ML20212F158

Safety Evaluation Supporting Mods Re Axial Power Shape Issue,Large Break LOCA ECCS Evaluation Model,Phase I,For Cycle 10 Operation

ML20212F158
Person / Time
Site:	Maine Yankee
Issue date:	01/06/1987
From:	Office of Nuclear Reactor Regulation
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ML20212F155	List: ML20212F149 ML20212F158
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NUDOCS 8701090519
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SAFETY EVALUATION BY OFFICE OF NUCLEAR REACTOR REGULATION MAINE YANKEE ATOMIC POWER COMPANY MAINE YANKEE ATOMIC POWER STATION DOCKET NO. 50-309 LARGE BREAK ECC5 EVALUATION MODEL MODIFICATIONS RELATED TO AXIAL POWER SHAPE ISSUE PHASE I

1. INTRODUCTION In a telecon on September 2, 1986, the licensee, Maine Yankee Atomic Power Company (MYAPCo) informed us of a potential non-conservatism in their large break LOCA analysis. In a meeting on September 9, 1986, Yankee Atomic, which performs Maine Yankee's fuel reload analysis, described how they had discovered that the highly peaked axial power distribution used in Maine Yankee's large break LOCA analysis since 1977 is not the bounding shape.

Appendix K to 10 CFR Part 50 requires that the axial shape which results in the most severe consequences should be used in the ECCS evaluation model LOCA calculations. It was determined by Yankee Atomic that the bounding shape is a flattened shape which results in higher peak cladding temperatures, especially later in the life of the reload. For the balance of Cycle 9, administrative limits on power peaking factors are being implemented to assure that results of LOCA analyses done with the current model comply with the limits of 10 CFR 50.46. For Cycle 10, MYAPCo has proposed changes in their approved ECCS evaluation model (EM) to recover the margin lost by utilizing the more severe flattened axial shape. The modifications, review, and analysis will be a two-step process. This SE addresses the changes in Phase I which are applicable to the analysis to be performed for the first half of Cycle 10.

The changes involve the selection of appropriate power shapes, and modification of the injection AP penalty.

l 2.0 POWER SHAPE SELECTION l In Reference ?, the licensee proposed a method for selecting appropriate l radial and axial power distributions to be used in LOCA analyses to assure l compliance with paragraph I. A. of Appendix K which states:

A range of power distribution shapes and peaking factors representing power distributions that may occur over the core lifetime shall be studied and the one selected should be that which results in the most severe calculated consequences, for the spectrum of postulated breaks and single failures analyzed.

Power shapes must also be selected so as not to violate Specified Acceptable Fuel Design Limits (SAFDLs) and they must also be calculated based on nuclear design parameters and possible operating conditions for the particular cycles under consideration. The radial power distribution (maximum pin power) is 10 hD 519 870106 p OCK 05000309 PDR

i determined for a variety of cycle conditions for thermal margin considerations.

, The maximum value is selected for the hot-test pin in the LOCA analysis. The corresponding radial power for the hot assembly is selected, and all uncer-tainties are included to maximize hot pin, hot assembly, and average core power

which are used in LOCA analysis.

With the pin power established for all LOCA analyses as a single maximum value, several axial shapes are next selected for thermal margin based on core power and symmetric offset considerations. This results in a symetric offsetcurveoflinearheatgenerationrate(LEGR)versuscoreheight.

This curve results in fairly limiting kw/ft values at elevations below the mid plane.

Using a representative selection of axial power shapes determined for the thermal analysis, several large break LOCA analyses are performed to assure compliance with 10 CFR 50.46 and Appendix K. It is verified that these shapes include flatter, more symetric shapes than were previously considered. LOCA analyses performed with this set of axial power shapes result in a LOCA limit curve which is more limiting than the thermal margin curve at higher. elevations.

At lower elevations the thermal margin curve is so clearly limiting that bottom peaked LOCA analyses are not required. This is particularly true since periods of core uncovery for both large and small breaks are much longer at higher elevations, thus making top skewed or symmetric power shapes more limiting than bottom skewed for LOCA analysis.

Appropriate monitoring is performed to verify that the radial and axial components of power distribution are maintained below the limiting values as described above, 3.0 INJECTION AP PENALTY Section I.D.4 of Appendix K to 10 CFR Part 50 requires that the thermalhydraulic interaction between steam and all emergency core cooling water shall be taken into account when calculating core reflooding rates. The currently approved ECCS evaluation rodel for the Maine Yankee plant utilizes an additional frictional pressure drop (AP penalty) to account for the steam-water inter-

, action effect. A AP penalty of 1.5 psid is utilized during the accumulator injection period; a penalty of 0.8 psid is used during the pumped injection

phase.

In its November 10, 1986 letter, MYAPCo proposed to modify the ECCS evaluation model for the Maine Yankee plant by reducing the AP penalty from 0.8 psid to 1

0.15 psid during the pumped injection phase. This section presents the staff's findings on the proposed modification.

i 3.1 EVALUATION OF AP PENALTY Since the time that the AP penalty used in the ECCS evaluation model for Maine l Yankee was approved, tests have been performed by EPRI to examine the effects of steam-water interaction in the cold legs of a PWR. These tests were per-

'i formed with 1/14 and 1/3 scale cold leggeometries and are discussed in EPRI reports EPRI-294-2, " Mixing of Emergency Core Cooling Water With Steam: 1/14 Scale Testing Phase," dated January 1975, and EPRI-294-2, " Mixing of Emergency Core Cooling Water With Steam: 1/3 Scale Test and Sumary," dated June 1975.

The testing program examined the cold leg pressure drop during both the accumulator injection phase and the pumped injection phase of the reflood portion of a large break LOCA.

In its November 10, 1986 letter, MYAPCo reported the results from all the EPRI tests performed to examine the pumped injection phase. Using the measured cold leg pressure drop data, the AP penalty associated with the steam-water inter-action effect was derived by subtracting out the piping frictional pressure drop. MYAPCo's examination of the data indicated that the AP penalty could be bounded by 0.15 psid. Only one of the 131 data points was above 0.15 psid, and it did not replicate; the duplicate run for that data point had a pressure loss of 0.06 psid.

4 MYAPCo also examined the calculated reflooding parameters for the Maine Yankee plant and compared them to the EPRI test ranges. All the calculated conditions -

were within the range of conditions examined during the EPRI tests.

The staff has previously reviewed the use of the EPRI test data for determining the AP penalty as part of the staff's review of the Exxon Nuclear Company ENC-WREM-IIA ECCS evaluation model and found use of the EPRI data acceptable.

Since the test conditions examined by the EPRI tests envelop the calculated Maine Yankee plant reflooding conditions and the EPRI test results indicate that the AP penalty can be bounded by a value of 0.15 psid, the staff finds the proposed model modification acceptable, f

4.0 CONCLUSION

S Selection of the neximum allowable pin power for use in LOCA analyses is always conservative and ac:eptable. A sufficient sample of axial shapes has been proposed for LOCA an.alyses. This in combination with the envelope of axial shapes for limiting thermal margin is acceptable for detemining axial power distribution and is in compliance with Section I.A of Appendix K to 10 CFR Part 50. For each reload, the licensee should address break spectrum con-siderations especially as relates to axial shape as required by Appendix K.

In particular, the possibility exists that small breaks may be limiting for the maximum kw/ft at the top 1.5 ft. of the core. Also since a full spectrum a

is not always analyzed for each axial shape selected and a completely consis-tent calculation is not always done for each break analyzed, the spectrum chosen and methods used must be justified for each reload.

Based upon the above, the staff finds that the proposed AP penalty of 0.15 psid during the pumped injection period meets the requirements of Section I.D.4 of Appendix K to 10 CFR Part 50. Accordingly, the staff finds the proposed modification to the ECCS evaluation model for the Maine Yankee plant acceptable.

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5.0 REFERENCES

1. Letter from G. D. Whittier (PYAPCo) to A. 1. Thadani (NRC), " Maine Yankee LOCA Analysis," GDW-86-212, dated September 15, 1986

2. Letter from G. D. Whittier (MYAPCo) to A. T. Thadani (NRC) " Maine Yankee LOCA Analysis," GDW-86-267, dated November 10, 1986 Principal Contributors:

N. Lauben R. Jones

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