ML17114A526

APR1400-F-A-NR-14003-NP, Rev. 1 Post-LOCA Long Term Cooling Evaluation Model.

ML17114A526
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Issue date:	03/31/2017
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APR1400-F-A-NR-14003-NP, Rev 1
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Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 Post-LOCA Long Term Cooling Evaluation Model Revision 1 Non-Proprietary March 2017 Copyright 2017 Korea Electric Power Corporation &

Korea Hydro & Nuclear Power Co., Ltd All Rights Reserved KEPCO & KHNP

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 REVISION HISTORY Revision Date Page Description September 0 All First Issue 2014 March 1 All Revised to reflect RAI-8457 2017 This document was prepared for the design certification application to the U.S. Nuclear Regulatory Commission and contains technological information that constitutes intellectual property of Korea Hydro & Nuclear Power Co., Ltd. Copying, using, or distributing the information in this document in whole or in part is permitted only to the U.S. Nuclear Regulatory Commission and its contractors for the purpose of reviewing design certification application materials. Other uses are strictly prohibited without the written permission of Korea Electric Power Corporation and Korea Hydro & Nuclear Power Co., Ltd.

KEPCO & KHNP i

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 ABSTRACT This report is to establish adequacy of the LTC (long-term cooling) of the reactor vessel following the LOCA (loss of coolant accident) for the APR1400 design. This report provides the overview of the applicable methodology and the description of specific assumptions incorporated into the codes used to analyze the LTC (long-term cooling), as well as a discussion of the bases for applying these codes and methods to the APR1400 design. The validation of principle models of these codes is presented through comparisons with computer codes that have been approved by the USNRC. The following codes are used in the LTC analysis:

The CEPAC (Reference 1) computer program is used to calculate the secondary system temperature, the NATFLOW (Reference 1) computer program is used to calculate the RCS (reactor coolant system), core and loop natural circulation flow rates and temperatures after RCS has refilled for small breaks, and the CELDA (Reference 1) computer program is used to calculate the long-term depressurization and refill of the RCS for small breaks. Boric acid concentration in the core is calculated by using the BORON (Reference 1) computer program.

This report also provides history of changes in methodology for LTC analysis. For the calculation of boric acid concentration, the new mixing volume assumption based on the Westinghouse LTC analysis of Waterford 3 (Reference 2) is used.

Based on the results of this analysis, it is concluded that the APR1400 SIS (safety injection system) satisfies all of 10 CFR 50.46 (Reference 3) acceptance criteria for LTC.

KEPCO & KHNP ii

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 TABLE OF CONTENTS 1 INTRODUCTION............................................................................... 1 2 BASIC LONG-TERM COOLING PLAN ............................................... 2 2.1 Functional Requirement ................................................................................................................. 2 2.2 Operational Sequence .................................................................................................................... 2 2.3 Basis of Plan ................................................................................................................................... 2 2.4 Emergency Core Cooling System Alignments ............................................................................... 3 3 ANALYTICAL APPROACH ................................................................ 6 3.1 General Description ........................................................................................................................ 6 3.2 LTC Boric Acid Precipitation Analysis ............................................................................................. 6 3.3 LTC Cooldown Analysis .................................................................................................................. 6 3.4 Codes Used in the LTC Analysis .................................................................................................... 7 3.5 Changes in Methodology ................................................................................................................ 7 3.5.1 Interim Approach Used at Waterford 3........................................................................................... 7 3.5.2 Modification of the BORON Code for Application of IRWST ......................................................... 8 3.5.3 Steam Flow Rate Calculation Using the Decay Heat Model (ANS 1971) ..................................... 8 3.5.4 Core Flush Flow ............................................................................................................................. 9 3.5.5 Calculation Method and Result for the Mixing Volume .................................................................. 9 3.6 Major Assumptions and General System Parameters ................................................................. 11 4 RESULTS OF LTC ANALYSIS ......................................................... 22 5 CONCLUSIONS .............................................................................. 27 6 REFERENCES ................................................................................ 28 KEPCO & KHNP iii

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 LIST OF TABLES Table 3-1 Major Variables for Calculation of the Mixing Volume ........................................................... 13 Table 3-2 Calculation of (Top of the Core Region) ......................................................................... 14 Table 3-3 Calculation of at each Time ............................................................................................ 15 Table 3-4 Calculation of the Boil-off Rate .............................................................................................. 16 Table 3-5 Calculation Results of Mixing Volume Using the Interim Methodology ................................. 17 Table 3-6 Solubility Limit of Boric Acid Solution .................................................................................... 18 Table 3-7 General System Parameters and Initial Conditions .............................................................. 18 Table 3-8 NOMENCLATURE LIST ........................................................................................................ 19 LIST OF FIGURES Figure 2-1 Long Term Cooling Plan .......................................................................................................... 4 Figure 2-2 Overlap Range of Cold-Leg Break Area .................................................................................. 5 Figure 3-1 Mixing Volume Change ......................................................................................................... 20 Figure 3-2 The Mixing Volume Calculated by Interim Methodology ....................................................... 21 Figure 4-1 RCS Refill Time vs. Break Area............................................................................................. 23 Figure 4-2 RCS Pressure after Refill vs. Break Area .............................................................................. 24 Figure 4-3 Inner Vessel Boric Acid Concentration vs. Time.................................................................... 25 Figure 4-4 BORON Calculated Results of Mixing Volume Change ........................................................ 26 KEPCO & KHNP iv

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 ACRONYMS AND ABBREVIATIONS CE Combustion Engineering CFR Code of Federal Regulation DVI direct-vessel injection ECCS emergency core cooling system FAP fuel alignment plate HF Henry-Fauske IRWST in-containment refueling water storage tank LOCA loss of coolant accident LTC long-term cooling NPP nuclear power plant PWR pressurized water reactor RCS reactor coolant system RWT refueling water tank SBLOCA small break LOCA SCS shutdown cooling system SDC shutdown cooling SG steam generator SI safety injection SIS safety injection system USNRC U.S. Nuclear Regulatory Commission KEPCO & KHNP v

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 1 INTRODUCTION The post-LOCA long-term cooling is defined as beginning at the time that the core is reflooded and ending when the plant is secured. During the long-term, operator action is needed to assure that core cooling is maintained until the plant can be brought to a cold shutdown condition. The LTC (long-term cooling) plan culminates in a secured plant with a minimum number of decisions and actions on the part of plant operators.

The purpose of this technical report is to present the LTC computer codes and methodologies for the analysis of LTC events in the APR1400 design control document (Reference 4) Chapter 15, except for the dose evaluation. The LTC methodology used for the APR1400 design is very similar to the conventional LTC methodology (Reference 1) used for currently operating U.S. Combustion Engineering (CE)-fleet PWRs (Pressurized Water Reactors).

In Chapter 2, the basic LTC plan for the APR1400 design is described. The details of analytical approach and changes in methodology are described in Chapter 3. Through the results of LTC analysis in Chapter 4, it is confirmed that the LTC methodology is applicable in the APR1400 design successfully.

KEPCO & KHNP 1

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 2 BASIC LONG-TERM COOLING PLAN 2.1 Functional Requirement The basic function of long-term cooling plan is to maintain the core at safe temperature levels while avoiding the precipitation of boric acid in the RCS.

2.2 Operational Sequence The LTC plan makes provision for maintaining core cooling and boric acid flushing by simultaneous hot-leg and DVI (direct-vessel injection) line injection for large break LOCA, for initiating shutdown cooling, if the break is small enough to assure successful operation of SCS. The knowledge of pressurizer pressure gives an idea to the plant operator for decision of the procedure.

Figure 2-1 show the basic sequence of events and timing of operator actions in the long-term cooling plan, as applied to the APR1400 design.

Major assumptions on operator actions in CENPD-254 (Reference 1) methodology are as follows:

- At one hour after LOCA, the operator has started operation of the steam generator cooldown.

- At two hours after LOCA, the operator has started operation of hot-leg and DVI line injection simultaneously.

- At eight hours after LOCA, if the RCS pressure remains above 450 psia, the reactor coolant system has been filled with liquid water.

2.3 Basis of Plan The LTC plan is based on the following reasoning.

1) Small and large break LOCA bring for distinctly different responses in the long-term cooling plan.

2) It is possible to determine from the pressurizer pressure whether the break is large or small.

The simultaneous hot-leg and DVI line injection from the SI (safety injection) pumps is prescribed by the LTC plan following any LOCA. This simultaneous injection prevents boric acid precipitation for an extensive range of large and intermediate sized breaks in either hot-leg or DVI line. For extremely small breaks, where reactor coolant system pressure remains high, the simultaneous injection flow is too small to provide effective flushing of the boric acid however, with extremely small breaks the system refills and the boric acid concentration remains low due to its dispersal throughout the RCS by natural circulation. As indicated in Figure 2-2, there is a range of break sizes where boric acid precipitation is prevented by either flushing by SI pump injection or by dispersal by natural circulation.

In addition to boric acid precipitation, the cooling of the RCS must be considered. The SI pump injection is capable of adequate cooling of the RCS for all but the smallest breaks. For the smallest breaks, the steam generators are initially employed to cool the RCS, with subsequent activation of the SCS (shutdown cooling system). There is a range of break sizes for which the SI pump injection alone can cool the RCS after the initial period of steam generator heat removal, but which also yields system conditions such that eventual successful entry into shutdown cooling is assured. Therefore, there is an overlap in which either of the two different core cooling modes is satisfactory as shown in figure 2-2.

KEPCO & KHNP 2

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 2.4 Emergency Core Cooling System Alignments The different alignments of the ECCS (emergency core cooling system) used in the LTC plan are as follows:

- Initial recirculation mode The injection by SI pumps from the IRWST (in-containment refueling water storage tank) has been secured.

- Simultaneous injection mode One or one half of the SI pump flow has been realigned to the RCS hot-legs. The LTC plan calls for a shift to this mode at about two hours after any LOCA.

- Shutdown cooling mode A small break is indicated by reactor coolant system pressure above 450 psia, at about eight hours after the LOCA. In this case, the reactor coolant system is entirely refilled. The SI pumps maintain the system pressure, and the RCS liquid level is sufficient for entry into SDC mode. The reactor coolant system temperature is then checked to assure that steam generator cooling has reduced it to the shutdown cooling entry value. Then, the SI pumps are realigned to discharge entirely into the DVI line; then they are throttled to reduce the RCS pressure to the SDC entry value. The shift is then made to the SDC mode.

KEPCO & KHNP 3

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 LOCA  : Loss of Coolant Accident LOCA SG  : Steam Generator SIT  : Safety Injection Tank SIAS SI  : Safety Injection OSP  : Off Site Power SI Pumps actuated(DVI)

P  : Primary Pressure Auto AFAS  : Auxiliary Feedwater Actuation Signal AFAS SCS  : Shutdown Cooling System SIAS  : Safety Injection Actuation Signal Auxiliary Feed Water Flow Actuated t  : Time after LOCA, hrs Auto Yes No OSP Available Activate Activate Turbine Bypass t < 1.0 Atmos. Dump Valve Manual Manual Isolate or Vent 1.0 < t < 3.0 the SITs Note :

Pressurizer The values for pressure shown are those Depressurization Initiated 1.0 < t < 4.0 indicated by the instrumentation. Manual They account for Instrumentation uncertainty. Align SI Flow to Hot Legs & DVI Nozzles 1.0 < t < 2.0 Manual Yes No P > 450 psia (31.6 kg/cm2A) 8.0 < t < 9.0 Yes No SCS Operable Maintain SI Injection to Hot and DVI Nozzles Yes No Maintain SG Existing SCS Entry Condition Heat Removal Existing Secure Steam Generators Manual Align All SI Flow Throttle SI Flow to DVI Nozzles Manual Manual Actuate SCS Manual Secure Steam Generators Manual Figure 2-1 Long Term Cooling Plan KEPCO & KHNP 4

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 RCS Pressure Break Area at 8 Hours cm2 (ft2) kg/cm2A (psia) 464.5 (0.500) 2.8 (38)

Simultaneous Hot Leg/DVI Nozzles 92.9 (0.100) 5.3 (75)

Injection Cools Core and Flushes 46.5 (0.050) 5.3 (76)

Boric Acid from Vessel.

37.2 (0.040) 5.3 (76) 35.3 (0.038) 5.3 (76) 34.4 (0.037) 6.0 (86) 27.9 (0.030) 7.6 (108) 18.6 (0.020) 11.3 (161)

Refill of RCS Disperses Boric 9.3 (0.010) 25.9 (368)

Acid throughout System and 4.6 (0.005) 57.4 (816)

SGs are able to cool RCS to 3.7 (0.004) 69.6 (990)

SDC Entry Temperature.

2.8 (0.003) 83.2 (1184)

Figure 2-2 Overlap Range of Cold-Leg Break Area KEPCO & KHNP 5

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 3 ANALYTICAL APPROACH 3.1 General Description The basic objective of the LTC analysis is to demonstrate the long-term coolability of core. The analysis procedures account for single-failures to assure that the performance objectives are consistent with this assumption.

It is important to recognize the difference in behavior between large and small break LOCAs in long-term cooling. The difference is that the RCS will remain at high pressure for small breaks and the injection flow rate will be too low for effective cooling; thus, small breaks require the SGs to cool the RCS until SDC can be initiated. In contrast, large breaks are adequately cooled by the injection flow because this flow is large due to the low RCS pressure. However, large breaks must utilize simultaneous hot-leg and DVI line injection to flush boric acid from the vessel. Thus, the LTC large-break and small-break analyses are different from each other.

Another issue to be considered is the effect of break location. For any large hot-leg break, the short-term ECCS injection flow through DVI lines will fill the annulus and provide the elevation head necessary to force flow through the core and out of the hot-leg break. Liquid flow which is in excess of the core boil-off will provide a substantial flushing flow through the core, and it will decrease and maintain the core boric acid concentration similar to that of the low levels at initial IRWST.

For large cold-leg breaks, however, boric acid concentrates in the core as long as the cold-leg injection is continued. If it is determined that the break occurred in the cold-leg (and hot-leg injection is possible.), the ECCS injection flow would be switched to hot-leg injection only. When sufficient elevation head builds up on the hot-leg, the core flushing flow will move down through the core and up through the annulus to the break.

In case of slot break at top of the cold-leg, the margin will be reduced because loop-seal refilling will increase the pressure drop of core-to-break steam flow. However, water in the cold-leg can be credited.

Therefore, the margin will be increased because crediting water in the cold-leg will increase the hydrostatic head of the downcomer. The additional pressure drop will be covered by this margin.

Therefore, long-term loop-seal refilling with a slot break at the cold-leg does not significantly affect the boric acid precipitation analysis.

Since the break location may not be easily determined, the long-term ECCS alignment should be able to cope with breaks in other locations. This ability is achieved by converting from short-term ECCS injection of cold-leg to long-term injection of simultaneous DVI and hot-leg. The intact side of RCS will build up the elevation head necessary to transfer flow through the core and out of the break. The cold-leg injection flow will continue through the cold-leg ECCS injection nozzles while the hot-leg injection point will be through the hot-leg suction lines of shutdown cooling system.

3.2 LTC Boric Acid Precipitation Analysis The LTC analysis boric acid precipitation (Reference 1) is chiefly concerned with control of the boric acid concentration. The timely initiation of simultaneous DVI line and hot-leg SI pump injection enables the intact side of the vessel to accumulate liquid elevation head. Once sufficient head has been established, a flushing flow of liquid through the core and out through the break will be provided thereby cooling the core and removing boric acid.

3.3 LTC Cooldown Analysis The post-LOCA procedures (Reference 1) for LTC cooldown is similar to those used during a normal cooldown to a cold shutdown condition. Long-term cooling of the core can be performed by the SDC KEPCO & KHNP 6

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 system since the RCS will be refilled by the SDC system where the simultaneous DVI and hot-leg injection is unable to cool and flush the core.

3.4 Codes Used in the LTC Analysis The LTC calculations are performed by using the LTC codes, which is described down below.

The CELDA (Reference 1) is used to describe the long-term primary system depressurization process and to determine whether the refilling of RCS is achieved for small breaks. The analysis is initialized from the CEFLASH-4AS analysis that is performed for the early part of accident. The steam generator secondary temperature as a function of time is input from the CEPAC analysis.

The NATFLOW (Reference 1) calculates the natural circulation flow rates in the core, and primary system pressure and temperature that occur in the absence of a primary system break. The code is run in an iterative sequence with the CEPAC code which provides the secondary system temperature as a function of time.

The BORON (Reference 1) is used to compute the boric acid concentration in the core and determines if the core flow is sufficient to prevent the solubility limit of boric acid from being exceeded.

The CEPAC (Reference 1) models the steam generators, including the operation of steam generator atmospheric dump valves, and provides the secondary system temperature as a function of time is used for input of the NATFLOW and CELDA codes. NATFLOW and CELDA codes do not contain independent steam generator models.

3.5 Changes in Methodology The following sections of this report describe the method of analysis and the method of updated licensing basis of boric acid precipitation analysis.

3.5.1 Interim Approach Used at Waterford 3 Some non-conservatism has been identified in the previous methodology (Reference 1), and as a result, in 2005, the USNRC suspended approval of CENPD-254 (Reference 5), which is the old LTC methodology for CE-designed nuclear power plants. The major issues related to the suspension are described as follows:

Void effect: The mixing volume must be justified and the void fraction must be taken into account when computing the boric acid concentration.

Time-varying mixing volume: The analysis to determine boric acid concentration needs to account for the variation in the mixing region while considering the pressure drop in loop.

Decay heat: The decay heat model in appendix K with a multiplier of 1.2 has to be used at all times.

Boric acid solubility limit: The solubility limit must be justified.

The interim approach is to reflect resolutions of four issues above to CENPD-254, and the methodology applying such interim approach is the interim methodology. The updated analysis for the APR1400 design utilized the post-LOCA LTC methodology with the interim approach. The two items described below in more detail are related to void effect and time-varying mixing volume issues.

(1) The liquid volume in the core and upper plenum mixing volumes (based on the void fraction) was calculated by applying the CEFLASH-4AS (Reference 6) phase-separation model to this region. The KEPCO & KHNP 7

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 phase-separation model used in CEFLASH-4AS was originally approved by USNRC for computing the mixture level in the core following all SBLOCAs. This model was shown to accurately predict the void fraction and, hence, the two-phase level in regions experiencing high rates of heat addition following SBLOCAs.

(2) The mixing volume was increased to include 50 percent of the reactor vessel lower plenum. The BACCHUS test (Mitsubishi Heavy Industries) (Reference 7) employed to simulate post-LOCA boric acid mixing in the lower plenum, and in the core of a Westinghouse and CE-designed PWR, was cited as justification for expanding the mixing volume to include a portion of lower plenum. The tests showed that the entire lower plenum volume contributed mixing, and thus only 50 percent of this volume to be credited is conservative.

3.5.2 Modification of the BORON Code for Application of IRWST The APR1400 design adopts IRWST instead of the refueling water tank (RWT) used in previous CE-type plants. In CE-type plants, ECC is injected from the RWT for a certain amount of time and is changed to the sump when the RWT is emptied. In the APR1400 design, however, ECC is injected from IRWST from the beginning. Therefore, the BORON code for APR1400 was modified to model the IRWST.

3.5.3 Steam Flow Rate Calculation Using the Decay Heat Model (ANS 1971)

The decay heat fraction (DHF) at one hour post-LOCA is determined by using the BORON computer code decay heat model described in CENPD-254-P-A, which is reproduced below.

TS where DECAY = normalized decay heat fraction including 1.1 conservative multiplier T = time (seconds).

When using a time of 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> or 3600 seconds, the DHF is:

TS DECAY = = 0.016143 Consistent with NRC imposed restrictions on the acceptability of the boric acid precipitation methodology, a decay heat multiplier of 1.2 was applied below:

Decay Heat Fraction (including 1.2 decay heat multiplier)

= 0.016143

• 1.2 / 1.1

= 0.01761 KEPCO & KHNP 8

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 The core power level, including power measurement uncertainty, is 4063 MWt. Therefore, using the above data, the core water boil-off rate (WBO) at 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> post-LOCA is equal to the core power times the decay heat fraction divided by the heat of vaporization, as shown below:

WBO = 4063 MWt

• 948.04 Btu/sec-MWt

• 0.01761 / (1150.28 180.18) Btu/lbm WBO = 69.95 lbm/sec 3.5.4 Core Flush Flow Core flush begins two hours after the ECCS is realigned according to the LTC plan for DVI line/hot-leg at two hours post LOCA. The core flushing flow is 1 SI pump flow or 1/2 SI pump flow for both cold-leg break and DVI line break. This report used 1/2 SI pump flow for the core flushing flow, for conservatism. The core flushing flow obtained is thus shown below.

W flush = 1/2 W SI W boiloff where, W flush = core flush flow rate W SI = SI injection flow rate into the reactor vessel W boiloff = water boiled off rate in the reactor vessel The BORON code is applied to calculate the boric acid concentration of double-ended guillotine breaks.

Thus, the RCS pressure is reduced to the containment pressure. However, 0.45 W SI was used instead of 1/2 W SI for analytical flexibility.

W flush = 0.45 W SI W boiloff 3.5.5 Calculation Method and Result for the Mixing Volume The major variables for calculation of the mixing volume are shown in Table 3-1. Figure 3-1 shows the mixing volume change from CENPD-254 to interim. Each part of the mixing volume is shown in Figure 3-

2. The atmospheric conditions (14.7 psia, 212 °F) assumed for LTC analysis were used to perform the following calculations.

A flat axial power shape was selected as a reasonably conservative representation of the axial power distribution. Therefore, in this report multiple core regions were included, as shown in Table 3-2. The void fraction of each core region was calculated by using the equations below and the results are shown in Table 3-2 and Table 3-3. In this report, the average void fraction shown in Table 3-3 was used to calculate the mixing volume.

The mixing volume used in the interim methodology was calculated by applying the CEFLASH-4AS phase separation model to this region, using the following equation. The variables in equations 1 through 13 were defined in Table 3-8.:

KEPCO & KHNP 9

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 TS Equation 1 TS where, PN = bubble production rate =

W N = flow bubbles from the lower sub-region P

VD = drift velocity = 3.0EXP 0.751000.0 TS Equation 2 B and C can be obtained from Equation 1, TS Equation 3 Equation 4 and, Equation 4 is divided by V AN ZN k PZ +k+WN B = 1 ln N . Equation 5 PN ZN k+WN If we assume the condition is subcooled, WN = 0, Wflashing = 0 k PZ +k B = 1 ln N Equation 6 PN ZN k k m+k B = 1 ln Equation 7 m k where, k = VD V AN TS Q

PN ZN = = m hlv KEPCO & KHNP 10

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 VD AC C = PN ZN +WN Equation 8 1C kC

= m Equation 9 1C m

C = . Equation 10 k+m The summarized results of the void fraction () are:

A = 0 Equation 11 k m(t)+k B =1 ln Equation 12 m(t) k m(t)

C = . Equation 13 k+m(t)

The variables used in the above equations are summarized in Table 3-1 and, the values for the time-dependent-boil-off rate are calculated in Table 3-4. In this report, we assumed a core of ten regions. The mixture height vs. time and the core and upper plenum void distributions at three time points of time are shown in Table 3-2 and Table 3-3, respectively.

The final mixing volume, based on a void fraction that corresponds to the above three points of time, is shown in Table 3-5. Void fraction C was calculated considering the core-area to outlet-plenum ratio. The result of C is shown in Table 3-3.

3.6 Major Assumptions and General System Parameters The major assumptions used in performing the LTC analysis are as follows:

a. No offsite power is available.

b. The worst single failure is the loss of two SI pump trains with additional conservativeness. This results in the following:

1) Two SIPs are operable.

KEPCO & KHNP 11

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1

2) One motor-driven auxiliary feedwater pump is operable.

c. One atmospheric dump valve on each SG is available to cool down the RCS.

d. RCS cooldown begins at 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> post-LOCA.

e. The SITs are vented or isolated before establishing shutdown cooling conditions for the small-break LTC procedure.

f. The pressurizer is depressurized to establish shutdown cooling conditions for the small-break LTC procedure.

g. RCS cooldown is terminated when the hot leg temperature is below the maximum shutdown cooling entry temperature including instrument uncertainty.

h. Pump flow rates and initial water source inventories used in the large-break LOCA boric acid precipitation analysis are selected to maximize the boric acid concentration in the core.

i. The solubility limit (29.3 wt %) (Reference 8) at the boiling temperature of boric acid at 1.03 kg/cm 2A is conservatively assumed as shown in Table 3-6. The real pressure is higher than 1.03 kg/cm2A considering the downcomer head and RCS flow resistance. Therefore, it is conservative limit.

Significant core and system parameters used in the post-LOCA long-term cooling analysis are presented in Table 3-7.

KEPCO & KHNP 12

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 Table 3-1 Major Variables for Calculation of the Mixing Volume Value Reference Power (Q) 4063 MWt 3983*1.02 Enthalpy (hfg) 970.05 Btu/lb 14.7 psia TS Acore AOutlet Plenum g 0.03732 lb/ft3 14.7 psia 14.7 VD 2.967 ft/sec 3.0EXP 0.751000.0 TS K=(VD* g *Acore) s KEPCO & KHNP 13

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 Table 3-2 Calculation of (Top of the Core Region)

Calculated Boil-off rate for each Mixing Void Fractions Height, Region, lbm/sec Volume ft Region B B B 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> (at 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />) (at 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />) (at 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br />)

TS 1 1.25 2 1.25 3 1.25 4 1.25 5 1.25 6 1.25 7 1.25 8 1.25 9 1.25 10 1.25 KEPCO & KHNP 14

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 Table 3-3 Calculation of at each Time C

Time (hours)

TS 0.0083 0.56 1.0 1.39 1.5 2.0 2.78 3.0 4.0 5.0 8.0 13.0 KEPCO & KHNP 15

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 Table 3-4 Calculation of the Boil-off Rate Time (hours) Decay Heat (lbm/sec) 0.0083 0.058345 231.74 0.56 0.020358 80.86 1.0 0.017611 69.95 1.39 0.016219 64.42 1.5 0.015913 63.20 2.0 0.014809 58.82 2.78 0.013638 54.17 3.0 0.013381 53.15 4.0 0.012453 49.46 5.0 0.011777 46.78 8.0 0.010471 41.59 13.0 0.009274 36.84 24.0 0.007956 31.60 KEPCO & KHNP 16

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 Table 3-5 Calculation Results of Mixing Volume Using the Interim Methodology Volume Final Volume Region Void Fraction (ft3) (ft3) TS Crediting 50%

participation of the Bottom inactive core lower plenum volume (top of lower support is conservative Lower structure to bottom relative to the Plenum of active core)

BACCHUS test (A) results.

Flow skirt to top of lower support Only liquid structure The void fraction in Core the core is calculated Core, guide tube, using the CEFLASH-(B) core shroud 4AS phase separation model.

Bottom of FAP to bottom of hot-leg The liquid volume in the outlet plenum is Outlet calculated by Plenum applying the core-to-(C) outlet plenum area ratio to the core exit Top inactive core void fraction (top of active core to bottom of FAP)

Total Volume, ft3 KEPCO & KHNP 17

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 Table 3-6 Solubility Limit of Boric Acid Solution Temperature, °F Temperature, °C Pressure, (psia) /atm Solubility, wt%

32 0.0 14.7 / 1.0 2.70 50 10.0 14.7 / 1.0 3.51 68 20.0 14.7 / 1.0 4.65 86 30.0 14.7 / 1.0 6.34 104 40.0 14.7 / 1.0 8.17 122 50.0 14.7 / 1.0 10.23 140 60.0 14.7 / 1.0 12.97 158 70.0 14.7 / 1.0 15.75 176 80.0 14.7 / 1.0 19.06 194 90.0 14.7 / 1.0 23.27 212 100.0 14.7 / 1.0 27.53 217.9 103.3 14.7 / 1.0 29.27 Table 3-7 General System Parameters and Initial Conditions Quantity Value Reactor Power Level (102 % of Nominal), MWt 4,062.66 SCS Entry Temperature, °C (°F) 193 (380)

SCS Entry Pressure, kg/cm2A (psia) 28.1 (400)

Atmospheric Dump Valve Capacity, per Valve 430,900 (950,000) (min) at 70.3 kg/cm2A (1,000 psia), kg/hr (lbm/hr)

Auxiliary Feedwater Storage Tank Capacity, per 1,870,000 (494,000) (min) tank, L (gal)

Boric Acid Concentration, wt% RCS 0.94 (1,650) (max)

(ppm)

IRWST 2.52 (4,400) (max)

SIT 2.52 (4,400) (max)

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Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 Table 3-8 NOMENCLATURE LIST VD drift velocity N region index (Z) the void fraction as a function of axial position v vapor density AN cross-sectional area of region N N the bubble production rate WN the in-flow bubbles from the lower sub-region hlv latent heat of vaporization flashing the linear flashing steam rate Z2ø two-phase mixture height GB,SS the steady state bubble mass of region N ZN either the height of region N or the two-phase mixture height in region N.

Q the total energy deposition rate for region N A void fraction in the lower plenum B void fraction in the core region C void fraction in the outlet plenum KEPCO & KHNP 19

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 Figure 3-1 Mixing Volume Change KEPCO & KHNP 20

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 Figure 3-2 The Mixing Volume Calculated by Interim Methodology KEPCO & KHNP 21

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 4 RESULTS OF LTC ANALYSIS The objective of this technical report is to describe improvements arising from changes in the methodology and the LTC codes. For the analysis of LTC, the new mixing-volume-calculation and cooldown methodology is the same as that used for the Westinghouse LTC analysis of Waterford 3 (Reference 2) at extended power uprate.

The LTC analysis predicts that the RCS will be refilled at various times depending on break sizes, as shown in Figure 4-1. As shown in the figure, for a break size as large as 0.037 ft2, the RCS is refilled within eight hours. In addition, the LTC analysis determines that the time over 14 hours1.62037e-4 days <br />0.00389 hours <br />2.314815e-5 weeks <br />5.327e-6 months <br /> is required to exhaust all auxiliary feedwater during cooldown of the RCS. Therefore, to allow a sufficient margin of time to avoid exhaustion of auxiliary feedwater, a period of eight to nine hours is selected for the operator to decide whether the small break LTC procedure is appropriate. These results demonstrate that SCS can be used for breaks as large as 0.037 ft2 for long-term cooling and flushing of the core. The LTC analysis also determines that the large-break procedures can flush the core for break sizes down to 0.004 ft2.

The operator chooses the appropriate procedure based upon the RCS pressure indicated at between eight and nine hours. Figure 4-2 presents the RCS pressure at eight hours for a wide range of break sizes. A decision point pressure of 450 psia is chosen. 450 psia decision point allows sufficient margin of

+/-300 psia more conservative than an instrument error. The reasonable assurance is provided for the operator to select the proper procedure for any break size.

The BORON code calculates the transient boric acid concentration in the RCS. The results are shown in Figure 4-3. As shown in the figure, flushing flow of 30 gpm can prevent the boric acid precipitation. The designed hot leg injection is 455.85 gpm. The boil-off rate is 441.00 gpm at the time of simultaneous injection and the flushing flow is 14.85 gpm. Thus it can be concluded that the sufficient amount of margin exists in the core flush flow. Moreover, as shown in Figure 4-4, the interim methodology is more conservative than the traditional CENPD-254 methodology from the perspective of boron precipitation time.

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Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 50 0.05 ECCS flow = 1 SI Pump RCS/SG Cooldown Begins at 2 Hours 40 0.04 30 2

Break Area, cm 0.03 Break Area, ft2 20 0.02 10 0.01 0 0.00 0 2 4 6 8 10 12 RCS Refill Time, Hours Figure 4-1 RCS Refill Time vs. Break Area KEPCO & KHNP 23

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 2

Break Area, ft 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 80 Time = 8 Hours Post-LOCA 70 Power = 4,062.66 MWt 1000 60 800 RCS Pressure after Refill, kg/cm A 2

50 RCS Pressure after Refill, psia 600 40 30 400 RCS is Filled RCS is not Filled at 8 Hours or Earlier at 8 Hours 20 200 10 0 0 0 10 20 30 40 50 60 70 Break Area, cm2 Figure 4-2 RCS Pressure after Refill vs. Break Area KEPCO & KHNP 24

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 60 Simultaneous DVI / Hot Leg injection Initiated at 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> 50 Power = 4,062.66 MWt RCS temperature = 212 oF No Core Flush 40 Boric Acid Concentration(wt %)

Solubility Limit

= 29.3 wt %

30 At P=14.7 psia 30 gpm Flush 20 10 Core Flush = SI Injection - Boil off Rate 0

0 2 4 6 8 10 Time(hours)

Figure 4-3 Inner Vessel Boric Acid Concentration vs. Time KEPCO & KHNP 25

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 60 CENPD-254 Interim Simultaneous DVI / Hot Leg injection Initiated at 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> after LOCA 50 Power = 4,062.66 MWt RCS temperature = 212 oF 40 Boric Acid Concentration(wt %)

30 Solubility Limit

= 29.3 wt %

20 At P=14.7 psia 10 0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Time(hours)

Figure 4-4 BORON Calculated Results of Mixing Volume Change KEPCO & KHNP 26

Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 5 CONCLUSIONS On the basis of the information in this technical report, the mixing volume is evaluated by applying interim methodology. Boric acid precipitation does not occur when simultaneous injection is started two hours after the accident. It is concluded that the existing codes and methodologies are appropriate for the APR1400 LTC analyses. In addition, it is concluded that the information provided in this technical report supports its purpose to provide key technical information related to the computer codes, methods, and models applicable to the regulatory requirements.

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Non-Proprietary Post-LOCA Long Term Cooling Evaluation Model APR1400-F-A-NR-14003-NP, Rev.1 6 REFERENCES

1. CENPD-254-P-A, "Post-LOCA LTC Evaluation Model", June 1980 (Proprietary).

2. W3F1-2005-0012, Supplement to Amendment Request NPF-38-249 Extended Power Uprate Waterford Steam Electric Station, Unit 3, February 16, 2005.

3. Acceptance Criteria for Emergency Core Cooling Systems for Light Water-Cooled Nuclear Power Reactors, 10 CFR 50.46, October 1988.

4. APR1400-K-X-FS-14002, APR 1400 Design Control Document Tier2, September 2014.

5. NRC letter dated Aug. 1, 2005, R. A. Gramm to A. Gresham, Suspension of NRC approval for use of Westinghouse topical report CENPD-254-P due to discovery of non-conservative modeling assumptions during calculation audit, ADAMS Access No. ML051920310.

6. CEFLASH-4AS, A Computer Program for Reactor Blowdown Analysis of the Small Break Loss-of-Coolant Accident,, CENPD-133P, Supplement 1, August 1974

7. WCAP-16317-P, Review and Evaluation of MHI BACCHUS PWR Vessel Mixing Tests, November 2004.

8. Optibor Boric Acids - U.S. Borax, December 2007.

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