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Accident analysis on LOCA in HCCR-TBS towards CCWS-1
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Accident analysis on LOCA in HCCR-TBS towards CCWS-1 Mu-Young Ahn a,∗ , Hyung Gon Jin b , Youngmin Lee a , Seungyon Cho a , Yi-Hyun Park a , Dong Won Lee b a b
National Fusion Research Institute, Daejeon, Republic of Korea Korea Atomic Energy Research Institute, Daejeon, Republic of Korea
h i g h l i g h t s • • • •
Accident analysis on LOCA in the HCCR-TBS HCS to CCWS-1was performed to assess impact on Component Cooling Water System-1. The analysis reveals that impact due to the accident is limited in terms of pressure increase in CCWS-1 and tritium ingress to CCWS-1. However, chronical release of tritium from the HCS to CCWS-1 through the heat exchanger should be further clarified. This aspect should be reflected in the design of the heat exchanger in future.
a r t i c l e
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Article history: Received 30 September 2016 Received in revised form 6 April 2017 Accepted 15 May 2017 Available online xxx Keywords: Accident analysis Helium Cooled Ceramic Reflector (HCCR) Loss of Coolant Accident (LOCA) Printed Circuit Heat Exchanger (PCHE) GAMMA-FR
a b s t r a c t Korean Helium Cooled Ceramic Reflector (HCCR) Test Blanket System (TBS) will be operated at elevated temperature with high pressure helium coolant during normal operation in ITER. One of the main ancillary systems of HCCR-TBS is Helium Cooling System (HCS) which play an important role to extract heat from HCCR Test Blanket Module (TBM) by the helium coolant to keep the operational temperature and the extracted heat is finally transferred to ITER CCWS-1 (Component Cooling Water System) by a Printed Circuit Heat Exchanger (PCHE) in the HCS. In such circumstances if Loss Of Coolant Accident (LOCA) occurs in the PCHE, the high pressure helium coolant in the primary side goes into the lower pressure water in the secondary side thus pressurizing CCWS-1. In addition, since the helium coolant contains tritium due to permeation from the TBM, tritium migrates into CCWS-1, a non-nuclear system. In this paper, accident analysis for LOCA in the heat exchanger is presented. For the analysis, GAMMA-FR code which has been developed for fusion applications was used. Main components in the HCS and CCWS-1 were modelled as volume and junctions. The accident analysis was performed for the reference case with ten channels rupture and sensitivity study was also performed by changing the crack size. The results show that pressure and tritium requirement of CCWS-1 can be met in spite of LOCA in the heat exchanger of the HCCR-TBS HCS. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Various design activities and supporting R&Ds have been performed for Korean Helium Cooled Ceramic Reflector (HCCR) Test Blanket System (TBS) aiming at demonstration of possibility of tritium self-sufficiency and heat extraction capable of producing electricity in ITER [1–3]. Since ITER is a basic nuclear installation, INB-174, according to French nuclear licensing procedures and the HCCR-TBS is operated at high pressure and/or high temperature with physical and functional interfaces to ITER, safety features of the HCCR-TBS should be demonstrated not only during normal
∗ Corresponding author. E-mail address: [email protected] (M.-Y. Ahn).
operation but also for accidental events to confirm that relevant ITER requirements and guidelines are met [4]. While most of failure modes of the HCCR-TBS and their accidental consequences are addressed in [5,6] respectively, this paper presents accident analysis on Loss Of Coolant Accident (LOCA) in Helium Cooling System (HCS) toward ITER Component Cooling Water System-1 (CCWS-1). The HCS is a main cooling system of the HCCR-TBS in order to transfer extracted heat from the Test Blanket Module (TBM) to CCWS-1 via Printed Circuit Heat Exchanger (PCHE). If LOCA occurs inside the PCHE, 8 MPa helium coolant in the primary side goes into 0.4 MPa water in the secondary side thus pressurizing CCWS-1. Furthermore, tritium permeated from the TBM to the helium coolant can migrate to CCWS-1 in which tritium is strictly restricted as a non-nuclear system. To analyze thermo-hydraulic transient behavior during the accident, GAMMA-
http://dx.doi.org/10.1016/j.fusengdes.2017.05.069 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: M.-Y. Ahn, et al., Accident analysis on LOCA in HCCR-TBS towards CCWS-1, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.069
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Fig. 1. Nodalization for the analysis. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
FR, a domestic system code for fusion applications, was used. GAMMA-FR, as a branch of GAMM+ code which has been adopted for conceptual design of the Next Generation Power Plant by Korea Atomic Energy Research Institute under a subcontract with General Atomics [7], has the capability to handle thermal hydraulic behaviors with chemical reaction in a multi-component mixture system as well as heat transfer within solid components [8], and it inherits general thermal hydraulic validation of the mother code [9]. As a further effort to use GAMMA-FR on HCCR-TBS safety analysis for ITER, the code validation is on-going under Korea-US collaboration program by comparing fusion system related experimental data and by comparing with MELCOR analysis results [10]. Main components of the HCS and CCWS-1 were modelled and the accident analysis was performed for the reference case having ten channels rupture with sensitivity study by changing the crack size. 2. Concerned system description 2.1. HCS of HCCR-TBS The HCS provides the primary helium coolant at the characteristic pressure, temperature and mass flow rate required by the HCCR TBM for testing and extraction of the heat produced. The HCS mainly consists of a circulator, a recuperator, a pre-heater, gas mixers, filters and a PCHE [11]. The hot helium coolant returned from the TBM is cooled down by the recuperator and the PCHE to room temperature before the circulator, and the coolant is re-heated up to the inlet temperature condition after the gas mixer where the helium coolant from the recuperator and the bypass line merges. In normal operation conditions, the pre-heater is turned off and the pre-heater bypass valves are opened. It is installed to heat up the helium coolant and the TBM up to operational temperature during start-up, commissioning and maintenance states. 2.2. CCWS-1 CCWS-1, one of sub-systems of CCWS, consists of horizontal circulation pumps, plate heat exchangers, pressurizer with relief valves and chemical addition system [12]. The pressurizer is connected to the suction side of the pump. It will accommodate the water volume expansion and keep the pressure in the system not only during operation but also during start-up or maintenance. The pressurizer is occupied by water initially at 12% level while rest 88%
is filled with nitrogen in order to avoid the contact of water with air. The water will expand during the operation. The safety relief valves are provided on the pressurizer to avoid any excess pressure increase in the system leading to accident. 3. Modelling Since amount of discharged helium is the key parameter for the analysis, only the circulator and the PCHE are modelled among the HCS components as shown in Fig. 1. And radiation heat transfer and temperature distribution along the pipes are not taken into account. Due to the same reason, flow network inside of TBM is simplified as one fluid volume (FB1300). The PCHE, which is inside of the red color rectangle in Fig. 1, has many narrow channels. FB1500 and 1510 are HCS side, and FB401 and 402 are for CCWS-1 side. FB1510 and 401 represent ruptured channels, which are connected when accident happens. And FB1500 and 402 are unbroken channels. The isolation valves are modeled at CCWS-1 side inlet and outlet, respectively, in order to mitigate the consequence of the accident. Currently, they are assumed to be closed when the HCS pressure decreases to 6 MPa, which should be investigated further to find the optimum detection values. The right hand side of Fig. 1 represents the nodalization of CCWS-1. The pump, pressurizer with relief valve and pipes going to various clients are modelled for this analysis. While FB403 in the blue rectangle is for cooling circuits for the other 5 TBSs, fluid blocks between the header (FB350) and collector (FB355) represents the pipes going to clients other than TBSs. Total mass flow rate of CCWS1 is 6230 kg/s and mass flow rate for 6 TBSs is 96 kg/s, i.e. 16 kg/s per each TBS cooling circuit. The relief valve opens when the pressure (FB600) exceeds 0.41 MPa and it is assumed to take one second to be fully opened. And it is closed when the pressure decreases below 0.39 MPa. As an initial condition, 88% of the pressurizer volume is filled with nitrogen. 4. Failure modes and accident description PCHE is adopted for the HCS heat exchanger considering its advantages such as high heat transfer efficiency, compactness, etc. The name comes from the process for fabrication of the thin plate with narrow channels on the surface, which is done by chemical etching. There are two patterns of printed circuits (cold side plate
Please cite this article in press as: M.-Y. Ahn, et al., Accident analysis on LOCA in HCCR-TBS towards CCWS-1, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.069
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Fig. 2. Fabrication procedure of PCHE.
with RV 1510 without RV 1510
8
Pressure (MPa)
7
6
5
4
3
2 0
100
200
300
400
500
600
time (second)
with RV 401 without RV 401
8
Pressure (MPa)
7
6
5
4
3
2 0
100
200
300
400
500
600
time (second) Fig. 3. Pressure at ruptured channel in the helium side FB1510 (upper) and in the water side FB401 (lower).
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with RV 350 without RV 350 0.455 0.450
Pressure (MPa)
0.445 0.440 0.435 0.430 0.425 0.420 0.415 0.410 0.405 0
200
400
600
800
time (second) Fig. 4. Pressure at CCWS-1 pipe (FB350).
with RV 600 without RV 600 0.435 0.430
Pressure (MPa)
0.425 0.420 0.415 0.410 0.405 0.400 0.395 0.390 0
200
400
600
800
time (second) Fig. 5. Pressure at CCWS-1 pressurizer (FB600).
and hot side plate). Then, these thin plates are diffusion bonded together to form the core of heat exchanger. Fig. 2 illustrates fabrication procedure of PCHE. Due to features of PCHE, failure modes can be different compared to conventional heat exchangers. Table 1 summarizes failure modes foreseen in PCHE. Damage level of failure mode #1 and #2 are considered to be limited and they do not give direct impact on the CCWS-1 while the others give. Failure mode #5 gives severe impact, however, it is expected to be negligible in terms of pressurization of CCWS-1 and tritium migration from the PCHE to CCWS-1 because both coolants will come out to the ambient due to high pressure. Actually, this event is considered same as the reference accident, ex-vessel LOCA to Tokamak Cooling Water System (TCWS) Vault Annex (VA) in [5,6], except that the water coolant from CCWS1 breaches out together with the helium coolant. The consequence of failure mode #3 and #4 are the same in that two fluids, helium from the HCCR-TBS HCS and water from CCWS-1, are mixed and propagates to CCWS-1 due to higher pressure of the helium than
Table 1 Expected failure modes for PCHE. No.
Description
Consequence
1
Channel fouling
2
Channel clogging
3
Channel cracking (rupture) Diffusion bonding failure Crack in welding line between block and block, or block and header
Heat transfer performance decrease Pressure drop increase, heat transfer performance decrease Two fluids mixed
4 5
Two fluids mixed Leakage to outside
the water, although transient behavior of thermo-hydraulic parameters could be different due to different propagation speed. In this paper, channel cracking (rupture) is adopted as a reference event,
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Table 2 Parameters for the analysis. Parameters
Value
CCWS-1 total loop volume CCWS-1 pressurizer volumea CCWS-1 loop temperaturea CCWS-1 loop pressurea CCWS-1 total mass flow rate HCS loop total volumea HCS loop temperaturea HCS loop pressure HCS total mass flow rate Rupture size (corresponds to 10 channels break)
3100 m3 [13] 40 m3 [13] 75 ◦ C [13] 0.4 MPa [13] 6230 kg/s [13] 0.667 m3 [14] 200 ◦ C [14] 8 MPa [14] 1.14 kg/s [14] 1.8E-6 m2
a
Assumed value based on the reference.
however sensitive study for rupture size is also given in order to qualitatively foresee possible consequence due to failure mode #4. The reference accident is initiated by failure of ten cooling channels of the PCHE in the HCCR-TBS HCS, which is cooled by CCWS-1. Then, 8 MPa helium coolant ingresses into water side of the PCHE, thus pressurizing CCWS-1. If pressure of the pressurizers in CCWS1 is over 0.41 MPa, then the relief valve is open to the boundary volume. If pressure of the pressurizers is relieved below 0.39 MPa, then the relief valve is closed. Main parameters for the accident are given in Table 2. 5. Results of accident analysis In order to verify the nodalization and parameters, steady-state result was obtained and it was compared with [13], which shows good agreement within 5% deviation of mass flow rate in all regions of CCWS-1. Then the accident analysis was performed for the reference accident with sudden rupture of ten cooling channels from the steady-state. 5.1. Pressure transient Fig. 3 shows pressure transient in the ruptured channels inside the PCHE. From Figs. 3–6 “with RV” denotes operation of the relief valve according to the pressure transient in the pressurizer (FB600) while “without RV” means no operation, i.e. close, of the relief valve regardless of pressure in the pressurizer. In the helium side, pressure decreases gradually from 8 MPa while pressure in the water side sharply increases from 0.4 MPa to over 7 MPa as the high pressure helium ingresses through the leak path, then it drops with time as the pressure propagates to the neighboring volumes. It is observed that chocked flow happens at the ruptured channels and it makes hard to have high pressure build-up at the volumes nearby which are comparatively much bigger than channel volume. Fig. 4 shows typical pressure trends of this accident in CCWS-1. Most of all fluid volumes have the same pattern. As a representative one, the red line indicates pressure curve without relief valve operation at FB350 which represents the pipes between the pump discharging side and clients. After the accident breaks, it oscillates a lot by the effect of discharging high pressure helium to CCWS-1. It is much severe in a small volume. For example, the pressurizer FB600 (40 m3 ) does not have pressure oscillation in Fig. 5 The pressure continuously increases over time for without-relief valve operation. For the more realistic approach, the relief valve operation and nitrogen in pressurizer (88% of pressurizer volume) are taken into account (black line). Pressure trend is the same as ‘without-relief valve’ case until the valve opens. If a fluid volume is away from the relief valve, maximum pressure slightly exceeds 0.41 MPa but there is no such a volume which is over the safety limit of 1 MPa given from CCWS-1, except the ruptured channels themselves. While valve opens, pressure drops and rebounds near
Fig. 6. Crack size sensitivities with 20 channels (upper), 40 channels (middle) and 60 channels (low) rupture.
0.39 MPa after valve closes. This type of wavy trends can be found in all CCWS-1 components. 5.2. Sensitivity study for rupture size Sensitive study is given here with respect to the crack size (from 20 to 60 channels) in order to qualitatively foresee possible consequence due to failure mode #4 mentioned in Table 1, diffusion bonding failure which results in larger rupture size. Fig. 6 indicates that the first pressure peak time in CCWS-1 pressurizer comes faster
Please cite this article in press as: M.-Y. Ahn, et al., Accident analysis on LOCA in HCCR-TBS towards CCWS-1, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.069
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0.05
Crack
Mass Flow Rate (kg/s)
0.04
0.03
0.02
0.01
0.00
0
100
200
300
400
500
600
time (second) Fig. 7. Mass flow rate of the helium ingress to CCWS-1.
with crack size increasing. It is to be noted that the accident pattern such as the maximum pressure is almost the same with each other due to the relief valve operation in the pressurizer except the peak time. Due to two phase chock flow at the end of ruptured channels, the peak time is roughly inversely proportional to the crack size. It can be concluded that overall impact to CCWS-1 by the reference accident #3 and #4 is almost the same due to the relief valve operation in the pressurizer except that the larger rupture size results in faster peak time.
the HCS to CCWS-1 through the PCHE is foreseen by tritium permeation and by helium leak containing tritium in it.
5.3. Tritium release to CCWS-1 due to accident
References
At the moment, detecting pressure of this accident is temporarily set to 6 MPa in the HCS and it takes 71.6 s to be detected as shown in Fig. 3. The helium ingress to CCWS-1 is estimated to be 0.96 kg during that period as in Fig. 7, which means total tritium ingress is only 1.08 GBq when applying 0.8 Pa tritium concentration in the helium coolant, which is the design value for the HCCR-TBS Coolant Purification System (CPS) [14]. The tritium ingress to CCWS-1 is far less than 370 GBq (considering factor 1000 for tritium) which is the requirement to remain as non-nuclear system according to French regulation Équipements sous Pression Nucléaires, known as ESPN [15]. And in terms of tritium density in CCWS-1, it is equivalent with 0.35 MBq/m3 , which is also far less than CCWS-1 requirement (10 MBq/m3 ). It is to be noted that actual tritium concentration in the helium coolant is considered to be far less than 0.8 Pa.
[1] M.-Y. Ahn, S. Cho, D.W. Lee, C.W. Lee, K.I. Shin, D.Y. Ku, et al., Design change of Korean HCCR TBM to vertical configuration, Fusion Eng. Des. 88 (2013) 2284–2288. [2] S. Cho, M.-Y. Ahn, D.W. Lee, Y.-H. Park, E.H. Lee, H.G. Jin, et al., Design and R&D progress of Korean HCCR TBM, Fusion Eng. Des. 89 (2014) 1137–1143. [3] D.W. Lee, H.G. Jin, E.H. Lee, J.S. Yoon, S.K. Kim, C.W. Lee, et al., Integrated design and performance analysis of the KO HCCR TBM for ITER, Fusion Eng. Des. 98–99 (2015) 1821–1824. [4] S. Cho, HCCR-TBS CD Accident Analysis Report, Personal communication. [5] M.-Y. Ahn, S. Cho, H.G. Jin, D.W. Lee, Y.-H. Park, Y. Lee, Preliminary failure modes and effects analysis on Korean HCCR TBS to be tested in ITER, Fusion Eng. Des. 98–99 (2015) 1715–1718. [6] M.-Y. Ahn, H.G. Jin, S. Cho, D.W. Lee, D.Y. Ku, Y.-H. Park, et al., Current status of accident analysis for Korean HCCR TBS, Fusion Eng. Des. 89 (2014) 1289–1293. [7] GA NGNP−Gas Cooled Reactor Design and Demonstration Projects Subcontract Statement of Work, May 7 (2010). [8] H.S. Lim, H.C. No, GAMMA multidimensional multicomponent mixture analysis to predict air ingress phenomena in an HTGR, Nuclear Sci. Eng. 152 (2006) 87–97. [9] H.S. Lim, N.I. Tak, S.N. Lee, GAMMA+ Verification and Validation for Basic Thermo-fluid Problems, 2015, KAERI/TR-6166/2015. [10] S. Cho, HCCR-TBS GAMMA-FR validation strategy for CDR accident analysis, Personal communication. [11] M.-Y. Ahn, S. Cho, E.H. Lee, Y.-H. Park, Y. Lee, Pipe stress analysis on HCCR-TBS ancillary systems in conceptual design, Fusion Eng. Des. 109–111 (2016) 1169–1173. [12] A. Kumar, System description document of CCWS, Personal communication. [13] L&T, Hydraulic analysis report for CCWS, Personal communication. [14] M.-Y. Ahn, et al., HCCR-TBS conceptual design description, Personal communication. [15] Order of 12/12/2005 on Nuclear Pressure Equipment.
6. Conclusions Accident analysis on LOCA in the HCCR-TBS HCS toward CCWS-1 was performed to assess the impact on CCWS-1. The results show that the pressure build-up in CCWS-1 by the LOCA is negligible due to vast size of CCWS-1 compared to small rupture size of the PCHE, and due to the relief valve operations in CCWS-1. It is also confirmed that the tritium requirement of CCWS-1 is satisfied in spite of tritium ingress due to the accident, and that CCWS-1 remains as a non-nuclear system defined by ESPN regulation. While conservative approach and assumptions were made as much as possible for the analysis, the results may have limitations due to early design stage of the concerned systems. In particular, emphasis should be focused on the PCHE design because chronical tritium release from
Acknowledgments This work was supported by R&D Program through National Fusion Research Institute (NFRI) funded by the Ministry of Education, Science and Technology of the Republic of Korea (NFRIIN1603-5).
Please cite this article in press as: M.-Y. Ahn, et al., Accident analysis on LOCA in HCCR-TBS towards CCWS-1, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.069
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Contents lists available at ScienceDirect
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Accident analysis on LOCA in HCCR-TBS towards CCWS-1 Mu-Young Ahn a,∗ , Hyung Gon Jin b , Youngmin Lee a , Seungyon Cho a , Yi-Hyun Park a , Dong Won Lee b a b
National Fusion Research Institute, Daejeon, Republic of Korea Korea Atomic Energy Research Institute, Daejeon, Republic of Korea
h i g h l i g h t s • • • •
Accident analysis on LOCA in the HCCR-TBS HCS to CCWS-1was performed to assess impact on Component Cooling Water System-1. The analysis reveals that impact due to the accident is limited in terms of pressure increase in CCWS-1 and tritium ingress to CCWS-1. However, chronical release of tritium from the HCS to CCWS-1 through the heat exchanger should be further clarified. This aspect should be reflected in the design of the heat exchanger in future.
a r t i c l e
i n f o
Article history: Received 30 September 2016 Received in revised form 6 April 2017 Accepted 15 May 2017 Available online xxx Keywords: Accident analysis Helium Cooled Ceramic Reflector (HCCR) Loss of Coolant Accident (LOCA) Printed Circuit Heat Exchanger (PCHE) GAMMA-FR
a b s t r a c t Korean Helium Cooled Ceramic Reflector (HCCR) Test Blanket System (TBS) will be operated at elevated temperature with high pressure helium coolant during normal operation in ITER. One of the main ancillary systems of HCCR-TBS is Helium Cooling System (HCS) which play an important role to extract heat from HCCR Test Blanket Module (TBM) by the helium coolant to keep the operational temperature and the extracted heat is finally transferred to ITER CCWS-1 (Component Cooling Water System) by a Printed Circuit Heat Exchanger (PCHE) in the HCS. In such circumstances if Loss Of Coolant Accident (LOCA) occurs in the PCHE, the high pressure helium coolant in the primary side goes into the lower pressure water in the secondary side thus pressurizing CCWS-1. In addition, since the helium coolant contains tritium due to permeation from the TBM, tritium migrates into CCWS-1, a non-nuclear system. In this paper, accident analysis for LOCA in the heat exchanger is presented. For the analysis, GAMMA-FR code which has been developed for fusion applications was used. Main components in the HCS and CCWS-1 were modelled as volume and junctions. The accident analysis was performed for the reference case with ten channels rupture and sensitivity study was also performed by changing the crack size. The results show that pressure and tritium requirement of CCWS-1 can be met in spite of LOCA in the heat exchanger of the HCCR-TBS HCS. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Various design activities and supporting R&Ds have been performed for Korean Helium Cooled Ceramic Reflector (HCCR) Test Blanket System (TBS) aiming at demonstration of possibility of tritium self-sufficiency and heat extraction capable of producing electricity in ITER [1–3]. Since ITER is a basic nuclear installation, INB-174, according to French nuclear licensing procedures and the HCCR-TBS is operated at high pressure and/or high temperature with physical and functional interfaces to ITER, safety features of the HCCR-TBS should be demonstrated not only during normal
∗ Corresponding author. E-mail address: [email protected] (M.-Y. Ahn).
operation but also for accidental events to confirm that relevant ITER requirements and guidelines are met [4]. While most of failure modes of the HCCR-TBS and their accidental consequences are addressed in [5,6] respectively, this paper presents accident analysis on Loss Of Coolant Accident (LOCA) in Helium Cooling System (HCS) toward ITER Component Cooling Water System-1 (CCWS-1). The HCS is a main cooling system of the HCCR-TBS in order to transfer extracted heat from the Test Blanket Module (TBM) to CCWS-1 via Printed Circuit Heat Exchanger (PCHE). If LOCA occurs inside the PCHE, 8 MPa helium coolant in the primary side goes into 0.4 MPa water in the secondary side thus pressurizing CCWS-1. Furthermore, tritium permeated from the TBM to the helium coolant can migrate to CCWS-1 in which tritium is strictly restricted as a non-nuclear system. To analyze thermo-hydraulic transient behavior during the accident, GAMMA-
http://dx.doi.org/10.1016/j.fusengdes.2017.05.069 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: M.-Y. Ahn, et al., Accident analysis on LOCA in HCCR-TBS towards CCWS-1, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.069
G Model FUSION-9553; No. of Pages 6
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2
Fig. 1. Nodalization for the analysis. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
FR, a domestic system code for fusion applications, was used. GAMMA-FR, as a branch of GAMM+ code which has been adopted for conceptual design of the Next Generation Power Plant by Korea Atomic Energy Research Institute under a subcontract with General Atomics [7], has the capability to handle thermal hydraulic behaviors with chemical reaction in a multi-component mixture system as well as heat transfer within solid components [8], and it inherits general thermal hydraulic validation of the mother code [9]. As a further effort to use GAMMA-FR on HCCR-TBS safety analysis for ITER, the code validation is on-going under Korea-US collaboration program by comparing fusion system related experimental data and by comparing with MELCOR analysis results [10]. Main components of the HCS and CCWS-1 were modelled and the accident analysis was performed for the reference case having ten channels rupture with sensitivity study by changing the crack size. 2. Concerned system description 2.1. HCS of HCCR-TBS The HCS provides the primary helium coolant at the characteristic pressure, temperature and mass flow rate required by the HCCR TBM for testing and extraction of the heat produced. The HCS mainly consists of a circulator, a recuperator, a pre-heater, gas mixers, filters and a PCHE [11]. The hot helium coolant returned from the TBM is cooled down by the recuperator and the PCHE to room temperature before the circulator, and the coolant is re-heated up to the inlet temperature condition after the gas mixer where the helium coolant from the recuperator and the bypass line merges. In normal operation conditions, the pre-heater is turned off and the pre-heater bypass valves are opened. It is installed to heat up the helium coolant and the TBM up to operational temperature during start-up, commissioning and maintenance states. 2.2. CCWS-1 CCWS-1, one of sub-systems of CCWS, consists of horizontal circulation pumps, plate heat exchangers, pressurizer with relief valves and chemical addition system [12]. The pressurizer is connected to the suction side of the pump. It will accommodate the water volume expansion and keep the pressure in the system not only during operation but also during start-up or maintenance. The pressurizer is occupied by water initially at 12% level while rest 88%
is filled with nitrogen in order to avoid the contact of water with air. The water will expand during the operation. The safety relief valves are provided on the pressurizer to avoid any excess pressure increase in the system leading to accident. 3. Modelling Since amount of discharged helium is the key parameter for the analysis, only the circulator and the PCHE are modelled among the HCS components as shown in Fig. 1. And radiation heat transfer and temperature distribution along the pipes are not taken into account. Due to the same reason, flow network inside of TBM is simplified as one fluid volume (FB1300). The PCHE, which is inside of the red color rectangle in Fig. 1, has many narrow channels. FB1500 and 1510 are HCS side, and FB401 and 402 are for CCWS-1 side. FB1510 and 401 represent ruptured channels, which are connected when accident happens. And FB1500 and 402 are unbroken channels. The isolation valves are modeled at CCWS-1 side inlet and outlet, respectively, in order to mitigate the consequence of the accident. Currently, they are assumed to be closed when the HCS pressure decreases to 6 MPa, which should be investigated further to find the optimum detection values. The right hand side of Fig. 1 represents the nodalization of CCWS-1. The pump, pressurizer with relief valve and pipes going to various clients are modelled for this analysis. While FB403 in the blue rectangle is for cooling circuits for the other 5 TBSs, fluid blocks between the header (FB350) and collector (FB355) represents the pipes going to clients other than TBSs. Total mass flow rate of CCWS1 is 6230 kg/s and mass flow rate for 6 TBSs is 96 kg/s, i.e. 16 kg/s per each TBS cooling circuit. The relief valve opens when the pressure (FB600) exceeds 0.41 MPa and it is assumed to take one second to be fully opened. And it is closed when the pressure decreases below 0.39 MPa. As an initial condition, 88% of the pressurizer volume is filled with nitrogen. 4. Failure modes and accident description PCHE is adopted for the HCS heat exchanger considering its advantages such as high heat transfer efficiency, compactness, etc. The name comes from the process for fabrication of the thin plate with narrow channels on the surface, which is done by chemical etching. There are two patterns of printed circuits (cold side plate
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Fig. 2. Fabrication procedure of PCHE.
with RV 1510 without RV 1510
8
Pressure (MPa)
7
6
5
4
3
2 0
100
200
300
400
500
600
time (second)
with RV 401 without RV 401
8
Pressure (MPa)
7
6
5
4
3
2 0
100
200
300
400
500
600
time (second) Fig. 3. Pressure at ruptured channel in the helium side FB1510 (upper) and in the water side FB401 (lower).
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with RV 350 without RV 350 0.455 0.450
Pressure (MPa)
0.445 0.440 0.435 0.430 0.425 0.420 0.415 0.410 0.405 0
200
400
600
800
time (second) Fig. 4. Pressure at CCWS-1 pipe (FB350).
with RV 600 without RV 600 0.435 0.430
Pressure (MPa)
0.425 0.420 0.415 0.410 0.405 0.400 0.395 0.390 0
200
400
600
800
time (second) Fig. 5. Pressure at CCWS-1 pressurizer (FB600).
and hot side plate). Then, these thin plates are diffusion bonded together to form the core of heat exchanger. Fig. 2 illustrates fabrication procedure of PCHE. Due to features of PCHE, failure modes can be different compared to conventional heat exchangers. Table 1 summarizes failure modes foreseen in PCHE. Damage level of failure mode #1 and #2 are considered to be limited and they do not give direct impact on the CCWS-1 while the others give. Failure mode #5 gives severe impact, however, it is expected to be negligible in terms of pressurization of CCWS-1 and tritium migration from the PCHE to CCWS-1 because both coolants will come out to the ambient due to high pressure. Actually, this event is considered same as the reference accident, ex-vessel LOCA to Tokamak Cooling Water System (TCWS) Vault Annex (VA) in [5,6], except that the water coolant from CCWS1 breaches out together with the helium coolant. The consequence of failure mode #3 and #4 are the same in that two fluids, helium from the HCCR-TBS HCS and water from CCWS-1, are mixed and propagates to CCWS-1 due to higher pressure of the helium than
Table 1 Expected failure modes for PCHE. No.
Description
Consequence
1
Channel fouling
2
Channel clogging
3
Channel cracking (rupture) Diffusion bonding failure Crack in welding line between block and block, or block and header
Heat transfer performance decrease Pressure drop increase, heat transfer performance decrease Two fluids mixed
4 5
Two fluids mixed Leakage to outside
the water, although transient behavior of thermo-hydraulic parameters could be different due to different propagation speed. In this paper, channel cracking (rupture) is adopted as a reference event,
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Table 2 Parameters for the analysis. Parameters
Value
CCWS-1 total loop volume CCWS-1 pressurizer volumea CCWS-1 loop temperaturea CCWS-1 loop pressurea CCWS-1 total mass flow rate HCS loop total volumea HCS loop temperaturea HCS loop pressure HCS total mass flow rate Rupture size (corresponds to 10 channels break)
3100 m3 [13] 40 m3 [13] 75 ◦ C [13] 0.4 MPa [13] 6230 kg/s [13] 0.667 m3 [14] 200 ◦ C [14] 8 MPa [14] 1.14 kg/s [14] 1.8E-6 m2
a
Assumed value based on the reference.
however sensitive study for rupture size is also given in order to qualitatively foresee possible consequence due to failure mode #4. The reference accident is initiated by failure of ten cooling channels of the PCHE in the HCCR-TBS HCS, which is cooled by CCWS-1. Then, 8 MPa helium coolant ingresses into water side of the PCHE, thus pressurizing CCWS-1. If pressure of the pressurizers in CCWS1 is over 0.41 MPa, then the relief valve is open to the boundary volume. If pressure of the pressurizers is relieved below 0.39 MPa, then the relief valve is closed. Main parameters for the accident are given in Table 2. 5. Results of accident analysis In order to verify the nodalization and parameters, steady-state result was obtained and it was compared with [13], which shows good agreement within 5% deviation of mass flow rate in all regions of CCWS-1. Then the accident analysis was performed for the reference accident with sudden rupture of ten cooling channels from the steady-state. 5.1. Pressure transient Fig. 3 shows pressure transient in the ruptured channels inside the PCHE. From Figs. 3–6 “with RV” denotes operation of the relief valve according to the pressure transient in the pressurizer (FB600) while “without RV” means no operation, i.e. close, of the relief valve regardless of pressure in the pressurizer. In the helium side, pressure decreases gradually from 8 MPa while pressure in the water side sharply increases from 0.4 MPa to over 7 MPa as the high pressure helium ingresses through the leak path, then it drops with time as the pressure propagates to the neighboring volumes. It is observed that chocked flow happens at the ruptured channels and it makes hard to have high pressure build-up at the volumes nearby which are comparatively much bigger than channel volume. Fig. 4 shows typical pressure trends of this accident in CCWS-1. Most of all fluid volumes have the same pattern. As a representative one, the red line indicates pressure curve without relief valve operation at FB350 which represents the pipes between the pump discharging side and clients. After the accident breaks, it oscillates a lot by the effect of discharging high pressure helium to CCWS-1. It is much severe in a small volume. For example, the pressurizer FB600 (40 m3 ) does not have pressure oscillation in Fig. 5 The pressure continuously increases over time for without-relief valve operation. For the more realistic approach, the relief valve operation and nitrogen in pressurizer (88% of pressurizer volume) are taken into account (black line). Pressure trend is the same as ‘without-relief valve’ case until the valve opens. If a fluid volume is away from the relief valve, maximum pressure slightly exceeds 0.41 MPa but there is no such a volume which is over the safety limit of 1 MPa given from CCWS-1, except the ruptured channels themselves. While valve opens, pressure drops and rebounds near
Fig. 6. Crack size sensitivities with 20 channels (upper), 40 channels (middle) and 60 channels (low) rupture.
0.39 MPa after valve closes. This type of wavy trends can be found in all CCWS-1 components. 5.2. Sensitivity study for rupture size Sensitive study is given here with respect to the crack size (from 20 to 60 channels) in order to qualitatively foresee possible consequence due to failure mode #4 mentioned in Table 1, diffusion bonding failure which results in larger rupture size. Fig. 6 indicates that the first pressure peak time in CCWS-1 pressurizer comes faster
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0.05
Crack
Mass Flow Rate (kg/s)
0.04
0.03
0.02
0.01
0.00
0
100
200
300
400
500
600
time (second) Fig. 7. Mass flow rate of the helium ingress to CCWS-1.
with crack size increasing. It is to be noted that the accident pattern such as the maximum pressure is almost the same with each other due to the relief valve operation in the pressurizer except the peak time. Due to two phase chock flow at the end of ruptured channels, the peak time is roughly inversely proportional to the crack size. It can be concluded that overall impact to CCWS-1 by the reference accident #3 and #4 is almost the same due to the relief valve operation in the pressurizer except that the larger rupture size results in faster peak time.
the HCS to CCWS-1 through the PCHE is foreseen by tritium permeation and by helium leak containing tritium in it.
5.3. Tritium release to CCWS-1 due to accident
References
At the moment, detecting pressure of this accident is temporarily set to 6 MPa in the HCS and it takes 71.6 s to be detected as shown in Fig. 3. The helium ingress to CCWS-1 is estimated to be 0.96 kg during that period as in Fig. 7, which means total tritium ingress is only 1.08 GBq when applying 0.8 Pa tritium concentration in the helium coolant, which is the design value for the HCCR-TBS Coolant Purification System (CPS) [14]. The tritium ingress to CCWS-1 is far less than 370 GBq (considering factor 1000 for tritium) which is the requirement to remain as non-nuclear system according to French regulation Équipements sous Pression Nucléaires, known as ESPN [15]. And in terms of tritium density in CCWS-1, it is equivalent with 0.35 MBq/m3 , which is also far less than CCWS-1 requirement (10 MBq/m3 ). It is to be noted that actual tritium concentration in the helium coolant is considered to be far less than 0.8 Pa.
[1] M.-Y. Ahn, S. Cho, D.W. Lee, C.W. Lee, K.I. Shin, D.Y. Ku, et al., Design change of Korean HCCR TBM to vertical configuration, Fusion Eng. Des. 88 (2013) 2284–2288. [2] S. Cho, M.-Y. Ahn, D.W. Lee, Y.-H. Park, E.H. Lee, H.G. Jin, et al., Design and R&D progress of Korean HCCR TBM, Fusion Eng. Des. 89 (2014) 1137–1143. [3] D.W. Lee, H.G. Jin, E.H. Lee, J.S. Yoon, S.K. Kim, C.W. Lee, et al., Integrated design and performance analysis of the KO HCCR TBM for ITER, Fusion Eng. Des. 98–99 (2015) 1821–1824. [4] S. Cho, HCCR-TBS CD Accident Analysis Report, Personal communication. [5] M.-Y. Ahn, S. Cho, H.G. Jin, D.W. Lee, Y.-H. Park, Y. Lee, Preliminary failure modes and effects analysis on Korean HCCR TBS to be tested in ITER, Fusion Eng. Des. 98–99 (2015) 1715–1718. [6] M.-Y. Ahn, H.G. Jin, S. Cho, D.W. Lee, D.Y. Ku, Y.-H. Park, et al., Current status of accident analysis for Korean HCCR TBS, Fusion Eng. Des. 89 (2014) 1289–1293. [7] GA NGNP−Gas Cooled Reactor Design and Demonstration Projects Subcontract Statement of Work, May 7 (2010). [8] H.S. Lim, H.C. No, GAMMA multidimensional multicomponent mixture analysis to predict air ingress phenomena in an HTGR, Nuclear Sci. Eng. 152 (2006) 87–97. [9] H.S. Lim, N.I. Tak, S.N. Lee, GAMMA+ Verification and Validation for Basic Thermo-fluid Problems, 2015, KAERI/TR-6166/2015. [10] S. Cho, HCCR-TBS GAMMA-FR validation strategy for CDR accident analysis, Personal communication. [11] M.-Y. Ahn, S. Cho, E.H. Lee, Y.-H. Park, Y. Lee, Pipe stress analysis on HCCR-TBS ancillary systems in conceptual design, Fusion Eng. Des. 109–111 (2016) 1169–1173. [12] A. Kumar, System description document of CCWS, Personal communication. [13] L&T, Hydraulic analysis report for CCWS, Personal communication. [14] M.-Y. Ahn, et al., HCCR-TBS conceptual design description, Personal communication. [15] Order of 12/12/2005 on Nuclear Pressure Equipment.
6. Conclusions Accident analysis on LOCA in the HCCR-TBS HCS toward CCWS-1 was performed to assess the impact on CCWS-1. The results show that the pressure build-up in CCWS-1 by the LOCA is negligible due to vast size of CCWS-1 compared to small rupture size of the PCHE, and due to the relief valve operations in CCWS-1. It is also confirmed that the tritium requirement of CCWS-1 is satisfied in spite of tritium ingress due to the accident, and that CCWS-1 remains as a non-nuclear system defined by ESPN regulation. While conservative approach and assumptions were made as much as possible for the analysis, the results may have limitations due to early design stage of the concerned systems. In particular, emphasis should be focused on the PCHE design because chronical tritium release from
Acknowledgments This work was supported by R&D Program through National Fusion Research Institute (NFRI) funded by the Ministry of Education, Science and Technology of the Republic of Korea (NFRIIN1603-5).
Please cite this article in press as: M.-Y. Ahn, et al., Accident analysis on LOCA in HCCR-TBS towards CCWS-1, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.05.069