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Validation of CATHARE V2.5 thermal-hydraulic code against full-scale PERSEO tests for decay heat removal in LWRs
Nuclear Engineering and Design 241 (2011) 4662–4671
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Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Validation of CATHARE V2.5 thermal-hydraulic code against full-scale PERSEO tests for decay heat removal in LWRs Giacomino Bandini ∗ , Paride Meloni, Massimiliano Polidori, Calogera Lombardo Agenzia Nazionale per le Nuove Tecnologie, L’Energia e lo Sviluppo Economico Sostenibile (ENEA), Centro Ricerche “Ezio Clementel”, Via Martiri di Monte Sole, 4, 40129 Bologna, Italy
a r t i c l e
i n f o
Article history: Received 29 December 2010 Received in revised form 16 February 2011 Accepted 25 February 2011
a b s t r a c t The PERSEO experimental program was performed in the framework of a domestic research program on innovative safety systems with the purpose to increase the reliability of passive decay heat removal systems implementing in-pool heat exchangers. The conceived system was tested at SIET laboratories by modifying the existing PANTHERS IC-PCC facility utilized in the past for testing a full scale module of the GE-SBWR in-pool heat exchanger. Integral tests and stability tests were conducted to verify the operating principles, the steadiness and the effectiveness of the system. Two of the more representative tests have been analyzed with CATHARE V2.5 for code validation purposes. The paper deals with the comparison of code results against experimental data. The capabilities and the limits of the code in simulating such kind of tests are highlighted. An improvement in the modeling of the large water reserve pool is suggested trying to reduce the discrepancies observed between code results and test measurements. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Within the frame of research activities on innovative safety systems for light water reactors (LWRs), particular attention has been addressed to increase the reliability of decay heat removal systems which implement in-pool heat exchangers. In the past two examples of energy removal systems using inpool heat exchangers were proposed to be installed in the GE-SBWR and the Westinghouse AP600: the Isolation Condenser (IC) and the Passive Residual Heat Removal (PRHR), respectively. In both systems the heat transfer was actuated by opening a valve installed on the primary side. The first proposal of moving the primary side valve (high pressure and temperature) to the pool side (low pressure and temperature) was studied by CEA and ENEA in the Thermal Valve (TV) concept (Bianchi et al., 1997; Meloni and Pignatel, 1998). In this case
Abbreviations: AP600, Advanced Plant 600; BWR, Boiling Water Reactor; CATHARE, Code for Analysis of THermalhydraulic during an Accident of Reactor and safety Evaluation; CEA, Commissariat à l’Énergie Atomique; CFD, Computational Fluid Dynamics; ENEA, Agenzia Nazionale per le Nuove Tecnologie, L’Energia e lo Sviluppo Economico Sostenibile; GE-SBWR, General Electric Simplified Boiling Water Reactor; HX, Heat eXchanger; IC, Isolation Condenser; OP, Overall Pool; PANTHERS, Performance Analysis and Testing of Heat Removal System; PCC, Passive Containment Condenser; PERSEO, In-Pool Energy Removal System for Emergency Operation; PRHR, Passive Residual Heat Removal; PWR, Pressurized Water Reactor; TV, Thermal Valve. ∗ Corresponding author. Tel.: +39 051 6098 669; fax: +39 051 6098 279. E-mail address: [email protected] (G. Bandini). 0029-5493/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2011.02.034
the valve was located steam side at the top of a bell covering the pool immersed heat exchanger. The opening of the valve in emergency condition led to the discharge of the insulating steam formed under the bell in nominal condition and the beginning of heat transfer from the primary side to the pool. Unluckily, in this case a valve with a large flow area is needed to avoid flow instabilities with consequent operating and constructive problems. The PERSEO (In-Pool Energy Removal System for Emergency Operation) project proposed by ENEA and SIET (Achilli et al., 2002) is an evolution of the TV concept where the triggering valve is installed liquid side, on a line connecting two pools at the bottom, in order to try to resolve the problems encountered with the valve located steam side. A new experimental facility was design and built at the SIET laboratories (Piacenza, Italy) by modifying the existing PANTHERS IC-PCC facility (Performance Analysis and Testing of Heat Removal System Isolation Condenser – Passive Containment Condenser) utilized in the past for testing a full scale module of the GE-SBWR in-pool heat exchanger. A test campaign was conducted on the PERSEO facility in order to verify the operating principles, the steadiness and the effectiveness of the decay heat removal system. Two of the more representative PERSEO tests have been analyzed with CATHARE for code validation purposes. 2. The PERSEO test facility The PERSEO test facility was built at SIET by modifying the existing PANTHERS IC-PCC facility utilized in the past for testing a full
G. Bandini et al. / Nuclear Engineering and Design 241 (2011) 4662–4671 Table 1 Main PERSEO test parameters.
HX POOL
Boil-off
Injector OVERALL POOL
Triggering valve
4663
HEAT EXCHANGER
Water discharge
Parameter
Unit
Test n. 7
Test n. 9
Primary side pressure Primary side temperature Primary steam flowrate HX extracted power HX Pool side pressure HX Pool side temperature HX Pool steam flowrate
MPa ◦ C kg/s MW MPa ◦ C kg/s
7 285 13 20 0.12 105 8.91
4 250 8 14 0.12 105 6.24
VESSEL
Feed line
Drain line
Steam supply Condensate discharge line Fig. 1. Scheme of the PERSEO facility.
scale module of the GE-SBWR in-pool heat exchanger. The scheme of the PERSEO facility is depicted in Fig. 1. The PERSEO facility mainly consists of: • the primary side containing the pressure vessel (43 m3 volume and 13 m height) and the heat exchanger (two cylindrical headers and 120 vertical pipes) interconnected by the steam feed line and the condensate drain line; • the pool side consisting of two pools connected at the top by a steam duct ending with an injector flowing into the Overall Pool (OP), and at the bottom by a water line with triggering valve. The Heat eXchanger (HX) is contained in the HX Pool (29 m3 volume and 5.7 m height); the OP (173 m3 volume and 5.8 m height) represents the water reservoir. The operation of the PERSEO facility is described as follows. The pressure vessel is maintained in saturation conditions typical for BWRs or secondary side of PWR steam generators (P = 7 MPa) by supplying properly de-superheated steam coming from a nearby power station. The pressure is kept constant by controlling the steam supply valve, while the water level in the vessel is maintained at the specified value for the test by discharge of water through the condensate discharge line at the vessel bottom. At the beginning of the test the HX, the feed line and the drain line are full of saturated steam; the HX Pool is full of air or steam depending on the test; the OP is full of cold water at the nominal level and the triggering valve is closed. Once reached the initial test conditions according to the test matrix, the triggering valve is opened and the HX Pool is flooded by cold water leading to steam condensation inside the HX tubes with power transfer from the primary side to the pool side. As soon as pool water boiling starts, the steam produced in the HX Pool is driven to the OP through the steam duct. The injector flowing about 1.3 m below the water level contributes to mix the OP water, thus limiting temperature stratification within the pool. The condensation of steam inside the OP leads to progressive cold water heatup until reaching the boiling point. The steam produced in the OP flows outside at atmospheric pressure through the boil-off pipe. When the injector is uncovered, no condensation is present anymore in
the OP; the steam flows outside directly through the boil-off pipe and the water reserve in the OP decreases according to the heat transfer rate in the HX Pool. During the system operation, the natural circulation which stabilizes on both primary and secondary sides determines the power evacuated by the system. In some tests, water is discharged from the OP bottom to accelerate the transient phase with water level decreasing. 3. Analyzed PERSEO tests Two different kinds of tests have been performed during the experimental campaign on the PERSEO facility (Ferri et al., 2002): • the stability tests to address particular critical problems happening in case of sudden steam condensation at the steam–water interface in the injector or within the HX Pool, following triggering valve re-opening with cold water injection in presence of steam in the HX Pool; • the integral tests aimed at demonstrating the behavior and performances of the system during all phases of a long accidental transient. The stability and integral test n. 7 and the integral test n. 9 have been analyzed with CATHARE code in the present study. The main PERSEO test parameters at full power operation during the transient phase are listed in Table 1. The test n. 7 foresees the system actuation with partial HX Pool fill-up (Phase 1), followed by the HX Pool total fill-up with reaching of boiling conditions in the OP and water level decreasing (Phase 2), with primary side pressure of 7 MPa. This test was devoted to investigate the following items: 1. the system actuation and the trend of power with a low HX Pool level; 2. the presence of instabilities due to steam condensation at the interface between water and steam in the injector; 3. the system re-actuation consequent to the HX Pool fill-up and reaching of thermal regime in both pools; 4. the effectiveness of the injector in mixing the OP water; 5. the power and flow regime variation after the OP level decreases below the injector outlet; 6. the trend of power as a function of the water level in the pools, decreasing for the loss of mass through the boil-off. The test n. 9 foresees the system actuation with total HX Pool fill-up followed by reaching of boiling conditions and pool level decreasing, with the primary side pressure of 4 MPa. This test was devoted to investigate the following items: 1. the system actuation consequent to the HX Poll fill-up and reaching of the thermal regime in both pools; 2. items 4, 5 and 6 like for previous test n. 7.
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G. Bandini et al. / Nuclear Engineering and Design 241 (2011) 4662–4671 15
P = 1 bar
DP-J001 kPa DP-J002 kPa DP injector (cathare)
HX Pool
DP inj-pool (cathare) Media Mobile per. (DP-J001 kPa) Average valuesuof100 DP-J001 kPa Average valuesuof100 DP-J002 kPa Media Mobile per. (DP-J002 kPa)
P steam
5
DP injector (J001)
Feed Line
DP (kPa)
Steam Line
10
0
IC HX Overall Pool
-5 Trigger. Valve
DP measurement Q drain
Water Line
Drain Line
DP inj-pool (J002)
-10 0
1000
2000
3000
Vessel
4000
5000
6000
Time (s)
Fig. 3. Test n. 7 (Ph. 2): Injector differential pressure. Q liquid
Fig. 2. CATHARE nodalization scheme.
4. CATHARE modeling of PERSEO facility The CATHARE code system (Geffraye et al., 2009) developed at CEA/Grenoble is a lumped parameter thermal-hydraulic code for transient analysis in LWRs. The CATHARE modeling scheme used for the analysis of PERSEO tests with the previous V1.5 version of the code (Bianchi et al., 2004; Meloni et al., 2005) has been refined according with more recent model developments and improvements introduced in the last V2.5 mod8.1 version of CATHARE employed in the present test analysis. The CATHARE nodalization scheme of the PERSEO facility is depicted in Fig. 2. The 0-D two-node module (volume) is used to represent the primary vessel, the HX collectors, the HX Pool lower and upper plenum and the OP (complete mixing). The volumes are interconnected by axial elements (pipe) representing the HX tube bundle, the water lines and the steam lines. Axial elements with cross-flow junctions are used to best simulate 2-D recirculation within the HX Pool and HX submerging and uncovery during the transient phase. Empirical correlations specifically derived at atmospheric pressure: (1) EPICE correlation for boiling heat transfer in the HX Pool and (2) SUPERCLAUDIA correlation for direct contact condensation in the OP have been implemented in the code (Bianchi et al., 2004), in order to overcome some deficiencies evidenced using the standard models of CATHARE and better reproduce the experimental data. Contrary to the test conduct, a pressure boundary condition at the HX Pool top was maintained active for 100 s during startup to avoid large instabilities calculated by the code, which were not observed in the tests. These instabilities are likely generated by natural circulation overestimation within the HX Pool leading to immediate strong condensation of steam in the upper plenum when cold water is injected at the pool bottom. The right evaluation of pressure drop through the steam injector is needed to well reproduce the HX Pool relative pressure trend and thus the overall system behavior. Therefore, friction losses through the conic-shape injector are calibrated on differential pressure measurements. To this aim, appropriate singular pressure loss coefficients are taken into account in the injector trying to reproduce the test measurements, as shown in Fig. 3 for the analysis of Phase 2 of Test n. 7. 5. Analysis of PERSEO tests The Test n. 7 (Phase 1 and Phase 2) and the Test n. 9 have been calculated with CATHARE V2.5 mod8.1 and the results are com-
Table 2 Initial conditions of OP and HX Pool. Parameter
Unit
Test 7 Ph. 1
Test 7 Ph. 2
Test 9
OP water level OP water temperature HX Pool water level HX Pool water temperature
m ◦ C m ◦ C
4.64 15 0.2 80
4.52 60 1.125 100
4.50 24.5 1.222 47
pared with the experimental data for code validation purposes. In the comparison figures the calculated and measured values are distinguished by their plotting line thickness: the thicker lines represent the code results, while the thinner lines represent the test measurements. The initial conditions (water level and temperature) of OP and HX Pool taken into account in the CATHARE calculations are summarized in Table 2 for the two analyzed tests. 5.1. Test n. 7 – Phase 1 The Phase 1 of Test n. 7 foresees the system actuation with partial HX Pool fill-up without reaching of boiling conditions in the OP. This test phase is aimed at verifying the system behavior during start-up. The chronology of main events characterizing the conduct of the test is shown in Table 3. The overall behavior of the system is illustrated from Figs. 4–9. After successive partial triggering valve openings, the HX Pool water level increases provoking a corresponding decrease of the OP level according with the respective pool area. As soon as the HX tubes are covered by water after about 900 s from the beginning of the transient, the power exchange becomes much more significant leading to boiling conditions with steam release to the OP through the steam duct and the injector, with consequent OP water temperature progressive increase. The definitive triggering valve closure at t = 1085 s leads to progressive decrease of the HX water level and of the extracted power. Once the steam flow through the injector Table 3 Chronology of main events. Event
Time (s)
Beginning of the test 1st partial triggering valve opening Triggering valve closure 2nd partial triggering valve opening Triggering valve closure 3rd partial triggering valve opening Triggering valve closure Primary side depressurization End of the test
0 300 433 446 480 864 1085 900–1400 4609
G. Bandini et al. / Nuclear Engineering and Design 241 (2011) 4662–4671
4
25000
3.5
L_VQ 0.195
HX tubes
1.5
Power (kW)
2.5 2
W_IC 2517.85
20000
COLLVHX
3 Level (m)
4665
POWERHX
15000
10000
1
5000 0.5
Valve closed
0
0 0
1000
2000
3000
4000
5000
0
1000
2000
Time (s) Fig. 4. Test n. 7 (Ph. 1): HX Pool collapsed water level.
4000
5000
Fig. 7. Test n. 7 (Ph. 1): HX exchanged power.
5
80 70
4
Temperature (°C)
Injector level Level (m)
3000
Time (s)
3 L_VP 4.646
2
MIXLEVCP COLLEVCP
60 50 T-P021
40
T-P022 T-P023
30
T-P025
1
T-P028
20
TEMPWAT
0
10 0
1000
2000 3000 Time (s)
4000
5000
0
1000
2000
3000
4000
5000
Time (s)
Fig. 5. Test n. 7 (Ph. 1): Overall Pool collapsed water level.
Fig. 8. Test n. 7 (Ph. 1): Overall Pool temperatures.
reduces, after about t = 2000 s, some temperature stratification is observed in the OP. Unless of a slight underestimation of level decrease in the second part of the transient, the collapsed water level rise and drop in the HX Pool is well predicted by the code (Fig. 4), so as for the OP water level (Fig. 5). Oscillations of the OP level observed in the test, likely provoked by free surface fluctuations, cannot be taken into account by the code. The calibration of the injector differential pressure carried out for the Phase 2 of the same test (see Section 4) allows a good representation of the HX Pool relative pressure for the Phase 1 too (Fig. 6). At the beginning of the test, the injector is full of water and the HX relative pressure against the OP pressure is close to zero.
Around t = 400 s the injector empties and the HX relative pressure stabilizes at 12 kPa according to OP water level. The largest discrepancy is observed at t = 1200 s, when the saturation conditions are reached at the pool bottom, owing to strong water recirculation in the HX Pool in boiling conditions. Indeed, the sudden vaporization calculated at the bottom determines a pressure spike that propagates in the pool, the steam duct and through the injector, since the triggering valve is already closed by that time. On the contrary, the transition to boiling at the HX Pool bottom is likely much more smooth in the test and thus such a peak pressure is not observed. The 2 kPa deviation around t = 2000 s is in line with the underestimation of extracted power leading to reduced 300
25
Temperature (°C)
20
DP (kPa)
15 10
Saturation at pool bottom
5 0
250
Tube middle 200
TW-C006 TW-C007 TW-C008
150
P-Q001 0.604
WALLT10
DP_CPHX
-5
100
-10 0
1000
2000
3000
4000
Time (s) Fig. 6. Test n. 7 (Ph. 1): HX Pool relative pressure.
5000
0
1000
2000 3000 Time (s)
4000
Fig. 9. Test n. 7 (Ph. 1): HX tube wall temperatures.
5000
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G. Bandini et al. / Nuclear Engineering and Design 241 (2011) 4662–4671
Table 4 Chronology of main events.
6 L_VP m
5
Time (s)
Beginning of the test Triggering valve opening OP water discharge opening Onset of OP water boiling Triggering valve closure Primary side depressurization End of the test
0 300 (in 26 s) 1150 ∼1400 3338 (in 123 s) 4685 5736
MIXLEVCP COLLEVCP
4 Level (m)
Event
Injector level 3 2 1
Water discharge = 16.2 kg/s 0 0
1000
2000
3000
4000
5000
6000
Time (s) Fig. 11. Test n. 7 (Ph. 2): Overall Pool collapsed water level.
The overall behavior of the system is illustrated from Figs. 10–15. After full opening of the triggering valve at t = 326 s the water level in the HX Pool quickly increases. When boiling is reached in the pool at t = 500 s, the collapsed water level reduces, stabilizing around 0.25 m above the middle HX tube plane. A progressive OP water level decrease is initiated at t = 1150 s by water discharge opening at the pool bottom. Once boiling is reached in the OP at about t = 1400 s the level decrease rate is enhanced due to mass loss through the boil-off pipe. Around t = 3400 s the triggering valve is closed and the HX Pool level starts to rapidly decrease, until boiling is terminated and the HX tube bundle is fully uncovered towards the end of the test. 25 P-Q001 kPa
20
DP (kPa)
steam flowrate through the injector during the same period of time (1800–2200 s). Towards the end of the test, the injector is full of saturated steam and the differential pressure of 12.4 kPa equalizes the hydraulic head of the water column between the injection level and the pool free surface. Since the final OP level, which slightly increases after triggering valve closure at t = 1085 s due to water thermal expansion, is well predicted, the HX Pool relative pressure is well predicted too. The power extracted by the HX, which reaches a maximum of about 20 MW when the HX tubes are fully submerged by water–steam mixture (collapsed level just above the middle tube plane), is well simulated (Fig. 7). The temporary power reduction during primary side depressurization between t = 900 and 1400 s is captured by the code. The progressive decrease of exchanged power with the decreasing HX Pool water level is quite well reproduced by CATHARE. The mean OP temperature increase is well predicted by the code, but the temperature stratification following steam flow reduction through the injector cannot be simulated with the 0-D two-node volume of CATHARE representing the OP (Fig. 8). The onset and the end of wetting at the middle plane of HX tube bundle is well predicted by CATHARE as shown in Fig. 9. Under wetting condition, the external wall temperature of HX tubes is over predicted by about 20 ◦ C. However, this quite large discrepancy is believed to be mainly caused by uncertainties in the thermocouple measurements rather than by wrong unbalance between boiling and condensation heat transfer coefficients calculated by CATHARE.
DP_CPHX
15
10
5.2. Test n. 7 – Phase 2 5
The Phase 2 of Test n. 7 foresees the system actuation with total HX Pool fill-up followed by reaching of boiling in the OP and water level decreasing. This phase of the test is aimed at demonstrating the correct system behavior during long term accidental transient. The chronology of main events characterizing the conduct of the test is shown in Table 4.
0 0
1000
2000
3000 Time (s)
4000
5000
6000
Fig. 12. Test n. 7 (Ph. 2): HX Pool relative pressure.
3.5
25000 L_VQ m
3
W_IC kW
COLLVHX
20000
POWERHX
HX tubes
2 1.5
Power (kW)
Level (m)
2.5 15000
10000
1 5000
0.5
Valve closed 0
0
0
1000
2000
3000
4000
5000
Time (s) Fig. 10. Test n. 7 (Ph. 2): HX Pool collapsed water level.
6000
0
1000
2000
3000 Time (s)
4000
5000
Fig. 13. Test n. 7 (Ph. 2): HX exchanged power.
6000
G. Bandini et al. / Nuclear Engineering and Design 241 (2011) 4662–4671 Table 5 Chronology of main events.
120
Temperature (°C)
4667
100 T-P008 °C T-P021 °C
80
T-P022 °C T-P023 °C T-P025 °C T-P028 °C
60
Event
Time (s)
Beginning of the test Triggering valve opening OP water discharge opening Onset of OP water boiling OP water discharge closure Triggering valve closure End of the test
0 142 (in 21 s) 2790 ∼3200 4840 4887 (in 93 s) 7708
TEMPWAT
0
1000
2000
3000 Time (s)
4000
5000
6000
Fig. 14. Test n. 7 (Ph. 2): Overall Pool temperatures.
The initial water level increase in the HX Pool is very well captured by CATHARE (Fig. 10). Afterwards, the level is initially overestimated until t = 1400 s and then underestimated until triggering valve closure (t = 3338 s), when the calculated level well matches the measured one. Towards the end of the transient the level is slightly overestimated like during the Phase 1 of the test. The OP water level behavior is well calculated by the code (Fig. 11). Large oscillations of the OP level are observed in the test after onset of steam flow through the injector at t = 500 s. They could be provoked by fluctuations of the free surface which cannot be reproduced by the 0-D module used to represent the OP. Starting from the initial value of 12 kPa with water-empty injector, the HX Pool relative pressure decrease after triggering valve opening at t = 300 s is roughly estimated (Fig. 12). The large pressure underestimation after onset of HX Pool boiling, likely associated to contemporary OP level oscillations, determines the HX Pool level mismatch evidenced in Fig. 10 between t = 500 and 1400 s. For similar reason, the relative pressure over prediction between t = 1400 and 3200 s causes the discrepancy in HX Pool level during the same period of time. The 20 MW power extracted by the HX is well predicted by the code until 1600 s (Fig. 13). Afterwards, an earlier power diminution is calculated likely due to HX Pool level under prediction. The power level decrease after triggering valve closure around t = 3400 s is well reproduced. Starting from stratified conditions reached at the end of Phase 1, the OP temperatures are mixed by the injector until onset of boiling is calculated around t = 1600 s, that is about 200 s later than in the test (Fig. 14). The calculated saturation temperature is some degrees above the measured value of 100 ◦ C at atmospheric
pressure, since the OP pressure and corresponding saturation temperature are calculated by CATHARE at the middle of water pool height, where the pressure is up to 20 kPa above the atmospheric value. Like in the Phase 1, the onset and the end of wetting at the middle HX tube plane is very well predicted by the code (Fig. 15). Furthermore, the same discrepancy is evidenced in the calculation of external wall temperature of HX tubes. 5.3. Test n. 9 Like in Test n. 7, the Test n. 9 foresees the system actuation with total HX Pool fill-up followed by reaching of boiling in the OP and water level decreasing. This test is aimed at demonstrating the correct system behavior during long term accidental transient under reduced primary pressure (4 MPa). The chronology of main events characterizing the conduct of the test is shown in Table 5. The overall behavior of the system is illustrated from Figs. 16–21. After full opening of the triggering valve at t = 163 s, the collapsed 3.5 3 2.5 Level (m)
40
HX tubes
2 1.5 L_VQ m
1
COLLVHX
0.5
Valve closed 0 0
1000
2000
3000
4000 5000 Time (s)
6000
7000
8000
Fig. 16. Test n. 9: HX Pool collapsed water level.
300
6
Tube middle
4
TW-C006 °C
Level (m)
Temperature (°C)
5
250
TW-C007 °C
200
TW-C008 °C WALLT10
Injector level 3 L_VP m
2
MIXLEVCP
150
COLLEVCP
1
Water discharge = 18.5 kg/s 0
100 0
1000
2000
3000 Time (s)
4000
5000
Fig. 15. Test n. 7 (Ph. 2): HX tube wall temperatures.
6000
0
1000
2000
3000
4000
5000
6000
7000
Time (s) Fig. 17. Test n. 9: Overall Pool collapsed water level.
8000
4668
G. Bandini et al. / Nuclear Engineering and Design 241 (2011) 4662–4671
300
20
DP_CPHX
Temperature (°C)
DP (kPa)
TW-C006 °C
P-Q001 kPa
15
10
5
TW-C007 °C
250
TW-C008 °C WALLT10
Tube middle
200
150
0 100
-5
0
0
1000
2000
3000
4000 5000 Time (s)
6000
7000
8000
1000
2000
3000
4000 5000 Time (s)
6000
7000
8000
Fig. 21. Test n. 9: HX tube wall temperatures. Fig. 18. Test n. 9: HX Pool relative pressure.
water level in the HX Pool quickly increases stabilizing close to the top of tube bundle. The progressive decrease of the OP level is started by water discharge from the bottom at t = 2790 s, approximately 400 s before the onset of boiling. The water discharge is stopped at t = 4840 s, that is 140 s before complete triggering valve closure. As a consequence, the OP level decrease is terminated in conjunction with the onset of diminution of extracted power which tends to zero according with full uncovery of HX tube bundle reached at the end of the test. The initial water level increase in the HX Pool is very well captured by CATHARE (Fig. 16). Similarly to what happened during Test n. 7, the level is overestimated in the first part of the transient (t = 900–2100 s) and then largely underestimated from t = 2900 s 18000 W_IC kW
Power (kW)
15000
POWERHX
12000 9000 6000 3000 0 0
1000 2000 3000 4000 5000 6000 7000 8000 Time (s) Fig. 19. Test n. 9: HX exchanged power.
6. Analysis with 3-volume OP modeling
120
100 Temperature (°C)
until the triggering valve is closed at about t = 5000 s. The decrease of the HX Pool level during the last phase of the transient is quite well reproduced until the end of the test. The OP collapsed water level behavior is well predicted by CATHARE during the whole transient (Fig. 17). The oscillatory behavior observed in the test before OP water boiling might be provoked by sudden steam condensation at the injector outlet, inducing fluctuations at the pool free surface. The lack of oscillations in the calculated OP level reflects also in much less enhanced fluctuations in the HX Pool level and relative pressure values calculated by the code between t = 500 and 3000 s (Fig. 18). The large HX Pool relative pressure overestimation between t = 2700 and 5000 s (Fig. 18) is consistent with the corresponding HX Pool level underestimation shown in Fig. 16. The power extracted by the HX reduces with the decreasing primary system pressure mainly because of: (1) enhanced pressure losses in the feed line with consequent reduction of natural circulation primary mass flowrate, and (2) the lower saturation temperature resulting in reduced temperature difference between primary and secondary sides (see Table 1 in Section 3). The calculated extracted power in Fig. 19 is 15 MW, i.e. slightly higher than the measured one (14 MW). The power diminution predicted by the code after triggering valve closure at about t = 5000 s is slightly more accelerated than the one observed in the test. The onset of boiling in the OP, occurring around t = 3200 s, is very well captured by the code (Fig. 20). As expected and observed in Test n. 7 also, the temperature stratification phenomena in the OP disappear as soon as boiling is reached in the pool. The onset of HX tubes wetting is well predicted by the code, while its end is slightly anticipated owing to earlier calculated power decrease (Fig. 21). Once more the HX tube wall temperature is over predicted under wetting conditions.
T-P006 °C
80
T-P008 °C T-P021 °C
60
T-P022 °C T-P023 °C T-P025 °C
40
T-P028 °C T-P030 °C TEMPWAT
20 0
1000
2000
3000
4000 5000 Time (s)
6000
Fig. 20. Test n. 9: Overall Pool temperatures.
7000
8000
The previous test analyses have demonstrated the general capability of CATHARE to evaluate main natural circulation and heat transfer phenomena occurring during the PERSEO tests. Apart from temperature stratification and free level oscillation effects in the OP which of course need a detailed CFD modeling to be taken in to account, significant discrepancies have been found in the calculation of the HX Pool relative pressure and collapsed water level trend, after boiling is reached in the OP. The HX Pool relative pressure against the OP pressure (atmospheric value) strongly depends on the hydraulic head of the water column above the steam injector outlet in the OP. This hydraulic head increases under OP boiling conditions depending on void formation in the pool and swollen level effect (see Figs. 11 and 17). Likely this swollen level effect is overestimated by the 0-D module
G. Bandini et al. / Nuclear Engineering and Design 241 (2011) 4662–4671
4669
3.5
P = 1 bar
L_VQ m
3 HX Pool
Q steam
Feed Line
Level (m)
Steam Line
1.5 1 0.5
Overall Pool
Valve closed
Trigg. Valve Q drain
HX tubes
2
IC HX H
COLLVHX
2.5
0 0
Drain Line
Water Line
1000
2000
Vessel
Test n. 7 Test n. 9
H = 1.0 m H = 2.2 m
3000 Time (s)
4000
5000
6000
Fig. 24. Test n. 7 (Ph. 2): HX Pool collapsed water level. Q liquid
of CATHARE representing the OP, because of homogeneous distribution of voids considered in the boiling water pool, which could be in contrast with non uniform radial and axial distribution of voids expected in the tests. The use of a 3-volume OP modeling has been proposed in this study in order to verify the OP swollen level effect and try to reduce the discrepancies with the test measurements. The implementation of the new OP modeling in the original nodalization scheme is illustrated in Fig. 22. The height of the OP bottom volume is different for the two tests according to the respective extracted power value and then different boiling rate within the OP. The Phase 2 of the Test n. 7 and the Test 9 in which boiling conditions are reached in the OP have been re-calculated with the new modeling and the obtained results are presented in the following sections. 6.1. Test n. 7 – Phase 2
in general reduced, allowing the best simulation of all phenomena concerned in the test. In particular, the HX Pool relative pressure is very well predicted after onset of boiling (Fig. 23). The reduced pressure value reflects in a more precise evaluation of the HX Pool collapsed level which slightly increases in better agreement with the test measurement (Fig. 24). On its turn, the even small HX Pool level difference permits to better predict the exchanged power diminution curve observed in the test (Fig. 25) during the OP level decreasing phase. Owing to enhanced HX exchanged power between t = 1600 and 3500 s with corresponding increase of OP boiling rate and thus of steam mass loss through the boil-off, the re-calculated 25000 W_IC kW
20000 Power (kW)
Fig. 22. 3-Volume Overall Pool modeling scheme.
POWERHX
15000
10000
The new code results for the Phase 2 of the Test n. 7 regarding the HX Pool relative pressure, the water level in the two pools and the HX exchanged power are shown from Figs. 23–26, in comparison with the test measurements. As expected, the differences with previous results become significant only after the onset of OP boiling around t = 1400 s. After this time onwards, the deviations from the experimental trend are
5000
0 0
1000
2000
3000 Time (s)
4000
5000
6000
Fig. 25. Test n. 7 (Ph. 2): HX exchanged power.
25
6 L_VP m
P-Q001 kPa
20
5
MIXLEVCP
DP_CPHX
COLLEVCP
15
Level (m)
DP (kPa)
4
10
Injector level 3 2
5
1
Water discharge = 16.2 kg/s 0
0 0
1000
2000
3000 Time (s)
4000
5000
Fig. 23. Test n. 7 (Ph. 2): HX Pool relative pressure.
6000
0
1000
2000
3000 Time (s)
4000
5000
Fig. 26. Test n. 7 (Ph. 2): Overall Pool collapsed water level.
6000
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G. Bandini et al. / Nuclear Engineering and Design 241 (2011) 4662–4671
6
18000
5
15000
Level (m)
4
Injector level 3 2
L_VP m
Power (kW)
W_IC kW POWERHX
12000 9000 6000
MIXLEVCP
3000
COLLEVCP
1
Water discharge = 18.5 kg/s 0
0 0
1000
2000
3000
4000 5000 Time (s)
6000
7000
8000
0
1000 2000 3000 4000 5000 6000 7000 8000 Time (s)
Fig. 27. Test n. 9: Overall Pool collapsed water level.
Fig. 30. Test n. 9: HX exchanged power.
OP collapsed water level perfectly matches the measured one (Fig. 26).
OP swollen level (Fig. 27), the HX Pool relative pressure is much better predicted (Fig. 28). On its turn, the right pressure trend reflects in more precise calculation of the HX Pool collapsed level (Fig. 29). No significant change is evidenced in the HX exchanged power (Fig. 30).
6.2. Test n. 9 The new code results for the Test n. 7 regarding the water level in the two pools, the HX Pool relative pressure and the HX exchanged power are shown from Figs. 27–30, in comparison with the test measurements. As expected, the differences with previous results become significant only after onset of OP water boiling around t = 3200 s. After this time onwards, owing to significant reduction of the calculated 20 P-Q001 kPa
DP (kPa)
15
The CATHARE V2.5 code has been successfully employed for the analysis of two of the more representative tests carried out on the full scale PERSEO facility that allowed the validation of important thermal-hydraulic aspects without the uncertainties usually connected with the scaling effect. The systematic comparison of code results with the test measurements has highlighted the general good capability of CATHARE to simulate all relevant phenomena governing the processes involved in the experimental transients, in particular:
DP_CPHX
• the start-up of the system and the stabilization of natural circulation on the primary circuit and pool side; • the power removal from the primary side by steam condensation and water boiling through in-pool heat exchangers; • the steam condensation with direct contact at atmospheric pressure by the implementation of an appropriate heat transfer correlation.
10
5
0
-5 0
1000
2000
3000
4000
5000
6000
7000
8000
Time (s) Fig. 28. Test n. 9: HX Pool relative pressure.
3.5 3 2.5 Level (m)
7. Conclusions
HX tubes
2 1.5 L_VQ m
1
COLLVHX
Some code limitations have been recognized which mainly concern mixing and temperature stratification effects in large water pools like the OP, owing to the simplified 0-D model used. In particular, stratification before boiling, non-homogeneous void distribution under boiling, and free level oscillations are likely the main causes for discrepancies observed between code results and test measurements. The applicability of the 3-D module called ‘TREED’ available in CATHARE for the best simulation of the OP could be verified. The re-calculation of the tests with a modified 3-volume OP model has allowed the overall best simulation of the experimental transients after reaching of boiling conditions in the OP, owing to the importance of OP swollen level effect on the HX Pool relative pressure value. The need of more detailed 3-D models for large water pool simulation is thus emphasized.
0.5
Valve closed
Acknowledgments
0 0
1000
2000
3000
4000 5000 Time (s)
6000
Fig. 29. Test n. 9: HX Pool collapsed water level.
7000
8000
The authors appreciate the support of the CATHARE team and in particular of Dr. G. Lavialle for the use of CATHARE code under extensive and fruitful ENEA-CEA bilateral collaborations.
G. Bandini et al. / Nuclear Engineering and Design 241 (2011) 4662–4671
References Achilli, A., Cattadori, G., Ferri, R., Rigamonti, M., Bianchi, F., Meloni, P., 2002. PERSEO project: experimental facility set-up and RELAP5 code calculations. In: Proc. 2nd EMSI and 40th European Two-Phase Flow Group Meeting , Stockholm, Sweden, June 10–13. Bianchi, F., Meloni, P., Pignatel, J.F., Gautier, G.M., 1997. Thermal valve system for LWR applications. In: Proc. Post-SMIRT14 Seminar , Pisa, Italy. Bianchi, F., Meloni, P., Ferri, R., Achilli, A., 2004. Assessment of RELAP5 MOD3.3 and CATHARE 2 V1.5a against a full scale test of PERSEO device. In: Proc. 12th International Conference on Nuclear Engineering (ICONE12) , Arlington, Virginia, USA, April 25–29. Ferri, R., Achilli, A., Gandolfi, S., 2002. PERSEO Project Experimental Data Report SIET 01 014 RP 02, Piacenza, Italy.
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Geffraye, G., Antoni, O., Farvacque, M., Kadri, D., Lavialle, G., Rameau, B., Ruby, A., 2009. CATHARE 2 V2.5 2: a single version for various applications. In: Proc. 13th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics (NURETH-13) , Kanazawa, Japan, September 27–October 2. Meloni, P., Pignatel, J.F., 1998. Theoretical design and assessment of isolation condenser system controlled with thermal valve. In: Proc. 6th International Conference on Nuclear Engineering (ICONE6) , San Diego, USA, May 10–14. Meloni, P., Bianchi, F., Ferri, R., Achilli, A., 2005. Experimental campaign and numerical analysis for the In-Pool Energy Removal System for Emergency Operation. In: Proc. 13th International Conference on Nuclear Engineering (ICONE13) , Beijing, China, May 16–20.
Contents lists available at ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Validation of CATHARE V2.5 thermal-hydraulic code against full-scale PERSEO tests for decay heat removal in LWRs Giacomino Bandini ∗ , Paride Meloni, Massimiliano Polidori, Calogera Lombardo Agenzia Nazionale per le Nuove Tecnologie, L’Energia e lo Sviluppo Economico Sostenibile (ENEA), Centro Ricerche “Ezio Clementel”, Via Martiri di Monte Sole, 4, 40129 Bologna, Italy
a r t i c l e
i n f o
Article history: Received 29 December 2010 Received in revised form 16 February 2011 Accepted 25 February 2011
a b s t r a c t The PERSEO experimental program was performed in the framework of a domestic research program on innovative safety systems with the purpose to increase the reliability of passive decay heat removal systems implementing in-pool heat exchangers. The conceived system was tested at SIET laboratories by modifying the existing PANTHERS IC-PCC facility utilized in the past for testing a full scale module of the GE-SBWR in-pool heat exchanger. Integral tests and stability tests were conducted to verify the operating principles, the steadiness and the effectiveness of the system. Two of the more representative tests have been analyzed with CATHARE V2.5 for code validation purposes. The paper deals with the comparison of code results against experimental data. The capabilities and the limits of the code in simulating such kind of tests are highlighted. An improvement in the modeling of the large water reserve pool is suggested trying to reduce the discrepancies observed between code results and test measurements. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Within the frame of research activities on innovative safety systems for light water reactors (LWRs), particular attention has been addressed to increase the reliability of decay heat removal systems which implement in-pool heat exchangers. In the past two examples of energy removal systems using inpool heat exchangers were proposed to be installed in the GE-SBWR and the Westinghouse AP600: the Isolation Condenser (IC) and the Passive Residual Heat Removal (PRHR), respectively. In both systems the heat transfer was actuated by opening a valve installed on the primary side. The first proposal of moving the primary side valve (high pressure and temperature) to the pool side (low pressure and temperature) was studied by CEA and ENEA in the Thermal Valve (TV) concept (Bianchi et al., 1997; Meloni and Pignatel, 1998). In this case
Abbreviations: AP600, Advanced Plant 600; BWR, Boiling Water Reactor; CATHARE, Code for Analysis of THermalhydraulic during an Accident of Reactor and safety Evaluation; CEA, Commissariat à l’Énergie Atomique; CFD, Computational Fluid Dynamics; ENEA, Agenzia Nazionale per le Nuove Tecnologie, L’Energia e lo Sviluppo Economico Sostenibile; GE-SBWR, General Electric Simplified Boiling Water Reactor; HX, Heat eXchanger; IC, Isolation Condenser; OP, Overall Pool; PANTHERS, Performance Analysis and Testing of Heat Removal System; PCC, Passive Containment Condenser; PERSEO, In-Pool Energy Removal System for Emergency Operation; PRHR, Passive Residual Heat Removal; PWR, Pressurized Water Reactor; TV, Thermal Valve. ∗ Corresponding author. Tel.: +39 051 6098 669; fax: +39 051 6098 279. E-mail address: [email protected] (G. Bandini). 0029-5493/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2011.02.034
the valve was located steam side at the top of a bell covering the pool immersed heat exchanger. The opening of the valve in emergency condition led to the discharge of the insulating steam formed under the bell in nominal condition and the beginning of heat transfer from the primary side to the pool. Unluckily, in this case a valve with a large flow area is needed to avoid flow instabilities with consequent operating and constructive problems. The PERSEO (In-Pool Energy Removal System for Emergency Operation) project proposed by ENEA and SIET (Achilli et al., 2002) is an evolution of the TV concept where the triggering valve is installed liquid side, on a line connecting two pools at the bottom, in order to try to resolve the problems encountered with the valve located steam side. A new experimental facility was design and built at the SIET laboratories (Piacenza, Italy) by modifying the existing PANTHERS IC-PCC facility (Performance Analysis and Testing of Heat Removal System Isolation Condenser – Passive Containment Condenser) utilized in the past for testing a full scale module of the GE-SBWR in-pool heat exchanger. A test campaign was conducted on the PERSEO facility in order to verify the operating principles, the steadiness and the effectiveness of the decay heat removal system. Two of the more representative PERSEO tests have been analyzed with CATHARE for code validation purposes. 2. The PERSEO test facility The PERSEO test facility was built at SIET by modifying the existing PANTHERS IC-PCC facility utilized in the past for testing a full
G. Bandini et al. / Nuclear Engineering and Design 241 (2011) 4662–4671 Table 1 Main PERSEO test parameters.
HX POOL
Boil-off
Injector OVERALL POOL
Triggering valve
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HEAT EXCHANGER
Water discharge
Parameter
Unit
Test n. 7
Test n. 9
Primary side pressure Primary side temperature Primary steam flowrate HX extracted power HX Pool side pressure HX Pool side temperature HX Pool steam flowrate
MPa ◦ C kg/s MW MPa ◦ C kg/s
7 285 13 20 0.12 105 8.91
4 250 8 14 0.12 105 6.24
VESSEL
Feed line
Drain line
Steam supply Condensate discharge line Fig. 1. Scheme of the PERSEO facility.
scale module of the GE-SBWR in-pool heat exchanger. The scheme of the PERSEO facility is depicted in Fig. 1. The PERSEO facility mainly consists of: • the primary side containing the pressure vessel (43 m3 volume and 13 m height) and the heat exchanger (two cylindrical headers and 120 vertical pipes) interconnected by the steam feed line and the condensate drain line; • the pool side consisting of two pools connected at the top by a steam duct ending with an injector flowing into the Overall Pool (OP), and at the bottom by a water line with triggering valve. The Heat eXchanger (HX) is contained in the HX Pool (29 m3 volume and 5.7 m height); the OP (173 m3 volume and 5.8 m height) represents the water reservoir. The operation of the PERSEO facility is described as follows. The pressure vessel is maintained in saturation conditions typical for BWRs or secondary side of PWR steam generators (P = 7 MPa) by supplying properly de-superheated steam coming from a nearby power station. The pressure is kept constant by controlling the steam supply valve, while the water level in the vessel is maintained at the specified value for the test by discharge of water through the condensate discharge line at the vessel bottom. At the beginning of the test the HX, the feed line and the drain line are full of saturated steam; the HX Pool is full of air or steam depending on the test; the OP is full of cold water at the nominal level and the triggering valve is closed. Once reached the initial test conditions according to the test matrix, the triggering valve is opened and the HX Pool is flooded by cold water leading to steam condensation inside the HX tubes with power transfer from the primary side to the pool side. As soon as pool water boiling starts, the steam produced in the HX Pool is driven to the OP through the steam duct. The injector flowing about 1.3 m below the water level contributes to mix the OP water, thus limiting temperature stratification within the pool. The condensation of steam inside the OP leads to progressive cold water heatup until reaching the boiling point. The steam produced in the OP flows outside at atmospheric pressure through the boil-off pipe. When the injector is uncovered, no condensation is present anymore in
the OP; the steam flows outside directly through the boil-off pipe and the water reserve in the OP decreases according to the heat transfer rate in the HX Pool. During the system operation, the natural circulation which stabilizes on both primary and secondary sides determines the power evacuated by the system. In some tests, water is discharged from the OP bottom to accelerate the transient phase with water level decreasing. 3. Analyzed PERSEO tests Two different kinds of tests have been performed during the experimental campaign on the PERSEO facility (Ferri et al., 2002): • the stability tests to address particular critical problems happening in case of sudden steam condensation at the steam–water interface in the injector or within the HX Pool, following triggering valve re-opening with cold water injection in presence of steam in the HX Pool; • the integral tests aimed at demonstrating the behavior and performances of the system during all phases of a long accidental transient. The stability and integral test n. 7 and the integral test n. 9 have been analyzed with CATHARE code in the present study. The main PERSEO test parameters at full power operation during the transient phase are listed in Table 1. The test n. 7 foresees the system actuation with partial HX Pool fill-up (Phase 1), followed by the HX Pool total fill-up with reaching of boiling conditions in the OP and water level decreasing (Phase 2), with primary side pressure of 7 MPa. This test was devoted to investigate the following items: 1. the system actuation and the trend of power with a low HX Pool level; 2. the presence of instabilities due to steam condensation at the interface between water and steam in the injector; 3. the system re-actuation consequent to the HX Pool fill-up and reaching of thermal regime in both pools; 4. the effectiveness of the injector in mixing the OP water; 5. the power and flow regime variation after the OP level decreases below the injector outlet; 6. the trend of power as a function of the water level in the pools, decreasing for the loss of mass through the boil-off. The test n. 9 foresees the system actuation with total HX Pool fill-up followed by reaching of boiling conditions and pool level decreasing, with the primary side pressure of 4 MPa. This test was devoted to investigate the following items: 1. the system actuation consequent to the HX Poll fill-up and reaching of the thermal regime in both pools; 2. items 4, 5 and 6 like for previous test n. 7.
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G. Bandini et al. / Nuclear Engineering and Design 241 (2011) 4662–4671 15
P = 1 bar
DP-J001 kPa DP-J002 kPa DP injector (cathare)
HX Pool
DP inj-pool (cathare) Media Mobile per. (DP-J001 kPa) Average valuesuof100 DP-J001 kPa Average valuesuof100 DP-J002 kPa Media Mobile per. (DP-J002 kPa)
P steam
5
DP injector (J001)
Feed Line
DP (kPa)
Steam Line
10
0
IC HX Overall Pool
-5 Trigger. Valve
DP measurement Q drain
Water Line
Drain Line
DP inj-pool (J002)
-10 0
1000
2000
3000
Vessel
4000
5000
6000
Time (s)
Fig. 3. Test n. 7 (Ph. 2): Injector differential pressure. Q liquid
Fig. 2. CATHARE nodalization scheme.
4. CATHARE modeling of PERSEO facility The CATHARE code system (Geffraye et al., 2009) developed at CEA/Grenoble is a lumped parameter thermal-hydraulic code for transient analysis in LWRs. The CATHARE modeling scheme used for the analysis of PERSEO tests with the previous V1.5 version of the code (Bianchi et al., 2004; Meloni et al., 2005) has been refined according with more recent model developments and improvements introduced in the last V2.5 mod8.1 version of CATHARE employed in the present test analysis. The CATHARE nodalization scheme of the PERSEO facility is depicted in Fig. 2. The 0-D two-node module (volume) is used to represent the primary vessel, the HX collectors, the HX Pool lower and upper plenum and the OP (complete mixing). The volumes are interconnected by axial elements (pipe) representing the HX tube bundle, the water lines and the steam lines. Axial elements with cross-flow junctions are used to best simulate 2-D recirculation within the HX Pool and HX submerging and uncovery during the transient phase. Empirical correlations specifically derived at atmospheric pressure: (1) EPICE correlation for boiling heat transfer in the HX Pool and (2) SUPERCLAUDIA correlation for direct contact condensation in the OP have been implemented in the code (Bianchi et al., 2004), in order to overcome some deficiencies evidenced using the standard models of CATHARE and better reproduce the experimental data. Contrary to the test conduct, a pressure boundary condition at the HX Pool top was maintained active for 100 s during startup to avoid large instabilities calculated by the code, which were not observed in the tests. These instabilities are likely generated by natural circulation overestimation within the HX Pool leading to immediate strong condensation of steam in the upper plenum when cold water is injected at the pool bottom. The right evaluation of pressure drop through the steam injector is needed to well reproduce the HX Pool relative pressure trend and thus the overall system behavior. Therefore, friction losses through the conic-shape injector are calibrated on differential pressure measurements. To this aim, appropriate singular pressure loss coefficients are taken into account in the injector trying to reproduce the test measurements, as shown in Fig. 3 for the analysis of Phase 2 of Test n. 7. 5. Analysis of PERSEO tests The Test n. 7 (Phase 1 and Phase 2) and the Test n. 9 have been calculated with CATHARE V2.5 mod8.1 and the results are com-
Table 2 Initial conditions of OP and HX Pool. Parameter
Unit
Test 7 Ph. 1
Test 7 Ph. 2
Test 9
OP water level OP water temperature HX Pool water level HX Pool water temperature
m ◦ C m ◦ C
4.64 15 0.2 80
4.52 60 1.125 100
4.50 24.5 1.222 47
pared with the experimental data for code validation purposes. In the comparison figures the calculated and measured values are distinguished by their plotting line thickness: the thicker lines represent the code results, while the thinner lines represent the test measurements. The initial conditions (water level and temperature) of OP and HX Pool taken into account in the CATHARE calculations are summarized in Table 2 for the two analyzed tests. 5.1. Test n. 7 – Phase 1 The Phase 1 of Test n. 7 foresees the system actuation with partial HX Pool fill-up without reaching of boiling conditions in the OP. This test phase is aimed at verifying the system behavior during start-up. The chronology of main events characterizing the conduct of the test is shown in Table 3. The overall behavior of the system is illustrated from Figs. 4–9. After successive partial triggering valve openings, the HX Pool water level increases provoking a corresponding decrease of the OP level according with the respective pool area. As soon as the HX tubes are covered by water after about 900 s from the beginning of the transient, the power exchange becomes much more significant leading to boiling conditions with steam release to the OP through the steam duct and the injector, with consequent OP water temperature progressive increase. The definitive triggering valve closure at t = 1085 s leads to progressive decrease of the HX water level and of the extracted power. Once the steam flow through the injector Table 3 Chronology of main events. Event
Time (s)
Beginning of the test 1st partial triggering valve opening Triggering valve closure 2nd partial triggering valve opening Triggering valve closure 3rd partial triggering valve opening Triggering valve closure Primary side depressurization End of the test
0 300 433 446 480 864 1085 900–1400 4609
G. Bandini et al. / Nuclear Engineering and Design 241 (2011) 4662–4671
4
25000
3.5
L_VQ 0.195
HX tubes
1.5
Power (kW)
2.5 2
W_IC 2517.85
20000
COLLVHX
3 Level (m)
4665
POWERHX
15000
10000
1
5000 0.5
Valve closed
0
0 0
1000
2000
3000
4000
5000
0
1000
2000
Time (s) Fig. 4. Test n. 7 (Ph. 1): HX Pool collapsed water level.
4000
5000
Fig. 7. Test n. 7 (Ph. 1): HX exchanged power.
5
80 70
4
Temperature (°C)
Injector level Level (m)
3000
Time (s)
3 L_VP 4.646
2
MIXLEVCP COLLEVCP
60 50 T-P021
40
T-P022 T-P023
30
T-P025
1
T-P028
20
TEMPWAT
0
10 0
1000
2000 3000 Time (s)
4000
5000
0
1000
2000
3000
4000
5000
Time (s)
Fig. 5. Test n. 7 (Ph. 1): Overall Pool collapsed water level.
Fig. 8. Test n. 7 (Ph. 1): Overall Pool temperatures.
reduces, after about t = 2000 s, some temperature stratification is observed in the OP. Unless of a slight underestimation of level decrease in the second part of the transient, the collapsed water level rise and drop in the HX Pool is well predicted by the code (Fig. 4), so as for the OP water level (Fig. 5). Oscillations of the OP level observed in the test, likely provoked by free surface fluctuations, cannot be taken into account by the code. The calibration of the injector differential pressure carried out for the Phase 2 of the same test (see Section 4) allows a good representation of the HX Pool relative pressure for the Phase 1 too (Fig. 6). At the beginning of the test, the injector is full of water and the HX relative pressure against the OP pressure is close to zero.
Around t = 400 s the injector empties and the HX relative pressure stabilizes at 12 kPa according to OP water level. The largest discrepancy is observed at t = 1200 s, when the saturation conditions are reached at the pool bottom, owing to strong water recirculation in the HX Pool in boiling conditions. Indeed, the sudden vaporization calculated at the bottom determines a pressure spike that propagates in the pool, the steam duct and through the injector, since the triggering valve is already closed by that time. On the contrary, the transition to boiling at the HX Pool bottom is likely much more smooth in the test and thus such a peak pressure is not observed. The 2 kPa deviation around t = 2000 s is in line with the underestimation of extracted power leading to reduced 300
25
Temperature (°C)
20
DP (kPa)
15 10
Saturation at pool bottom
5 0
250
Tube middle 200
TW-C006 TW-C007 TW-C008
150
P-Q001 0.604
WALLT10
DP_CPHX
-5
100
-10 0
1000
2000
3000
4000
Time (s) Fig. 6. Test n. 7 (Ph. 1): HX Pool relative pressure.
5000
0
1000
2000 3000 Time (s)
4000
Fig. 9. Test n. 7 (Ph. 1): HX tube wall temperatures.
5000
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G. Bandini et al. / Nuclear Engineering and Design 241 (2011) 4662–4671
Table 4 Chronology of main events.
6 L_VP m
5
Time (s)
Beginning of the test Triggering valve opening OP water discharge opening Onset of OP water boiling Triggering valve closure Primary side depressurization End of the test
0 300 (in 26 s) 1150 ∼1400 3338 (in 123 s) 4685 5736
MIXLEVCP COLLEVCP
4 Level (m)
Event
Injector level 3 2 1
Water discharge = 16.2 kg/s 0 0
1000
2000
3000
4000
5000
6000
Time (s) Fig. 11. Test n. 7 (Ph. 2): Overall Pool collapsed water level.
The overall behavior of the system is illustrated from Figs. 10–15. After full opening of the triggering valve at t = 326 s the water level in the HX Pool quickly increases. When boiling is reached in the pool at t = 500 s, the collapsed water level reduces, stabilizing around 0.25 m above the middle HX tube plane. A progressive OP water level decrease is initiated at t = 1150 s by water discharge opening at the pool bottom. Once boiling is reached in the OP at about t = 1400 s the level decrease rate is enhanced due to mass loss through the boil-off pipe. Around t = 3400 s the triggering valve is closed and the HX Pool level starts to rapidly decrease, until boiling is terminated and the HX tube bundle is fully uncovered towards the end of the test. 25 P-Q001 kPa
20
DP (kPa)
steam flowrate through the injector during the same period of time (1800–2200 s). Towards the end of the test, the injector is full of saturated steam and the differential pressure of 12.4 kPa equalizes the hydraulic head of the water column between the injection level and the pool free surface. Since the final OP level, which slightly increases after triggering valve closure at t = 1085 s due to water thermal expansion, is well predicted, the HX Pool relative pressure is well predicted too. The power extracted by the HX, which reaches a maximum of about 20 MW when the HX tubes are fully submerged by water–steam mixture (collapsed level just above the middle tube plane), is well simulated (Fig. 7). The temporary power reduction during primary side depressurization between t = 900 and 1400 s is captured by the code. The progressive decrease of exchanged power with the decreasing HX Pool water level is quite well reproduced by CATHARE. The mean OP temperature increase is well predicted by the code, but the temperature stratification following steam flow reduction through the injector cannot be simulated with the 0-D two-node volume of CATHARE representing the OP (Fig. 8). The onset and the end of wetting at the middle plane of HX tube bundle is well predicted by CATHARE as shown in Fig. 9. Under wetting condition, the external wall temperature of HX tubes is over predicted by about 20 ◦ C. However, this quite large discrepancy is believed to be mainly caused by uncertainties in the thermocouple measurements rather than by wrong unbalance between boiling and condensation heat transfer coefficients calculated by CATHARE.
DP_CPHX
15
10
5.2. Test n. 7 – Phase 2 5
The Phase 2 of Test n. 7 foresees the system actuation with total HX Pool fill-up followed by reaching of boiling in the OP and water level decreasing. This phase of the test is aimed at demonstrating the correct system behavior during long term accidental transient. The chronology of main events characterizing the conduct of the test is shown in Table 4.
0 0
1000
2000
3000 Time (s)
4000
5000
6000
Fig. 12. Test n. 7 (Ph. 2): HX Pool relative pressure.
3.5
25000 L_VQ m
3
W_IC kW
COLLVHX
20000
POWERHX
HX tubes
2 1.5
Power (kW)
Level (m)
2.5 15000
10000
1 5000
0.5
Valve closed 0
0
0
1000
2000
3000
4000
5000
Time (s) Fig. 10. Test n. 7 (Ph. 2): HX Pool collapsed water level.
6000
0
1000
2000
3000 Time (s)
4000
5000
Fig. 13. Test n. 7 (Ph. 2): HX exchanged power.
6000
G. Bandini et al. / Nuclear Engineering and Design 241 (2011) 4662–4671 Table 5 Chronology of main events.
120
Temperature (°C)
4667
100 T-P008 °C T-P021 °C
80
T-P022 °C T-P023 °C T-P025 °C T-P028 °C
60
Event
Time (s)
Beginning of the test Triggering valve opening OP water discharge opening Onset of OP water boiling OP water discharge closure Triggering valve closure End of the test
0 142 (in 21 s) 2790 ∼3200 4840 4887 (in 93 s) 7708
TEMPWAT
0
1000
2000
3000 Time (s)
4000
5000
6000
Fig. 14. Test n. 7 (Ph. 2): Overall Pool temperatures.
The initial water level increase in the HX Pool is very well captured by CATHARE (Fig. 10). Afterwards, the level is initially overestimated until t = 1400 s and then underestimated until triggering valve closure (t = 3338 s), when the calculated level well matches the measured one. Towards the end of the transient the level is slightly overestimated like during the Phase 1 of the test. The OP water level behavior is well calculated by the code (Fig. 11). Large oscillations of the OP level are observed in the test after onset of steam flow through the injector at t = 500 s. They could be provoked by fluctuations of the free surface which cannot be reproduced by the 0-D module used to represent the OP. Starting from the initial value of 12 kPa with water-empty injector, the HX Pool relative pressure decrease after triggering valve opening at t = 300 s is roughly estimated (Fig. 12). The large pressure underestimation after onset of HX Pool boiling, likely associated to contemporary OP level oscillations, determines the HX Pool level mismatch evidenced in Fig. 10 between t = 500 and 1400 s. For similar reason, the relative pressure over prediction between t = 1400 and 3200 s causes the discrepancy in HX Pool level during the same period of time. The 20 MW power extracted by the HX is well predicted by the code until 1600 s (Fig. 13). Afterwards, an earlier power diminution is calculated likely due to HX Pool level under prediction. The power level decrease after triggering valve closure around t = 3400 s is well reproduced. Starting from stratified conditions reached at the end of Phase 1, the OP temperatures are mixed by the injector until onset of boiling is calculated around t = 1600 s, that is about 200 s later than in the test (Fig. 14). The calculated saturation temperature is some degrees above the measured value of 100 ◦ C at atmospheric
pressure, since the OP pressure and corresponding saturation temperature are calculated by CATHARE at the middle of water pool height, where the pressure is up to 20 kPa above the atmospheric value. Like in the Phase 1, the onset and the end of wetting at the middle HX tube plane is very well predicted by the code (Fig. 15). Furthermore, the same discrepancy is evidenced in the calculation of external wall temperature of HX tubes. 5.3. Test n. 9 Like in Test n. 7, the Test n. 9 foresees the system actuation with total HX Pool fill-up followed by reaching of boiling in the OP and water level decreasing. This test is aimed at demonstrating the correct system behavior during long term accidental transient under reduced primary pressure (4 MPa). The chronology of main events characterizing the conduct of the test is shown in Table 5. The overall behavior of the system is illustrated from Figs. 16–21. After full opening of the triggering valve at t = 163 s, the collapsed 3.5 3 2.5 Level (m)
40
HX tubes
2 1.5 L_VQ m
1
COLLVHX
0.5
Valve closed 0 0
1000
2000
3000
4000 5000 Time (s)
6000
7000
8000
Fig. 16. Test n. 9: HX Pool collapsed water level.
300
6
Tube middle
4
TW-C006 °C
Level (m)
Temperature (°C)
5
250
TW-C007 °C
200
TW-C008 °C WALLT10
Injector level 3 L_VP m
2
MIXLEVCP
150
COLLEVCP
1
Water discharge = 18.5 kg/s 0
100 0
1000
2000
3000 Time (s)
4000
5000
Fig. 15. Test n. 7 (Ph. 2): HX tube wall temperatures.
6000
0
1000
2000
3000
4000
5000
6000
7000
Time (s) Fig. 17. Test n. 9: Overall Pool collapsed water level.
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300
20
DP_CPHX
Temperature (°C)
DP (kPa)
TW-C006 °C
P-Q001 kPa
15
10
5
TW-C007 °C
250
TW-C008 °C WALLT10
Tube middle
200
150
0 100
-5
0
0
1000
2000
3000
4000 5000 Time (s)
6000
7000
8000
1000
2000
3000
4000 5000 Time (s)
6000
7000
8000
Fig. 21. Test n. 9: HX tube wall temperatures. Fig. 18. Test n. 9: HX Pool relative pressure.
water level in the HX Pool quickly increases stabilizing close to the top of tube bundle. The progressive decrease of the OP level is started by water discharge from the bottom at t = 2790 s, approximately 400 s before the onset of boiling. The water discharge is stopped at t = 4840 s, that is 140 s before complete triggering valve closure. As a consequence, the OP level decrease is terminated in conjunction with the onset of diminution of extracted power which tends to zero according with full uncovery of HX tube bundle reached at the end of the test. The initial water level increase in the HX Pool is very well captured by CATHARE (Fig. 16). Similarly to what happened during Test n. 7, the level is overestimated in the first part of the transient (t = 900–2100 s) and then largely underestimated from t = 2900 s 18000 W_IC kW
Power (kW)
15000
POWERHX
12000 9000 6000 3000 0 0
1000 2000 3000 4000 5000 6000 7000 8000 Time (s) Fig. 19. Test n. 9: HX exchanged power.
6. Analysis with 3-volume OP modeling
120
100 Temperature (°C)
until the triggering valve is closed at about t = 5000 s. The decrease of the HX Pool level during the last phase of the transient is quite well reproduced until the end of the test. The OP collapsed water level behavior is well predicted by CATHARE during the whole transient (Fig. 17). The oscillatory behavior observed in the test before OP water boiling might be provoked by sudden steam condensation at the injector outlet, inducing fluctuations at the pool free surface. The lack of oscillations in the calculated OP level reflects also in much less enhanced fluctuations in the HX Pool level and relative pressure values calculated by the code between t = 500 and 3000 s (Fig. 18). The large HX Pool relative pressure overestimation between t = 2700 and 5000 s (Fig. 18) is consistent with the corresponding HX Pool level underestimation shown in Fig. 16. The power extracted by the HX reduces with the decreasing primary system pressure mainly because of: (1) enhanced pressure losses in the feed line with consequent reduction of natural circulation primary mass flowrate, and (2) the lower saturation temperature resulting in reduced temperature difference between primary and secondary sides (see Table 1 in Section 3). The calculated extracted power in Fig. 19 is 15 MW, i.e. slightly higher than the measured one (14 MW). The power diminution predicted by the code after triggering valve closure at about t = 5000 s is slightly more accelerated than the one observed in the test. The onset of boiling in the OP, occurring around t = 3200 s, is very well captured by the code (Fig. 20). As expected and observed in Test n. 7 also, the temperature stratification phenomena in the OP disappear as soon as boiling is reached in the pool. The onset of HX tubes wetting is well predicted by the code, while its end is slightly anticipated owing to earlier calculated power decrease (Fig. 21). Once more the HX tube wall temperature is over predicted under wetting conditions.
T-P006 °C
80
T-P008 °C T-P021 °C
60
T-P022 °C T-P023 °C T-P025 °C
40
T-P028 °C T-P030 °C TEMPWAT
20 0
1000
2000
3000
4000 5000 Time (s)
6000
Fig. 20. Test n. 9: Overall Pool temperatures.
7000
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The previous test analyses have demonstrated the general capability of CATHARE to evaluate main natural circulation and heat transfer phenomena occurring during the PERSEO tests. Apart from temperature stratification and free level oscillation effects in the OP which of course need a detailed CFD modeling to be taken in to account, significant discrepancies have been found in the calculation of the HX Pool relative pressure and collapsed water level trend, after boiling is reached in the OP. The HX Pool relative pressure against the OP pressure (atmospheric value) strongly depends on the hydraulic head of the water column above the steam injector outlet in the OP. This hydraulic head increases under OP boiling conditions depending on void formation in the pool and swollen level effect (see Figs. 11 and 17). Likely this swollen level effect is overestimated by the 0-D module
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3.5
P = 1 bar
L_VQ m
3 HX Pool
Q steam
Feed Line
Level (m)
Steam Line
1.5 1 0.5
Overall Pool
Valve closed
Trigg. Valve Q drain
HX tubes
2
IC HX H
COLLVHX
2.5
0 0
Drain Line
Water Line
1000
2000
Vessel
Test n. 7 Test n. 9
H = 1.0 m H = 2.2 m
3000 Time (s)
4000
5000
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Fig. 24. Test n. 7 (Ph. 2): HX Pool collapsed water level. Q liquid
of CATHARE representing the OP, because of homogeneous distribution of voids considered in the boiling water pool, which could be in contrast with non uniform radial and axial distribution of voids expected in the tests. The use of a 3-volume OP modeling has been proposed in this study in order to verify the OP swollen level effect and try to reduce the discrepancies with the test measurements. The implementation of the new OP modeling in the original nodalization scheme is illustrated in Fig. 22. The height of the OP bottom volume is different for the two tests according to the respective extracted power value and then different boiling rate within the OP. The Phase 2 of the Test n. 7 and the Test 9 in which boiling conditions are reached in the OP have been re-calculated with the new modeling and the obtained results are presented in the following sections. 6.1. Test n. 7 – Phase 2
in general reduced, allowing the best simulation of all phenomena concerned in the test. In particular, the HX Pool relative pressure is very well predicted after onset of boiling (Fig. 23). The reduced pressure value reflects in a more precise evaluation of the HX Pool collapsed level which slightly increases in better agreement with the test measurement (Fig. 24). On its turn, the even small HX Pool level difference permits to better predict the exchanged power diminution curve observed in the test (Fig. 25) during the OP level decreasing phase. Owing to enhanced HX exchanged power between t = 1600 and 3500 s with corresponding increase of OP boiling rate and thus of steam mass loss through the boil-off, the re-calculated 25000 W_IC kW
20000 Power (kW)
Fig. 22. 3-Volume Overall Pool modeling scheme.
POWERHX
15000
10000
The new code results for the Phase 2 of the Test n. 7 regarding the HX Pool relative pressure, the water level in the two pools and the HX exchanged power are shown from Figs. 23–26, in comparison with the test measurements. As expected, the differences with previous results become significant only after the onset of OP boiling around t = 1400 s. After this time onwards, the deviations from the experimental trend are
5000
0 0
1000
2000
3000 Time (s)
4000
5000
6000
Fig. 25. Test n. 7 (Ph. 2): HX exchanged power.
25
6 L_VP m
P-Q001 kPa
20
5
MIXLEVCP
DP_CPHX
COLLEVCP
15
Level (m)
DP (kPa)
4
10
Injector level 3 2
5
1
Water discharge = 16.2 kg/s 0
0 0
1000
2000
3000 Time (s)
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Fig. 23. Test n. 7 (Ph. 2): HX Pool relative pressure.
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0
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3000 Time (s)
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Fig. 26. Test n. 7 (Ph. 2): Overall Pool collapsed water level.
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18000
5
15000
Level (m)
4
Injector level 3 2
L_VP m
Power (kW)
W_IC kW POWERHX
12000 9000 6000
MIXLEVCP
3000
COLLEVCP
1
Water discharge = 18.5 kg/s 0
0 0
1000
2000
3000
4000 5000 Time (s)
6000
7000
8000
0
1000 2000 3000 4000 5000 6000 7000 8000 Time (s)
Fig. 27. Test n. 9: Overall Pool collapsed water level.
Fig. 30. Test n. 9: HX exchanged power.
OP collapsed water level perfectly matches the measured one (Fig. 26).
OP swollen level (Fig. 27), the HX Pool relative pressure is much better predicted (Fig. 28). On its turn, the right pressure trend reflects in more precise calculation of the HX Pool collapsed level (Fig. 29). No significant change is evidenced in the HX exchanged power (Fig. 30).
6.2. Test n. 9 The new code results for the Test n. 7 regarding the water level in the two pools, the HX Pool relative pressure and the HX exchanged power are shown from Figs. 27–30, in comparison with the test measurements. As expected, the differences with previous results become significant only after onset of OP water boiling around t = 3200 s. After this time onwards, owing to significant reduction of the calculated 20 P-Q001 kPa
DP (kPa)
15
The CATHARE V2.5 code has been successfully employed for the analysis of two of the more representative tests carried out on the full scale PERSEO facility that allowed the validation of important thermal-hydraulic aspects without the uncertainties usually connected with the scaling effect. The systematic comparison of code results with the test measurements has highlighted the general good capability of CATHARE to simulate all relevant phenomena governing the processes involved in the experimental transients, in particular:
DP_CPHX
• the start-up of the system and the stabilization of natural circulation on the primary circuit and pool side; • the power removal from the primary side by steam condensation and water boiling through in-pool heat exchangers; • the steam condensation with direct contact at atmospheric pressure by the implementation of an appropriate heat transfer correlation.
10
5
0
-5 0
1000
2000
3000
4000
5000
6000
7000
8000
Time (s) Fig. 28. Test n. 9: HX Pool relative pressure.
3.5 3 2.5 Level (m)
7. Conclusions
HX tubes
2 1.5 L_VQ m
1
COLLVHX
Some code limitations have been recognized which mainly concern mixing and temperature stratification effects in large water pools like the OP, owing to the simplified 0-D model used. In particular, stratification before boiling, non-homogeneous void distribution under boiling, and free level oscillations are likely the main causes for discrepancies observed between code results and test measurements. The applicability of the 3-D module called ‘TREED’ available in CATHARE for the best simulation of the OP could be verified. The re-calculation of the tests with a modified 3-volume OP model has allowed the overall best simulation of the experimental transients after reaching of boiling conditions in the OP, owing to the importance of OP swollen level effect on the HX Pool relative pressure value. The need of more detailed 3-D models for large water pool simulation is thus emphasized.
0.5
Valve closed
Acknowledgments
0 0
1000
2000
3000
4000 5000 Time (s)
6000
Fig. 29. Test n. 9: HX Pool collapsed water level.
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The authors appreciate the support of the CATHARE team and in particular of Dr. G. Lavialle for the use of CATHARE code under extensive and fruitful ENEA-CEA bilateral collaborations.
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References Achilli, A., Cattadori, G., Ferri, R., Rigamonti, M., Bianchi, F., Meloni, P., 2002. PERSEO project: experimental facility set-up and RELAP5 code calculations. In: Proc. 2nd EMSI and 40th European Two-Phase Flow Group Meeting , Stockholm, Sweden, June 10–13. Bianchi, F., Meloni, P., Pignatel, J.F., Gautier, G.M., 1997. Thermal valve system for LWR applications. In: Proc. Post-SMIRT14 Seminar , Pisa, Italy. Bianchi, F., Meloni, P., Ferri, R., Achilli, A., 2004. Assessment of RELAP5 MOD3.3 and CATHARE 2 V1.5a against a full scale test of PERSEO device. In: Proc. 12th International Conference on Nuclear Engineering (ICONE12) , Arlington, Virginia, USA, April 25–29. Ferri, R., Achilli, A., Gandolfi, S., 2002. PERSEO Project Experimental Data Report SIET 01 014 RP 02, Piacenza, Italy.
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Geffraye, G., Antoni, O., Farvacque, M., Kadri, D., Lavialle, G., Rameau, B., Ruby, A., 2009. CATHARE 2 V2.5 2: a single version for various applications. In: Proc. 13th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics (NURETH-13) , Kanazawa, Japan, September 27–October 2. Meloni, P., Pignatel, J.F., 1998. Theoretical design and assessment of isolation condenser system controlled with thermal valve. In: Proc. 6th International Conference on Nuclear Engineering (ICONE6) , San Diego, USA, May 10–14. Meloni, P., Bianchi, F., Ferri, R., Achilli, A., 2005. Experimental campaign and numerical analysis for the In-Pool Energy Removal System for Emergency Operation. In: Proc. 13th International Conference on Nuclear Engineering (ICONE13) , Beijing, China, May 16–20.