ML16357A147
| ML16357A147 | |
| Person / Time | |
|---|---|
| Site: | Limerick  |
| Issue date: | 09/19/2016 |
| From: | Exelon Generation Co |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML16357A167 | List:
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| References | |
| Download: ML16357A147 (8) | |
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LGS UFSAR APPENDIX 15A - CONTINUOUS CONTROL ROD WITHDRAWAL TRANSIENT ANALYSIS TABLE OF CONTENTS 15A.1 INTRODUCTION 15A.2 METHODS OF ANALYSIS 15A.3 RESULTS 15A.4 CONCLUSIONS 15A.5 REFERENCES APPENDIX 15A 15A-i REV. 13, SEPTEMBER 2006
LGS UFSAR APPENDIX 15A - CONTINUOUS CONTROL ROD WITHDRAWAL TRANSIENT ANALYSIS LIST OF TABLES Table Title 15A-1 Sequence of Events for Continuous Rod Withdrawal During Reactor Startup 15A-2 Summary of Results for Detailed and Point Kinetics Evaluations of Continuous Rod Withdrawal in the Startup Range APPENDIX 15A 15A-ii REV. 13, SEPTEMBER 2006
LGS UFSAR APPENDIX 15A - CONTINUOUS CONTROL ROD WITHDRAWAL TRANSIENT ANALYSIS LIST OF FIGURES Figure Title 15A-1 Point Kinetics Control Rod Reactivity Insertion 15A-2 P/A versus Rod Worth NEDO-10527 Supplement 1 and Detailed Analysis 15A-3 Continuous RWE in the Startup Range, Core Average Power versus Time for 1.6%, 2.0%, and 2.5% Worth's (Point Model Kinetics) 15A-4 Continuous Control Rod Withdrawal from Hot Startup APPENDIX 15A 15A-iii REV. 13, SEPTEMBER 2006
LGS UFSAR APPENDIX 15A - CONTINUOUS CONTROL ROD WITHDRAWAL TRANSIENT ANALYSIS 15A.1 INTRODUCTION The continuous control rod withdrawal transient analysis in the startup range (Section 15.4.1.2) was performed to demonstrate that the licensing basis criteria for fuel failure will not be exceeded when an out-of-sequence control rod is withdrawn at the maximum allowable normal drive speed.
The sequence and timing assumed in this special analysis are shown in Table 15A-1.
The RWM constraints on rod sequences will prevent the continuous withdrawal of an out-of-sequence rod. This analysis was performed to demonstrate that, even for the unlikely event where the RWM fails to block the continuous withdrawal of an out-of-sequence rod, the licensing basis criterion for fuel failure is still satisfied.
The methods and design basis used for performing the detailed analysis for this event are similar to those previously approved for the control rod-drop accident (References 15A.1 through 15A.3).
Additional simplified point model kinetics calculations were performed to evaluate the dependence of peak fuel enthalpy on the control blade worth. For the detailed calculation, the 50% control rod density pattern was selected as the initial starting condition, which is consistent with the approved design basis for the control rod-drop accident (References 15A.1 through 15A-3).
The licensing basis criterion for fuel failure is the contained energy of a fuel pellet located in the peak power region of the core which shall not exceed 170 cal/gm-UO2.
15A.2 METHODS OF ANALYSIS Because the rod worth calculations using the approved design basis methods (References 15A.1 through 15A-3) use three-dimensional geometry, it is not practical to do a detailed analysis of this event parameterizing control rod worths. Therefore, the methods of analysis employed were to perform a detailed evaluation of this event for a typical BWR and control rod worth (1.6% k) and to use a point model calculation to evaluate the results over the expected ranges of out-of-sequence control rod worths. The detailed calculations are performed to demonstrate (1) the consequences of this event over the expected power operating range and (2) the validity of the approximate point model calculation. The point model calculation will demonstrate that the licensing criterion for fuel failure is satisfied over the range of expected out-of-sequence control rod worths. These methods are described in detail below.
The methods used to perform the detailed calculation are identical to those used to perform the design basis control rod-drop accident with the following exceptions:
- a. The rod withdrawal rate is 3.6 in/sec (0.3 fps) rather than the blade drop velocity of 3.11 fps.
- b. Scram is initiated either by the IRM or 15% APRM scram in the startup range. The IRM system is assumed to be in the worst bypass condition allowed by Technical Specifications.
- c. The blade being withdrawn inserts along with remaining drives at Technical Specification insertion rates upon initiation of scram signal.
Examination of a number of rod withdrawal transients in the low power startup range, using a two-dimensional R-Z model, has shown that higher fuel enthalpy addition would result from transients starting at the 1% power level rather than from lower power levels. The analysis further shows that for continuous rod withdrawal from these initial power levels (1% range) the APRM 15%
APPENDIX 15A 15A.1 REV. 13, SEPTEMBER 2006
LGS UFSAR power level scram is likely to be reached as soon as the degraded (worst bypass condition) IRM scram. Consequently, credit is taken for either the IRM or APRM 15% scram in meeting the consequences of this event. The transients for this response were initiated at 1% of power and were performed using the 15% APRM scram.
An initial point kinetics calculation was run to determine the time to scram based on an APRM scram setpoint of 15% power and an initial power level of 1%. From this time and the maximum allowable rod withdrawal speed, it is possible to show the degree of rod withdrawal before reinsertion due to the scram. From this information, Figure 15A-1, showing the modified effective reactivity shape, was constructed.
The point model kinetics calculations use the same equations employed in the adiabatic approximation described on page 4-1 of Reference 15A.1. The rod reactivity characteristics and scram reactivity functions are input identically to the adiabatic calculations, and the Doppler reactivity is input as a function of core average fuel enthalpy. The Doppler reactivity feedback function input to the point model calculations was derived from the detailed analysis of the 1.6%
rod worth case described above. This is a conservative assumption for higher rod worths because the power peaking and therefore spatial Doppler feedback will be larger for higher rod worths. As will be seen in Section 15A.3, maximum enthalpies resulted from cases initiated at 1% of rated power. In this power range, the APRM will initiate a scram at 15% of power; therefore, the APRM 15% power scram was used for these calculations, thereby eliminating the need to perform the spatial analysis required for the IRM scram. All other inputs are consistent with the detailed transient calculation.
The point model kinetics calculations result in core average enthalpies. The peak enthalpies were calculated using the following equation:
h = ho + (P/A)T ( h f - ho) (EQ. 15A-1) where:
h = Final peak fuel enthalpy ho = Initial fuel enthalpy hf = Final core average fuel enthalpy (P/A)T = Total peaking factor (radial peaking
- axial peaking
- local fuel pin peaking).
For these calculations, the (radial
- axial) peaking factors as a function of rod worth were obtained from the calculations performed in section 3.6 of Reference 15A-2 and are shown in Figure 15A-2.
It was conservatively assumed that no power flattening due to Doppler feedback occurred during the course of the transient.
15A.3 RESULTS The reactivity insertion resulting from moving the control rod is shown in Figure 15A-1 for the point kinetics calculations. The core average power versus time and the global peaking factors from section 3.6 of Reference 15A-2 are shown in Figures 15A-3 and 15A-2, respectively. The results of the point kinetics calculation are summarized in Table 15A-2 along with the results of the detailed analysis.
APPENDIX 15A 15A.2 REV. 13, SEPTEMBER 2006
LGS UFSAR From Figure 15A-3 and Table 15A-2, it is shown that the core average energy deposition is insensitive to control rod worth; therefore, the only change in peak enthalpy as a function of rod worth will result from differences in the global peaking which increases with rod worth. Comparison of the global peaking factors shown in Figure 15A-2 with the values used in the detailed calculations demonstrates that the Reference 15A-2 values are reasonable for their application in this study. For all cases, the peak fuel enthalpy is well below the licensing design criteria of 170 cal/gm.
Cases 4 and 5 of Table 15A-2 show that the point kinetics calculations give conservative results relative to the detailed evaluations. The primary difference is that the global peaking will flatten during the transient due to Doppler feedback. This is accounted for in the detailed calculation, but the point kinetics calculations conservatively assumed that the peaking remains constant at its initial value.
The differences in core average and peak enthalpy between cases 1 and 5 are due to the fact that for case 1 the scram was initiated by the 15% APRM scram setpoint, whereas in case 5 the scram was initiated by the IRMs. As seen by Figure 15A-4, this occurred at a core average power of 21%. Because the APRM trip point will be reached first, it is reasonable to take credit for the APRM scram.
15A.4 CONCLUSIONS From this study, the following conclusions can be stated:
- a. The resultant peak fuel enthalpies due to the continuous withdrawal of an out-of-sequence rod in the startup range results in peak fuel enthalpies that are significantly less than the licensing basis criteria of 170 cal/gm.
- b. The point model calculations used to assess the sensitivity of peak enthalpy as a function of control rod worth are in good agreement with, and slightly conservative relative to the more detailed design basis model which is employed to evaluate the continuous rod withdrawal transient in the startup range.
15A.5 REFERENCES 15A-1 C.J. Paone et al, "Rod-Drop Accident Analysis For Large Boiling Water Reactors",
NEDO-10527, (March 1972).
15A-2 R.C. Stirn et al, "Rod-Drop Accident Analysis For Large Boiling Water Reactors",
NEDO-10527, Supplement 1, (July 1972).
15A-3 R.C. Stirn, "Rod-Drop Accident Analysis For Large Boiling Water Reactors, Addendum 2, Exposed Cores", NEDO-10527, Supplement 2, (January 1973).
APPENDIX 15A 15A.3 REV. 13, SEPTEMBER 2006
LGS UFSAR Table 15A-1 SEQUENCE OF EVENTS FOR CONTINUOUS ROD WITHDRAWAL DURING REACTOR STARTUP TIME (sec) EVENT 0 The reactor is critical and operating in the startup range.
>0 The operator selects and withdraws an out-of-sequence control rod at the maximum normal drive speed of 3.6 in/sec.
4 sec (Approx) The RWM fails to block the selection (selection error) and continuous withdrawal (withdraw error) of the out-of-sequence rod.
4-8 sec The reactor scram is initiated by the IRM system or the APRM system.
5-9 sec The prompt power burst is terminated by a combination of Doppler and/or scram feedback.
10 sec The transient is finally terminated by the scram of all rods, including the control rod being withdrawn. Scram insertion times are assumed to be technical specification insertion rates.
APPENDIX 15A 15A.4 REV. 13, SEPTEMBER 2006
SUMMARY
OF RESULTS FOR DETAILED AND POINT KINETICS EVALUATIONS OF CONTINUOUS ROD WITHDRAWAL IN THE STARTUP RANGE Control Rod Case Worth (%k) hf(cal/gm) P/A(1) h(cal/gm) 1 1.6 17.3 24.2 42.7 2 2.0 17.3 30.9 50.0 3 2.5 17.2 46.0 58.5 4 1.6(2) 18.3 19.7(3) 56.2 5 1.6(4) 18.3 19.7 59.6 (1)
P/A = global peaking factor (Radial
- Axial).
(2)
Detailed transient calculation. All other data reported are for point kinetics calculations.
(3)
The P/A = 19.7 is the initial value. For the detailed analysis, this value will decrease during the course of the transient because the power shape will flatten due to Doppler feedback.
(4)
Point kinetics calculation with IRM initiated scram and three-dimensional simulator global peaking.
APPENDIX 15A 15A.5 REV. 13, SEPTEMBER 2006