ML20058M082

Transient Analysis Methodology for Wolf Creek Generating Station Technical Evaluation

ML20058M082
Person / Time
Site:	Wolf Creek
Issue date:	09/20/1993
From:	WOLF CREEK NUCLEAR OPERATING CORP.
To:	Office of Nuclear Reactor Regulation
Shared Package
ML20058M085	List: ML20058M082 ML20058M980 ML20058M985
References
CON-FIN-L-1318 ITS-NRC-93-3, NUDOCS 9309280330
Download: ML20058M082 (13)

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ATTACHMENT l j

ITS/NRC/93-3 l

IECHNICAL EVALUATION: l t

" TRANSIENT ANALYSIS METHODOLOGY FOR THE WOLF CREEK GENERATING STATION"  !

WOLF CREEK NUCLEAR OPERATING CORPORATION j i

1.0 INTRODUCTION

i The topical report entitled " Transient Analysis Methodology for the Wolf '

i Creek Generating Station," dated January 1991 (Ref. 1), describes ' reload transient safety evaluation methodology developed by Wolf Creek Nuclear  ;

Operating Corporation (WCNOC) for the Wolf Creek Generating Station -(WCGS) in  !

order to obtain NRC approval for their use of RETRAN-02 MOD 03, an NRC i reviewed and approved code (Ref. 2). Additional information was provided in i References 3, 4 and 5.  !

i The stated objective of the topical report is for WCNOC to demonstrate its i ability to perform the safety analyses required for licensing, operation, i testing and surveillance of a WCGS cycle using the WCNOC analytical approach  ;

and models based upon the RETRAN-02 MOD 03 computer code.-  !

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In support of future reload safety evaluations, WCNOC redefined the analysis 1 envelope for WCGS and reanalyzed the resulting enveloping Chapter 15 USAR j transients, excluding the rod ejection and LOCA transients. l This review focused upon evaluation of. acceptability of the WCNOC transient }

methodology using the RETRAN code for' reload analysis through determination j of adequacy of qualification of such models against WCGS plant test data and l other NRC approved analyses. The review was not performed to determine  ;

adequacy or accuracy of transient initial conditions, assumptions, trip setpoints and their respective uncertainties. Transient analyses were  ;

reviewed strictly from the perspective ~ of adequacy of the RETRAN WCNOC model .

for such applicatior.s. Although the WCNOC's core ' thermal-hydraulic  ;

methodology utilizing the VIPRE-Cl code is briefly described in Appendix C t (Ref. 1), its review is beyond the scope of this review since a separate i topical report on that subject has been submitted to the NRC for review and j approval. Similarly, acceptability of actual computation of the DNBR for the  ;

limiting transients or the methodology used to determine the thermal design i limit was not reviewed. ,

1 2.0 Summarv l

The topical report describes the WCNOC safety analysis and evaluation j methodology for selected Chapter 15 USAR analyses.

In addition WCNOC stated  !

that the objective of that effort was to demonstrate its capability and l l

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competence to use the RETRAN computer code.

WCNOC developed three WCGS RETRAN models for qualification in support of reload analysis. A "best-estimate" model was a modification of the WCGS generic based model consisting of a two-loop asymmetric model developed for reload transient analyses. Three sets of benchmark calculations against plant test data were performed to validate the WCGS RETRAN model.

A USAR " Comparative" model was developed to demonstrate that WCNOC has the capability and understanding to perform USAR type transients. The Comparative model was created by modifying the base model and input to the code to better match the current USAR analysis results, including incorporation of the Jens-Lottes heat transfer correlation. However, since the vendor's USAR analysis is based upon the Model D steam generator data instead of Model F, WCNOC used the WCGS specific data for input.

The WCNOC " evaluation" model is intended for licensing type analysis. The evaluation model utilizes a combination of built-in RETRAN heat transfer models instead of the Jens-Lottes correlation. WCT further changed some transient assumptions and component modeling to be more " realistic" reflecting performance of the plant rather than those used in the Comparative analyses. The transient assumptions were further modified to reflect WCNOC's current DNBR methodology. Seven transients were analyzed in support of the WCNOC reload methodology.

In both USAR comparative and WCGS evaluation transient analyses, the same set of transients which would be required to be reanalyzed prior to reload were presented. Three of these transients rely upon reactor scram on the low-low steam generator level trip actuation. Because WCNOC's steam generatcr model does not permit computation of the mixture level, WCNOC used the same methodology to correlate the liquid mass to the mixture level in SG used by the vendor in the USAR.

Analysis results were presented and discussed. i 3.0 EVALUATION WCNOC's methodology for performance of Chapter 15-type licensing analysis was reviewed. .

3.1 WCGS RETRAN Plant Models The Wolf Creek Generating Station (WCGS) is a 3411 MWt Westinghouse faur-loop pressurized water reactor. In order to develope a WCGS evaluation model, WCNOC developed a best-estimate mooel for benchmark against three startup tests. Qualification of the USAR Comparative model was 'further conducted against the seven current USAR transients through comparison with vendor's ,

analyses. These are the same transients which WCNOC considers as bounding and representative of " enveloping" transients which would be re-analyzed in support of reload safety evaluation. Finally, the RETRAN WCGS evaluation models are developed to be used for licensing-type analysis.

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r These three RETRAN WCGS plant models are described below.

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3.1.1 RETRAN Best-Estimate Model A "best-estimate" model was a modification of the WCGS generic based model described in Appendix A of the topical report and consists of a two-loop asymmetric model developed for reload transient analyses. Some key features ,

at a described below. {

Plant Nodalization A two-loop asymmetric plant model was used for the best-estimate benchmark >

calculations.

Steam Generators the WCGS RETRAN model contains Model F steam generator information. Modeled after the Westinghouse SG nodalization scheme for the current USAR analysis, WCNOC uses a four-volume primary and a single volume secondary steam '

generator.

Pressurizer The pressurizer (PZR) model is a single node non-equilibrium volume including the PZR spray and the relief and safety valves, but not the heaters.

Reactor Kinetics Model A point kinetics model was used to determine the core power response together with specific reactivity forcing functions and thermal feedback effects from the moderator and fuel temperature coefficients in the three core regions, where a choppad cosine power distribution profile is assumed. In the best-  ;

estimate calculations, WCNOC used the reactivity coefficients used in the USAR.

Reactor Trio loaic and Control System Models RETRAN trip functions are utilized to simulate trip logic required for '

transient analysis such as the following: reactor protection functions,.

control system bistable element logic, and general problem control.

Four main control systems (SG level, PZR pressure, PZR level and rod moveme ~) are simulated in the RETRAN base model.

3.1.2 RETRAN USAR Comparative Mpdgl In order to gain further proficiency . in, and an understanding of the +

mechanics of, the analytical processes in performance of USAR-type analyses and the computer codes used in such analyses, WCNOC developed RETRAN comparative models. It was WCNOC's belief that the comparative analyses i using input and boundary conditions similar or identical to those used in the USAR analyses would assist in identi fying and interpreting causes of ,

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differences between two approaches. In addition, WCNOC intended that these g analyses would validate the RETRAN models since the current USAR analyses were performed using approved methods.

WCNOC incorporated many of the assumptions of the USAR models (as described in Section B.O of Appendix B of Reference 1) to make the comparison meaningful. Features shared by the Comparative model to match the vendor's models and assumptions include key initial conditions, reactivity characteristics, the use of the Jens-Lottes heat transfer correlation, transient specific setpoints, time delays and trip parameters and control .

system model i r.g . However, in some instances, matching modeling was not possible due te differences in analytical thermal-hydraulic models built into the computer codes used, more accurate component representation, and setpoint value differences, all of which would result in computed differences. "

Certain selected differences are discussed in detail below.

The vendor's reactivity feedback modeling was modified for the Comparative analysis. Doppler reactivity feedback is modeled differently in RETRAN than in the vendor's LOFTRAN code. RETRAN uses the average fuel temperature computed as a volume weighted average of the internal nodal temperature within the fuel pellet. The LOFTRAN code does ne. contain a fuel rod conduction model and uses a power defect coefficient for Doppler. For the comparative analyses, the Doppler feedback used in the USAR was converted to a form consistent with RETRAN.

A delay time assumed in the OTDT trip by RETRAN is different from that used in the vendor's analysis and is consistent with the current practice: this is being reviewed under a separate topical report. A decay heat multiplier of 1.2 was chosen for analysis of loss of normal feedwater transient instead of 1.0 used by the vendor in the USAR analysis.

For most of the analyses, WCNOC used an asymmetric 2-loop base model where one loop represents a single loop and the second loop represents the other l three loops. A variation of this is a two-loop symmetric model used for the l loss of normal feedwater analysis in which each loop in the model represents '

two loops.

A non-equilibrium pressurizer model was incorporated into the RETRAN model instead of the two-region variable control volume model used by the vendor. .

, The safety valve modeling is different between the USAR and RETRAN analyses.

The valve actuation for the RETRAN models is based on steam generator pressure and does not necessarily result in the steam generators being maintained at saturation conditions as was done in the vendor's calculations as stated by WCNOC.

Steam Generator Model The Model F steam generator was modeled for WCGS by the licensee in the Comparative model, instead of the Model D steam generator used by the vendor, in the order to more accurately represent the component. Modeling of the ,

steamline in the RETRAN plant model included a common header, permitting i 4

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communication between steam generators until the MSIVs closed.

A single bubble rise volume for the entire SG vessel shell side was used in all transients presented. However, for the three transients where tracking of the mixture level was important, the local conditions heat transfer model was utilized to simulate tube uncovery and its associated effect on heat transfer. The use of the local conditions heat transfer model precludes use of more nodes, including a separator model, in the shell side. Therefore, it was not used in the multi-node SG model. Lack of a separator (and thus lack of modeling of recirculation flow) in the WCNOC SG model was justified by stating that once the SGs are isolated in the single-volume mode, the mass flux past the SGs approaches zero. Furthermore, because the peak SG pressure is limited by SG safety valve actuation in three undercooling transients analyzed by WCNOC, WCNOC stated that the use of a nonequilibrium separator would not produce a higher SG pressure.

WCNOC performed a limited scope of sensitivity analyses to show that the single-volume secondary SG with the local heat transfer option yields conservative results for selected transients when compared with a multi-node homogeneous steam generator model. While the impact of use of a single-node steam generator model on the primary side pressure was slightly more conservative in that higher peak pressure and/or faster pressurization was predicted in one SG nodalization study with the feedwater line break event, the steam generator pressure was predicted to be lower. Therefore it is recommended that for transients where prediction of higher SG pressure is important, WCNOC be required to perform sensitivity studies to ensure that the selection of a particular SG model results in conservative prediction.

SG Mixture level Calculation Alaorithm LOFTRAN tracks water volume and RETRAN tracks mixture level. According to the licensee, heat transfer is not degraded by LOFTRAN until the tubes are completely uncovered while RETRAN computes heat transfer coefficients according to the local conditions. For those transients for which accurate computation of the mixture level is important, a comparison is made between the liquid volume and the minimum liquid volume required to cover the tubes.

Based upon Westinghouse approach used in the USAR analysis, a liquid volume is defined corresponding to the level expected in the downcomer at the low- l low level SG level trip setpoint in the Comparative analyses. In the three - '

transients for which the low-low SG level trip is modeled, the initial SG masses were adjusted in a conservative direction.

Boron Transport Model In response to the RETRAN SER requiring each user to develop and qualify a boron transport model, WCNOC (in Reference 3) described and qualified its boron transport model to be used in analysis of the main steamline break transient. Various time delays and lags are built into WCNOC's model which is based upon the volumetric flow though the local component of the RCS thereby taking into account the transient behavior of the flow and fluid density.

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The response of the RETRAN boron transpcrt model was compared to that in the USAR steam line break analysis. In both cases analyzed, with and without offsite power, the boron reached the core much earlier in the USAR analysis than in the RETRAN comparative and evaluation analyses. Therefore, WCNOC's boron transport model is conservative and acceptable.

3.1.3 RETRAN Evaluation Model The RETRAN USAR " evaluation" model was developed for use in licensing-type analysis.

WCNOC presented its approaches to licensing-type analysis in Section 2.0 of the topical report.

RCS Modeling 4% SG tube plugging was assumed in the six transients resulting in RCS heatup, while no plugging was assumed in the cooldown event.

The thermal design flow of 372,800 gpm used in the enveloping transient analysis is less than the value of 382,800 gpm used in the USAR analyses.

Safety Valve Modelina SG safety valve closing setpoints were changed to accommodate 14% blowdown.

In addition, these valves were allowed to cycle, reflecting, for example, the five banks of SG SVs permitted to open at one pressure and close at a pressure lower than the opening value.

Steam Generators Model WCNOC used the RETRAN built-in heat transfer package instead of the Jens-Lottes heat transfer correlation which was used by the vendor in the USAR analyses.

Auxiliary Feedwater WCNOC changed the modeling of the AFV injection actuation delay time as well as the assumed flowrate. WCNOC's times represent the time to start the pumps .

and purge the pipe volume between the main feedwater isolation valve and the SGs according to the plant configuration.

The AFW flow was also changed in certain transients. These differences are-highlighted in later discussions.

Reactivity Feedbath ,

WCNOC intends to recompute reactivity coefficients with each cycle to er.3ure that the values are bounded by the values used in the evaluation analyses. -

The Doppler coefficient, as in the RETRAN Comparative analyses, is given in terms of a tsmperature coefficient rather than a power coefficient as used by the vendor.

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- 3.2 Comparison with Startuo Tests WCNOC attempted to demonstrate its ability to use the code through test data comparison analyses rather than to qualify individual models against the startup tests. In these analyses, WCNOC attempted to capture the global trends rather than to qualify individual models.

Three startup tests were selected for this purpose: (1) the complete loss of RCS flow; (2) large load reduction and (3) turbine trip without steam dump.

Two changes were made to the base model for the test comparison analysis:

the turbine and steam condensate volumes were modeled and the recirculation path in the steam generator secondary side was also simulated.

The complete loss of RCS flow at hot zero power analysis was revised using better test data which became available after the initial analysis was performed. The predicted coastdown curve agreed well with the measured data.

This was an expected result since RETRAN pump coastdown predictions generally

- match well with plant data.

The large load reduction test was reanalyzed in Reference 3 by specifying actual steam dump demand values as boundary conditions. This revision resulted in only slightly better agreement between the test data and the predicted parameter values. Generally, peaks in SG header pressure, RCS cold leg temperatures and PZR level were not captured well; however, the global trends agreed better.

Analysis of the turbine trip without steam dump was also revised in Reference '

3 by initially matching test data for the steam flow, main feedwater flow and adjusting RCS flow to yield the desired RC delta T. Although after these adjustments initial conditions matched closely, the computed behavior in key system parameters did not result in better agreement.

3.3 Transient Analyses Comparison between the vendor's and WCNOC's results for the USAR analyses is discussed below. Since evaluation calculations were not compared to other transients, only the significant differences are highlighted in the following discussion. .

3.3.1 Uncontrolled RCCA Bank Withdrawal at Power Two reactivity insertion cases were analyzed for the purpose of comparing Vendor's USAR analyses with WCNOC's RETRAN analyses. The RETRAN computed key parameters agreed well with the vendor's USAR results for the maximum positive reactivity insertion case. Differences between two sets of calculations in the latter half of the transient for the minimum positive reactivity insertion case are attributed to the difference in assumed delay times in the OTDT trips and modeling of PZR sprays. This indicates that the RETRAN PZR spray modeling in the WCNOC model is more effective in pressure control than the model in the current USAR analysis.

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Two reactivity insertion cases were also analyzed for evaluation analyses.

The RETRAN computed key parameters were comparable to those computed in the comparative analyses although slightly more conservative results were

• predicted due to the selection of initial conditions. The reactor trip occurred about 6 seconds earlier in the evaluation analysis of the minimum reactivity case.

3.3.2 Comolete loss of Reactor Coolant Flow As expected, the RETRAN computed RCS flow coastdown curve matched well with the USAR data. The current USAR core power was slightly higher than the RETRAN prediction. This was attributed to the difference in Doppler interpolation scheme utilized by two codes. Because of the slightly higher power computed in the USAR analysis. WCNOC stated that the PZR pressure peaked earlier, and due to a difference in SG heat transfer the PZR pressure remained higher in the RETRAN analysis after reactor scram. This occurred after the expected time of MDNBR.

Comparable predictions were obtained in the evaluation analysis.

3.3.3 Locked Rotor Two cases were analyzed: with and without offsite power. All four RCPs were assumed to be initially in operation due to a Technical Specification requirement.

RCS pressure responses were different between the two sets of data: RETRAN predicted pressure was consistently lower than that of the USAR. WCNOC attributed the differences to (1) a different treatment of the Doppler reactivity table, (2) greater cooldown capability of the RETRAN SG safety valve modeling, and (3) different primary-to-secondary heat transfer characteristics.

A sensitivity study was performed to determine the cause of the oscillatory behavior in the primary system pressure exhibited in the USAR comparative analysis. It was attributed to the inertial effect of the fluid in the pressurizer surge line. When a low value of junction inertia was input, the peak pressure became smother and the peak slightly higher.

For conservatism in the evaluation analysis, WCNOC assumed an additional 0.5 ~

second delay before scram after receipt of low flow trip signal. Comparable results were predicted for same two cases in the evaluation analyses. '

3.3.4 Loss of Load / Turbine Trio WCNOC analyzed four parametric cases with respect to pressure control operation and reactivity feedback.

Although in all cases the PZR pressure peaked slightly higher in the USAR cases than it did in RETRAN analysis, agreement between the RETRAN and USAR analyses was adequate. ,

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Similar results were obtained in the evaluation analyses. Minor differences were due to the differences in transient assumptions between the evaluation '

and USAR analyses discussed earlier.

3.3.5 Loss of Normal Feedwater The transient analysis objective was to demonstrate the capability of the AFW system to provide long-term heat removal and to prevent overpressurization of the RCS or PZR water solid condition. To that end, WCNOC assumed that the PZR PORV and spray were operable, and, in addition, selected a high PZR initial liquid level.

WCNOC stated that a difference in the PZR spray model between two USAR '

analyses contributed to the difference in PZR pressure change from 10 to 40 seconds into the transient when reactor scram occurred. Between roughly 50 to 1000 seconds, the primary system was predicted in the Comparative analysis to be hotter than that in the USAR analysis indicating less effective heat removal by the steam generators.

AFW flow was actuated upon a low-low SG water level signal. Only one motor driven AFW pump was assumed available and no credit was taken for a turbine driven AFW pump. In the USAR comparative analysis, 500 gpm flow was assumed. .

A 60 second delay was assumed after receipt of trip signal. In the evaluation model, 1000 gpm flow was assumed. WCNOC stated that this ',

corresponded to a minimum flow rate from one of two motor driven AFW pumps plus credit for the turbine driven AFW pump. WCNOC's position is that the assumption of one motor driven pump failure would satisfy the single failure criterion. A 295.0 second delay was assumed after trip signal receipt.

This, according to WCNOC, represented the time to purge the feed line volume.

WCNOC stated that this was conservative since it did not take credit for the warm water in the pipe.

In this transient, the actuation of the low-low SG level was computed to be earlier by WCNOC than predicted by the vendor for both the USAR and evaluation cases. A sensitivity study was perfcnned to investigate the impact of early trip actuation by forcing the trip to occur at the same time as it did in the vendor's USAR analysis. As expected, because it permitte' more heat to be generated, the timing of the PZR pressure peak was earl' a but was limited by the PORVs and earlier repressurization was also predici d. -

However, the overall impact on the transient consequences was minor.

As demonstrated in a sensitivity study, the use of the RETRAN heat transfer package in the evaluation model rather than the Jens-Lottes heat transfer correlation resulted in a significantly different prediction in the long-term. In the near-term, there was little difference.

The major difference between the evaluation and USAR analyses is that in the evaluation analysis two out of three pumps were assumed available to deliver AFW flow resulting in twice the flow rate which was more than sufficient to remove the decay heat. Therefore, SG inventory was restored enough to '

prevent the heat-up of the primary side and this event became benign. In the USAR analysis, with half AFW flow, it took several thousand seconds before 9

a 4 the heat removal capability equilibrated with the decay heat.

3.3.6 Feedwater line Break In addition to the differences mentioned earlier: (1) In the USAR analysis, AFW delay of 432.0 seconds' was assumed to be consistent with the vendor's analysis while in the evaluation analysis, it was 374 seconds; and (2) heat transfer to the faulted SG was not permitted in the USAR analysis while it was permitted in the evaluation analysis.

Two cases, with and without offsite power, were analyzed. Because of the steam generator type, this is considered a heat-up transient for WCGS. A '

difference in the SG models, and therefore the height of the FW connection in the SG, caused a significant difference in the amount of the mass inventory left in the SGs and in the heat-up characteristics. In both RETRAN cases, more energy was removed by the discharge causing more depressurization of.the primary side and a delayed heat-up, while in the USAR analyses the depressurization was truncated by a heat-up caused by the dried SG.

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The reactor tripped on actuation of the low-low SG level setpoint. Both RETRAN USAR comparative and evaluation cases predicted later actuation than was predicted in the vendor's analysis.

The differences in the transient assumptions resulted in significant differences in the predicted plant behavior, especially in the primary pressure responses. Early MSIV closure in the evaluation model reduced the RCS heatup computed in the USAR analysis. In addition, the heat transfer through the faulted SG helped cool the primary longer in the evaluation analysis.

In the SG nodalization sensitivity study, the SG heat transfer degradation was computed to occur earlier in the single-node SG model than in the multi-node case. The long-term responses differed significantly due to the different quality computed for the break flow. Since the volume mixture level was below the feedwater line elevation at the time of break, the <

initial break flow was vapor in the single-node SG model case while two-phase break flow was predicted for the first 14 seconds using the multi-node homogeneous SG model.

3.3.7 Main Steamline Break A split-core nodalization replaced the core model in the two-loop base model to better model the asymmetric cooldown resulting from the steamline break.

The WCGS RETRAN core was split into two flow channels. Cross-flow between channels within the core was not allowed. A coolant (enthalpy) mixing model  ;

was implemented based upon the Westinghouse model used in the current USAR analysis: this model consisted of removing energy from the non-faulted plenum volume and adding it to the corresponding faulted plenum. This approach was used in both USAR comparative and evaluation analyses.

Cases with and without offsite power at hot zero power were analyzed. Higher return to power was predicted with RETRAN than in the vendor's USAR analysis .

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s for the case with offsite power available even though the core average temperature was computed to be higher than the USAR analysis. This indicates that there was more moderator reactivity feedback modeled in the RETRAN model. Other parameters agreed favorably.

Agreement was mixed for the case without offsite power available. Initially there was higher core power increase (at C; seconds); however, the rate of increase decreased shortly afterwards and the peak core power was not reached until 155 seconds while in the USAR analysis the peak core power was reached by 100 seconds.

Similar sets of predictions were obtained in the evaluation analysis for the same cases.

4.0 SONCLUSIONS AND RECOMMENDATIONS Based upon the submitted materials through analysis of licensing-type transient behavior, WCNOC's transient analysis methodology using the RETRAN computer code for the Wolf Creek Generating Station is generally acceptable subject to the following:

1. At the time of reload, selection criteria for the initial conditions, transient assumptions and trip actuation logic and related parameters should be reviewed to assure that predictions are acceptably conservative. Furthermore, if any of the above is changed from that in the current USAR, the licensee must demonstrate that such changet do not result in less conservative predictions and are based upon sound analytical foundation.

2. The WCNOC DNBR methodology is an integral part of transient methodology, since it affects the selection of the initial conditions and assumptions for certain transient analyses.

Therefore, at the time the first set of reload analyses is submitted by WCNOC, it is recommended that RETRAN transient data be reviewed for consistency with the range of applicability of the DNBR methodology.

3. Since there are a significant number of changes and revisions in the submitted materials, it is recommended that the subject topical .

report be revised to reflect those changes and new materials submitted by WCNOC during this review.

5.0 REFERENCES

1. " Transient Analysis Methodology for the Wolf Creek Generating Station,"

Wolf Creek Nuclear Operating Corporation, January 1991.

2. "RETRAN-02-A Program for Transient Thermal-Hydraulic Analysis of Complex Fluid Flow Systems, EPRI NP-1850-CCM Revision 2, EPRI, November 1984.

3. Letter from F.T. Rhodes (WCNOC) to USNRC, " Response to Request for 11

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Additional Information on the Transient Analysis Methodology for the Wolf Creek Generating Station," July 16, 1992.

4. Letter from B.D. Withers (WCNOC) to USNRC, " Response to Request for Additional Information Regarding Transient Analysis Methodology Topical Report for the Wolf Creek Generating Station," January 15, 1993.

5. Letter from B.D. Withers (WCNOC) to USNRC, " Response to Request for Additional Information Regarding Transient Analysis Methcdology Topical Report for the Wolf Creek Generating Station," February 25, 1993.

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