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Short-term pressure and temperature MSLB response analyses for large dry containment of the Maanshan nuclear power station
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Short-term pressure and temperature MSLB response analyses for large dry containment of the Maanshan nuclear power station Liang-Che Dai ∗ , Yen-Shu Chen, Yng-Ruey Yuann Nuclear Engineering Division, Institute of Nuclear Energy Research, No. 1000, Wenhua Road, Jiaan Village, Longtan Township, Taoyuan County 32546, Taiwan, ROC
h i g h l i g h t s • • • •
The GOTHIC code is used for the PWR dry containment pressure and temperature analysis. Boundary conditions are hot standby and 102% power main steam line break accidents. Containment pressure and temperature responses of GOTHIC are similar with FSAR. The capability of the developed model to perform licensing calculation is assessed.
a r t i c l e
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Article history: Received 29 January 2014 Received in revised form 29 August 2014 Accepted 1 September 2014
a b s t r a c t Units 1 and 2 of the Maanshan nuclear power station are the typical Westinghouse three-loop PWR (pressurized water reactor) with large dry containments. In this study, the containment analysis program GOTHIC is adopted for the dry containment pressure and temperature analysis. Free air space and sump of the PWR dry containment are individually modeled as control volumes. The containment spray system and fan cooler unit are also considered in the GOTHIC model. The blowdown mass and energy data of the main steam line break (hot standby condition and various reactor thermal power levels) are tabulated in the Maanshan Final Safety Analysis Report (FSAR) 6.2 which could be used as the boundary conditions for the containment model. The calculated containment pressure and temperature behaviors of the selected cases are in good agreement with the FSAR results. In this study, hot standby and 102% reactor thermal power main steam line break accidents are selected. The calculated peak containment pressure is 323.50 kPag (46.92 psig) for hot standby MSLB, which is a little higher than the FSAR value of 311.92 kPag (45.24 psig). But it is still below the design value of 413.69 kPag (60 psig). The calculated peak vapor temperature inside the containment is 187.0 ◦ C (368.59 F) for 102% reactor thermal power MSLB, which is lower than the FSAR result of 194.42 ◦ C (381.95 F). The effects of the containment spray system and fan cooler units could be clearly observed in the GOTHIC analysis. The calculated containment pressure and temperature behaviors of the selected cases are in good agreement with the FSAR results. Since the results are similar with the Mananshan FSAR, and the applicability of the GOTHIC dry containment model is assessed. © 2014 Elsevier B.V. All rights reserved.
1. Introduction
Abbreviations: PWR, pressurized water reactor; FSAR, Final Safety Analysis Report; MSLB, main steam line break; NSSS, nuclear steam supply system; LOCA, loss of coolant accident; SRP, Standard Review Plan; LOFT, Loss-of-Fluid Test; RPV, reactor pressure vessel; RCS, reactor coolant system; RCP, reactor coolant pump; DBA, design basis accident; BWR, boiling water reactor; RWST, refueling water storage tank; OLTP, original licensing thermal power. ∗ Corresponding author. Tel.: +886 3 4711400 6088; fax: +886 3 4711404. E-mail address: [email protected] (L.-C. Dai).
The containment structure contains the nuclear steam supply system (NSSS). When the NSSS pipe break or loss of coolant accident (LOCA) occurs, the high energy fluid would directly release to the containment space from the break and instantly pressurize the containment structure. Therefore, the integrity of the containment may be compromised by the pressurization and may cause the radiation release. The acceptance criteria in section 6.2.1.1.A of Standard Review Plan (SRP) (SRP, 2007) require that the peak calculated containment pressure following a postulated loss-of-coolant
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accident, or a steam or feedwater line break, should be less than the containment design pressure. Section 6.2 of the Final Safety Analysis Report (FSAR) (FSAR, 2005) of the Maanshan nuclear power station indicates that the containment pressure analyses are performed by the Bechtel COPATTA (Performance and Sizing of Dry Pressure Containments, 1975) computer program that was derived from the CONTEMPT (Richardson et al., 1967) program written for the NRC Loss-ofFluid Test (LOFT) program. Since the COPATTA computer program has never been used in Taiwan, the general thermal-hydraulic code GOTHIC (GOTHIC Containment Analysis Package User Manual, 2006; GOTHIC Containment Analysis Package Qualification Report, 2006; GOTHIC Containment Analysis Package Technical Manual, 2006) is adopted for developing the dry containment thermalhydraulic analysis model and methodology. 2. Brief PWR system descriptions Maanshan nuclear power station has two identical Westinghouse three-loop PWR units. The original rated thermal core power is 2775 MWt. The total rated thermal power is 2785 MWt with 10 MW reactor coolant pump heat power included. The design and build of the containment should fully withstand the liquid blowdown during the loss of coolant accident (LOCA) and steam line break accident without jeopardizing the safety of the public. The design pressure of the dry containment of Maanshan plant is 413.69 kPag (60 psig). When design basis accident (DBA) occurs, the maximum pressure should not exceed the design pressure. The dry containment of Maanshan plant is a cylindrical concrete building with a hemisphere dome on top. The cutaway diagram of the dry containment of Maanshan plant is shown in Fig. 1. Two recirculation sumps are on the bottom floor of the containment. Suction inlets of residual heat removal system (RHR) and containment spray system are in the bottom of the sumps. The containment heat removal system includes containment spray system and fan cooler units. They can remove the energy of high energy fluid that blowdown into the containment in order to reduce the containment air space pressure. Fan cooler units also have the function maintaining the ambient temperature and relative humidity inside the containment in the normal operation. There is no passive pressure suppression system inside the dry containment of PWR that can cool or condense the blowdown fluid by directing it into a pool of water via the vent pipe such as the suppression pool of the boiling water reactor (BWR). For dry containment, the containment spray system reduces the containment pressure by condensing the vapor in the containment air space. There are two independent containment spray trains in Maanshan plant. Each train has its own spray pump and spray header. The water supply of the containment spray system is initially from the refueling water storage tank (RWST) when the pipe break accident occurs in the containment and the water is transferred to the spray nozzles on the upper part of the containment. These nozzles spray droplets throughout the containment air space in order to cool down the vapor and reduce the containment pressure. 3. Gothic containment model descriptions 3.1. GOTHIC code introduction The name “GOTHIC” is abbreviated from Generation of Thermal Hydraulic Information for Containment. GOTHIC is a thermal hydraulic analysis code of nuclear power plant containment (GOTHIC Containment Analysis Package User Manual, 2006; GOTHIC Containment Analysis Package Qualification Report, 2006; GOTHIC Containment Analysis Package Technical Manual, 2006). It
Table 1 Principal containment design parameters. Containment internal design pressure, kPag (psig) Containment peak calculated pressure (DBA), kPag (psig) Containment peak superheat temperature (DBA), ◦ C (F) Containment internal dimensions Cylindrical wall diameter, m (ft) Cylindrical wall height, m (ft) Containment height, m (ft) Net free internal volume, m3 (ft3 )
413.69 (60) 311.92 (45.24) 197.56 (387.6)
39.62 (130) 39.62 (130) 59.28 (194.5) 5.75 × 104 (2.03 × 106 )
is developed by NAI (Numerical Applications, Inc.) with the fund of EPRI (Electric Power Research Institute). GOTHIC is an international recognized code. There are many containment analyses of both BWR and PWR using GOTHIC code. Since GOTHIC code has large flexibility and versatility, it can be applied to various types of reactors or containments. There are many nuclear power plants using GOTHIC to perform the containment safety evaluations, such as the containment pressure and temperature response analyses after LOCA and steam pipe rupture events. Kewaunee, Grand Gulf, Catawba, McGuire, Waterford, Clinton, Brunswick, Calvert Cliffs and Nine Mile Point are the nuclear plants that have conducted the containment safety evaluations with GOTHIC code and submitted their topical reports for NRC licensing approval, FSAR or test specification update. GOTHIC code is very versatile and can be applied to different analyses in power plants. For example, Columbia, Clinton power plants and KEPCO had been conducted the containment analyses of station blackout (SBO) events with GOTHIC code. GOTHIC can also be applied for equipment qualification (EQ) analyses in order to verify critical equipment installed in the reactor building or turbine building that could withstand the pressure and temperature transients during pipe break events. Oconee, Ontario, Shearon Harris, Oyster Creek and Calvert Cliffs are the utilities that had used GOTHIC to perform the EQ analyses for critical components. Besides, GOTHIC is also commonly used in thermal hydraulic analyses of high energy line break (HELB) in turbine or auxiliary building. These examples address the versatility of GOTHIC code. 3.2. Maanshan containment short term model There were only two thermal hydraulic nodes in the Maanshan containment short term model of FSAR (FSAR, 2005). They represented the internal air space of the dry containment and sump volume on the bottom of the containment respectively. For the analysis of the main steam line break (MSLB) inside the containment, blowdown mass and energy data tables were treated as the boundary conditions of the model. In order to make the comparison with the Maanshan FSAR, the containment air space and the sump volume were also modeled by two control volumes respectively. The important design parameters of the containment were listed in Table 1. The net free volume of the containment was 5.75 × 104 m3 . Two containment recirculation sumps were on the bottom of the containment floor. Total volume of two sumps was 26.5 m3 (7000 gal). The GOTHIC nodal diagram was shown in Fig. 2. The containment air space was modeled as control volume 1. Two sumps were modeled as a single volume and it was control volume 2. The net volume of control volume 1 was the net free volume minus the sump volume which was 5.747 × 104 m3 . On the other hand, the net volume of control volume 2 was 26.5 m3 which equaled 7000 gal as described previously. The containment constituting structures and system components inside the containment building were modeled as eighteen
Please cite this article in press as: Dai, L.-C., et al., Short-term pressure and temperature MSLB response analyses for large dry containment of the Maanshan nuclear power station. Nucl. Eng. Des. (2014), http://dx.doi.org/10.1016/j.nucengdes.2014.09.007
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Fig. 1. Cutaway diagram of Maanshan containment.
individual heat structures. The material properties, thickness and heat transfer area of these heat structures were already given in FSAR data and these data were directly applied for the heat conductor model in the GOTHIC Maanshan containment model. In MSLB analysis, the condensation heat transfer between the vapor and wall surface was calculated by Uchida experimental equation that was specified in section 6.2.1.1.3.2.7 of FSAR. The convection heat transfer coefficient of the heat conductor surrounding the sumps was specified as 2.27 W/m2 K (0.4 Btu/hr ft2 F) in section 6.2.1.1.3.5 of FSAR. The event time span of a short term pipe break analysis was 2400 s. The containment spray system still operated in injection phase and it did not switch to recirculation phase during the short term analysis. The water source of spray pumps in injection phase was refueling water storage tank (RWST). In Table 6.2-27 of FSAR, the water temperature of RWST was specified as 36.7 ◦ C (98 F). The initial conditions of containment analyses were summarized in Table 2. There was only one train of containment sprays available based on the conservative single failure assumption. The flow rate of the spray pump was specified in Table 6.2-28 of FSAR, and its value was 170.3 l/s (2700 gpm). The droplet size from the spray
Table 2 Summary of the results (FSAR vs. GOTHIC). FSAR 6.2
GOTHIC
Hot standby MSLB Peak containment pressure, kPag (psig) Peak containment temperature, ◦ C (F)
311.92 (45.24) 194.42 (381.95)
323.50 (46.92) 180.59 (357.07)
102% OLTP MSLB Peak containment pressure, kPag (psig) Peak containment temperature, ◦ C (F)
270.27 (39.20) 197.56 (387.6)
277.86 (40.30) 186.99 (368.59)
nozzle was conservatively assumed to be 1000 m to reduce the total heat transfer area of the sprayed droplets. The initiation signal of the containment spray system was containment high pressure (Hi-3). Technical specification requires that Hi-3 signal initiated when the containment pressure had reached 124.8 kPag (18.1 psig), but a higher set point 155.1 kPag (22.5 psig) was specified in section 6.2.1.4.8 of FSAR for further conservative consideration. One of the critical functions of fan cooler units was to remove the energy of the containment internal air space after the pipe
Please cite this article in press as: Dai, L.-C., et al., Short-term pressure and temperature MSLB response analyses for large dry containment of the Maanshan nuclear power station. Nucl. Eng. Des. (2014), http://dx.doi.org/10.1016/j.nucengdes.2014.09.007
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B.C 1F Broken S/G Blowdown
1 4 2
B.C 2F MS Header Blowdown
CS Spray Nozzle 1N
Fan Cooler Cooler 1C
C.V 1 CTMT
B.C 3F RWST
3
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Fig. 2. Nodal diagram for Maanshan dry containment GOTHIC model.
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MSLB Break flow rate (kg/sec) (Broken S/G side & MS Header side)
break event in order to reduce the containment temperature and pressure. There was only one fan cooler unit available in containment response analyses for single failure conservatism. The air volumetric flow rate of the fan cooler unit was 934.5 m3 /min (33,000 ft3 /min). Technical specification states that the initiation set point of fan cooler units was 21.4 kPag (3.1 psig), but a more conservative value 31.0 kPag (4.5 psig) was applied in the analysis. The heat removal capacity per fan cooler unit was specified as a function of containment vapor temperature and component cooling water (CCW) temperature. The component cooling water supply to the fan cooler was conservatively assumed to be at its maximum supply temperature of 48.9 ◦ C (120 F) for less heat removal.
Hot Standby MSLB
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Fig. 3. Blowdown mass flow rates for hot standby MSLB. 4500
MSLB Break flow rate (kg/sec) (Broken S/G side & MS Header side)
The analyses in section 6.2 of FSAR showed that the most severe transient of the containment pressure was main steam line break (MSLB) under hot standby condition. The containment peak pressure reached 311.92 kPag (45.24 psig). The MSLB blowdown mass and energy data under hot standby condition were listed in FSAR and they could be directly applied as the boundary condition in Maanshan GOTHIC model. Another selected case was MSLB under 102% reactor thermal power and the blowdown mass and energy data were also listed in FSAR. The results of these two cases showed that the most severe pressure transient of the containment for GOTHIC code remained the same as FSAR. Thus, the consistency of both GOTHIC analyses and FSAR were verified. The MSLB blowdown mass flow rates of selected cases from both broken steam generator side and main steam header side were plotted in Figs. 3 and 4. In Figs. 3 and 4, the independent variable was the elapsed time after the event in logarithm scale. When MSLB event occurred, the inventory inside broken loop steam generator would directly blowdown into the containment
10
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Fig. 4. Blowdown mass flow rates for 102% power MSLB.
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3.3.1. Assumptions for short term MSLB analysis Assumptions for Maanshan short term MSLB analysis were listed below: • Main steam line guillotine break occurred at 0 s under hot standby condition or 102% original licensed thermal power (OLTP). • Blowdown mass and energy data from FSAR for hot standby condition MSLB were adopted as the boundary conditions in GOTHIC model. • Blowdown mass and energy data from FSAR for 102% OLTP MSLB were adopted as the boundary conditions in GOTHIC model. • Only one train of containment sprays available based on the conservative single failure assumption. The flow rate of one train spray pump was 170.3 l/s (2700 gpm). • The Hi-3 set point of the containment spray initiation was 155.1 kPag (22.5 psig). The delay time was 134 s for hot standby MSLB and it was 94 s for 102% OLTP MSLB. • The volumetric air flow rate of the fan cooler unit was 934.5 m3 /min (33,000 ft3 /min). • The heat removal capacity per fan cooler unit was specified as a function of containment vapor temperature and component cooling water (CCW) temperature. The component cooling water supply to the fan cooler was conservatively assumed to be at its maximum supply temperature of 48.9 ◦ C (120 F). • The Hi-1 set point of the fan cooler unit initiation was 31.0 kPag (4.5 psig) and the delay time was 33 s considering the restoration time of CCW after the event. • The event time span of a short term MSLB analysis was 2400 s. 3.3.2. Initial conditions for Maanshan GOTHIC model Initial conditions of Maanshan GOTHIC model were summarized below: • • • • • •
Containment initial pressure was 101.35 kPa (14.7 psia). Containment initial temperature was 48.9 ◦ C (120 F). Containment initial relative humidity was 100%. Containment sump volume was initially dry without any water. The fan cooler was not in operation initially. The atmosphere temperature outside the containment structure was 36.7 ◦ C (98 F).
350 300 Containment Pressure (kPag)
through the nearest break. On the opposite side of the steam line break, blowdown flow came out by the reverse flow from intact loop steam generators through the main steam header and it would then be terminated by the closure of main steam isolation valves. These phenomena could be easily observed in both Figs. 3 and 4.
250 200 150 100 Hot Standby MSLB FSAR GOTHIC
50 0 1
10
100
1000
10000
Time (sec) Fig. 5. Containment pressure responses for the hot standby MSLB.
larger than in GOTHIC. It indicated that the condensation effect of FSAR was stronger than GOTHIC. The magnitude of decrease of the containment pressure and temperature of FSAR were both larger than GOTHIC model. Figs. 5 and 6 showed that the pressure and temperature responses were consistent between FSAR and GOTHIC. The rate of change of the pressure and temperature profiles decreased at 10 s after MSLB, and it was caused by the decrease of the blowdown flow from the broken steam generator side after 10 s and the termination of the blowdown flow from the steam header side with the closure of main steam isolation valves (MSIV) at 7.58 s. The combining effect reduced the gradient of the pressure and temperature curves. The temperature and pressure curves of FSAR and GOTHIC had a similar appearance before the initiation of the spray. The initiation time of the containment spray system had a 134 s delay after the containment pressure rising over the set point Hi-3. The initiation time of FSAR and GOTHIC were at 167 s and 163 s respectively. The difference of the spray initiation sequence between both of them was little. The pressure response in Fig. 5 also indicated that the pressure response in FSAR analysis dropped immediately after the spray initiation and GOTHIC model had a slower pressure response corresponding to the spray initiation. The pressure response of GOTHIC continued to rise about 23 s after the spray initiation and then dropped. The pressure drop of GOTHIC due to the droplet condensation was also less than FSAR. This indicated that the spray condensation effect in GOTHIC analysis was
4. MSLB analysis results and discussion
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Fig. 5 showed the containment pressure transient during the hot standby MSLB of Maanshan GOTHIC model analysis. In FSAR, the fan cooler and the containment spray started at time 34 s and 167 s respectively. For GOTHIC analysis, the fan cooler and the containment spray started at time 34.1 s and 163 s respectively. The initiation sequence of the containment heat removal systems was nearly consistent between GOTHIC and FSAR. When the containment spray system initiated, containment pressure immediately dropped because of the sprayed droplets condensed the blowdown vapor in the containment. Fig. 6 was the containment temperature transient result for the same event. It also indicated that the vapor temperature immediately dropped by vapor condensation when the containment spray system initiated. However, it could be observed in Fig. 6, temperature drop of the FSAR profile was
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160 140 120 100 Hot Standby MSLB FSAR GOTHIC
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Time (sec) Fig. 6. Containment temperature responses for the hot standby MSLB.
Please cite this article in press as: Dai, L.-C., et al., Short-term pressure and temperature MSLB response analyses for large dry containment of the Maanshan nuclear power station. Nucl. Eng. Des. (2014), http://dx.doi.org/10.1016/j.nucengdes.2014.09.007
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less than FSAR. However, because the blowdown continued, the pressure rose again and a concave shaped pressure profile was then formed. The pressure profiles of FSAR and GOTHIC were almost overlapped after 250 s and the pressurization continued until 334 s. After that time, due to the sudden decrease of the blowdown flow (Fig. 3), the containment pressure began to drop. With the steam still flowed out to the containment air space, the containment pressure increased again about 300 s after the drop. Because the blowdown mass flow rate reduced to 25% of the original value, the pressurization rate was much slower than the pressure response curve before 334 s. The containment pressure reached its maximum at 1800 s and the increasing tendency was then reversed due to the stop of blowdown. The maximum containment pressure of FSAR was 311.92 kPag (45.24 psig) and GOTHIC was 323.50 kPag (46.92 psig) for hot standby MSLB analysis. The difference between them was only 11.58 kPa (1.68 psi). The containment temperature response was shown in Fig. 6. The response of the temperature transient of GOTHIC analysis was quite similar with FSAR. The slope of the temperature response curve of GOTHIC reduced at 8 s after the event. It indicated that the temperature increment after that time was smaller than previous. The reason was the MSIV closure stopping the steam header side blowdown. However, the temperature response of FSAR did not have such apparent gradient change like GOTHIC but it still had a reduced temperature increment caused by the MSIV closure. The containment temperature response had an apparent discrepancy between GOTHIC and FSAR at 20 s and it was caused by the previously described temperature increment difference. It resulted that FSAR had a higher containment temperature than GOTHIC. This discrepancy was due to the theoretical assumption of the COPATTA code used by FSAR. The COPATTA code assumed that both vapor and liquid phases were separable. The liquid blowdown to the containment would directly go to the recirculation sumps without energy exchange between the air and vapor in order to gain a higher vapor temperature. On the other hand, GOTHIC code had a complete two phase mass and energy exchange model to adequately simulate the mass and energy transfer between liquid and vapor phases. Therefore, GOTHIC had a lower temperature profile than FSAR during the time interval spanning from 20 s after the start of the event to the spray initiation. The available fan cooler unit initiated at 34.1 s after MSLB and did affect the containment temperature response. The containment temperature rise of GOTHIC decreased after the fan cooler unit initiation and produced a temperature profile with a milder gradient change before the spray initiation. The fan cooler unit had less effect on the FSAR temperature profile and it caused the FSAR temperature profile before the spray initiation diverging from GOTHIC ones. After the spray initiation, the droplet condensation effect caused the containment air temperature of both FSAR and GOTHIC to drop steeply and it could be observed clearly that FSAR had a larger temperature drop. Then the containment air temperature rose again owing to the persistence of the outward flow till time 334 s. After time 334 s, the containment air temperature dropped again because the blowdown flow rate decrease but the descending tendency would not last long. The pressure and temperature of both GOTHIC and FSAR decreased when the break flow was terminated at 1800 s. Fig. 7 compared the containment temperature and saturation temperature calculated with GOTHIC. It was found that before the spray initiation, there was superheat vapor in the containment because the vapor temperature exceeded the saturation temperature. The pressure and temperature continued to increase till the spray initiation. As mentioned in previous section, pressure and temperature dropped almost immediately when the spray was activating. The vapor temperature and the saturation temperature were overlapped at 200 s which was about 33 s after the spray activation. It indicated that the state of the containment
140 120 100 Hot Standby MSLB Saturation Temperature Vapor Temperature
80 60 1
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1000
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Time (sec) Fig. 7. Comparison of the containment temperature and the saturation temperature calculated with GOTHIC for the hot standby MSLB.
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Containment Pressure (kPag)
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Time (sec) Fig. 8. Containment pressure responses for the 102% OLTP MSLB.
atmosphere had transited from superheat vapor to two phase saturation by condensation. After that time, the containment temperature equaled the saturation temperature till the end of the analysis. The containment temperature of FSAR reached its maximum value 194.42 ◦ C (381.95 F) at the moment of the spray initiation and the containment temperature of GOTHIC reached its maximum value 180.59 ◦ C (357.07 F) 6 s after the spray initiation. 4.2. 102% OLTP MSLB short term analysis Fig. 8 was the containment pressure transient during the 102% OLTP MSLB of Maanshan GOTHIC model analysis. FSAR Fig. 6.2-1 sheet 7 indicated that the fan cooler and the containment spray started at time 34.2 s and 125.2 s. In GOTHIC analysis, t the fan cooler and the containment spray started at time 34.2 s and 114 s respectively. The initiation sequences of the fan cooler of GOTHIC and FSAR were consistent but the initiation time of the containment spray system of GOTHIC was 11 s ahead of FSAR. The reason was shown clearly in Fig. 8, time required for the pressure response curve exceeding the Hi-3 set point of GOTHIC analysis was less than FSAR. The comparison of the containment temperature transient during the 102% OLTP MSLB of GOTHIC analysis and FSAR Fig. 6.2-1 sheet 7 was shown in Fig. 9. The containment atmosphere temperature reduces after the spray initiation due to the vapor condensation effect. The temperature drop of GOTHIC
Please cite this article in press as: Dai, L.-C., et al., Short-term pressure and temperature MSLB response analyses for large dry containment of the Maanshan nuclear power station. Nucl. Eng. Des. (2014), http://dx.doi.org/10.1016/j.nucengdes.2014.09.007
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140 120 100
102% Power MSLB FSAR GOTHIC
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Time (sec) Fig. 9. Containment temperature responses for the 102% OLTP MSLB.
occurred at 120 s which was 6 s after the spray initiation and the containment atmosphere temperature also reached its maximum value 186.99 ◦ C (368.59 F). However, the maximum containment temperature of FSAR analysis was 197.56 ◦ C (387.6 F) at 125.2 s and it was greater than in GOTHIC analysis. The comparison of the containment temperature transient showed that the spray condensation effect in FSAR analysis was more significant than in GOTHIC analysis. Since the containment pressure and temperature drops of FSAR caused by the spray were both larger than GOTHIC. The overall tendencies of the pressure and temperature response curves of GOTHIC in Figs. 8 and 9 were roughly consistent with FSAR curves. In Figs. 8 and 9, a clearly gradient change of the pressure and temperature response of GOTHIC could be observed at 8 s after the MSLB event. The turbine side blowdown flow in Fig. 4 was terminated at 7.55 s due to the closure of MSIVs and then the pressurization and heat up rate of the containment were reduced. However, the gradient change of FSAR temperature response occurs at about 10 s, which lagged behind GOTHIC. As previously mentioned, the initiation time of the containment spray of GOTHIC was 11 s ahead FSAR. Comparing with FSAR, the GOTHIC analysis of containment pressure and temperature response after the spray initiation was different. The containment pressure of FSAR dropped immediately after the spray initiation. In contrast, the containment pressure of GOTHIC still mildly rose and last about 11 s even the spray had already initiated. The containment pressure of FSAR dropped 20.68 kPa (3 psi) immediately after the spray initiation. However, the containment pressure of GOTHIC analysis remained a nearly flat gradient in the duration ranging from 125 to 158 s. Then, the pressure responses of both FSAR and GOTHIC rose again with the continuation of blowdown flow. But the increasing tendency last shortly, both the pressure responses dropped at 175 s in correspond with the step decrease of forward blowdown flow shown in Fig. 4. After 200 s, both the pressure response curves of FSAR and GOTHIC fitted well. The cause of the 102% MSLB forward blowdown flow step decrease at 175 s shown in Fig. 4 was the inventory dryout in the broken loop steam generator that was described in FSAR 6.2.1.4. In contrast, the inventory dryout time of hot standby MSLB extended to 333.7 s shown in Fig. 3 that was caused by the power level difference. After the steam generator inventory dryout, the forward blowdown was only supplied by the auxiliary feedwater to the broken loop steam generator. The auxiliary feedwater continuously operated till 30 min or 1800 s after the MSLB. It was manually isolated at that time and then terminated the forward blowdown flow. The concave pressure response curves during this duration shown in Fig. 8 illustrated that the containment pressure descended
102% Power MSLB Saturation Temperature Vapor Temperature
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10000
Time (sec) Fig. 10. Comparison of the containment temperature and the saturation temperature calculated with GOTHIC for the 102% OLTP MSLB.
first by the inventory dryout and gradually rose again by the continuation of the blowdown flow. The containment pressure reached its maximum value just at 1800 s and then the ascending tendency reversed by the termination of the steam blowdown. The peak pressure of FSAR was 270.27 kPag (39.20 psig) and the peak value of GOTHIC analysis was 277.86 kPag (40.30 psig). The difference of peak pressure of these two analyses was only 7.59 kPa (1.10 psi). Fig. 9 was the containment temperature transient during the 102% OLTP MSLB of Maanshan GOTHIC model analysis. The overall response of the temperature transient of GOTHIC analysis was similar with FSAR. The gradient of the temperature response curve of GOTHIC reduced at about 8 s after the event and it was caused by the termination of the reverse blowdown with MSIV closure shown in Fig. 4. The gradient change by MSIV closure of FSAR analysis occurred at about 10 s which was 2 s later than GOTHIC. Before the initiation of the fan cooler, both curves of FSAR and GOTHIC almost overlapped with each other. But there was still a small discrepancy between two response curves. The containment temperature of GOTHIC shown in Fig. 9 lied above the FSAR ones before the fan cooler initiation. However, the temperature gradient of GOTHIC changed by the reduction of the temperature increment when the fan cooler initiated. Therefore, the temperature response curve of GOTHIC intersected with FSAR ones at about 50 s and the containment temperature of GOTHIC fell below FSAR one after that time. The initiation of fan cooler did reduce the peak value of GOTHIC with the reduction of temperature gradient. Although the gradient change of FSAR was not as significant as GOTHIC analysis due to the logarithmic time scale. But after a carefully examination, the temperature gradient of FSAR actually reduced after the initiation of fan cooler. The peak containment temperature occurred at the spray initiation. The peak value of GOTHIC was 186.99 ◦ C (368.59 F) and it is less than 197.56 ◦ C (387.6 F) of FSAR. After the spray initiation, the containment temperature suddenly dropped. The containment temperature of FSAR dropped to 121.67 ◦ C (251 F) in 40 s from the initiation of spray, while GOTHIC analysis drops to 123.89 ◦ C (255 F) in 100 s from the initiation of spray. The condensation cooling effect of GOTHIC analysis was clearly less than FSAR because it required longer time for GOTHIC to cool the containment atmosphere as shown in Fig. 9. Fig. 10 depicted the containment temperature and saturation temperature of GOTHIC. The containment temperature was superheated before the spray initiation. The containment vapor temperature and saturation temperature of GOTHIC analysis became overlapped after 220 s as shown in Fig. 10. It indicated that the spray droplets cooled down the superheated containment
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atmosphere to the saturation state after that time. Once the containment atmosphere reached saturated, the gradient of containment temperature sharply changed. It was a nearly rectangular turn in containment temperature curve. The containment temperature descended slightly first and then ascended due to the continuation of blowdown till 1800 s. The isolation of the broken steam generator at 1800 s terminated the steam blowdown flow. The slowly ascending tendency of temperature also stopped by the termination and began to drop till the end of analysis. In conclusion, after the spray initiation, the containment temperature of both GOTHIC and FSAR fell below 148.89 ◦ C (300 F) and never exceeded 148.89 ◦ C (300 F) till the end of analysis. 5. Conclusions The Maanshan dry containment model in this study was built by the advanced generic containment thermal hydraulic code GOTHIC. Two thermal hydraulic nodes were used to model the internal air space and bottom sumps for the short term MSLB analysis which was similar to the containment model of COPATTA program described in FSAR. The initial conditions, material properties, thickness, heat transfer area, heat transfer mode and heat transfer coefficients for heat conductors were all taken directly from FSAR. The most severe containment pressure transient was specified as hot standby MSLB by the Maanshan FSAR section 6.2. To verify it in the analyses of GOTHIC, 102% OLTP MSLB was chosen in contrast to the hot standby MSLB. The input data bases of both GOTHIC and FSAR models were almost identical because the same data listed in FSAR was adopted by both models. The results of pressure and temperature responses of both GOTHIC and FSAR were summarized in Table 2. In Table 2, it indicated that the containment peak pressure during hot standby MSLB of both FSAR and GOTHIC analyses were all greater than 102% OLTP MSLB cases. It assessed that the most severe containment pressure transient of GOTHIC analysis was also hot standby MSLB. The result of hot standby MSLB showed that the pressure response of both GOTHIC and FSAR were quite consistent. There was little discrepancy of the temperature response of both GOTHIC and FSAR but the overall tendency remained quite reasonable. Furthermore, as shown in Table 2, it could also be identified that the most severe containment temperature transient was 102% OLTP MSLB for both FSAR and GOTHIC analyses. Although the pressure and temperature responses were not addressed in detail in Maanshan FSAR, the post-accident responses inside the containment could be explored with the GOTHIC model of large dry containment that was already assessed with FSAR in this study. For example, in Figs. 7 and 10, the containment atmosphere temperatures were above the saturation temperature from the beginning of the MSLB event to the initiation of containment spray which meant that the containment atmosphere was heated to superheat state by the main steam from both ends of breaks. The gradient of the containment atmosphere temperature curves remained nearly constant for about 8 s but the rate of the containment atmosphere temperature rise actually decreased by the logarithmic scale of time. The containment atmosphere temperature rise was governed by the main steam blowdown rates and the rate of temperature rise was reduced by decreased break flow.
The discontinuous gradient change of the containment temperature at about 8 s corresponded to the stop of the header side break flow. The second discontinuous gradient change of the containment temperature occurred at the point that the fan cooler unit was just started. It indicated that part of the energy of the containment atmosphere was removed by the fan cooler unit but it did not capable enough to fully cope with the energy addition by the broken steam generator side break flow. Although the rate of the containment atmosphere temperature rise was reduced by the fan cooler unit, the containment atmosphere maintained superheat state because the containment atmosphere temperature was still above the saturation temperature. In Figs. 7 and 10, the containment atmosphere reached saturated state about 100 s after the spray initiation but the atmosphere temperature dropped immediately after the spray initiation. The superheat vapor in the containment atmosphere was condensed by tiny droplets produced by the spray nozzles. The fan cooler unit only removed vapor phase energy from the containment atmosphere and the containment pressure increase could not be mitigated by fan cooler units. The containment spray nozzles provided thousands of tiny droplets which were 1000 m in diameter. Therefore these droplets had a very large heat transfer area to interact with the superheat vapor and then the superheat vapor was condensed. In Figs. 5 and 8, the containment pressure increase was terminated because the containment atmosphere containing the superheat vapor was condensed and changed to saturated liquid. The superheat vapor content of the containment atmosphere was removed by the condensation process and thus reduced the vapor partial pressure. With the vapor partial pressure decrease, the total pressure also decreased. Consequently, the containment atmosphere pressure increase was suppressed by the condensation interfacial mass transfer. In this study, the dry containment GOTHIC model was built and compared with FSAR. The assessed GOTHIC model could be used in many containment related issues with reasonable physical responses. Acknowledgement Supports and advices from my colleagues of the Institute of Nuclear Energy Research are acknowledged. References Final Safety Report, 2005. Maanshan Nuclear Power Station Units 1 and 2, Revision 39. Taiwan Power Company. 2006. GOTHIC Containment Analysis Package Qualification Report, Version 7.2a(QA), Rev. 9, EPRI, NAI 8907-09. 2006. GOTHIC Containment Analysis Package Technical Manual, Version 7.2a(QA), Rev. 16, EPRI, NAI 8907-06. 2006. GOTHIC Containment Analysis Package User Manual, Version 7.2a(QA), Rev. 17, EPRI, NAI 8907-02. 1975. Performance and Sizing of Dry Pressure Containments. BN-TOP-3, Revision 3. Bechtel Power Corporation. Richardson, L.C., et al., 1967. CONTEMPT: A Computer Program for Predicting the Containment Pressure Temperature Response to a Loss-of-Coolant Accident. Phillips Petroleum Company. 2007. Standard Review Plan 6.2.1.1.A, NUREG-0800, Revision 3. NRC, USA.
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Short-term pressure and temperature MSLB response analyses for large dry containment of the Maanshan nuclear power station Liang-Che Dai ∗ , Yen-Shu Chen, Yng-Ruey Yuann Nuclear Engineering Division, Institute of Nuclear Energy Research, No. 1000, Wenhua Road, Jiaan Village, Longtan Township, Taoyuan County 32546, Taiwan, ROC
h i g h l i g h t s • • • •
The GOTHIC code is used for the PWR dry containment pressure and temperature analysis. Boundary conditions are hot standby and 102% power main steam line break accidents. Containment pressure and temperature responses of GOTHIC are similar with FSAR. The capability of the developed model to perform licensing calculation is assessed.
a r t i c l e
i n f o
Article history: Received 29 January 2014 Received in revised form 29 August 2014 Accepted 1 September 2014
a b s t r a c t Units 1 and 2 of the Maanshan nuclear power station are the typical Westinghouse three-loop PWR (pressurized water reactor) with large dry containments. In this study, the containment analysis program GOTHIC is adopted for the dry containment pressure and temperature analysis. Free air space and sump of the PWR dry containment are individually modeled as control volumes. The containment spray system and fan cooler unit are also considered in the GOTHIC model. The blowdown mass and energy data of the main steam line break (hot standby condition and various reactor thermal power levels) are tabulated in the Maanshan Final Safety Analysis Report (FSAR) 6.2 which could be used as the boundary conditions for the containment model. The calculated containment pressure and temperature behaviors of the selected cases are in good agreement with the FSAR results. In this study, hot standby and 102% reactor thermal power main steam line break accidents are selected. The calculated peak containment pressure is 323.50 kPag (46.92 psig) for hot standby MSLB, which is a little higher than the FSAR value of 311.92 kPag (45.24 psig). But it is still below the design value of 413.69 kPag (60 psig). The calculated peak vapor temperature inside the containment is 187.0 ◦ C (368.59 F) for 102% reactor thermal power MSLB, which is lower than the FSAR result of 194.42 ◦ C (381.95 F). The effects of the containment spray system and fan cooler units could be clearly observed in the GOTHIC analysis. The calculated containment pressure and temperature behaviors of the selected cases are in good agreement with the FSAR results. Since the results are similar with the Mananshan FSAR, and the applicability of the GOTHIC dry containment model is assessed. © 2014 Elsevier B.V. All rights reserved.
1. Introduction
Abbreviations: PWR, pressurized water reactor; FSAR, Final Safety Analysis Report; MSLB, main steam line break; NSSS, nuclear steam supply system; LOCA, loss of coolant accident; SRP, Standard Review Plan; LOFT, Loss-of-Fluid Test; RPV, reactor pressure vessel; RCS, reactor coolant system; RCP, reactor coolant pump; DBA, design basis accident; BWR, boiling water reactor; RWST, refueling water storage tank; OLTP, original licensing thermal power. ∗ Corresponding author. Tel.: +886 3 4711400 6088; fax: +886 3 4711404. E-mail address: [email protected] (L.-C. Dai).
The containment structure contains the nuclear steam supply system (NSSS). When the NSSS pipe break or loss of coolant accident (LOCA) occurs, the high energy fluid would directly release to the containment space from the break and instantly pressurize the containment structure. Therefore, the integrity of the containment may be compromised by the pressurization and may cause the radiation release. The acceptance criteria in section 6.2.1.1.A of Standard Review Plan (SRP) (SRP, 2007) require that the peak calculated containment pressure following a postulated loss-of-coolant
http://dx.doi.org/10.1016/j.nucengdes.2014.09.007 0029-5493/© 2014 Elsevier B.V. All rights reserved.
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accident, or a steam or feedwater line break, should be less than the containment design pressure. Section 6.2 of the Final Safety Analysis Report (FSAR) (FSAR, 2005) of the Maanshan nuclear power station indicates that the containment pressure analyses are performed by the Bechtel COPATTA (Performance and Sizing of Dry Pressure Containments, 1975) computer program that was derived from the CONTEMPT (Richardson et al., 1967) program written for the NRC Loss-ofFluid Test (LOFT) program. Since the COPATTA computer program has never been used in Taiwan, the general thermal-hydraulic code GOTHIC (GOTHIC Containment Analysis Package User Manual, 2006; GOTHIC Containment Analysis Package Qualification Report, 2006; GOTHIC Containment Analysis Package Technical Manual, 2006) is adopted for developing the dry containment thermalhydraulic analysis model and methodology. 2. Brief PWR system descriptions Maanshan nuclear power station has two identical Westinghouse three-loop PWR units. The original rated thermal core power is 2775 MWt. The total rated thermal power is 2785 MWt with 10 MW reactor coolant pump heat power included. The design and build of the containment should fully withstand the liquid blowdown during the loss of coolant accident (LOCA) and steam line break accident without jeopardizing the safety of the public. The design pressure of the dry containment of Maanshan plant is 413.69 kPag (60 psig). When design basis accident (DBA) occurs, the maximum pressure should not exceed the design pressure. The dry containment of Maanshan plant is a cylindrical concrete building with a hemisphere dome on top. The cutaway diagram of the dry containment of Maanshan plant is shown in Fig. 1. Two recirculation sumps are on the bottom floor of the containment. Suction inlets of residual heat removal system (RHR) and containment spray system are in the bottom of the sumps. The containment heat removal system includes containment spray system and fan cooler units. They can remove the energy of high energy fluid that blowdown into the containment in order to reduce the containment air space pressure. Fan cooler units also have the function maintaining the ambient temperature and relative humidity inside the containment in the normal operation. There is no passive pressure suppression system inside the dry containment of PWR that can cool or condense the blowdown fluid by directing it into a pool of water via the vent pipe such as the suppression pool of the boiling water reactor (BWR). For dry containment, the containment spray system reduces the containment pressure by condensing the vapor in the containment air space. There are two independent containment spray trains in Maanshan plant. Each train has its own spray pump and spray header. The water supply of the containment spray system is initially from the refueling water storage tank (RWST) when the pipe break accident occurs in the containment and the water is transferred to the spray nozzles on the upper part of the containment. These nozzles spray droplets throughout the containment air space in order to cool down the vapor and reduce the containment pressure. 3. Gothic containment model descriptions 3.1. GOTHIC code introduction The name “GOTHIC” is abbreviated from Generation of Thermal Hydraulic Information for Containment. GOTHIC is a thermal hydraulic analysis code of nuclear power plant containment (GOTHIC Containment Analysis Package User Manual, 2006; GOTHIC Containment Analysis Package Qualification Report, 2006; GOTHIC Containment Analysis Package Technical Manual, 2006). It
Table 1 Principal containment design parameters. Containment internal design pressure, kPag (psig) Containment peak calculated pressure (DBA), kPag (psig) Containment peak superheat temperature (DBA), ◦ C (F) Containment internal dimensions Cylindrical wall diameter, m (ft) Cylindrical wall height, m (ft) Containment height, m (ft) Net free internal volume, m3 (ft3 )
413.69 (60) 311.92 (45.24) 197.56 (387.6)
39.62 (130) 39.62 (130) 59.28 (194.5) 5.75 × 104 (2.03 × 106 )
is developed by NAI (Numerical Applications, Inc.) with the fund of EPRI (Electric Power Research Institute). GOTHIC is an international recognized code. There are many containment analyses of both BWR and PWR using GOTHIC code. Since GOTHIC code has large flexibility and versatility, it can be applied to various types of reactors or containments. There are many nuclear power plants using GOTHIC to perform the containment safety evaluations, such as the containment pressure and temperature response analyses after LOCA and steam pipe rupture events. Kewaunee, Grand Gulf, Catawba, McGuire, Waterford, Clinton, Brunswick, Calvert Cliffs and Nine Mile Point are the nuclear plants that have conducted the containment safety evaluations with GOTHIC code and submitted their topical reports for NRC licensing approval, FSAR or test specification update. GOTHIC code is very versatile and can be applied to different analyses in power plants. For example, Columbia, Clinton power plants and KEPCO had been conducted the containment analyses of station blackout (SBO) events with GOTHIC code. GOTHIC can also be applied for equipment qualification (EQ) analyses in order to verify critical equipment installed in the reactor building or turbine building that could withstand the pressure and temperature transients during pipe break events. Oconee, Ontario, Shearon Harris, Oyster Creek and Calvert Cliffs are the utilities that had used GOTHIC to perform the EQ analyses for critical components. Besides, GOTHIC is also commonly used in thermal hydraulic analyses of high energy line break (HELB) in turbine or auxiliary building. These examples address the versatility of GOTHIC code. 3.2. Maanshan containment short term model There were only two thermal hydraulic nodes in the Maanshan containment short term model of FSAR (FSAR, 2005). They represented the internal air space of the dry containment and sump volume on the bottom of the containment respectively. For the analysis of the main steam line break (MSLB) inside the containment, blowdown mass and energy data tables were treated as the boundary conditions of the model. In order to make the comparison with the Maanshan FSAR, the containment air space and the sump volume were also modeled by two control volumes respectively. The important design parameters of the containment were listed in Table 1. The net free volume of the containment was 5.75 × 104 m3 . Two containment recirculation sumps were on the bottom of the containment floor. Total volume of two sumps was 26.5 m3 (7000 gal). The GOTHIC nodal diagram was shown in Fig. 2. The containment air space was modeled as control volume 1. Two sumps were modeled as a single volume and it was control volume 2. The net volume of control volume 1 was the net free volume minus the sump volume which was 5.747 × 104 m3 . On the other hand, the net volume of control volume 2 was 26.5 m3 which equaled 7000 gal as described previously. The containment constituting structures and system components inside the containment building were modeled as eighteen
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Fig. 1. Cutaway diagram of Maanshan containment.
individual heat structures. The material properties, thickness and heat transfer area of these heat structures were already given in FSAR data and these data were directly applied for the heat conductor model in the GOTHIC Maanshan containment model. In MSLB analysis, the condensation heat transfer between the vapor and wall surface was calculated by Uchida experimental equation that was specified in section 6.2.1.1.3.2.7 of FSAR. The convection heat transfer coefficient of the heat conductor surrounding the sumps was specified as 2.27 W/m2 K (0.4 Btu/hr ft2 F) in section 6.2.1.1.3.5 of FSAR. The event time span of a short term pipe break analysis was 2400 s. The containment spray system still operated in injection phase and it did not switch to recirculation phase during the short term analysis. The water source of spray pumps in injection phase was refueling water storage tank (RWST). In Table 6.2-27 of FSAR, the water temperature of RWST was specified as 36.7 ◦ C (98 F). The initial conditions of containment analyses were summarized in Table 2. There was only one train of containment sprays available based on the conservative single failure assumption. The flow rate of the spray pump was specified in Table 6.2-28 of FSAR, and its value was 170.3 l/s (2700 gpm). The droplet size from the spray
Table 2 Summary of the results (FSAR vs. GOTHIC). FSAR 6.2
GOTHIC
Hot standby MSLB Peak containment pressure, kPag (psig) Peak containment temperature, ◦ C (F)
311.92 (45.24) 194.42 (381.95)
323.50 (46.92) 180.59 (357.07)
102% OLTP MSLB Peak containment pressure, kPag (psig) Peak containment temperature, ◦ C (F)
270.27 (39.20) 197.56 (387.6)
277.86 (40.30) 186.99 (368.59)
nozzle was conservatively assumed to be 1000 m to reduce the total heat transfer area of the sprayed droplets. The initiation signal of the containment spray system was containment high pressure (Hi-3). Technical specification requires that Hi-3 signal initiated when the containment pressure had reached 124.8 kPag (18.1 psig), but a higher set point 155.1 kPag (22.5 psig) was specified in section 6.2.1.4.8 of FSAR for further conservative consideration. One of the critical functions of fan cooler units was to remove the energy of the containment internal air space after the pipe
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B.C 1F Broken S/G Blowdown
1 4 2
B.C 2F MS Header Blowdown
CS Spray Nozzle 1N
Fan Cooler Cooler 1C
C.V 1 CTMT
B.C 3F RWST
3
C.V 2 Sump
Fig. 2. Nodal diagram for Maanshan dry containment GOTHIC model.
5000
MSLB Break flow rate (kg/sec) (Broken S/G side & MS Header side)
break event in order to reduce the containment temperature and pressure. There was only one fan cooler unit available in containment response analyses for single failure conservatism. The air volumetric flow rate of the fan cooler unit was 934.5 m3 /min (33,000 ft3 /min). Technical specification states that the initiation set point of fan cooler units was 21.4 kPag (3.1 psig), but a more conservative value 31.0 kPag (4.5 psig) was applied in the analysis. The heat removal capacity per fan cooler unit was specified as a function of containment vapor temperature and component cooling water (CCW) temperature. The component cooling water supply to the fan cooler was conservatively assumed to be at its maximum supply temperature of 48.9 ◦ C (120 F) for less heat removal.
Hot Standby MSLB
4500
Broken S/G side MS Header side
4000 3500 3000 2500 2000 1500 1000 500 0 1
3.3. Maanshan short term MSLB analysis
100
1000
10000
Time (sec)
Fig. 3. Blowdown mass flow rates for hot standby MSLB. 4500
MSLB Break flow rate (kg/sec) (Broken S/G side & MS Header side)
The analyses in section 6.2 of FSAR showed that the most severe transient of the containment pressure was main steam line break (MSLB) under hot standby condition. The containment peak pressure reached 311.92 kPag (45.24 psig). The MSLB blowdown mass and energy data under hot standby condition were listed in FSAR and they could be directly applied as the boundary condition in Maanshan GOTHIC model. Another selected case was MSLB under 102% reactor thermal power and the blowdown mass and energy data were also listed in FSAR. The results of these two cases showed that the most severe pressure transient of the containment for GOTHIC code remained the same as FSAR. Thus, the consistency of both GOTHIC analyses and FSAR were verified. The MSLB blowdown mass flow rates of selected cases from both broken steam generator side and main steam header side were plotted in Figs. 3 and 4. In Figs. 3 and 4, the independent variable was the elapsed time after the event in logarithm scale. When MSLB event occurred, the inventory inside broken loop steam generator would directly blowdown into the containment
10
102% MSLB Broken S/G side
4000
MS Header side
3500 3000 2500 2000 1500 1000 500 0 1
10
100
1000
10000
Time (sec)
Fig. 4. Blowdown mass flow rates for 102% power MSLB.
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3.3.1. Assumptions for short term MSLB analysis Assumptions for Maanshan short term MSLB analysis were listed below: • Main steam line guillotine break occurred at 0 s under hot standby condition or 102% original licensed thermal power (OLTP). • Blowdown mass and energy data from FSAR for hot standby condition MSLB were adopted as the boundary conditions in GOTHIC model. • Blowdown mass and energy data from FSAR for 102% OLTP MSLB were adopted as the boundary conditions in GOTHIC model. • Only one train of containment sprays available based on the conservative single failure assumption. The flow rate of one train spray pump was 170.3 l/s (2700 gpm). • The Hi-3 set point of the containment spray initiation was 155.1 kPag (22.5 psig). The delay time was 134 s for hot standby MSLB and it was 94 s for 102% OLTP MSLB. • The volumetric air flow rate of the fan cooler unit was 934.5 m3 /min (33,000 ft3 /min). • The heat removal capacity per fan cooler unit was specified as a function of containment vapor temperature and component cooling water (CCW) temperature. The component cooling water supply to the fan cooler was conservatively assumed to be at its maximum supply temperature of 48.9 ◦ C (120 F). • The Hi-1 set point of the fan cooler unit initiation was 31.0 kPag (4.5 psig) and the delay time was 33 s considering the restoration time of CCW after the event. • The event time span of a short term MSLB analysis was 2400 s. 3.3.2. Initial conditions for Maanshan GOTHIC model Initial conditions of Maanshan GOTHIC model were summarized below: • • • • • •
Containment initial pressure was 101.35 kPa (14.7 psia). Containment initial temperature was 48.9 ◦ C (120 F). Containment initial relative humidity was 100%. Containment sump volume was initially dry without any water. The fan cooler was not in operation initially. The atmosphere temperature outside the containment structure was 36.7 ◦ C (98 F).
350 300 Containment Pressure (kPag)
through the nearest break. On the opposite side of the steam line break, blowdown flow came out by the reverse flow from intact loop steam generators through the main steam header and it would then be terminated by the closure of main steam isolation valves. These phenomena could be easily observed in both Figs. 3 and 4.
250 200 150 100 Hot Standby MSLB FSAR GOTHIC
50 0 1
10
100
1000
10000
Time (sec) Fig. 5. Containment pressure responses for the hot standby MSLB.
larger than in GOTHIC. It indicated that the condensation effect of FSAR was stronger than GOTHIC. The magnitude of decrease of the containment pressure and temperature of FSAR were both larger than GOTHIC model. Figs. 5 and 6 showed that the pressure and temperature responses were consistent between FSAR and GOTHIC. The rate of change of the pressure and temperature profiles decreased at 10 s after MSLB, and it was caused by the decrease of the blowdown flow from the broken steam generator side after 10 s and the termination of the blowdown flow from the steam header side with the closure of main steam isolation valves (MSIV) at 7.58 s. The combining effect reduced the gradient of the pressure and temperature curves. The temperature and pressure curves of FSAR and GOTHIC had a similar appearance before the initiation of the spray. The initiation time of the containment spray system had a 134 s delay after the containment pressure rising over the set point Hi-3. The initiation time of FSAR and GOTHIC were at 167 s and 163 s respectively. The difference of the spray initiation sequence between both of them was little. The pressure response in Fig. 5 also indicated that the pressure response in FSAR analysis dropped immediately after the spray initiation and GOTHIC model had a slower pressure response corresponding to the spray initiation. The pressure response of GOTHIC continued to rise about 23 s after the spray initiation and then dropped. The pressure drop of GOTHIC due to the droplet condensation was also less than FSAR. This indicated that the spray condensation effect in GOTHIC analysis was
4. MSLB analysis results and discussion
200
4.1. Hot standby MSLB short term analysis
180 Containment Temperature ( C)
Fig. 5 showed the containment pressure transient during the hot standby MSLB of Maanshan GOTHIC model analysis. In FSAR, the fan cooler and the containment spray started at time 34 s and 167 s respectively. For GOTHIC analysis, the fan cooler and the containment spray started at time 34.1 s and 163 s respectively. The initiation sequence of the containment heat removal systems was nearly consistent between GOTHIC and FSAR. When the containment spray system initiated, containment pressure immediately dropped because of the sprayed droplets condensed the blowdown vapor in the containment. Fig. 6 was the containment temperature transient result for the same event. It also indicated that the vapor temperature immediately dropped by vapor condensation when the containment spray system initiated. However, it could be observed in Fig. 6, temperature drop of the FSAR profile was
5
160 140 120 100 Hot Standby MSLB FSAR GOTHIC
80 60 1
10
100
1000
10000
Time (sec) Fig. 6. Containment temperature responses for the hot standby MSLB.
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less than FSAR. However, because the blowdown continued, the pressure rose again and a concave shaped pressure profile was then formed. The pressure profiles of FSAR and GOTHIC were almost overlapped after 250 s and the pressurization continued until 334 s. After that time, due to the sudden decrease of the blowdown flow (Fig. 3), the containment pressure began to drop. With the steam still flowed out to the containment air space, the containment pressure increased again about 300 s after the drop. Because the blowdown mass flow rate reduced to 25% of the original value, the pressurization rate was much slower than the pressure response curve before 334 s. The containment pressure reached its maximum at 1800 s and the increasing tendency was then reversed due to the stop of blowdown. The maximum containment pressure of FSAR was 311.92 kPag (45.24 psig) and GOTHIC was 323.50 kPag (46.92 psig) for hot standby MSLB analysis. The difference between them was only 11.58 kPa (1.68 psi). The containment temperature response was shown in Fig. 6. The response of the temperature transient of GOTHIC analysis was quite similar with FSAR. The slope of the temperature response curve of GOTHIC reduced at 8 s after the event. It indicated that the temperature increment after that time was smaller than previous. The reason was the MSIV closure stopping the steam header side blowdown. However, the temperature response of FSAR did not have such apparent gradient change like GOTHIC but it still had a reduced temperature increment caused by the MSIV closure. The containment temperature response had an apparent discrepancy between GOTHIC and FSAR at 20 s and it was caused by the previously described temperature increment difference. It resulted that FSAR had a higher containment temperature than GOTHIC. This discrepancy was due to the theoretical assumption of the COPATTA code used by FSAR. The COPATTA code assumed that both vapor and liquid phases were separable. The liquid blowdown to the containment would directly go to the recirculation sumps without energy exchange between the air and vapor in order to gain a higher vapor temperature. On the other hand, GOTHIC code had a complete two phase mass and energy exchange model to adequately simulate the mass and energy transfer between liquid and vapor phases. Therefore, GOTHIC had a lower temperature profile than FSAR during the time interval spanning from 20 s after the start of the event to the spray initiation. The available fan cooler unit initiated at 34.1 s after MSLB and did affect the containment temperature response. The containment temperature rise of GOTHIC decreased after the fan cooler unit initiation and produced a temperature profile with a milder gradient change before the spray initiation. The fan cooler unit had less effect on the FSAR temperature profile and it caused the FSAR temperature profile before the spray initiation diverging from GOTHIC ones. After the spray initiation, the droplet condensation effect caused the containment air temperature of both FSAR and GOTHIC to drop steeply and it could be observed clearly that FSAR had a larger temperature drop. Then the containment air temperature rose again owing to the persistence of the outward flow till time 334 s. After time 334 s, the containment air temperature dropped again because the blowdown flow rate decrease but the descending tendency would not last long. The pressure and temperature of both GOTHIC and FSAR decreased when the break flow was terminated at 1800 s. Fig. 7 compared the containment temperature and saturation temperature calculated with GOTHIC. It was found that before the spray initiation, there was superheat vapor in the containment because the vapor temperature exceeded the saturation temperature. The pressure and temperature continued to increase till the spray initiation. As mentioned in previous section, pressure and temperature dropped almost immediately when the spray was activating. The vapor temperature and the saturation temperature were overlapped at 200 s which was about 33 s after the spray activation. It indicated that the state of the containment
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atmosphere had transited from superheat vapor to two phase saturation by condensation. After that time, the containment temperature equaled the saturation temperature till the end of the analysis. The containment temperature of FSAR reached its maximum value 194.42 ◦ C (381.95 F) at the moment of the spray initiation and the containment temperature of GOTHIC reached its maximum value 180.59 ◦ C (357.07 F) 6 s after the spray initiation. 4.2. 102% OLTP MSLB short term analysis Fig. 8 was the containment pressure transient during the 102% OLTP MSLB of Maanshan GOTHIC model analysis. FSAR Fig. 6.2-1 sheet 7 indicated that the fan cooler and the containment spray started at time 34.2 s and 125.2 s. In GOTHIC analysis, t the fan cooler and the containment spray started at time 34.2 s and 114 s respectively. The initiation sequences of the fan cooler of GOTHIC and FSAR were consistent but the initiation time of the containment spray system of GOTHIC was 11 s ahead of FSAR. The reason was shown clearly in Fig. 8, time required for the pressure response curve exceeding the Hi-3 set point of GOTHIC analysis was less than FSAR. The comparison of the containment temperature transient during the 102% OLTP MSLB of GOTHIC analysis and FSAR Fig. 6.2-1 sheet 7 was shown in Fig. 9. The containment atmosphere temperature reduces after the spray initiation due to the vapor condensation effect. The temperature drop of GOTHIC
Please cite this article in press as: Dai, L.-C., et al., Short-term pressure and temperature MSLB response analyses for large dry containment of the Maanshan nuclear power station. Nucl. Eng. Des. (2014), http://dx.doi.org/10.1016/j.nucengdes.2014.09.007
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occurred at 120 s which was 6 s after the spray initiation and the containment atmosphere temperature also reached its maximum value 186.99 ◦ C (368.59 F). However, the maximum containment temperature of FSAR analysis was 197.56 ◦ C (387.6 F) at 125.2 s and it was greater than in GOTHIC analysis. The comparison of the containment temperature transient showed that the spray condensation effect in FSAR analysis was more significant than in GOTHIC analysis. Since the containment pressure and temperature drops of FSAR caused by the spray were both larger than GOTHIC. The overall tendencies of the pressure and temperature response curves of GOTHIC in Figs. 8 and 9 were roughly consistent with FSAR curves. In Figs. 8 and 9, a clearly gradient change of the pressure and temperature response of GOTHIC could be observed at 8 s after the MSLB event. The turbine side blowdown flow in Fig. 4 was terminated at 7.55 s due to the closure of MSIVs and then the pressurization and heat up rate of the containment were reduced. However, the gradient change of FSAR temperature response occurs at about 10 s, which lagged behind GOTHIC. As previously mentioned, the initiation time of the containment spray of GOTHIC was 11 s ahead FSAR. Comparing with FSAR, the GOTHIC analysis of containment pressure and temperature response after the spray initiation was different. The containment pressure of FSAR dropped immediately after the spray initiation. In contrast, the containment pressure of GOTHIC still mildly rose and last about 11 s even the spray had already initiated. The containment pressure of FSAR dropped 20.68 kPa (3 psi) immediately after the spray initiation. However, the containment pressure of GOTHIC analysis remained a nearly flat gradient in the duration ranging from 125 to 158 s. Then, the pressure responses of both FSAR and GOTHIC rose again with the continuation of blowdown flow. But the increasing tendency last shortly, both the pressure responses dropped at 175 s in correspond with the step decrease of forward blowdown flow shown in Fig. 4. After 200 s, both the pressure response curves of FSAR and GOTHIC fitted well. The cause of the 102% MSLB forward blowdown flow step decrease at 175 s shown in Fig. 4 was the inventory dryout in the broken loop steam generator that was described in FSAR 6.2.1.4. In contrast, the inventory dryout time of hot standby MSLB extended to 333.7 s shown in Fig. 3 that was caused by the power level difference. After the steam generator inventory dryout, the forward blowdown was only supplied by the auxiliary feedwater to the broken loop steam generator. The auxiliary feedwater continuously operated till 30 min or 1800 s after the MSLB. It was manually isolated at that time and then terminated the forward blowdown flow. The concave pressure response curves during this duration shown in Fig. 8 illustrated that the containment pressure descended
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first by the inventory dryout and gradually rose again by the continuation of the blowdown flow. The containment pressure reached its maximum value just at 1800 s and then the ascending tendency reversed by the termination of the steam blowdown. The peak pressure of FSAR was 270.27 kPag (39.20 psig) and the peak value of GOTHIC analysis was 277.86 kPag (40.30 psig). The difference of peak pressure of these two analyses was only 7.59 kPa (1.10 psi). Fig. 9 was the containment temperature transient during the 102% OLTP MSLB of Maanshan GOTHIC model analysis. The overall response of the temperature transient of GOTHIC analysis was similar with FSAR. The gradient of the temperature response curve of GOTHIC reduced at about 8 s after the event and it was caused by the termination of the reverse blowdown with MSIV closure shown in Fig. 4. The gradient change by MSIV closure of FSAR analysis occurred at about 10 s which was 2 s later than GOTHIC. Before the initiation of the fan cooler, both curves of FSAR and GOTHIC almost overlapped with each other. But there was still a small discrepancy between two response curves. The containment temperature of GOTHIC shown in Fig. 9 lied above the FSAR ones before the fan cooler initiation. However, the temperature gradient of GOTHIC changed by the reduction of the temperature increment when the fan cooler initiated. Therefore, the temperature response curve of GOTHIC intersected with FSAR ones at about 50 s and the containment temperature of GOTHIC fell below FSAR one after that time. The initiation of fan cooler did reduce the peak value of GOTHIC with the reduction of temperature gradient. Although the gradient change of FSAR was not as significant as GOTHIC analysis due to the logarithmic time scale. But after a carefully examination, the temperature gradient of FSAR actually reduced after the initiation of fan cooler. The peak containment temperature occurred at the spray initiation. The peak value of GOTHIC was 186.99 ◦ C (368.59 F) and it is less than 197.56 ◦ C (387.6 F) of FSAR. After the spray initiation, the containment temperature suddenly dropped. The containment temperature of FSAR dropped to 121.67 ◦ C (251 F) in 40 s from the initiation of spray, while GOTHIC analysis drops to 123.89 ◦ C (255 F) in 100 s from the initiation of spray. The condensation cooling effect of GOTHIC analysis was clearly less than FSAR because it required longer time for GOTHIC to cool the containment atmosphere as shown in Fig. 9. Fig. 10 depicted the containment temperature and saturation temperature of GOTHIC. The containment temperature was superheated before the spray initiation. The containment vapor temperature and saturation temperature of GOTHIC analysis became overlapped after 220 s as shown in Fig. 10. It indicated that the spray droplets cooled down the superheated containment
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atmosphere to the saturation state after that time. Once the containment atmosphere reached saturated, the gradient of containment temperature sharply changed. It was a nearly rectangular turn in containment temperature curve. The containment temperature descended slightly first and then ascended due to the continuation of blowdown till 1800 s. The isolation of the broken steam generator at 1800 s terminated the steam blowdown flow. The slowly ascending tendency of temperature also stopped by the termination and began to drop till the end of analysis. In conclusion, after the spray initiation, the containment temperature of both GOTHIC and FSAR fell below 148.89 ◦ C (300 F) and never exceeded 148.89 ◦ C (300 F) till the end of analysis. 5. Conclusions The Maanshan dry containment model in this study was built by the advanced generic containment thermal hydraulic code GOTHIC. Two thermal hydraulic nodes were used to model the internal air space and bottom sumps for the short term MSLB analysis which was similar to the containment model of COPATTA program described in FSAR. The initial conditions, material properties, thickness, heat transfer area, heat transfer mode and heat transfer coefficients for heat conductors were all taken directly from FSAR. The most severe containment pressure transient was specified as hot standby MSLB by the Maanshan FSAR section 6.2. To verify it in the analyses of GOTHIC, 102% OLTP MSLB was chosen in contrast to the hot standby MSLB. The input data bases of both GOTHIC and FSAR models were almost identical because the same data listed in FSAR was adopted by both models. The results of pressure and temperature responses of both GOTHIC and FSAR were summarized in Table 2. In Table 2, it indicated that the containment peak pressure during hot standby MSLB of both FSAR and GOTHIC analyses were all greater than 102% OLTP MSLB cases. It assessed that the most severe containment pressure transient of GOTHIC analysis was also hot standby MSLB. The result of hot standby MSLB showed that the pressure response of both GOTHIC and FSAR were quite consistent. There was little discrepancy of the temperature response of both GOTHIC and FSAR but the overall tendency remained quite reasonable. Furthermore, as shown in Table 2, it could also be identified that the most severe containment temperature transient was 102% OLTP MSLB for both FSAR and GOTHIC analyses. Although the pressure and temperature responses were not addressed in detail in Maanshan FSAR, the post-accident responses inside the containment could be explored with the GOTHIC model of large dry containment that was already assessed with FSAR in this study. For example, in Figs. 7 and 10, the containment atmosphere temperatures were above the saturation temperature from the beginning of the MSLB event to the initiation of containment spray which meant that the containment atmosphere was heated to superheat state by the main steam from both ends of breaks. The gradient of the containment atmosphere temperature curves remained nearly constant for about 8 s but the rate of the containment atmosphere temperature rise actually decreased by the logarithmic scale of time. The containment atmosphere temperature rise was governed by the main steam blowdown rates and the rate of temperature rise was reduced by decreased break flow.
The discontinuous gradient change of the containment temperature at about 8 s corresponded to the stop of the header side break flow. The second discontinuous gradient change of the containment temperature occurred at the point that the fan cooler unit was just started. It indicated that part of the energy of the containment atmosphere was removed by the fan cooler unit but it did not capable enough to fully cope with the energy addition by the broken steam generator side break flow. Although the rate of the containment atmosphere temperature rise was reduced by the fan cooler unit, the containment atmosphere maintained superheat state because the containment atmosphere temperature was still above the saturation temperature. In Figs. 7 and 10, the containment atmosphere reached saturated state about 100 s after the spray initiation but the atmosphere temperature dropped immediately after the spray initiation. The superheat vapor in the containment atmosphere was condensed by tiny droplets produced by the spray nozzles. The fan cooler unit only removed vapor phase energy from the containment atmosphere and the containment pressure increase could not be mitigated by fan cooler units. The containment spray nozzles provided thousands of tiny droplets which were 1000 m in diameter. Therefore these droplets had a very large heat transfer area to interact with the superheat vapor and then the superheat vapor was condensed. In Figs. 5 and 8, the containment pressure increase was terminated because the containment atmosphere containing the superheat vapor was condensed and changed to saturated liquid. The superheat vapor content of the containment atmosphere was removed by the condensation process and thus reduced the vapor partial pressure. With the vapor partial pressure decrease, the total pressure also decreased. Consequently, the containment atmosphere pressure increase was suppressed by the condensation interfacial mass transfer. In this study, the dry containment GOTHIC model was built and compared with FSAR. The assessed GOTHIC model could be used in many containment related issues with reasonable physical responses. Acknowledgement Supports and advices from my colleagues of the Institute of Nuclear Energy Research are acknowledged. References Final Safety Report, 2005. Maanshan Nuclear Power Station Units 1 and 2, Revision 39. Taiwan Power Company. 2006. GOTHIC Containment Analysis Package Qualification Report, Version 7.2a(QA), Rev. 9, EPRI, NAI 8907-09. 2006. GOTHIC Containment Analysis Package Technical Manual, Version 7.2a(QA), Rev. 16, EPRI, NAI 8907-06. 2006. GOTHIC Containment Analysis Package User Manual, Version 7.2a(QA), Rev. 17, EPRI, NAI 8907-02. 1975. Performance and Sizing of Dry Pressure Containments. BN-TOP-3, Revision 3. Bechtel Power Corporation. Richardson, L.C., et al., 1967. CONTEMPT: A Computer Program for Predicting the Containment Pressure Temperature Response to a Loss-of-Coolant Accident. Phillips Petroleum Company. 2007. Standard Review Plan 6.2.1.1.A, NUREG-0800, Revision 3. NRC, USA.
Please cite this article in press as: Dai, L.-C., et al., Short-term pressure and temperature MSLB response analyses for large dry containment of the Maanshan nuclear power station. Nucl. Eng. Des. (2014), http://dx.doi.org/10.1016/j.nucengdes.2014.09.007