ML20136G304
| ML20136G304 | |
| Person / Time | |
|---|---|
| Site: | 05200003 |
| Issue date: | 03/06/1997 |
| From: | Doug Garner WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | |
| Shared Package | |
| ML20057G451 | List: |
| References | |
| SAE-APS-97-20, NUDOCS 9703170260 | |
| Download: ML20136G304 (6) | |
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SAE-APS 97 20 WC/T Validation for AP600 Long Term Cooling D.C. Garner March 6,1997 4
AP600.Ltr.Rpt 1
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WC/T Validation for AP600 Long Term Cooling l
- 1. Introduction To validate WC/T for the long term core' cooling analysis of AP600 a series of i-calculabons were made to demonstrate the behavior of the code and applicability of the code by j
comparison to OSU tong term coolir;g test data. The calculations were documented in WCAP-
- 14776, Reference 1. The lack of sensitivity of the quasi-equilibrium solution to the initial vessel j
conditions <of liquid level and downcomer temperature was shown by a set of comparitive calculations which were run for a test simulation time of 1000 seconds. In each case, the same quasi-equilibrium solution was obtained independent of the initial conditions. For the calculational i
comparisons with OSU test data, the window periods were selected at conditions which challenge i
the AP600 long term cooling design, i.e_. at a times when the IRWST and sump levels were low, providing the minimum driving heads to the DVI flows and the minimum vessel liquid levels.
I These calculations were also run for a test simulation time of 1000 seconds. Good comparisons l
were obtained with the reactor vessel liquid levels and reasonable comparisons were obtained for most other significant parameters. This was documented in Section 5 of Reference 1, i
Subsequent to the issuance of Reference 1 and in response to discussions with the NRC regarding solution divergence at extended calculation times, the convergence of WC/T was j
evaluated with comparitive long and short calculations using the for Oregon State University Test Simulatioris for reference. It was found that the sama quasi equilibrium solution was obtained at a calculation time of 3000 seconds, as obtained for calculations started 2000 seconds later and run for a 1000 second period. This is discussed in Section 2.1 below.
i Also to address possible divergence, the effect of boundary condition variations were evaluated in a set of calculations. Perturbations were applied to boundary conditions, including IRWST l
level, S.G. secondary temperature, and core decay heat, for a period of 200 seconds at the start j
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- of the calculation and the values subsequently retumed to the correct levels for the remainder of i
the window. The results are discussed in Section 2.2.
Three data comparison calculations were also performed for periods of at least 3000 seconds of test data simulation and are compared to OSU test data in Sections 2.3,2.4, and 2.5 of this report.,Three different window periods were evaluated during IRWST draindown and sump operation covering the time period of 1260 seconds to 16,500 seconds, i.
- 2. Additional WC/T Convergence Calculations 2.1 WC/T Extended Time Calculation Convergence Subsequent to issuance of WCAP-le 776, a transient window calculation was extended from the normal 1000 second length to n length of more than 3000 seconds, in order to demonstrate solution convergence for calculations at extended times. This was performed for the 2" Cold Leg Break Test (SB01) s' tarting at initiation of IRWST injection (1260 seconds) and running to a test time of 4600 seconds. For the reference calculation, the initial vessel conditions were taken from the test data at 1260 seconds. The comparison calculation was pedormed using identical initial vessel conditions (test data at 1260 seconds) but was started at a transient tima AP6001tr.Rpt 2
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of 3600 seconds and run with the test boundary conditions occuring during the 3600 second to 4600 second period.
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~ The results of this comparison are demonstrated most clearly in the companson of the reactor vessel liquid levels, Figures 2.1-2 through 2.1-4, where identical levels are established well before j
the end of the calculations at 4600 seconds. Similiar correspondence is shown in the DVI line j
flows, Figures 2.1-7 through 2.1-10 where comparable flows are obtained for the last 700 seconds i-of the comparison. Finally, good correspondence is obtained for the ADS-4 flows as shown by Figures 2.1-14 and 2.1-16. The results indicate that extended time calculations with WCff, for j
l a boundary value problem such as long term cooling, do not result in error accumulation and i
solution divergence.
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2.2 WC/T Boundary Condition Convergence Also, subsequent to issuance of WCAP-14776, it was decided to demonstrate that the solution would converge to the quasi equilibrium values if the boundary conditions were perturbed i
and subsequentily retumed to the correct _ values. For the 2" Cold Leg Break transient (SB01),
a 1000 second window was selected which extended from 8000 to 9000 seconds. The vessel initial conditions were set at values occuring at 8000 seconds. The boundary conditions were set at values occuring earlier in the transient, at 3600 seconds. The IRWST level was set at 2.5 feet above the test data level for 200 seconds and then returned to the correct values for the remainder of the window. For the same period, the steam generator secondary side temperature was increased by 45 F and the core decay heat was increased by 30%. While the early portion of the solutions reflected the differences in IRW3T levels, steam generator temperatures, and core decay heat values, identical results were obtained for the reference and the comparison calculations well before the end of the 1000 second window.
The results of this comparison are demonstrated in the comparison of the reactor vessel liquid levels, Figures 2.2-2 through 2.2-4, where identical levels are established well before the end of the calculations at 9000 seconds. Similiar correspondence is shown in the DVI line flows, Figures 2.2 7 through 2.210 where comparable flows are obtained for the last 300 seconds of the comparison. Finally, good correspondence is obtained for the ADS 4 flows as shown by Figures 2.214 and 2.2-16 where again comparable flows are obtained for the last 300 seconds of the comparison. The results indicate that perturbations to the boundary conditions, whether real or artificial, are subsequently dissipated with additional calculations and are eliminated from the quasi-equilibrium solution after a sufficient length of time.
2.3 WC/T Extended Time Calculations, SB01 from 1260 sec. to 4600 sec.
To demonstrate the adequacy of WC/T to accurately perform extended time LTC calculations, an additional set of data comparison calculations was performed subsequent to the issuance of WCAP-14776. Two additional windows were selected for the 2" Cold Leg Break (SB01) and the sump switchover window "tt 13,500 seconds for the CMT Balance Une Break (SB10) was extended to 16,500 seconds. The three calculations were performed for periods of at least 3000 seconds and were compared to OSU data.
l AP600.Ltr.Rpt 3
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t The first window selected was for test SB01 from the start of IRWST injection at 1260 seconds and extending to a time of 4600 seconds. This window included a significant period of vessel liquid level _ increases following the RCS depressurization and a subsequent period of low core steaming rates due to the high DVI line flow from the nearly full IRWST. These conditions were calculated with reasonable accuracy as demonstrated in Figures 2.3-22 through 2.3-24 and Figure 2.3-21. The total DVI line flow rates compared well with the test data during this extended time period as indicated in Figures 2.3-7 through 2.3-10. It is noted that no attempt was made to simulate the CMT refill phenomenon since it was relatively brief and resulted in an insignificant reduction in the IRWST inventory as indicated in Figure 2.31. Also, the effects on the vessel levels were brief and had no lasting impact on the quasi-equilibrium solution; Figures 2.3 22 through 2.3 24. The ADS-4 flows and the break flow also compared reasonably well to the data, Figures 2.3-17 through 2.3-19 and Figures 2.313 and 2.314, with no indication of accumulating flow rate errors with the extended calculation times.
2.4 WC/T Extended Time Calculations, SB01 from 8000 sec. to 11000 sec.
The second window selected was for test SB01 during IRWST injection from a time of 8000 seconds to11,000 seconds. This window included a vessel liquid level decreases following re-initiation of steaming in the core and a subsequent period of low core steaming rates due to high DVI line flow rates. The calculated vessel levels were conservatively low with respect to the measured values as demonstrated in Figure A 2.4 22 through 2.4-24. Both downcomer and upper plenum liquid levels showed the trend o' Wereasing level but the initial magnitude of the decrease was over predicted. As the trinsient progressed, this trend tended to become less significant. The values of core steaming rate were well predicted during this period as indicated in Figure 2.4 21. The total DVI line flow rates compared well with the test data during this extended time period as indicated in Figures 2.4-7 through 2.4-10. It is noted that no attempt was made to simulate the system flow oscillations since they are believed to require extremely fine nodalization in the downcomer to calculate the condensation effects and the current noding system predicted the mean value of the flows and the related vessel liquid inventories with reasonable accuracy; Figures 2.4 22 through 2.4-24. The ADS-4 flows and the break flow also compared reasonably well to the data, Figures 2.4-17 through 2.4 20 and Figures 2.4-13 and 2.4-14, with no indication of accumulating flow rate errors with the extended calculation times.
2.5 WC/T Extended Time Calculations, SB10 from 13,500 sec. to 16,500 sec.
- The third window selected was from test SB10. It was initiated at 13,500 seconds, near the end of IRWST injection and extending to a time of 16,500 seconds at which time both sump check valves and sump isolation valves were fully open. This calculation is an extension of calculation 5.2 in WCAP-14776. The window included a period of upper plenum liquid level decrease following the switchover to sump flow and a subsequent period of increased core steaming rates due to lower DVI line flow rates and higher DVI line liquid temperatures. These conditions were calculated with reasonable accuracy as demonstrated in Figures 2.5-23 and Figure 2.3-21. The total DVI line flow rates into the vessel compared well with the test data during this extended time period as indicated in Figures 2.3 7 through 2.3-10. It is noted that the sump and IRWST flow i
. rates and directions are also reasonable well predicted during this period except in portions of line 1 in the period following opening of the sump isolation valve, Figures 2.5-3 and 2.5-5. 'In this AP600.Ltr.Rpt 4
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t period, the calculated flow from the sump to the IRWST is higher than measured flow. Of particular significance, however, is the fact that the correct quasi-equilibrium solution is obtained near the end of the transient. This demonstrates, that after a significant perturbation to the boundary conditions (opening of the sump isolation valves), the solution will adjust the flow conditions and retum to the correct values. It is further noted that, during this period, the reactor vessel levels were not significantly influenced.
- 3. Summary In November 1996, Westinghouse submitted WCAP 14776 with a set of WC/T long term cooling f.alculations using the window mode and windows of 1000 seconds in length. In response to NRC questions and concems, additional calculations have been performed for windows of 3000 seconds in length or longer. Three data comparison calculations have been performed with OSU test eiata for the longer window lengths. In addition, an extended time convergence comparison calcunstion was performed as well as a boundary condition convergence calculation. The results provide no indication of long term divergence in the WC/T long term cooling calculations. Further, these calculations when combined with those in WCAP-14776 indicate that vessel liquid levels are conservatively predicted by WC/T, i.e. typically 3 to 6 inches below the measured OSU test values. Thus, the calculations are considered to provide considerable support for the applicability of the 'NC/T code for the AP600 long-term cooling calculations.
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References:
- 1. WCAP 14776, "y(COBRA / TRAC OSU Long-Term Cooling Final Verification Report", D.C.
Garner, et. al., November 1996 AP600.Ltr.Rpt 5
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All figures in SAE-APS-97-20 are Westinghouse Proprietary Class 2 O
AP600.Ltr.Rpt 6