ML20137Z711

Topical Rept Evaluation of Rev 1 to NEDO-20566-2, GE Analytical Model for LOCA Analysis in Accordance W/10CFR50, App K,Amend 2,One .... Rept Acceptable for LOCA Evaluations During single-loop Operation

ML20137Z711
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Issue date:	03/05/1986
From:	Office of Nuclear Reactor Regulation
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NUDOCS 8603130099
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,- ENCLOSURE 1

TOPICAL REPORT EVALUATION REPORT NUMBER: NEDO-20566-2, Rev-1 REPORT TITLE: General Electric Company Analytical Model For Loss-of-Coolant Accident Analysis in Accordance with 10 CFR 50 Appendix-K Amendment No. 2 One Recirculation Loop-Out-of-Service REPORT DATE: July 1978.

ORIGINATING ORGANIZATION: General Electric Company Nuclear Energy Business Operations REVIEWED BY: Reactor Systems Branch Division Of BWR Licensing Introduction The report describes the methodolgy that GE plans to use for emergency core cooling system (ECCS) performance analysis under conditions where one recircula-tion loop is out-of-service. The methodology for non-jet pump plants has been previously accepted and will not be addressed further here. The methodology for jet pump plants reduces current maximum average planar linear heat generation rate (MAPLHGR) Ifmits for Single Loop Operation (SLO) and is the subject of this safety evaluation.

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TOPRP E % y 02/11/86 1 GE TOPICAL RPT NE00-20566-2

i 2.0

SUMMARY

OF TOPICAL REPORT 2.1 Comparison of One and Two-Loop LOCA Analyses If two recirculation pumps are operating and a pipe break occurs in one of the two recirculation loops, the pump in the unbroken loop is assumed to immediately trip and-begin to coast down. The decaying core flow due to the pump coastdown results in very effective heat transfer during the initial phase.of the blowdown.

If only'one recirculation loop is operating, and the break occurs in the operat-ing loop, continued core flow is provided cnly by natural circulation because the vessel is blowing down to the containment through both sections of the broken loop. The core flow decreases more rapidly than in the two-loop operat-ing case, and the departure from nucleate boiling for the high power node night occur 1 to 2 seconds after the postulating accident, resulting in more severe cladding heatup.for the one-loop operating case.

2.2 Procedures for LOCA Analysis Following One-Loop Operation GE's methodology has been developed to account for the degraded blowdown heat transfer'and its impact on ECCS analysis. This ECCS methodology for SLO opera-tions conservatively assumes a time to boiling transition of 0.1 seconds, whereas actual calculations would predict times from 1 to 2 seconds. This as-sumption is input to the SAFE /REFLOOD code where water level and vessel pressure results are calculated for the largest possible break size. The one-loop SAFE /

REFLOOD analyses'are performed with the following assumptions.

(a) The staff approved ECCS computer codes are used for the calculation.

(b) The vessel blowdown and inventory calculation are performed assuming no coast-down recirculation flow.

(c) The reactor is assumed to be operating at 102% rated power with correspond-ing core flow, steam flow, pressure etc. This assumption is conservative for operation at lower power (as expected in SLO) in that calculations with the reactor operating at a reduced power level for SLO show later core uncovery and earlier core reflooding - both of which result in less severe cladding heatup.

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Reflood time is then compared to the reficod time for the two-loop ECCS analysis, and if they are similar or the two loop conservatively bounds the SLO value, i.e., within a range that would produce less than a 20 F change in peak cladding temperature, a generic procedure is used. If they do not satisfy this criterion, plant specific heatup and MAPLHGR calcula-tions are performed.

The generic procedure uses curves of MAPLHGR reduction factors for N-1 operation as a function of boiling transition and reflood times. These reduction factors are the ratio of the SLO MAPLHGR to the MAPLHGR for 2 loop operation where both MAPLHGRs result in the same PCT.

The SLO MAPLhGR was calculated with an assumed boiling transition time of 0.1 seconds for conservatively selected LOCA analyses (based on DNB and reflood characteristics). The ratio of these values to the corresponding 2 loop MAPLHGR was so calculated and established the reduction factor curves.

For plant-specific heatup and MAPLHGR calculation, the following assumptions are used.

(a) The use of the staff approved standard heatup analysis computer code CHASTE.

(b) In the heatup calculations, the use of 102% of bundle power in conformance with 10 CFR 50.46, Appendix K (c) Boiling transition occurs at 0.1 second after the accident.

(d) The heatup calculations are based on a typical fuel as the sensitivity of the calculated MAPLHGR reduction factor are essentially the same for all fuels.

2.3 Effect on Break Spectrum of One-Loop Operation The primary characteristics that determine the most limiting break for the one-loop analysis are:

(a) The hot node reflooding time 02/11/86 3 GE TOPICAL RPT NEDD-20566-2

, (b) The hot node uncovery time.

As all breaks are assumed to result in a calculated time to BT of 0.1 second, the break that results in the longest period during which the hot node remains uncovered will generally result in the highest calculated PCT for one-loop operation. If two breaks have similar times during which the hot node remains uncovered, then the larger of the two breaks will be more limiting, as it has an earlier uncovery time (resulting in relatively less removal of stored energy and a higher decay heat after uncovery).

For single-loop break spectrum analysis, a boiling transition time of 0.1 second is conservatively assumed for all breaks larger than 1 square foot, and the reflooding times and total uncovered times are compared to the times calculated for the two-loop analysis. The time to BT in the two-loop analysis is the same for all breaks. If they are similar, the most Ilmiting break for the single-loop analysis will be the same limiting break for the two-loop analysis. The majority of plants fall in this category.

For a few plants, the time to BT used in the two-loop analysis is offferent for different breaks and the effect of using the one-loop analysis assumption for the time to BT can have different effects on the calculated PCT for different breaks.

MAPLHGR reduction factors are determ'ned for the various potentially limiting breaks. The one loop MAPLHGRs are then determined using the lowest MAPLHGR reduction factor for any break and the two-loop MAPLHGRs. This procedure accounts for the fact that a different break might be more limiting for the one-loop analysis than was used in the two-loop analysis.

A representative calculation for a sample small (0.07 ft ).2 break was performed using the one-loop operation heatup assumptions, i.e., early boiling transition followed by Ellion pool boiling until high power node uncovery. The calculated PCTs for two-loop and one-loop operation were 1725F and 1760F respectively.

There is no significant difference in PCTs between the two modes of operation.

As is the case for two-loop operation in all BWRs, the calculated PCTs for small breaks remain well below the 2200 F limit.

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2.4 Worst Single Failure The single failure which is most limiting remains unchanged in going from two-loop to one-loop operation.

This is true because the limiting single failure for either case is that which generally results in the longest reflooding time.

Since the basic phenomena and relative reflooding times for various failures are the same for both one-loop and two-loop operation, the limiting failure is identical for both cases. The equalizer valve between loops will be kept closed during SLO, thus the same effective break area will be maintained.

3.0 SAFETY EVALUATION The General Electric evaluation model for two loop BWRs contains a detailed evaluation of system parameters to determine the blowdown heat transfer and subsequen,t fuel cladding temperature.

The significant reactor system parameters are determined with the LAM 8 computer program and input to the SCAT computer program which calculates the blowdown heat transfer. For two-loop operation, the blowdown heat transfer can be characterized by a period of high heat trans-fer until boiling transition occurs (5-9 seconds), a period of low heat transfer by pool boiling, and a subsequent period of flow film boiling heat transfer dur-ing lower plenum flashing. These heat transfer coefficients are input to the CHASTE computer program which calculates the fuel assembly hot plane temperature transients.

Following blowdown, CHASTE assumes the appropriate values for spray heat transfer after the time that rated core spray is calculated, and terminates the temperature calculation following core hot spot recovery. These times (rated core spray and hot spot recovery) are calculated with the SAFE and REFLOOD com-puter programs,. respectively.

For one-loop operation, General Electric has proposed a simplified model to evaluate the heat transfer during blowdown. The limiting condition would occur if one loop is assumed out of service and the postulated LOCA occurred in the second operating loop.

No core flow coastdown is assumed for this case as the only operating loop is assumed to be broken, whereas if the break is assumed in the idle loop, credit would be possible for flow coastdown in the operating loop.

To consider this condition, GE proposed that the CHASTE heatup calculation would assume that dryout occurs at 0.1 second for all postulated break sizes.

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" Following dryout, heat transfer by pool boiling is assumed until the hot spot is uncovered. Following the time of uncovery, a convection heat transfer co-efficient of zero is conservatively assumed until the time rated core spray.

Since detailed system parameters are not required to evaluate the blowdown heat transfer, the LAMB and SCAT computer programs are not used. A SAFE calculation is performed to determine the system pressure, core level during blowdown, and the time of rated core spray. This calculation is conservatively performed at a power level of 102 percent.

Following blowdown, the one-loop LOCA calculation is performed in a manner similar to the two loop model. That is, the CHASTE calculation is continued assuming the Appendix K specified values for spray heat transfer after the time of rated core spray, and is terminated folloviing core hot spot recovery. The time of core hot spot recovery for one-1 cop operation is calculated with the REFLOOD computer program, which is also used for the two-loop application. The major assumptions and codes used in the LOCA analysis for SLO are summarized in Table 1. As indicated in the Table, GE has used the staff approved codes and analytical models.

The staff finds that the proposed LOCA model for one-loop operation is accept-

, able and meets all requirements of Appendix K to 10 CFR 50.46.

The staff concludes that the licensing topical report NE00-20566-2, Rev-1 dated July 1978 is acceptable for LOCA evaluations during single-loop operation for General Electric cesigned reactors of the jet pump classes.

4.0 DOCUMENTATION REQUIRED The licensee should submit the following plant specific information for staff review with the request for SLO approval.

(a) Plants which do not use generic MAPLHGR curves Pressure vs time Water Level vs time MCPR vs Time 02/11/86 6 GE TOPICAL RPT NEDO-20566-2

Heat Transfer Coefft vs time PCT vs time Core average inlet flow vs time MAPLHGR reduction factor (b) Plants Which use Generic MAPLHGR Curves Pressure vs time To assure CS initiation time is conservative relative to generic curves.

Water level vs time To assure hot node uncovery time is conservative relative to generic curves. To assure reflood time of single loop case is less than or " equal" to two loop case and to see what reflood time is.

(c) Ref'lood area vs~ break area 2 loop and 1 loop (Safe /Reflood Cals) Disch: Breaks > 1 ft2 (d) Reflood area vs break

, area 2 loop and 1 loop (Safe /Reflood Cals) suction breaks > 1 ft2 (e) BT time from 2 loop analyses for both suction and discharge break analyzed.

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Table 1 Comparison of MAPLHGR and PCT calculation assumptions for one pump versus two pump operation Two-Pump One-Pump Operation Operation Core Pressure Calculation LAM 8 Not applicable

• Transient Core Flow Calculati.on LAM 8 Not applicable

• Lower Plenum Enthalpy Calculation LAM 8 Not applicable

• MCPR SCAT Not applicable

• Convective Heat Transfer Coefficient NE00-20566 NE00-20566**

Water Level Calculation SAFE /REFLOOD SAFE /REFLOOD Vessel Pressure Calculation SAFE /REFLOOD SAFE /REFLOOD Peak Cladding Temperature CHASTE CHASTE Calculation Break Spectrum Calculations CHASTE plus CHASTE plus SAFE /REFLOOD SAFE /REFLOOD MAPLHGR Calculation CHASTE CHASTE or Generic Alternative Procedure

• Boiling transition for LOCA from one-loop operation is assumed at 0.1 second; therefore, LAM 8 and SCAT calculations not required.

• • For one pump operation, loss of nucleate boiling assumed at 0.1 second after the LOCA.

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REFERENCES (1) " General Electric Company Analytical Model for Loss of Coolant Analysis in accordance with 10 CFR 50 Appendix K - Amendment No.2 One Recirculation Loop out of Service," NED0-20566-2, Revision 1, Class 1, July 1978.

(2) Memorandum for D. G. Eisenhut, Acting Assistant Director for Systems and Projects, 00R from R. L. Tedesco, Assistant Director for Reactor Safety, DSS. Subject "LOCA model for BWR operation with one recirculation loop out of service," dated Dec. 4,1978.

(3) Responses to Round I questions on one recirculation loop out-of-service.

Letter from R. E. Engel, Manager, GE to P. S. Check, Chief NRC. Dated October 17, 1979.

(4) Responses to Round 2 questions on one recirculation loop out of-service.

Note from R. T. Hill, GE to Marvin Mendonza, NRC, dated January 13, 1981.

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