ML20198S623

Forwards Open Items Associated w/AP600 SER on AP600 Containment Design & Accident Analyses

ML20198S623
Person / Time
Site:	05200003
Issue date:	01/16/1998
From:	Huffman W NRC (Affiliation Not Assigned)
To:	Liparulo N WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
References
NUDOCS 9801260241
Download: ML20198S623 (8)

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,%G t Mr. Nicholas J. Regulatory Liparulo,Analysis Manager? ..

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SUBJECT:

"OPEN ITEMS ASSOCIATED WITH THE AP600 SAFETY EVALUATION REPORTJ _ %i < i 3i "E\myijd,mfL -(SER)~ON THE AP600 CONTAINMENT DESIGN AND ACCIDENT ANALYSESsH. -r e-m -

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Dear Mr. Liparulo; ,

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The Containment Systems and Sever Accident Branch of the U.S. Nuclear Regulatory : ,

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Commission (NRC) has provided an SER input to the Standardization Project Directorate on the ~ , , , ,

AP600 containment design features and accident analyses, SER Chapter 6.2. The input has ;1l lesues which have already been identified and captured by existing FSER open items. The staff t' tbolieves that labeling these issues as separate FSER open items would result in unnecessary ;  ;

. iduplication of response. effort for Westinghouse.' However, a summary discussion of the staff's

, conoems with the AP600 containment analyses have been provided in the enclosure to this letter for Westinghouse's information to assist in its resolution of the open items in this area.-

if you have any questions regarding this matter, you may contact me at (301) 4151141. ,

Sincerely,

,. original signed by:

William C. Huffman, Froject Manager Standaroization Project Directorate  ;

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Division of Reactor Program Management

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Docket No.52-003 -

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Mr. Nicholas J. Liparulo. - Docket No.52-003 Westinghouse Electric Corporation AP600 c . cc: Mr. B. A. McIntyre Mr. Russ Bell -

Advanced Plant Safety & Licensing Senior Project Manager, Programs -

, Westinghouse Electric Corporation Nuclear Energy Institute .

Energy Systems Business Unit 1776 i Street, NW '

P.O. Box 355 Suite 300

< Pittsburgh, PA 15230 Washington, DC 20006-3706 Ms. Cindy L Haag Ms. Lynn Connor Advanced Plant Safety & Licensing Doc-Search Associates.

Westinghouse Electric Corporation Post Office Box 34 -

Energy Systems Business Unit Cabin John, MD 20818

Box 355 i Pittsburgh, PA 15230 Dr. Craig D. Sawyer, Manager Advanced Reactor Programs Mr. Stsrling Franks - GE Nuclear Energy >

U.S. Department of Energy 175 Curiner Avenue, MC-754 l NE-50 San Jose, CA 95125 19901 Germantown Road Germantown, MD 20874. Mr. Robert H. Buchholz GE Nuclear Energy Mr. Frank A. Ross 175 Curtner Avenue, MC-781 U.S. Department of Energy, NE 42 _ San Jose, CA 95125 Office of LWR Safety and Technology 19901 Germantown Road Barton Z. Cowan, Esq.

. Germantown, MD 20874 Eckert Seamans Cherin & Mellott 600 Grant Street 42nd Floor Mr. Charles Thompson, Nuclear Engineer Pittsburgh, PA 15219 AP600 Certification NE 50 ' Mr. Ed Rodwell, Manager 19901 Germantown Road PWR Design Certification Germantown, MD 20874 Electric Power Research Institute M12 Hillview Avenue Palo Alto, CA 94303 4

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SER CHAPTER 6.2 OPEN ITEMS ASSOCIATED WITH THE AP600 .

CONTAINMENT DESIGN AND ACCIDENT ANALYSES ,

I -- Open items related to the Development and use of the limited PCS water model to ,

pvaluate the performance of the PCS

< The AP600 WGOTHIC evaluation model, as used to support design certification,' does not have the capability to correctly model the variations in expected wetted surface coverage area as the PCS water flow rate is reduced over time. The treatment of the wetted surface area itself does not impact the peak pressure calculation for LOCA analyses. To account .

E for this modeling deficiency, Westinghouse has developed a limited PCS flow model to produce an input boundary condition, or forcing function, for WGOTHIC to describe the expected PCS behavior. However, the application of the limited PCS flow model, which artificially reduced the actual flow rate and therefore enhances evaporating cooling, may be misleading. In response to this concom, as identified in staff RAI 480.873, Westinghouse 4-ignored the specific issue. Further clarifications on the limited PCS flow model have been

, requested.

Issues concoming the limited PCS flow model are outlined in Table A below.

Table A - Evaluation of Westinghouse POS flow and heat transfer models Phenomena Effect Comment Coverage area Westinghouse assumes no These are small conservatisms.

- Height evaporation above second weir. Subcooling is usually eliminated *

- Stripe width Extent of subcooled region may be before second weir.

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overpredicted.

90% flow from cold WDT tests at Constant 90% " wet" area throughout 832.8 liters / min (220 gpm), ,

conservative for 1,665.6 liters / min

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event.

(440 gpm) (first 3 hrs). After 3 hrs, g

effect is not significant if above deck region is "well mixed".

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l' Heat convected Latent heat of PCS runoff is This is a small conservatism for

bywater film neglected. peak pressure period. May be significant for the 1200 second to

! 30 hour3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br /> period when the runoff fraction becomes larger.

i i Resistance of- PCS film thickness may be This non-conservatism may be extemal water - significantly underpredicted, significant after peak pressure film ~ period ( >1200 sec.) when the i Film may reach saturation runoff

• action is larger.

l temperature eartier.

Buoyancy driven Artificialwater coverage profile Skew expected only for post 3 air flow skews evaporation heat flux toward hour period. Effect is small, higher elevations conservative reduction in buoyant -

driving force.

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F 2-Phenomena Effect Comment Convection and Artificial water coverage profile Skew expected only for post 3 -

radiatiu heat - lowers dry clime convection and hour period. Errors may be self transfer from dry radiation from upper elevations; compensating.

clime increases convection and radiation i from lower elevations.

4 Westinghouse failed to identify, or at least account for, the crucial need to consider two-dimensional (2-D) heat transfer for the long-term containment pressure response (after three

' hours when the PCS flow rate is first cut back to about one-half its initial value) in the

. selection of their analysis methodology (GOTHIC) and in their development of a model for

, the PCS 00fGOTHIC). With the coverage area less than the initial assumed 90 percent,

, heat transfer from the hot, dry regions of the shell into the cooler, wet regions of the shell would occur. To account for this deficiency, Westinghouse performs an ancillary calculation 4

to credit more PCS water in the evaporation process, effectively generating a correction factor, and applies it to the limited PCS flow model. This change to the limited PCS flow model was first presented to the staff in May 1997, i

ll Open items related to the use of lumo-carameter model by Westinohouse to evaluate the

AP600 containment response to desian basis accidents (LOCAs and MSLBst These open

! item relates to the "well mixed" assumotion used by Westinohouse to model the above dagh reoions of the AP600 i

j Westinghouse uses the lumped-parameter modeling feature of the WGOTHIC computer program. There tre known limitations associated with this type of model. First, it must be demonstrated that the containment is "well mixed." This must be demonstrated for both LOCA and MSLB events. The lurr. ped-parameter model does not account for momentum

transfer in regions and solves only a simplified momentum equation for the network of L junctions. The calculated non-condensable distribution within a horizontal plain is therefore uniform, or homogenized, if the "well mixed" assumption is proven then it would be reasonable to use the calculated non-condensable distribution to evaluate the mass and heat transfer on the inside condensing surface of the containment shell. If the containment i

cannot be shown to be "well mixed," then a penalty must be applied to account for the

expected distiibution of non-condensables. Westinghouse has not provided sufficient d

information (experimental or analytical) for the staff to conclude that the containment is "well mixed" or tJ assess the possible magnitude or impact of attemative distributions of non-condensah..s.

issues related to the "well mixed" assumption are outlined in Table B.

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Table 8 - WOOTHic modela which tend to homogentre calculations Assumption Effect on "well mixed" or Tendency toward circulapn & stratification homogenization Lumood Parameter Overpredicts convective Artificially increases EM model has network of LP flows. Circulation artifcJally homogenization.

nodes; high.

few nodes below dock (1 or 2 layers). Underpredicts stratifcation.

- many stccked nodes above deck.

Heat Sink Utilization Reduces heat transfer. Artificially increases

- Elimination of horizontal Minimites energy storage in homogenization below deck.

surfaces and operating deck. containment structures and maximizes temperatures in Unknown effect on

- No heat structures in dead- pools. homogenization above deck. ,

ended compartments after {

blowdown (i.e., > 30 secs). Eliminates important heat  !

sinks for circulation.

Reduces stratification.

Maximizes atmospheric ,

energy content / steam i concentration for shell condensation. I Condensate Modelina - Hot condensate is Overpredicts

- 100% Stripping by polar crane instantaneously dumped to homogenization.

rail and intemal stiffeners. the sump, enhancing circulation and minimizing

- Chun and Seban model used stratification.

to predict effective thickness.

Distorts droplet / atmosphere irdaraction.

PCS Flow Modelina Artificially heats up dome Overpredicts

- Delayed initiation (to 337 shell and intemal homogenization by secs), atmosphere. Minimizes minimizing cooldown/ layering stratification at start of PCS of dense air on operating No evaporation above second flow. deck.

weir.

Provides ample time to mix

- Chun and Seban model used with air in dome.

to predict effective thickness.

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Assumption Effect on "well mixed" or Tendency toward l circulation & stratification homogenization i

Modeling of openings between Supports closed natural increases homogenization of above and below deck circulation loop (particularly dome and below d%* ,

compartments. These paths after start of 4th stage regions affectec Dy loop.

are essential to provide - ADS).

4 circulation from above to below l deck. Entrains cold fluid, i

Repositioning of MSLB break to Enhances circulation and overpredicts operating dock level, entrainment in the above homogenization.

! dock regions.

Provides additional access to below deck region. .

1 Removes actual asymmetry.

. Many of the arguments to support the "well mixed

• assumption, as presented by Westinghouse in Section g of WCAP-14407, *WGOTHIC Application to AP600," Revision 1, July 1997, are based on convectior, Jominated entrainment flows in combustible fires. This assessment is not directly applicable to the condensation dominated entrainment expected in the AP600 as documented in the PIRT. Other arguments are based on flow patterns in enclosures which are not characteristic of the expected conditions in the AP600.

4 The staff is unable to determine if a sufficient design margin exists in the AP600 containment 3

to conclude that the requirements of 10 CFR 50, Appendix A, General Design Criteria (GDC) 16,38 and 50 are met. The analysis methodology 00(GOTHIC) used by Westinghouse in support of design certification does not contain appropriate features and modeling

. capabilities to evaluate the "best estimate" performance caeracteristics of the passive containment cooling system (PCS). This approach would require the uncertainties in the 4

methodology be identified, assessed and property treated, instead, Westinghouse has

, _ elected to use a conservative, " bounding" evaluation model (EM) approach. - However, the AP600 WGOTHIC EM may not adequately assess, and may not properly treat, known limitations associated with the use of the lumped-parameter approach and the PCS flow and heat transfer models.

The staff will consider a smaller margin, less then 10%, for the AP600 if it is determined that the licensing analyses that support design certification are conservative and that an appropriate initial test program, in combination with satisfactory initial test and acceptance criteria (certified design material), is developed.

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111 Open item related to the location of the MSLB used for licensina analysis The second limitation of the lumped-parameter model concems break location. Because momentum is not transferred between regioes, an elevated break, as compared to a break at a lower elevation, will result in a higher calculated containment pressure because steam is not readily circulated to regions below the assumed break location and some of the condensing surface is lost in the calculation. This is a well-known limitation of the lumped-parameter network chosen by Westinghouse to perform the licensing analyses.

Westinghouse models the MSLB at the operating deck level, not at its actual elevated location. This is unacceptable. Arguments concoming steam concentrations as observed in the LST to justify the selected location are inadequate. The LST configuration is too different, particulady in the below deck region, and because of the continued outflow through the below deck region in the LST (to provide a means to measure the condensate inside the facility) it is difficult to infer or speculate on behavior in the AP600. The MSLB analysis needs to be performed with the break at its reallocation. The limitations of the selected methodology must be accounted for in the design basis analyses.

IV Open items related to the cuality of documentation provided by Westinahouse to support desian certification includina corrections to supportina technical reports and expected revisions to the SSAR.

The documentation provided to the staff to support the design certification of the AP600 PCS is poorly organized and contains errors, omissions and inconsistencies, in addition, the objectives of the analysis methodology have changed with time, it is difficult to determine what documents and portions of documents represent the current Westinghouse Evaluation Methodology. This has resulted in the expenditure of a significant amount of steff resources on the review of material which has been subsequent ly withdrawn, or significantly revised as a result of the staff's review.

Westinghouse has informed that staff, in letter NSD-NRC-97-5338,"AP600 PCS PIRT/ Scaling Closure Meeting Summary," from R. A. McIntyre, Westinghouse, to T. R. Quay, NRC, dated September 22,1997, that staff ider,tu%t errors in WCAP-14326, Revision 1, WCAP-14812, Revision 2, and WCAP 14845, Rehon 1, will be corrected and revisions submitted, in addition, there is an error in the pressure rate equation developed in the scaling study (WCAP-14845) that must be corrected. The normalization, dimensionless substitutions and the n groups need to De reevaluated after the pressure rate equation is corrected and the PlRT confirmed, or appropriately modified and, if necessary, additional sensitivity studies performed to support the PIRT and the evaluation model, The SSAR (based on Revision 18) needs to be updated to reflect current commitments related to the initial test program (ITP), the initial test and acceptance criteria (ITAAC) or AP600 certified design material, and the technical specifications. SSAR references needs to be revised to properly identify the WGOTHIC computer program and the computer programs used to determine the mass and energy releases for the design basis accidents.

The PIRT, scaling and application reports need to provide a clear and consistent description of the AP600 PCS evaluation model to document the licensing analyses that support design

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certification. The SSAR needs to be updated to determine if the ITP, ITAAC and technical -

specifications are consistent with, and support, those features that are necessary to confirm the licensing analyses that support design certification.

Westinghouse needs to develop an ITAAC requirement based on acceptable indu'stry standards for the liner and concrete fabrication process and for the concrete mixture to assure that the 5 mil air gap is appropriate and conservative for the licensing analyses.

Westinghouse is also considering other means to allow for a larger air gap in the licensing aniyses, an upper bound value, if the ITAAC becomes over burdening on a licensee.

V. Open items related to the containment Testinn prooram To satisfy the requirements of 10 CFR 52.47(b)(2)(1)(A), Westinghouse has developed test programs to investigste the passive containment safety systems. These programs include both component and phenomenological (separate-effects) tests and integral-systems tests.

The Large-Scale Test (LST) is the only integral test for the AP600 PCS. The cold, water distribution test (WDT) is the only full scale representation of the PCS flow characteristics.

Additional separate-effects tests have been performed to extend the range existing mass and heat transfer correlations in the AP600 analysis codes, to comply with the last of the three requirements above.

The shortcomings of the LST include the following major issues:

e- The tests were primarily focused on steady state conditions to assess the mass and heat transfer correlations to be used in WGOTHIC Very limited transient information is available. The LST is usefulin evaluating the applicability of the mass and heat transfer correlations used in WGOTHIC, WGOTHIC analyses of LST tests are usefulin verifying the model changes made to GOTHIC by Westinghouse by demonstrating that these changes have been properly coded.

• The intemal configuration (below deck region and heat sinks) and PCS water delivery systems (J-tubes and unsteady flow rat _es) are significantly different from the AP600. Therefore interpretation of the test results to address expected behavior in the AP600 is uncertain and reliance on LST data alone is generally insufficient to reach closure.

e- - Limited data available to assess mixing issues and non-condensable distributions.-

7 Westinghouse contends that the LST was always "well mixed," but has not been

- able to denionstrate that the AP600 is also "well mixed."

The staff in unable to determine if the testing program is sufficient to understand the i containment response to the PCS or to address the modeling deficiencMs in the WGOTHIC computer program and its application to the AP600. Understanding the distortions in the LST, combined with proper accounting of important phenomena, as identified through an appropriate scaling analysis, in the development of the AP600 evaluation modelis necessary

- to conclude that the evaluation model is acceptable.

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