ML20209H924

Topical Rept Evaluation Accepting WCAP-8575(P) & Suppls 1-3, Augmented Startup & Cycle 1 Physics Program, for Referencing in Licensing Submittals Re Verification of Design Methods Used to Calculate Transient Peaking Factors

ML20209H924
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Issue date:	10/30/1985
From:	Office of Nuclear Reactor Regulation
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ENCLOSURE SAFETY EVALUATION OF A WESTINGHOUSE LICENSING TOPICAL REPORT REPORT NO.: WCAP-8575(P) and Supplements 1, 2 and 3 REPORT TITLE: Augmented Startup and Cycle 1 Physics Program REPORT DATE: August 1975; Supplement 1. June 1976; Supplement 2, August 1977; and Supplement 3 June 1981 REPORT CLASSIFICATION: Proprietary -

.g. ORIGINATING ORGANIZATION:- Westinghouse Electric Corporation REVIEWED BY: Core Performance Branch

SUMMARY

OF REPORT Westinghouse Electric Corporation (W) developed the augmented startup test program described in the licensing topical report and its three supplements to satisfy NRC and ACRS concerns related to transient peaking factor mearurement and design methods verification for Festinghouse PWRs. The ACRS, for example, in its Trojan letter of November 20, 1974 (Ref. 1) stated its concerns as follows: "Because

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there is limited operating experience with very large, high power density reactors.

' the ACRS believes that a more cautious than normal approach to full power is prudent, with longer periods of operation at power levels in the range of 70 to 90% of full power, and with additional monitoring of core and system performance' throughout the life of the first core. The Comittee recomends that the Regulatory Staff evaluate the overall operating experience prior to sustained operation at full power." The staff concern was that Westinghouse had not. verified its design methods with transient peaking factor measurements.

This test program was made possible by a Westinghouse development in core mapping with the movable detector system. Core mapping during transient conditions (e.g., load following) is not usually done in Westinghouse reactors because a complete incore map takes about two hours during which time the xenon distribution 8511110299 851030 PDR TOPRP EPfVWEST C PDR rp c . . - _ - . . . - . - _ - . ._-

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- l changes sufficiently to affect the power distribution casurement. The development. 1 quarter core flux mapping, provides for scanning selected incore thimbles such that, upon reflection into a quarter segment of the core, approximately the same radial density of measurements as in a full incore map is achieved. A quarter core flux map takes less than 20 minutes. This time interval is short enough to minimize transient xenon effects on the power distribution. The quarter core flux mapping technique was compared to the full incore mapping technique in terms of reaking factors and axial offset (i.e., the axial offset is equal to the power in the top half of the core minus the power in the bottom half of the core and divided by the total power). The agreement in the results obtained for the two flux mapping techniques is very good. The staff has reviewed and approved a licensing topical report (Ref. 2) on quarter core flux mapping (approval g= letter dated November 11,.1977).

4 Three basic types of tests were perfomed as part of the augmented startup test program. These tests all followed the procedures of the Westinghouse power distribution control strategy, that is, the procedures of constant ax'ial offset control (CAOC). The Westinghouse licensing topical report on CAOC (Ref. 3) has been reviewed and approved by the staff (approval letter dated January 31,1978).

These tests are:

1. Step Load Swings From -10% to +10% ,

.. Step load swings of -10 percent followed by +10 percent are routinely l- perfomed at 75 and 90 percent power during plant startup. As a result, the axial flux difference (i.e., the axial flux difference is equal to the

• power in the top half of the core minus the power in the bottom of the core) moves within the band defined by CAOC procedures but does not exceed technical specification requirements. Quarter core flux maps were taken innediately before and after the 10 percent load changes in each i

direction to measure the peaking factor behavior during these load changes.

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2. Special Hot Channel Factor Surveillance l These tests were perfonned to investigate the effect on peaking factors of I

operating outside of the CAOC band for the maximum time o.' one hour as allowed by the technical specifications. Tests were performed at the most negative and most positive allowed axial flux differences and for the end of first cycle as well as beginning of first cycle for a number af plants.

Full and quarter core flux maps were taken to measure the behavior of the peaking factors during these tests.

! 3. Load Follow Tests f These tests-involve load swings of approximately 50 percent power using.

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CAOC procedures. Full and quarter core flux maps were taken to determine the peaking factors achieved under these transient load following conditions.

1 These tests are analyzed by using as input certain of the measured data as a

, function of time (e.g., core power, control rod bank position or axial flux difference) to simulate the test and other measured data as a function of time

-(e.g., total peaking factor, control rod bank position or axial flux difference) to provide verification of Westinghouse design methods for calculating the effects of transient xenon on the power distribution. These design methods are used to l predict the maximum peaking factor which can occur when using CAOC procedures.

Data for methods verification under steady-state operating' conditions exist but I -

there have been no data for methods verification under transient xenon conditions.

This augmented startup test program provides the data' required for design methods l verification for load following transients.

The licensing topical report and its supplements describe the detailed test i program and the reactors in which the tests were perfonned. The results obtained for the various tests are discussed. Results obtained using the design methods to simulate the tests are compared to test results.

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SUMMARY

OF EVALUATION 1

The staff evaluation of the report and its three supplements considered two major areas; that is, the measurements and the verification of the design methods for calculating transient peaking factors.

Measurements An extensive series of tests was performed at a number of Westinghouse PWRs.

Other tests were performed in addition to the three basic types of tests discussed previously. Table 6-1 of Supplement 3 provides a summary of the tests performed by Westinghouse. ,

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The results obtained by Westinghouse demonstrated conclusively that CAOC procedures can be followed in an effective manner by plant operators. These tests also demonstrated that measured values of the core peaking factors were below accident analysis limits even though some of the tests were in operational areas not normally considered for plant operation. Based on a review of the test data, the staff concludes that the tests demonstrate the efficacy of the Westinghouse power distribution control strategy and provide an acceptable data base for use in the verification of the Westinghouse design methods for calculating transient peaking factors.

Verification of Design Methods for Calculating Transient Peaking Factors

- Westinghouse used a one-dimensional, time-dependent, diffusion theory computer -

program to simulate the spatial and time behavior of a number of measured quantities for these transient tests that are primarily axial xenon transients.

Comparisons between the results obtained from the analysis of the tests and the results obtained from the test program provide, therefore, an assessment of the Westinghouse xenon model. These comparisons provide, in particular, an

assessment of the Westinghouse design methods for calculating transient peaking factors. i i The core axial power distribution is a function of many variables. It can vary rapidly as a result of control rod bank motion and load demand changes. It varies

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j slowly with changes in the xenon concentration distribution. Some of the parameters

which are significant in the analysis of the axial power distribution are

core power level, core height, coolant temperature distribution and flow, core i j average coolant temperature variation as a function of reactor power, fuel burnup

] distribution, control rod bank worths, and amount of overlap of the control rod '

l banks. For the tests, measurements were taken at specified times of the reactor j power, soluble boron concentration, control rod bank position, core power

! distribution, core average axial power distribution, and axial flux difference.

These measurements at discrete times were used to construct the time dependency l of the parameters of interest.  !

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, In addition to the quantities discussed above, Table 6.1 of Supplement 3 indicates Ig that a load following test was performed using part-length control rods. -

Part-length control rods can have a significant effect on the axial power l distribution. Since part-length control rods are no longer used in Westinghouse

. reactors, this test will not be considered in the staff's evaluation of

Westinghouse design methods for calculating transient peaking factors.

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At the beginning of a test (time zero) the reactor is at steady-state conditions:

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the reactor power is constant, the control rod bank position is constant, the

! spatial xenon concentration distribution is constant and the power distribution i is constant. Minor fluctuations in these quantities about their nominal values near time zero are not important for the ensuing discussion.- A power distribution j can be determined from movable incore detector measurements in instrument thimbles <

placed in selected fuel assemblies. The axial flux difference can then be determined from the power distribution measurement. An analysis at this point in time I would give a calculated power distribution comparable to the measurement within  !

I the biases and uncertainties for the quantities of interest. -The results provided in the supplements to the topical report and the updated Westinghouse topical ,

. report on nuclear uncertainties (Ref. 4) confirin the adequacy of the Westinghouse design methods for computing the time zero, initial conditions for the transients.

l If, from these initial test conditions, the reactor undergoes a series of power l

j . changes (load changes) over a time interval lasting several days, the reactor power, control rod bank position, and soluble boron concentration will vary as  !

l a function of time. Characteristics of the power distribution such as the total I

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peaking factor and the axial flux difference will also vary as a function of time. There is, consequently, a cause and effect relationship among the various quantities. For example, changing the control rod bank position will affect the axial flux difference. The power distribution in such a load following maneuver is controlled using CAOC procedures (or variants of CAOC procedures) to prevent violation of peaking factor limits. The CAOC procedures are somewhat complex and include control of the axial offset within a band about some target axial offset. Manual and automatic power level control can be used. Control to the center, negative, or positive positions in the axial offset band can be performed. Control for minimum soluble boron variation can be performed. Other reactor parameters may need to be followed in performing a load following maneuver.

For example, the core average temperature needs to correctly follow power variations

and the soluble boron concentration may need to be adjusted to accomplish this. ,

Westinghouse's first method of simulating the tests (Method 1) used the power.

control rod bank position and soluble boron concentration as a function of time as input to the calculations. The important power distribution parameter, the axial flux difference, was predicted as a function of time. Other power distribution characteristics of the tests were also predicted. The calculated axial flux difference, after a period of time somewhat longer than the period of a xenon oscillation, began to show differences from the measurements.

Westinghouse stated (page 3-10 of Supplement 1) that this difference was not a

- model deficiency but the result of modeling a continuously varying maneuver by hourly adjustments of the control rod bank position and hourly spatial xenon

_ depletion. In response to the staff's request (Ref. 5) that Westinghouse demonstrate

- that agreement between measurement and simulation improves as the interval between time steps is decreased, Westinghouse responded in a letter on February 28, 1978 (Ref. 6). The response stated that because of the theta-differencing method for treating xenon concentration calculations little improvement could be expected to result from decreasing the simulation time interval. The response stated, for example, that reducing the time step to, say, fifteen minutes could not significantly improve the simulations. Westinghouse states that modeling deficiencies, that are always present even in the most sophisticated analyses, cause a compounding inherent error in the simula'.an of these load following transients using Method 1. Additional infonnation on the Westinghouse calculations wasalsorequested(Ref.7).

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Westinghouse proposed another method (Method 2) for simulating the tests. In this method the axial flux difference as a function of time is used as an input quantity to the simulation and the control rod bank position as a function of time is computed and compared to measured control rod bank positions. Simulations of a number of tests using Method 2 yielded acceptable results. However, the staff, in letters from J. Stolz to T. Anderson on August 18, 1978 (Ref. 8) and from C. Thomas to E. P. Rahe on June 2,1983 (Ref. 9), stated its reservations as to the appropriateness of Method 2 for validating the Westinghouse transient peaking factor design methods. Westinghouse (Ref. 10) provided a response to the earlier request for additional information (Ref. 8) on the Method 2 simulation.

There has been considerable discussion between the NRC and Westinghouse staffs g on the merits of'the two methods of analyzing the tests. The t'.aff believes that Method 2, by forcing the analysis method to follow the measured axial flux difference as a function of time, forces the calculated axial power distribution to match the measured axial power distribution. This in turn forces the calculated axial xenon concentration distribution to follow, perforce, the experimental (although not measured) axial xenon concentration distribution. Method 2 has, in effect, adjusted the xenon model in the simulation methods, that is, the very model that is important in detennining the adequacy of the Westinghouse design methods for calculating transient peaking factors. Westinghouse has provided a limited comparison of the two methods in analyzing the same test data. A comparison is shown for two tests in L-tion 4 of Supplement 3 of

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l the report. The first of the two tests was for a test period of short duration as compared to the period of a xenon oscillation and, as ex'pected, both simulation methods provided acceptable test predictions. The second test was for a period of time that was much longer than the period of a xenon oscillation and, qualitatively, both simulation methods provided acceptable test predictions, that is, of the axial flux difference for Method 1 and 'of the control rod bank position for Method 2. For Method 2 quantitative results are presented for the core average axial power distribution, the elevation dependent radial peaking factor and the integrated assembly power distribution. The calculated results are in good agreement with results obtained from measured data. However, a quantitative assessment of the transient peaking factors was not presented for Method 1 for this second test.

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1 i To resolve the staff's concerns Westinghouse recently provided the staff with i f the results of a sensitivity study of the two simulation methods (Ref. 11). In j this study an axial xenon transient was calculated. It was then simulated by

both Methods 1 and 2 with small perturbations made to a number of parameters i important to the simulation. Only one parameter at a time was perturbed. For Method 1 it is evident that, for times longer than the period of an axial xenon l

oscillation, the change in the axial flux difference as a function of time l increases with each succeeding period of the transient for each type of g perturbation. This sensitivity study indicates that the Method I simulation of the extended load following tests in the augmented startup test program is j apparently unstable with respect to either small errors in modeling or input j data and, therefore, incapable of properly simulating the test data. As an .

f g- example Westinghouse indicated that a small change in one of the input parameters i to the second test, referred to previously, gave good results for the Method 1 I

. simulation as contrasted with the original simulation of the test. For Method l 2, the change in the control rod bank position as a function of time exhibited but small changes for each type of perturbation. In particular, the' oscillatory i nature of the transient was barely perceptible over the duration of the transient. '

This sensitivity study confirined the expected good properties of simulation Method 2. In addition, the peaking factor as a function of power and core height was relatively insensitive to any of the perturbations considered for Method 2.

a Based on our evaluation of the results presented in the topical report and the I sensitivity study, we conclude that the use of Method 2 for simulating the test data is acceptable and, in fact, necessary. We also concitide that the results

! obtained using the Method I simulation of the tests are not indicative of any

• l inherent errors in the Westinghouse transient xenon models. -

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l In Section 5 of Supplement 3 of the report Westinghouse presents the results obtained for peaking factors and hot channel factors for the extensive testing performed on the plant described in Section 4 of Supplement 3 of the report.

i The data were analyzed using simulation Method 2. For the hot channel factor, all of the analyzed points were within the Westinghouse uncertainty bands

! bounding the measured values except for two low power points. The data were

analyzed for cases where the reactor power was higher than 50%. The core

} averaged axial power distribution was also analyzed and compared with the

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4 measured data. Good agreement was obtained with the analyzed data being within the uncertainty bands for the measured data. The total peaking factor as a function of power and core height was also compared to the measured data. The

! results indicate that the calculated values of the total peaking factors are l more conservative than the corresponding measured data. Westinghouse also analyzed the maximum value of the elevation dependent total peaking factor as a t function of power in a manner consistent with design procedures. The results i indicate that (1) no test gave total peaking factors which exceeded the plant

FAC analysis and (2) the calculated maximum elevation dependent total peaking factors for the tests were always more conservative than the measurements. These 4

results demonstrated that the Westinghouse design methods provide acceptable results for the hot channel factor and the core averaged axial power distribution j when compared to-the measured data and the associated uncertainty bounds. Although

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no uncertainty bounds were used or established for the total peaking factor, the analyses demonstrated that acceptable results were obtained with the Westinghouse design methods since the calculations were always conservative when compared to the measured data.

In sunnary, we conclude our evaluation by stating that Westinghouse has met the two major objectives of the augmented startup test program in an j acceptable manner because

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. 1. The concerns by the ACRS (e.g., in its Trojan letter of November 20,1974) on limited experience with tery large, high power density reactors have

_ been resolved. The test data obtained in this augmented startup test program and over many cycles of operating experience have conclusively .

j demonstrated the safe operation at full rated power for this class of-reactor.

2. The verification of the Westinghouse design methods for calculating  ;

transient peaking factors, using the test data, has been established for l simulation Method 2.

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REGULATORY POSITION The staff concludes that the licensing topical report and its three supplements are acceptable for referencing in licensing submittals by Westinghouse with regards to the verification of the design methods used to calculate transient peaking factors and with regard to the extensive data base accumulated during the testing.

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REFERENCES

1. ACRS letter on the Trojan plant, November 20, 1974.

2. "Excore Detector Recalibration Using Quarter-Core Flux Maps," R. A. Kerr, WCAP-8648, June 1976.

3. " Power Distribution Control and Load Following Procedures " T. Morita.

L. R. Scherpereel, K. J. Dzikowski, R. E. Radcliffe, and D. M. Lucoff, WCAP-8385 (Proprietary) September 1974.

,~ f. 4. " Evaluation-of Nuclear Hot Channel Factor Uncertainties," E. M. Spier and T. Q. Nguyen, and update to WCAP-7308-L (Proprietary), March 1984.

5. Letter of January 26, 1978 from John F. Stolz (NRC) to C. Eicheldinger (W).

6. Letter (NS-CE-1707) of February 28, 1978 fromC.Eicheldinger(E)to i JohnF.Stolz(NRC).

l l 7. Letter of March 22, 1978 from John F. Stolz (NRC) to C. Eicheldinger (W).

_ 8. Letter of August 18, 1978 from John F. Stolz (NRC) to T. M. Anderson (W).

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_ 9. Letter of June 2,1983 from C. O. Thomas (NRC) to E. P. Rahe (W).

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10. Letter (NS-TMA-1963) of October 4,1978 from T. M. Anderson (W) to John F. Stolz (NRC).

11. Letter (NS-NRC-85-3069) of October 1, 1985 from 2. P. Rahe (W) to C.O. Thomas (NRC).

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