therefore, at the close of refueling when the reactor vessel is isolated from the refueling cavity, boron levels in the reactor coolant system (RCS) can be as high as 2600 ppm, prior to startup of the unit. The boron level of the RCS is diluted down to approximately 1800 -
1950 ppm prior to taking the reactor critical. Boron levels in the RCS continue to be diluted down as power operation continues throughout the operating cycle. This is done to allow the reactor to remain critical as xenon (a negative reactivity source) builds up during the operating cycle.
The RWST also serves as an alternative source of borated water to the reactor coolant system, should the water level in the volume control tank fall below its low level setpoint.
2.0 EVALUATION The licensee has performed preliminary evaluations of the effect of the proposed increased boron concentration limits on each of the Chapter 15 transients and accidents presented in the CPSES Final Safety Analysis Report (FSAR) and no adverse effects were observed. However, prior to operation with a revised core configuration, all of the accident analyses will be evaluated for continued applicability as part of the reload safety evaluation process.
The increased RCS boron concentration required to support extended length fuel cycles, for example, will result in a more positive (less negative) moderator temperature coefficient (MTC). Therefore, this effect on those events which are adversely affected by a more positive MTC will be re-evaluated for the cycle-specific core design during the normal reload analysis process.
The effect of an increased boron concentration on the loss of coolant accident (LOCA) was evaluated. Since the boron concentration is increased, the reactor will, of course, remain shut down during the reflood and long-term cooling phase with the minimum proposed boron concentration even with no control rods inserted in the core. However, one effect of boration during a LOCA is the progressive increase over time of the boron concentration in the core. This occurs because the water vaporizes out of the break and leaves behind the boron it originally contained.
If the concentration exceeds a critical value, boric acid can crystallize in the core and precipitate out of solution. The concern is that the precipitation of boric acid crystals could block core cooling. The boron solubility limit at 35*F coolant temperature is 4900 ppm.
To preclude the possibility of crystallization of the RWST contents, the TS require the RWST temperature to be no less than 40*F.
To assure boron solubility within the core, the licensee has also determined the required switchover time for the initiation of hot leg recirculation (which provides for proper core mixing) based on the proposed increased RWST and accumulator baron concentrations.
2.1 RWST and ECCS Accumulator Considerations The licensee's submittal indicates that the proposed increases in boron concentrations to the CPSES RWST and ECCS accumulators do not constitute an s
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unreviewed safety issue in regard to the plant design. The proposed changes involve an increase in boron concentration of approximately twenty percent.
As the insides of the RWST and ECCS accumulators are lined with stainless steel, the proposed increases in baron concentrations will not significantly increase the acidity (corrosiveness) of the borated solutions enough to threaten the structural integrity of the tanks.
2.2 Reactor Coolant System (RCS) Considerations As stated above, the proposed modification to the RWST boron concentration will increase the RCS boron concentration to the 1800-1950 ppm range just prior to power operation of the unit. However, operation of CPSES at RCS boron concentrations > 1600 ppm requires a modified (elevated) lithium (L1) control program in order to maintain the RCS coolant in the normal 6.9 - 7.4 pH range. The table below, illustrates the lithium - boron correlation for the Comanche Peak RCS coolant at a T., of 310.8 degrees C.
Boron Min. Li Min.
Target Target Max. Li Max.
ppm ppm pH Li ppm pH ppm pH 2500 3.86 6.90 4.01 6.92 4.16 6.93 2300 3.46 6.90 3.61 6.92 3.76 6.93 2100 3.07 5.90 3.22 6.92 3.37 6.94 1900 2.71 6.90 2.86 6.92 3.01 6.94 1800 2.54 6.90 2.69 6.92 2.84 6.95 1600 2.20 6.90 2.35 6.93 2.50 6.95 1400 2.05 6.93 2.20 6.96 2.35 6.99 The Electric Power Research Institute (EPRI), in conjunction with the Westinghouse Electric Corporation, has studied the effects of boren and lithium water chemistry on Primary Water Stress Corrosion Cracking (PWSCC) of Inconel 600. According to the studies, Inconel 600 specimens inaersed in aqueous solutions containing 3.5 ppm lithium have exhibited a significant increase in crack initiation rates when compared to identical specimens immersed in aqueous solutions containing 2.2 ppm lithium.
EPRI therefore recommends that the RCS coolant pH be limited to a range to 6.9 - 7.4, and the RCS coolant Li level to a maximum of 2.2 ppm, when the boron concentration in the coolant is 1200 ppm or less. EPRI does not recommend operating for extended periods with RCS coolant lithium levels above 2.2 ppm.
i As the table indicates, RCS coolant boron levels above 1600 ppm require the i
addition of elevated lithium levels in order to maintain the coolant pH in the 6.9 - 7.4 range. However, the CPSES RCS will contain elevated boron levels (above 1600 ppm) and elevated lithium levels (above 2.2 ppm) for a short time interval during initial startup and operation of the unit (approximately four
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effective full power days of operation). This time interval is typically less than the crack initiation times needed to promote PWSCC cracks in Inconel 600.
Therefore, the elevated lithium and boron levels in the RCS coolant should not significantly increase the probability of PWSCC in the steam generator tubes or reactor vessel internals fabricated from Inconel 600.
2.3 Effects on Operation durino toss of Coolant Accident Conditions The proposed technical specification changes will result in a decrease in containment sump volume pH from 8.5 to 8.25 during design basis accident (DBA) conditions.
This decrease in pH could change the value of the elemental iodine partition coefficient which is used in the licensee's calculations for determining offsite iodine doses following a DBA.
The licensee indicated in their October 19, 1992 submittal that an iodine partition factor of 5,000 was consistent with decontamination factor of 100 which was assumed in the CPSES Safety Evaluation Report. The licensee stated further that, even with the slightly lower pH and the new baron concentrations, the partition factor of 5,000 would remain valid. With this being the case, the licensee stated that iodine decontamination factor would also remcin unchanged. Since the decontamination factor of 100 remains unchanged, then the staff assessment of the radiological consequences of a loss of coolant accident also remains unchanged. Therefore, the staff is in agreement with the licensee's conclusion that the increase in boron concentration in the RWST and the ECCS will not alter the results of the CPSES accident analyses.
The licensee has submitted data which supports their position that an elemental iodine partition coefficient of 5000 remains conservative in spite of the drop in sump volume pH. The licensee's selection of the elemental iodine partition coefficient value agrees with values cited in industry literature for similar accident conditions. As the accident analyses still remain bounding, the proposed modifications do not significantly reduce the margins of any of the licensee's calculations of DBA offsite iodine doses which could be released following a design basis accident.
2.4 Precipitation of Boron in the RCS and Safety In.iection Paths The proposed technical specifications do not create an additional risk of precipitating boron in the RCS as the maximum boron concentration in the coolant would remain well within the boron solubility limit even at minimum coolant temperature (4600 ppm B at 35*F). Furthermore, the RWST and safety injection lines to the reactor vessel are heat traced where necessary to ensure that the boron will remain in solution in the tanks, and that precipitation does not occur during a postulated ECCS actuation during a DBA.
The proposed modifications do not extend the boron concentrations beyond the boron solubility limits during normal, shutdown, and accident operations of the plant.
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1 2.5 luggCy The staff has reviewed the effects of the proposed increased boron concentration limits on the non-LOCA and LOCA safety analyses and concludes that all pertinent safety criteria are satisfactorily met. Based on the above evaluation, the licensee's request to increase the boron concentration range in the RWST to 2400-2600 ppm and to 2300-2600 ppm in the accumulators is acceptable.
In addition, the proposed increase in the minimum boron concentration during refueling operations for the RCS and the refueling canal to 2400 ppm is acceptable. Nonetheless, prior to operation with a revised core configuration, the effects on the accident analyses of the higher boron concentrations in the RWST, ECCS accumulators, and RCS will be explicitly evaluated by the licensee during the reload safety evaluation process.
l The licensee has shown, through information provided in the licensee's original submittal and additional information in the subsequent submittals, that the proposed technical specification changes do not significantly change the corrosive nature of fluids in the RWST, ECCS Accumulators or RCS. The proposed modifications will not change the results of the licensee's calculations of the offsite iodine doses which are determined to be released following a design basis accident. Additionally, the licensee has properly evaluated the concern regarding boric acid crystallization.
The proposed modifications will cause elevated boron and lithium levels in the RCS; however, the elevated levels will only exist for a short time interval (during initial startup and operation of the unit). Therefore, the proposed modifications will not significantly increase the amount of PWSCC in the reactor vessel internals or steam generator components fabricated from Inconel 600. The staff, therefore, finds the technical specification changes acceptable.
3.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Texas State official was notified of the proposed issuance of the amendments.
The State official had no comments.
4.0 ENVIRONMENTAL CONSIDERATION
The amendments change a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20. The NRC staff has determined that the amendments involve no significant increase in the amounts, and r.o significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has previously issued a proposed finding that the amendments involve no significant hazards consideration, and there has been no public comment on such finding (58 FR 25865). Accordingly, the amendments s
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. meet the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9). Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendment.
5.0 CONCLUSION
The Commission has concluded, based on the consideration discussed above, that:
(1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendments will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributors: L. Kopp J. Medoff J. Hayes Date: October 5, 1993 i
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