NG-99-0928, Forwards Response to NRC 980902 Comments Re Design Basis NPSH Analyses as Requested by GL 97-04

Forwards Response to NRC 980902 Comments Re Design Basis NPSH Analyses as Requested by GL 97-04

ML20209B727
Person / Time
Site:	Duane Arnold
Issue date:	07/01/1999
From:	Peveler K IES UTILITIES INC., (FORMERLY IOWA ELECTRIC LIGHT
To:	NRC (Affiliation Not Assigned), NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
GL-97-04, GL-97-4, NG-99-0928, NG-99-928, NUDOCS 9907070417
Download: ML20209B727 (5)

Text

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• ALLI ANT l UTILITIES ins uone. inc.

. IES Utilities OIEINEEN"i Palo. I A 52324-9785 Office: 319.H51.7011 lax: 319.85 t.?JM6 www.alliant enngy.com July 1,1999 NG-99-0928 Office of Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission Attn: Document Control Desk l

Mail Station 0-Pl-17  !

Washington, DC 20555-0001 l

Subject:

Durae Arnold Energy Center (DAEC)

Docket No: 50-331 Op. License No: DPR-49 NRC Generic Letter (GL) 97-04, " Assurance Of 1 Sufficient Net Positive Suction Head for Emergency  ;

Core Cooling and Containment IIeat Removal Pumps,"

dated October 7,1997

References:

1. NG-97-2237, letter from J. Franz (IES) to NRC, dated January 5,1998, Response to NRC GL 97-04

2. NRC Request for AdditionalInformation Pertaining to GL 97-04, dated May 14,1998  ;

3. NG-98-1053, letter from K. Peveler to NRC, dated l June 12,1998, Response to Request for Additional Information Pertaining to NRC GL 97-04

4. Letter dated September 2,1998, to L. Liu (IES) from '

R. L.aufer (NRC), Discussion of Response to Request for Additional Information on Generic Letter 97-04 File: A-101b, A-105 By letter dated January 5,1998 (Reference 1), IES Utilities provided information regarding design basis net positive suction head (NPSil) analyses as requested by the subject generic letter. Reference 2 provided a request for additional information (RAl) from th: Staff, which was answered by letter dated June 12,1998 (Reference 3). By letter dated September 2,1998 l

(Reference 4), the NRC provided additional comments.

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g 9907070417 990701 PDR ADOCK 05000331

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July 1,1999

' NG-99-0928 Page 2 Several issues are identified in the Staff's September 2,1998 letter.

• The original IES response includes NPSil data for a system line-up that includes two residual heat removal (RIIR) pumps operating in the long-tenn case. This system line-up is different than the limiting case identified in the Updated Final Safety Analysis Report (UFSAR).

The Staff believes that the containment pressure margins discussed in the original SE and Supplement 1 are no longer applicable because of changes to the NPSil calculations and a new methodology was used in the containment pressure and temperature analysis. It appears that the DAEC's resolution of Bulletin 96-03 will result in increased reliance on containment pressure.

• A question exists over the acceptability of the DAEC's use of the General Electric (GE) analytical code SIIEX-04V for the NPSH containment analysis.

Additional information regarding these issues is provided in the attachment. Some of the issues concern the NPSil analysis conducted to support the modifications performed for Bulletin 96-03.

Those modifications involved replacing the R11R and core spray (CS) suction strainers with new {

GE stacked disc strainers. IES Utilities understands that the Staff has significant concerns over the use of additional containment pressure in NPSil analysis. Every effort to assure minimal pressure is required has been made. The final results of this analysis will be documented in accordance with the response to Bulletin 96-03. ]

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In summary, a new limiting configuration has not been created in the conduct of the revised NPSH analysis. No increase in reliance on overpressure was credited. The code S11EX-04V was used and benchmarked against the original results of the containment analysis at the DAEC.

Should you have any questions regarding this matter, please contact this office. .

Sincerely, w2f' 4 Kenneth E. Peveler Manager, Regulatory Performance 1

! Attachment l cc: C. Rushworth E. Protsch J. Franz D. Wilson B. Mozafari (NRC-NRR)

J. Dyer (Region 111)

NRC Resident Office DOCU

Attachment to NG-99-0928 Page1 of3 Clarification of"' 97-04 Issues IOIR System Configuration for Long Term NPSH Analysis In the letter of September 2,1998, the Staff notes that the scenario with two RHR pumps at runout or rated flow is different than the scenario evaluated in the original NRC Safety Evaluation (SE) of the DAEC (one CS pump and one RHR pump running continuously).

At the time of the DAEC's original response to GL 97-04, an Emergency Core Cooling System (ECCS) suction strainer modification was being planned, in support of the resolution of Bulletin 96-03. The January 5,1998 response to GL 97-04 provided the system configurations and the NPSH results from the analysis of record prior to changes to the strainer headioss resulting from these modifications. This analysis revision was the result of a detailed review of the existing NPSH calculations performed as preliminary work for the ECCS suction strainer modifications. >

Various corrections were made to assure that the analysis represented the existing plant configuration.

In the representation of the RHR system in Table 2 of the original GL response, the NPSH  ;

reflected is for 2 RHR pumps through one penetration. This was evaluated because of the )

resulting higher flow rates to show that the required NPSH was available in all operating conditions for the DAEC. However, the limiting NPSH design and licensing bases are for long-  ;

term cooling with only one RHR pump and one CS pump operable when evaluating the  !

capability of the low pressure coolant injection (LPCI) and CS systems to rellood and maintain core cooling following a loss-of-coolant-accident (LOCA). The DAEC has no intention of l changing this limiting licensing basis for core cooling and reflood. Therefore, the calculations of record showing that NPSH requirements are maintained for the existing system with two RHR pumps operating is an additional, non-limiting evaluation of the system NPSil capabilities. The representation of the NPSH requirements in the UFSAR will continue to show single pump operation of the RHR system for long-term cooling.

An additional point is the time frame for operator action in association with the reduction in flow for the CS and RHR systems. In the preliminary evaluations it was determined to be conservative to evaluate the performance of the system with respect to a delay of 30 minutes prior to operator action to reduce flow rates for the pumps. Although this time was used in the preliminary strainer calculations, there is no intent to change the existing licensing basis of operator action occurring after a ten minute delay. The final ECCS strainer calculations with respect to the NRC Bulletin 96-03 strainer installations will use the ten-minute delay to assure consistency with the design and licensing basis.

Credit for Containment Overpressure In the September 2,1998 letter, the Staff states that it appears that the DAEC will be outside its licensing basis with respect to credit for containment overpressure when the proposed resolution to Bulletin 96-03 is implemented.

Attachment to NG-99-0928 Page 2 of 3 This misconception may have arisen from the preliminary infonnation provided to the Staff at a presentation in February of 1998 regarding the DAEC's resolution of Bulletin 96 '03. In the DAEC's original response to GL 97-04, no data from the evaluations perfomied in association with NRC Bulletin 96-03 are included. GL 97-04 requested an evaluation of the existing (pre-Bulletin 96-03) analysis to assure that it was in accordance with the licensing basis.

Containment overpressure is credited in the original licensing basis for the DAEC. Section 6.3 of the original NRC Safety Evaluation of the DAEC dated January 23,1973, states that "The most limiting case occurs during the long term transient following the design basis LOCA when one core spray and one RHR pump will be running continuously. In this operating condition, the NPSH requirement for the spray pump is the limiting parameter. The analysis shows that a containment pressure margin of about 1.5 psi will be available throughout the long-term post-LOCA period to assure adequate NPSH for the core spray pumps for the above cited conditions.

Although the design does not fully meet the provisions of the safety guide [ Safety Guide No.1],

we have concluded that the applicant's analysis is conservative and that there should be adequate NPSH to the ECCS pumps,in the unlikely event of a LOCA." Supplement I to the SE (dated March 2,1973) revised the available containment pressure margin from 1.5 psi to 2.7 psi. This is depicted on UFSAR Figure 5.4-15," Containment Pressure Margin for Adequate Core Spray and RHR Pump NPSH." j l

in the original NPSH analysis that was reviewed by the NRC, via the SE and Supplement 1, the i CS system was evaluated to have adequate NPSH margin if the containment pressure was I maintained above a peak value of 17.8 psia at the peak torus temperature. The revised (pre-Bulletin 96-03 modification) analysis shows that the new peak pressure that must be maintained for CS is 16.5 psia at peak torus temperature. The RHR system was originally evaluated requiring 16.7 psia at the peak torus temperature with only one pump operating. The revised (pre-Bulletin 96-03 modification) analysis shows that with two pumps operating the peak pressure required is less than 15.2 psia. The results of this comparison show that by removing the various errors and as-built discrepancies from the original analysis the margins of safety originally reviewed and accepted by the NRC are increased. This evaluation is based on a comparison of the absolute pressures required for the NPSH (determined in CAL-M97-007, revision 1) and the containment pressure available identified in UFSAR Figure 5.4-15 (prior to any revisions for NRC Bulletin 96-03).

It can be seen from the above comparison that no additional reliance on containment pressure or e, ductions in the margin required for the pie-Bulletin 96-03 configuration as specified in the DAEC UFSAR was needed after the NPSH analy, sis was revised to remove any discrepancies from the initial plant analysis.

Containment Analysis New containment pressure and temperature response analyses were performed by GE in support of the new strainer design. The September 2,1998 letter indicates the Staff believes that the new containment analysis did not use the smne methodology as reviewed and approved during the original licensing of the DAEC and that this makes the containment pressure margins discussed in the original'NRC SE and Supplement I no longer applicable.

1 Attachment to

, , NG-99-0928 Page 3 of 3 While the containment analysis and the revised NPSH analysis including the new GE strainers are not part of the evaluations performed in accordance with GL 97-04, the following information provides additional clarification on the issues identified in the Staff's September 2,1998 letter.

This information demonstrates that the basic methodology used in the new containment analysis has not changed from that already accepted by the NRC.

The intent of the containment analysis, with respect to NPSH analysis, was to provide the most limiting suppression chamber pressure / suppression pool temperature conditions to determine if adequate NPSH margins are available for the ECCS pumps at the DAEC in the event of a LOCA.

Both short term and long term analyses were performed. The short-term analysis evaluated the most limiting conditions for the first 10 minutes following the LOCA before any operator actions are initiated (double-ended recirculation suction line break with all ECCS pumps running for a duration of 10 minutes). The long-term analysis provided the limiting conditions up to and past the time of peak suppression pool temperature several hours into the event. The long term analysis assumptions were intended to provide the most limiting suppression chamber airspace pressure and suppression pool temperature data for the NPSH evaluation.

The GE computer code SHEX-04V was used to perform the short-term and long-term containment analyses for the NPSH evaluation. The SHEX-04V code models the reactor vessel fluid as a single themiodynamic fluid node consisting of steam and water at saturated conditions. The conditions of the vessel fluid node are determined by mass and energy balances. The break flow used in the NPSH analysis is calculated internally by the SHEX-04V code based on the vessel fluid conditions and the break area. The break flow calculations during the initial phase of the blowdown are based on Moody's Homogeneous Equilibrium critical flow model. As the reactor vessel depressurizes and the break flow becomes subcritical, the break flow is then calculated by the Bernoulli's equation for single-phase flow.

GE perforned the new containment analyses for the DAEC using a plant-unique decay heat curve generated using the ANS 5.1-1979 decay heat model with 2 standard deviations (2c) of uncertainty in order to assure conservative results for suppression pool water ternperatures.

An analysis was performed to benchmark the SHEX-04V code for application to the DAEC l containment analysis. The purpose of the benchmark analysis was to demonstrate that the SHEX- )

04V code could conservatively duplicate the results of the original UFSAR analysis. For the benchmark analysis, the decay heat curve was based on the May-Witt model since that model 'was commonly used in the original UFSAR analyses. The calculated peak suppression pool temperature for the benchmark analysis was 197.0 F which matches the 197 F reported in the UFSAR. Thus, 1' the benchmark analysis demonstrates that the SHEX-04V code reproduces the original UFSAR results for the DAEC.

In addition, the use of the SHEX-04 code has previously been approved for use by the DAEC as part of the Station Blackout analysis. An NRC review of the applicability of the GE analysis using SHEX-04 for the DAEC was provided in the Station Blackout Rule Conformance Evaluation dated November 22,1991.

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