ML063070064

Response to Request for Additional Information Regarding License Amendment Request - 226: Increase in Technical Specification Minimum Required Refueling Water Storage Tank Boron Concentration

ML063070064
Person / Time
Site:	Kewaunee
Issue date:	11/02/2006
From:	Gerald Bichof Dominion Energy Kewaunee
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
06-497A
Download: ML063070064 (11)

Text

1)ominion Energy Kewaunee, Inc.

7 0 0 0 I l o r n l n ~ o nI3oulrvard, Glen Allcn, VA 2 3 0 0 0 1 Dominion' November 2 , 2006 U. S. Nuclear Regulatory Commission Serial No. 06-497A Attention: Document Control Desk NL&OS/CDS: R3 Washington, DC 20555 Docket No. 50-305 License No. DPR-43 DOMINION ENERGY KEWAUNEE. INC.

KEWAUNEE POWER STATION

SUBJECT:

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION REGARDING LICENSE AMENDMENT REQUEST - 226: INCREASE IN TECHNICAL SPECIFICATION MINIMUM REQUIRED REFUELING WATER STORAGE TANK BORON CONCENTRATION In accordance with 10 CFR 50.90, Dominion Energy Kewaunee, Inc. (DEK) submitted a request for an amendment to the Kewaunee Power Station (KPS) Technical Specifications (TS) (Reference 1). The proposed amendment would increase the minimum required boron concentration in the refueling water storage tank (RWST) from 2400 parts per million (pprn) to 2500 ppm. Subsequent to DEK's submittal, the NRC staff requested additional information to complete its review. provides the questions asked by the NRC staff with DEK's responses.

The RAI responses do not change the significant hazards determination for the proposed amendment discussed in Reference 1.

In Reference 1, DEK requested approval of the proposed amendment by September 15, 2006 to support implementation of the proposed change prior to the end of the KPS Fall 2006 refueling outage (Refueling Outage 28). Since the time of the original submittal, DEK has completed an analysis to validate that this change was not required to support startup of KPS following the Fall 2006 refueling outage. Therefore, DEK requests a change to the previously submitted approval date from September 15, 2006 to December 15, 2006. DEK also requests 60 days to implement this change once it is approved.

In accordance with 10 CFR 50.91(b), a copy of this letter, with attachments, is being provided to the designated Wisconsin official.

If you have any questions, please contact Mr. Craig Sly at 804-273-2784.

Very truly yours, Gerald T. Bischof Vice President - Nuclear Engineering

Serial No. 06- 497A LAR 226 Response to RAI Page 2 of 3 Reference

1. Letter from Eugene S. Grecheck (DEK) to Document Control Desk (NRC),

"License Amendment Request 226 - lncrease in Technical Specification Minimum Refueling Water Storage Tank Required Boron Concentration," dated June 28,2006. (ADAMS Accession No. ML061800307)

Attachment

1. Response to NRC Request for Additional Information Regarding License Amendment Request - 226, lncrease in Technical Specification Minimum Required Refueling Water Storage Tank Boron Concentration.

Commitments made in this letter: None cc: Regional Administrator U. S. Nuclear Regulatory Commission Region Ill 2443 Warrenville Road Suite 210 Lisle, Illinois 60532-4352 Mr. D. H. Jaffe Project Manager U.S. Nuclear Regulatory Commission Mail Stop 0-7-D-1 Washington, D. C. 20555 Mr. S. C. Burton NRC Senior Resident Inspector Kewaunee Power Station Public Service Commission of Wisconsin Electric Division P.O. Box 7854 Madison, WI 53707

Serial No. 06- 497A LAR 226 Response to RAI Page 3 of 3 COMMONWEEALTH OF VIRGINIA )

)

COUNTY OF HENRICO 1 The foregoinlg document was acknowledged before me, in and for the County and Commonwealth aforesaid, today by Gerald T. Bischof, who is Vice President, Nuclear Engineering of Dominion Energy Kewaunee, Inc. He has affirmed before me that he is duly authorized to execute and file the foregoing document in behalf of that Company, and that the statements in the document are true to the best of his knowledge and belief.

Acknowledged before me this alVd day of nl*wmcdul. , 2006.

My Commission Expires: 3/.&UB (SEAL)

Attachment 1 RESPONSE TO NRC REQUEST FOR ADDITIONAL INFORMATION REGARDING LICENSE AMENDMENT REQUEST - 226: INCREASE IN TECHNICAL SPECIFICATION MINIMUM REQUIRED REFUELING WATER STORAGE TANK BORON CONCENTRATION KEWAUNEE POWER STATION DOMINION ENERGY KEWAUNEE, INC.

Serial No. 06-497A Attachment 1 Page 1 of 7 ATTACHMENT 1 RESPONSE TO NRC REQUEST FOR ADDITIONAL INFORMATION REGARDING LlCElNSE AMENDMENT REQUEST - 226: INCREASE IN TECHNICAL SPECIFICATION MINIMUM REQUIRED REFUELING WATER STORAGE TANK BORON CONCENTRATION Pursuant to 10 CFR 50.90, Dominion Energy Kewaunee, Inc. (DEK) requested an amendment to the Kewaunee Power Station (KPS) Facility Operating License (D PR-43). The proposed amendment would change KPS TS 3.3.b.3.B and TS 3.3.b.4.A to increase the minimum required boron concentration in the refueling water storage tank (RWST) from 2400 parts per million (pprn) to 2500 ppm.

On August 22, 2006 DEK received five questions related to this amendment request.

Each question and DEK's response is provided below.

Question 1:

Please provide the analyses justifying why the current minimum boron concentration of 2400 pprn in the Refueling Water Storage Tank (RWST) is not sufficient and why 2500 pprn will be sufficient to ensure that the containment sump boron concentration following a postulated large break loss-of-coolant accident would be greater than that required to maintain subcriticality of the Cycle 28 reload core during recirculation of coolant from the containment sump.

Response

In Reference 1, DEK submitted a request for an amendment to the KPS Technical Specifications (TS) to increase the minimum allowable refueling water storage tank (RWST) boron concentration from 2400 pprn to 2500 ppm. Since the time of the original subrr~ittal,DEK has completed an analysis that supports the conclusion that this change was not required to support startup of KPS following the Fall 2006 refueling outage (Refueling Outage 28). Therefore, the current minimum boron concentration of 2400 pprn in the RWST is sufficient to ensure that the Cycle 28 reload core will remain subcritical in the event of a large break loss-of-coolant accident (LBLOCA). However, future reload cores will challenge this limitation, so increasing the minimum required RWST boron concentration remains advisable. Therefore, a detailed response to Question 1 is presented below to further clarify the justification for the proposed increase in RWST boron concentration.

Serial No. 06-497A Attachment 1 Page 2 of 7 The KPS Updated Safety Analysis Report (USAR) Section 14.3, Reactor Coolant System Pipe Ruptures (Loss-of-Coolant Accident), requires demonstration of the adequacy of the emergency core cooling system (ECCS) to provide long-term core cooling. Analyses that demonstrate the adequacy of long-term cooling provided by the ECCS assunie maintenance of subcriticality following a LOCA. USAR Section 14.3 states that, "An average RCSIsump mixed boron concentration is calculated to ensure that the post-LOCA core remains subcritical."

Post-LOCA subcriticality is verified for each reload core as part of the reload design and analysis process. The need to develop a calculation of the post-LOCA sump boron concentration, and to compare the results of this calculation to reload-specific critical boron concentration at post-LOCA conditions, was communicated to utilities by a Westinghouse technical bulletin. The bulletin described the need for utilities to develop reload proceldures to: (a) calculate the critical core boron concentration at conditions expected during the long-term recirculation phase following a LOCA, and; (b) calculate a mass-weighted average boron concentration in the sump which considers all credible sources of water that may discharge to the sump following a LOCA. The post-LOCA sump boron concentration analysis determines the minimum boron concentration that is predicted to be experienced following a LBLOCA, assuming tank volumes and solute concentratior~sare controlled in accordance with their respective limits. The predicted post-LOCA sump boron concentration is described as a function of hot full power (HFP) critical boron concentration.

Following a small or large break loss-of-coolant accident (SBLOCA or LBLOCA), fluid from various volumes accumulate in the containment sump. At KPS, these volumes include the RWST, the caustic addition standpipe, the safety injection accumulators (SIAs), the high head safety injection system piping (SI Piping), the low headlresidual heat removal piping (RHR Piping), the reactor coolant system (RCS), and potentially unborated water in the containment sump during normal operation. The RWST, SIAs, SI Piping, RI-IR Piping, and the RCS contain boric acid solution, whereas the caustic addition stanlclpipe contains sodium hydroxide solution.

Depending on the magnitude of the LOCA, some or all of the liquid contained in these volumes will be introduced into the containment, and will ultimately accumulate in the containment sump. In the design basis LBLOCA, all of the liquid in these volumes (less any unusable volume) is assumed to be transferred into the containment. It is necessary to have a sufficiently high boric acid concentration in the sump mixture to ensure that the reactor remains subcritical on boron alone. (For the design basis LBLOCA, the Westinghouse evaluation model assumes no Rod Control Cluster Assembly (RCCA) insertion.)

A mass-weighted average sump boron concentration is calculated by summing the product of ,the water mass (i.e., volume multiplied by density) and the boron concentratiori of each volume, and dividing by the sum of the masses. Uniform mixing of all tank contents in the sump is assumed. The limit for the maximum allowable

Serial No. 06-497A Attachment 1 Page 3 of 7 critical boron concentration at post-LOCA core conditions is presented as a function of the minimum critical boron concentration at pre-LOCA conditions. Conservative assumptions regarding pre-LOCA and post-LOCA coolant temperature, control rod position, xenon concentration, and cycle burnup are employed in the analysis.

In the preliminary Westinghouse post-LOCA sump boron concentration analysis, which was being prepared contemporaneous with Reference 1, an RWST boron concentration of 2400 ppm resulted in a calculated post-LOCA sump boron concentration that was lower than tk~ecalculated maximum critical boron concentration for the reload core at post-LOCA clore conditions (The calculation was performed at the low end-of-Cycle 27 burnup window. More favorable Cycle 28 post-LOCA subcriticality results would be predicted for the actual end-of-Cycle 27 burnup). However, when an RWST boron concentratior~~ of 2500 ppm was employed, the resulting calculated post-LOCA sump boron concentration was greater than the calculated maximum critical boron concentratiorl~ for the reload core at post-LOCA core conditions. Westinghouse estimates of Cycle 28 reload safety analysis results (low end-of-Cycle 27 burnup window), which were developed around the time Reference 1 was being prepared, are shown below for illustration.

Results (ppm boron) lEi:!re-LOCA 1419 Limit (2500 ppm RWST) 2260 (68' F) Core Critical 2208 m Post-LOCA (212' F) Core Critical 2241

+I9 Results (ppm boron) 1419 ppm RWST) 2185 F) Core Critical 2208 F) Core Critical 2241

-56*

Maximum post-LOCA core critical boron concentration exceeds the post-LOCA boror~concentration limit.

Serial No. 06-497A Attachment 1 Page 4 of 7 Question 2:

Please justkl how the safety analyses that use the minimum RWST boron concentratior~of 2400 pprn (e.g. post-LOCA subcriticality and MSLB accident shutdown margin) will be "unaffected and will remain bounding and valid" if the minimum RWST boron concer )tration is being increased from 2400 pprn to 2500 ppm.

Response

It is first nocessary to clarify Question 2, since the question cites post-LOCA subcriticality as among the safety analyses that are "unaffected and [that] will remain bounding" if :he minimum RWST boron concentration is increased from 2400 pprn to 2500 ppm. 14s described in Section 3.0, "Background," of Reference 1, "the effect of this change is to further constrain the allowable RWST boron concentration within the range of RVlST boron concentrations already considered in the plant design and licensing bases." Section 4.0 of Reference 1, "Technical Analysis," goes on to state that, "As a result of selecting the revised required minimum RWST boron concentration within the current minimum and maximum limits, the safety analyses that use the minimum (2400 ppm) or maximum (2625 ppm) RWST boron concentration as a design input will be unaffected and will remain bounding and valid exceDt for the ~ost-LOCA sumc, boron concentration analvsis." The reasons for this exception are discussed below.

For a given rldoad core, an increase in the minimum RWST boron concentration results in an increass in the margin by which the plant is calculated to be shutdown following a LBLOCA. Thus, an increase in the actual RWST boron concentration is a conservative change with 'espect to post-LOCA subcriticalty. However, post-LOCA subcriticality is a KPS design and licensing basis criterion governing the establishment of RWST volume and boron concentration (i.e., USAR Section 14 states that, "An average RCSIsump mixed boron concentration is calculated to ensure that the post-LOCA core remains subcritical"). Therefore, it is necessary to increase the minimum RWST boron concentration specified in Technical Specifications in order to credit an increased RWST boroi concentration in the post-LOCA subcriticality analysis. With this clarification, Question 2 can now be addressed in the context of safety analyses other than post-LOCA subcriticality.

Safety analyses that use the minimum RWST boron concentration of 2400 ppm (e.g.

LOCA and lllSLB accident analyses) are unaffected and remain valid if the minimum required RW ST boron concentration is increased from 2400 pprn to 2500 ppm. This is because these accident analyses require the injection of emergency core cooling system (ECCS) water for accident mitigation. The ECCS water is borated, and is injected into the reactor coolant system (RCS) to maintain core inventory for core cooling, anc to ensure that the reactor is adequately shutdown. If the boron concentratior~of the ECCS injection water is increased (e.g., to 2500 pprn), then the reactor cook nt system (i.e., the system that is receiving the injected ECCS water) will

Serial No. 06-497A Attachment 1 Page 5 of 7 have a high3r post-accident boron concentration than it would have if the ECCS injection wabr had a boron concentration of 2400 ppm. On this basis, it may be concluded thiit the safety analyses that use the minimum RWST boron concentration as a design i n p t~(e.g., LOCA and MSLB) will be unaffected and will remain bounding and valid if the rrinimum RWST boron concentration is increased from 2400 pprn to 2500 PPm-For the boror~precipitation analysis, higher assumed values of RCS and RWST boron concentratior are conservative; that is, boron precipitation is more likely to occur during a loss-of-coolant accident when higher initial boron concentrations are present in the RCS and R\VST. As described below in the response to Question 4, the boron precipitation analysis conservatively assumes injected ECCS fluid at the maximum RWST boror concentration (i.e., 2625 pprn). Further, the analysis assumes an at-power critica boron concentration (initial RCS condition prior to the accident) of 2400 ppm, which conservatively bounds the maximum plausible at-power critical boron concentratior for reload cores. Thus, in the case of the post-LOCA boron precipitation analysis, selection of a revised required minimum RWST boron concentration within the current range of allowable boron concentrations (i.e., 2400 pprn to 2625 ppm) ensures that the safe:y analysis will remain bounding and valid. The adequacy of the existing analysis of post-LOCA boron precipitation is also addressed in Question 4 (below).

The same logic applied to LBLOCA, MSLB, and boron precipitation is applied in the Reference 1 evaluation of the effect of the proposed change on post-LOCA containment spray and containment sump pH analysis, and on equipment environmental qualification. In each case, to "further constrain the allowable RWST boron concentratior within the range of RWST boron concentrations already considered in the plant design iind licensing bases" ensures acceptable safety analysis results.

In summary, the current accident analyses assume RWST boron concentration is controlled to within the range of 2400 pprn to 2625 ppm. If the minimum required RWST boror concentration is increased to 2500 ppm, such that the range becomes 2500-2625 p3m, the current accident analyses that assume a minimum RWST boron concentratior of 2400 pprn will remain valid and bounding. Because post-LOCA subcriticality s part of the KPS design and licensing basis, it is necessary to increase the Technica Specification minimum RWST boron concentration (Reference 1) in order to credit an increased RWST boron concentration in the post-LOCA subcriticality analysis.

Serial No. 06-497A Attachment 1 Page 6 of 7 Question 3:

Please provide the Post-LOCA Sump Boron Concentration Limit analysis mentioned in Section 4.4 'hat demonstrates that 2500 pprn is a sufficient value for the minimum RWST boron concentration to ensure post-LOCA core subcriticality with the increased core reactivit! expected for Cycle 28.

Response

See response to Question 1.

Question 4:

What is the reason for using 2400 pprn for boron entration in th actor coolant system (described in Section 4.3) ? 1s this value related, in any way, to the 2400 pprn value for the minimum RWST boron concentration? If so, please provide a new post-LOCA Boron Precipitation analysis that uses 2500 pprn for maximum full power critical boron concer rtration in the RCS instead of 2400 ppm.

Response

Question 4 requests clarification of the adequacy of the existing analysis of post-LOCA boron precipitation in light of the proposed increase in minimum RWST boron concentratior. For the boron precipitation analysis, higher RCS and RWST boron concentratior values are conservative. The analysis of post-LOCA boron precipitation conservatively assumes that fluid injected by the ECCS is at the maximum RWST boron concentratior (i.e., 2625 pprn). Furthermore, the analysis assumes an at-power critical boron conce~itration(initial RCS condition prior to the accident) of 2400 ppm, which conservatively bounds the maximum plausible at-power critical boron concentration for reload cores. This value was selected because the minimum RWST boron concentratior conservatively bounds achievable reload values of maximum at-power critical RCS boron concentration.

After the minimum RWST boron concentration is increased from 2400 pprn to 2500 pprn, the value of initial (pre-LOCA) at-power critical RCS boron concentration assumed in the post-LOCA boron precipitation analysis (i.e., 2400 ppm) will continue to conservatively bound reload values of maximum at-power critical boron concentration.

At-power crit cal boron concentrations for KPS reload core designs are typically less than 2100 ppm with no xenon, and less than 1700 pprn with equilibrium xenon. The reload safetp evaluation process will continue to confirm that at-power critical boron concentratior~sfor reload cores are less than the boron concentration assumed in the boron precipitation analysis (i.e., 2400 pprn). On this basis, it is concluded that there is no requirement for a new post-LOCA boron precipitation analysis to be performed using a higher a s s ~ m e dvalue of maximum full power critical boron concentration.

Serial No. 06-497A Attachment 1 Page 7 of 7 Question 5:

In the submit 'a1 (Section 4.5) a statement is made that "the containment spray and post-LOCA contai,?mentsump pH analysis limits continue to be satisfied with an increase in minimum RM'ST boron concentration to 2500 pprn': In this context the word "satisfied" means that the value of sump pH will create an environment which will produce release of optimal amount of elemental iodine from the sump water and the corrosion of metallic surfaces will 5e minimized. The licensee should provide the actual values for pH which allow creatiou of such an environment.

Response

The results of the current KPS analysis for post-LOCA containment sump pH demonstrate that sump pH will remain within the acceptable post-LOCA sump pH range of 7.0 to 9.5, assuming upper and lower bounding values of RWST boron concentration of 2625 pprn and 2400 ppm, respectively. The post-LOCA sump pH range of 7.0 to 9.5 was established to ensure that radioactive iodine forms chemical species that remain in solution and, therefore, do not adversely affect post-LOCA radiological dose analysis results. The acceptable range of post-LOCA sump pH was also established to minimize corrosion of metallic surfaces within containment. The proposed change (i.e.,

increasing the minimum RWST boron concentration to 2500 ppm) will have no adverse impact on the! results of the containment sump pH analysis, since the pH remains within the current cmtainment sump pH analysis range for RWST boron concentration (i.e.,

2500 pprn is ! ~ i t h i the n range of 2400 pprn to 2625 pprn).

As an example, the actual KPS RWST boron concentration could be controlled to a minimum bo~.onconcentration greater than 2400 pprn (e.g., 2550 ppm) in order to logistically si~pportthe minimum required refueling water boron concentration of 2500 pprn specified in the Core Operating Limits Report (COLR). This control would be consistent w th the existing KPS safety analyses, since it constrains the allowable RWST boror concentration within the range of RWST boron concentrations already considered in the plant design and licensing bases. Therefore, with the proposed increase in 3WST boron concentration from 2400 pprn to 2500 ppm, the current containment sump pH analyses will remain valid, and the acceptance criterion of a sump pH between 7.0 and 9.5 will be satisfied.

Reference:

1. Letter from E. S. Grecheck (DEK) to Document Control Desk (NRC), "License Amendmmt Request 226 - Increase in Technical Specification Minimum Refueling Water Stc rage Tank Required Boron Concentration," dated June 28, 2006. (ADAMS Accession No. ML061800307)