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Design and development of innovative passive valves for Nuclear Power Plant applications
Nuclear Engineering and Design 286 (2015) 195–204
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Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Design and development of innovative passive valves for Nuclear Power Plant applications M.K. Sapra ∗ , S. Kundu, A.K. Pal, P.K. Vijayan, K.K. Vaze, R.K. Sinha Reactor Design and Development Group, Bhabha Atomic Research Centre, Department of Atomic Energy, Trombay, Mumbai 400085, India
h i g h l i g h t s • • • • •
Passive valves are self-acting valves requiring no external energy to function. These valves have been developed for Advanced Heavy Water Reactor (AHWR) of India. Passive valves are core components of passive safety systems of the reactor. Accumulator Isolation Passive Valve (AIPV) has been developed and tested for ECSS. AIPV provided passive isolation and flow regulation in ECCS of Integral Test Loop.
a r t i c l e
i n f o
Article history: Received 4 August 2014 Received in revised form 26 February 2015 Accepted 27 February 2015
a b s t r a c t The recent Fukushima accident has resulted in an increased need for passive safety systems in upcoming advanced reactors. In order to enhance the global contribution and acceptability of nuclear energy, proven evidence is required to show that it is not only green but also safe, in case of extreme natural events. To achieve and establish this fact, we need to design, demonstrate and incorporate reliable ‘passive safety systems’ in our advanced reactor designs. In Nuclear Power Plants (NPPs), the use of passive safety systems such as accumulators, condensing and evaporative heat exchangers and gravity driven cooling systems provide enhanced safety and reliability. In addition, they eliminate the huge costs associated with the installation, maintenance and operation of active safety systems that require multiple pumps with independent and redundant electric power supplies. As a result, passive safety systems are preferred for numerous advanced reactor concepts. In current NPPs, passive safety systems which are not participating in day to day operation, are kept isolated, and require a signal and external energy source to open the valve. It is proposed to replace these valves by passive components and devices such as self-acting valves, rupture disks, etc. Some of these innovative passive valves, which do not require external power, have been recently designed, developed and tested at rated conditions. These valves are proposed to be used for various passive safety systems of an upcoming Nuclear Power Plant being designed by India. For example, the Hot Shutdown Passive Valves (HSPV), developed for the decay heat removal system keep the main heat transport system under hot conditions by passively sensing and controlling the system pressure. Another crucial and important valve which has been successfully developed is the Poison Injection Passive Valve (PIPV) for the Passive Poison Injection System. It not only provides higher reliability, but also ensures safe shutdown of the reactor in case of insider threats or malevolent acts in disabling active shutdown system of the reactor. Recently, an innovative valve called the Accumulator Isolation Passive Valve (AIPV) has been developed for the Emergency Core Cooling System (ECCS), which is engineered to mitigate the consequences of Loss of Coolant Accident (LOCA). During normal operation of the reactor, the pressurized accumulators (55 bar) are kept isolated from the reactor core (70 bar) by means of AIPVs. In case of a LOCA, these passive valves open when the main heat transport system pressure falls to a desired value. For prolonged cooling of the core, these passive valves regulate the discharge in a desired manner. These are non-standard, high pressure and high temperature valves, which are unavailable commercially and hence have to be indigenously designed and developed.
∗ Corresponding author. Tel.: +91 22 25591617; fax: +91 22 25505151. E-mail address: [email protected] (M.K. Sapra). http://dx.doi.org/10.1016/j.nucengdes.2015.02.022 0029-5493/© 2015 Elsevier B.V. All rights reserved.
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This paper primarily deals with the design, development and testing of Accumulator Isolation Passive Valves (AIPV) proposed to be used in the ECCS. A 25 NB size AIPV has been designed and successfully tested at Integral Test Loop (ITL) under simulated reactor conditions. It is a self-acting, ANSI 600 rating valve, which requires no external energy (i.e., neither air nor electrical power). It not only provides passive isolation but also passively controls high pressure liquid discharge through it. The design concept of the valve, functional performance, in situ valve testing methodology and the test results at simulated conditions are discussed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
2. Advanced Heavy Water Reactor (AHWR)
Following the Fukushima accident, which occurred on Friday, March 11, 2011, in the Fukushima Daiichi Nuclear Power Plant on the east coast of northern Japan, the safety features of Nuclear Power Plants (NPPs) are being re-examined to demonstrate their capabilities to cope with severe accidents. An increasing need is being recognized to provide systems with passive or intrinsic characteristics which would ensure the continued cooling of fuel and its containment systems (Vijayan et al., 2013a). The use of passive safety systems such as accumulators, condensation and evaporative heat exchangers, and gravity driven safety injection systems eliminate the costs associated with the installation, maintenance and operation of active safety systems that require multiple pumps with independent and redundant electric power supplies. As a result, passive safety systems are being considered for numerous reactor concepts (including in Generation III and III+ concepts) and are expected to find applications in the Generation-IV reactor concepts, as identified by the Generation IV International Forum (GIF). Another motivation for the use of passive safety systems is the potential for enhanced safety through increased safety system reliability (IAEA, 2009). Active safety systems have the advantages of easily allowing a power increase and using proven technologies, on the other hand, while passive safety systems are operated by means of natural phenomena such as gravity, natural circulation and pressure differences. Passive safety systems have the advantages of high reliability, minimal human errors, simplification and easy modularization. Based on advantages and disadvantages of active and passive safety systems, both active and passive safety systems must be installed in Nuclear Power Plants for enhanced safety (Chang et al., 2013). A large scale simplified pressurized water reactor as a candidate for the Japanese Next Generation Pressurized Water Reactor (PWR) has innovative features such as hybrid safety systems (an optimum combination of active and passive safety system) and horizontal steam generators. Here passive safety systems act as backups to prevent core damage if the active safety systems do not operate correctly due to operational or other errors (Yonezo Tujikura et al., 2000). The Advanced Heavy Water Reactor (AHWR) being designed in India incorporates several passive safety systems which will enable the plant to survive potential severe accidents without fuel damage and require no human intervention (Nayak and Sinha, 2007). This paper describes some of the important passive safety systems adopted in the AHWR design. These passive systems use innovative passive valves which are custom designed to suit process requirements and have been developed in-house. The development and testing of passive valves and experimental demonstrations of passive safety systems of the AHWR under simulated process conditions are included in this paper. The development and testing of one of the innovative passive valves called the Accumulator Isolation Passive Valve (AIPV) is discussed in detail.
Advanced Heavy Water Reactor (AHWR) shown schematically in Fig. 1 is a 300 MWe, vertical, pressure tube type, heavy watermoderated, boiling light-water-cooled reactor fueled by dual mixed oxide (MOX) consisting of (PUTh)O2 and (233 U-Th)O2 with a 100 year lifetime. The fuel cluster is designed to generate maximum energy from thorium, to maintain self-sufficiency in 233 U and to achieve a slightly negative void coefficient of reactivity. Natural circulation mode of coolant circulation is adopted during normal operation, transient and accident conditions, which eliminates all accident scenarios resulting from pump failure in addition to reducing capital and operating costs (Sinha and Kakodkar, 2006). 3. Passive safety systems The AHWR incorporates passive systems for core heat removal during normal operation, shutdown conditions and postulated accident conditions such as a Loss of Coolant Accident (LOCA). During normal operation, the core heat is removed by the natural circulation of coolant through hot fuel channels. In Station Black Out (SBO) conditions, the decay heat is removed in a passive mode by Isolation Condensers (ICs) immersed in a large pool of water in a Gravity Driven Water Pool (GDWP), located near the top of the reactor building. During a postulated accident such as a LOCA, high-pressure coolant is directly injected into fuel clusters in a passive mode initially from accumulators and later from GDWP for removal of decay heat. Passive containment cooling and isolation, core submergence, passive poison injection in the moderator by usage of system steam pressure during high pressure transients, and passive cooling of concrete structure in high temperature zones are the additional passive features of AHWR. As per IAEA-TECDOC – 626, the passive safety systems are defined as “Either a system which is composed entirely of passive components and structures or a system which uses active components in a very limited way to initiate subsequent passive operation” (IAEA, 1991). There are four categories to distinguish the different degrees of passivity, namely: Category A, B, C and D as described below;
Fig. 1. Schematic of AHWR.
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Fig. 2. Passive systems of AHWR.
Category A is characterized by: • • • •
No signal inputs of “intelligence” No external power sources or forces No moving mechanical parts, and No moving working fluid. Category B is characterized by:
• • • •
No signal inputs of “intelligence” No external power sources or forces No moving mechanical parts; but Moving working fluids. Category C is characterized by:
• No signal inputs of intelligence. • No external power sources or forces; but • Moving mechanical parts, whether or not moving working fluids are also present.
3.1. Passive core decay heat removal system The main objective is to provide removal of decay heat to an ultimate heat sink after safe shutdown of the reactor. The conventional active decay heat removal systems use water as a cooling medium whose availability cannot be ensured in the event of a Fukushima type long term SBO condition (Vijayan et al., 2013b). The passive decay heat removal system assures core decay heat removal for extended periods of SBO ensuring safety of the reactor. 3.2. Passive Poison Injection System Safe shut down of the reactor is an important safety function which must be ensured to avoid and mitigate accidents of a severe nature. Conventional wired shutdown system depends on active signals (electricity/air). Therefore, this system is vulnerable to the failure of the active signals, inability of plant operators to manage the events, or malevolent acts by an insider. The Passive Poison Injection System passively shuts down the reactor even with failure of wired shutdown systems. 3.3. Passive Emergency Core Cooling System
The examples of Category D safety systems are as follows: • Emergency core cooling/injection systems, based on gravitydriven or compressed nitrogen driven fluid circulation, initiated by fail-safe logic actuating battery-powered electric or electropneumatic valves; • Emergency Core Cooling Systems, based on gravity-driven flow of water, activated by valves which break open on demand (if a suitable qualification process of the actuators can be identified); and • Emergency reactor shutdown systems based on gravity-driven, or static pressure driven control rods, activated by fail-safe trip logic. The passive valves and devices developed for the AHWR fall under Category C (IAEA, 1991). Some of the important passive safety systems of the AHWR that use innovative passive valves are shown in Fig. 2 and are described as follows:
To mitigate the consequences of a LOCA, a passive Emergency Core Cooling System (ECCS) is provided in AHWR. This system provides direct injection of water into fuel cluster in a passive mode during LOCA and removes heat from the fuel. The ECCS water is injected initially from accumulators and later from the GDWP. 4. Passive valves The operation of the above mentioned three passive safety systems is made possible by innovative passive valves called the Hot Shutdown Passive Valves (HSPV), Poison Injection Passive Valve (PIPV) and Accumulator Isolation Passive Valve (AIPV). Since the valves used in these safety systems do not require active power supply (i.e. electrical or pneumatic) for operation, they are known as passive valves. These passive valves are indigenously designed and developed to suit different process requirements of the individual passive safety systems. The salient features of passive valves are as follows:
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• • • • •
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No external power – electrical or pneumatic. Uses process fluid pressure energy for actuation. Provides regulation or isolation as per the process requirement. Bellows sealed and bellows actuated. Double sealed pressure boundary.
Some of the passive valves required for AHWR application, which have been designed, developed and successfully tested and demonstrated at scaled experimental facilities, are listed below. • Hot Shutdown Passive Valve (HSPV) for passive core decay heat removal system, Poison Injection Passive Valve (PIPV) for Passive Poison Injection System, and • Accumulator Isolation Passive Valve (AIPV) for passive Emergency Core Cooling System. Presently, these valves are manufactured for experimental facility use and therefore the test conditions are based upon the facility requirement and not for operating Nuclear Power Plant. Series of tests conducted using these valves have given satisfactory performance and have met all the functional and process requirement of the reactor safety systems. This has given enough confidence to manufacture a new set of passive valves as per ASME Section III NB-3500 to meet in general the test requirement of ASME – QME-1-2012 and our technical specifications. These valves will be nuclear grade valves and will undergo stringent functional testing and qualification program including thermal cycling, pressure cycling, vibration and seismic testing. Since there are no rubber or non-metallic components; radiation aging is not envisaged at this stage.
5. Hot Shutdown Passive Valve (HSPV) AHWR has a large pool of water called Gravity Driven Water Pool (GDWP) at top of the reactor primary containment. The isolation condensers are immersed in the GDWP and connected to the Main Heat Transport System (MHTS), forming a natural circulation loop through a HSPV and active valve in parallel. During hot shutdown, the steam drum pressure rises due to decay heat. The HSPV opens on high steam drum pressure to establish communication between the steam drum and the IC, rejecting decay heat to the GDWP water. Once the steam drum pressure lowers to a normal value, the HSPV closes. Thus, hot shutdown condition can be maintained for prolonged periods of SBO. The schematic of the passive core decay heat removal system with HSPV is shown in Fig. 3.
Fig. 3. Decay heat removal system and Passive Poison Injection System of AHWR.
5.1.1. In situ calibration of HSPV The in situ calibration (i.e. online field calibration) of the HSPV was carried out by pressurizing its actuator from an external pump. Actuator pressure was increased and the valve movement was recorded with an LVDT output to generate valve opening and closing characteristics as shown in Fig. 5. The experimental data shows the linearity, controllability and hysteresis of the developed passive valve which was found to be within acceptable limits. 5.1.2. Simulation of hot shutdown and decay heat removal AHWR station blackout (i.e. class III and class IV power supply failure) will lead to the plant shutdown. Due to decay heat generation after reactor shutdown, the steam drum (SD) pressure rises. Under these conditions, decay heat will be removed using IC through the HSPV. This capability was verified during testing by simulating hot shutdown at 14 bar pressure. The HSPV was set at
5.1. HSPV testing and results In the process of developing a Hot Shutdown Passive Valve (HSPV) for AHWR, a prototype 2 in. size ANSI 600 passive valve, suitable for 28 bar pressure has been designed and successfully tested at 220 ◦ C in Integral Test Loop (ITL) at the institute shown in Fig. 4 (Sapra et al., 2007). Following tests were conducted: • • • • • • •
Station Black Out simulation. Functional testing of HSPV at rated conditions. Decay heat removal function. Controllability, linearity, hysteresis and repeatability of HSPV SD pressure control using HSPV Valve performance at different pressure and flow conditions. In situ calibration of HSPV.
Fig. 4. HSPV installed at ITL.
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110 100 valve opening valve closing
90
Full opening 21.6 bar
80
Start closing 21 bar
HSPV open (%)
70 60 50 40 30 20
Full reclosing 16.8 bar
10
Start opening 18 bar
0 -10 13
14
15
16
17
18
19
20
21
22
23
24
25
Steam drum pressure (bar) Fig. 5. In situ calibration of HSPV
Fig. 7. Bellows Test Facility (BTF) at IGCAR.
14 bar opening pressure and the operating power of the ITL was reduced to 50 kW. As shown in Fig. 6, HSPV opening occurred for 5 min duration. This maintained the primary main heat transport (MHT) of the ITL in the HOT condition and the plant in shutdown condition with SD pressure at 14 ± 0.5 bar. After 6 h, the pressure again rises above set point of 14 bars and the HSPV opens momentarily, which brings down the system pressure to acceptable levels. This simulation test successfully demonstrated the functional performance requirement for the HSPV and decay heat removal system of the AHWR.
Fig. 6. Experimental results for decay heat removal.
6. Qualification and testing of valve bellows Metallic bellows are widely used in the valve industry in lieu of dynamic seals or packing to permit stem travel and sealing with zero leakage. The passive valves described here also use bellows in their actuator for gland sealing. These bellows are generally made out of thin metallic sheets manufactured using the hydro-forming process. They can perform very well under high pressure and temperature conditions. Structural integrity of the bellows is required for overall system reliability. Bellows are generally designed based on guidelines given by Expansion Joint Manufacturer Association (EJMA). However, EJMA only supports standard U-shaped/toroidal bellows. The nuclear industry faces stringent requirements of high pressure and large displacement to match reactor operating conditions and required valve flow conditions. High pressure requires large thickness, but it introduces high axial stiffness, which is not desirable. For this diverse requirement, shape of the bellows needs to be optimized and multiple layer construction is favored to reduce the resulting alternating stress levels which govern the fatigue life of the bellows. Stainless steel grade 304L, if used as the material of construction, needs six plies of 0.5 mm thickness. But this introduces very high axial stiffness and cannot meet the required design criteria. The material of construction was changed to INCONEL-600 and was found to be satisfying both the requirements of stresses as well as axial stiffness with two plies of 0.5 mm thickness. The bellows design was evaluated using finite element method. A parametric finite element model was developed to quickly incorporate any dimensional or shape changes during the design stage. A procedure was developed to determine membrane and bending stresses from the component stresses computed using the finite element model. This procedure was validated against published literature values (Dureja et al., 2011). The shape of the bellows plays a major role in the componentdeveloped membrane and bending stresses. Various shapes were modeled and analyzed and it was found that an omega shaped bellows with different lobe radii outperform the conventional U shaped bellows. Based on the above studies, omega shaped bellows were procured from reputable suppliers and subjected to cyclic test to determine the fatigue life. A dedicated Bellows Test Facility (BTF) as shown in Fig. 7 has been designed, developed and commissioned to test 19 bellows at a time. A given test set of bellows is externally pressurized and heated to the design temperature. Axial
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Fig. 9. Experimental results for response time. Fig. 8. Test facility for PIPV
displacement of ±10.0 mm was applied for cyclic testing. The first bellows assembly consisting of three bellows has successfully passed the required number of 25,000 cycles. Further tests are planned to meet requirement of ASME Section III Appendix II for testing of 5 bellows assemblies subjected to 65,000 cycles. 7. Poison Injection Passive Valve (PIPV) The Passive Poison Injection System (PPIS) provides the passive means of reactor shutdown through a passive valve called the Poison Injection Passive Valve (PIPV). AHWR has two independent active shutdown systems, one comprising the mechanical shut off rods and the other employing injection of a liquid poison in the low pressure moderator, both requiring active signals for reactor shutdown. As shown in Fig. 3, during normal reactor operation, the PIPV isolates high pressure helium gas tank and poison tank. Failure of two active (wired) shutdown systems results in increased main heat transport (MHT) system pressure beyond a predetermined value that bursts rupture disks RD-1 and 2, which in-turn actuates the PIPV. The opening of the PIPV initiates an inrush of high pressure helium gas into the poison tank to push the poison into the moderator, thereby shutting down the reactor.
pressure upstream of the rupture discs was raised up to 86 bars of nitrogen pressure. As soon as the actuator pressure reached 84 bar, the two rupture disks in series prior to the PIPV actuator burst at 84 bar (RD-1) and 82 bar (RD-2). This action pressurized the valve actuator to open the valve instantaneously. A series of such tests were conducted to obtain the performance objectives mentioned above. One of the test profiles with upstream pressure, downstream pressure, actuator pressure and valve opening time is shown in Fig. 10. 8. Accumulator Isolation Passive Valve (AIPV) In the AHWR, the Emergency Core Cooling System (ECCS) provides direct injection of coolant into the fuel bundle to mitigate the consequences of a Loss of Coolant Accident (LOCA) in the Main Heat Transport (MHT) circuit. To achieve complete passive operation, a passive valve called the Accumulator Isolation Passive Valve (AIPV) is proposed to be incorporated in the High Pressure Injection System (HPIS) of the ECCS. The HPIS consists of four accumulators. There are four Emergency Core Cooling (ECC) Headers and each accumulator is connected through an injection pipe to one ECC Header. The ECC headers are connected to each fuel channel through ECC feeders. A typical sketch of the ECCS HPIS for the AHWR is shown in Fig. 11. The injection pipes between each accumulator and ECC header have isolation valves, check valves and an AIPV. The
7.1. PIPV testing and results
• • • •
Valve functional performance at rated conditions. Valve set pressure actuation. Opening time of the valve. Seat leak test, isolation capability and repeatability.
7.1.1. Opening time of PIPV The opening time of a normally closed (NC) PIPV is measured by LVDT movement attached to the stem of the valve. The high speed Oscilloscope records the LVDT output from valve full close to full open position. The opening time observed was 400 ms as shown in Fig. 9. 7.1.2. PIPV testing at simulated conditions The prototype PIPV is a bellows-actuated and normally closed (NC) valve. At the PPIS experimental test facility, the inlet of the PIPV from the gas tank was pressurized to 30 bar and then the
RD-1 bursting
RD-2 bursting
90
Pressure(bar), Valve opening(mm)
Toward the development of a PIPV for the AHWR, a prototype 2.5 in. size valve has been designed and successfully tested at AHWR simulated conditions at the passive valve test facility (Fig. 8) with the following important objectives;
80 70
Gas tank pr. PIPV position PIPV actuator pr. RD-1 inlet pr. RD-2 inlet pr.
60 50 40 30 20 10
PIPV opening 0 1080
1085
1090
1095
1100
Time (x125 ms) Fig. 10. Test profile at 30 bar gas tank pressure.
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201
Fig. 12. Variation of ECCH pressure.
8.2. AIPV design
Fig. 11. ECCS of AHWR.
ECCS HPIS is designed to provide coolant injection from the accumulators directly into the core for 15 min after a LOCA to ensure adequate core cooling during all phases of the accident.
8.1. Role of AIPV in ECCS During normal reactor operation the MHT/Emergency Core Cooling Header (ECCH) pressure will be 70 bars and accumulator pressure will be 55 bars. Motorized valves will be in an open condition (Fig. 11) and the AIPV will be in a closed condition. When a LOCA occurs, the MHT system pressure starts falling. At 50 bars, the AIPV located between the accumulator and the ECC Header will open. On AIPV opening, due to high nitrogen pressure in the accumulators, cold water enters into the ECC headers and directly into the core. During normal reactor operation, the AIPV serves as a passive isolation device between the accumulator and MHT system. The RELAP analysis for post LOCA scenario with 200% inlet header break is shown in Fig. 12 in terms of variation in ECCH pressure, accumulator pressure and accumulator flow. The analysis was carried out with actual AHWR process line size of 200 NB between the accumulator and ECCH (Srivastava et al., 2008). Initially large flow is required and subsequently the flow should reduce to cool the core for longer duration. The opening of AIPV is governed by resultant pressure difference between accumulator and ECC header acting across the actuator piston. Initially, it opens more as the difference between accumulator and header pressure is at a maximum. As the differential pressure reduces, it starts closing, thereby reducing the discharge flow rate. The discharge flow rate is further controlled through its non-linear plug profile as per process requirement. In this manner, apart from passive isolation of the accumulator and MHT system, the AIPV also serves as a passive flow control device to regulate the accumulator discharge to the MHT in a desired manner to provide longer cooling at reduced flow rate.
The required size of the AIPV for AHWR will be 200 NB. However, a prototype 25 NB size Accumulator Isolation Passive Valve (AIPV) has been designed and developed for qualification testing. It is ANSI 600 class rating, stainless steel bellows-sealed and bellows-actuated globe valve with spring loaded differential pressure actuator. The AIPV is a self-acting type of valve requiring no external energy. The energy in the process fluid pressure is utilized for valve operation thereby achieving its passive design features. Two feedback lines each from upstream and downstream of AIPV will be connected to its differential pressure actuator as schematically shown in Fig. 13. The force balance analysis carried out for design of the valve takes due care of all the pressure forces within valve body and actuator and also due to spring force, bellows force, and packing friction force. 8.3. AIPV testing The developed AIPV was installed at the Integral Test Loop (ITL), as shown in Fig. 14 between the existing accumulator and ECCS header to demonstrate its closed loop performance and operational capabilities under simulated process conditions (Sapra et al., 2010). Following are the important online testing objectives of AIPV at ITL. • Functional performance of the valve at rated simulated conditions.
Fig. 13. Schematic view of the AIPV.
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PI
PI
AIPV
TO ECCS HEADER
FROM ACCUMULATOR
CHECK VALVE
V-2
V-1 PI
V-3 HAND PUMP-2
HAND PUMP-1
Fig. 16. In situ calibration setup for AIPV.
are carbon steel material, ANSI 600 class rating. Upstream of the AIPV is connected to the Accumulator through a NRV. Downstream of the AIPV is connected to the ECCS header through a motor operated valve (MOV). The pressure difference between accumulator and ECCS header is communicated to differential pressure actuator of the AIPV. On ECCS header pressure falling below the accumulator pressure (55 bar), AIPV will start opening. This pressure difference is adjusted to 5 bar maximum using spring adjuster on the valve. The AIPV will continue to open as this differential pressure rises. The AIPV will close, once the pressure on two sides i.e. accumulator and ECCS header gets equalized. Fig. 14. AIPV test setup at Integral Test Loop.
• Verify set pressure opening and closing characteristics such as set point repeatability, accumulation, and blow down. • Demonstrate successful ECCS operation with AIPV. • Verify valve dynamic performance (i.e. travel vs. time measurement at rated conditions). • Characterize the flow regulation with differential pressure change across the valve and its actuator. • Prove in situ calibration capability of the AIPV 8.4. Test setup The piping layout of the AIPV test setup is shown in Fig. 15. The piping used for installing the AIPV in between the accumulator and ECCS header is carbon steel material, 25 NB Sch. 80 pipe. Flanges
8.5. Experiments and results 8.5.1. In situ calibration of AIPV The in situ calibration of installed AIPV was carried out to obtain a change in valve opening with variation in differential actuator pressure. As shown in Fig. 16, the AIPV was isolated from the accumulator and ECCS header by closing respective manual valves. For external pressurization, two portable hand pumps were used. The pressure by hand pump-2 simulates the accumulator pressure and hand pump-1 simulates the ECCS header pressure. By maintaining a constant accumulator pressure, process and ECCS header pressure were reduced from 70 bar to zero to observe the opening of the valve. This demonstrated the regular testing and calibration of the AIPV in the plant without disconnecting or removing the same. The extracted data from calibration results are graphically shown in Fig. 17. The experimental results provide the linearity, repeatability and valve opening characteristics of AIPV.
100 90 80
AIPV opening (%)
70 60
Accumulator pressure 55 bar Accumulator pressure 45 bar Accumulator pressure 40 bar Accumulator pressure 20 bar Accumulator pressure 5 bar Accumulator pressure 0 bar
50 40 30 20 10 0 -70 -60 -50 -40 -30 -20 -10
0
10
20
30
40
Differential actuator pressure (bar) Fig. 15. AIPV test setup piping layout at ITL.
Fig. 17. In situ calibration results of AIPV.
50
60
70
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QOV opens 70
QOV opens 60
50
40 QOV closed 30
20
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Accumulator pressure ECCS header pressure Flow Valve travel
60
50
1.0
40
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30 0.0 20 AIPV opens
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10
Valve opening (mm)
Accumulator pressure ECCS header pressure Valve opening Flow
Pressure (bar), Flow (lpm)
Pressure(bar), Flow(lpm), Valve opening(mm)
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AIPV opens -1.0
0 0
0 0
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400
200
400
600
800
1000
1200
1400
1600
1800
2000
Time (second)
600
Time (second) Fig. 19. High temperature LOCA experimental results. Fig. 18. Cold LOCA experimental results.
8.5.2. Linearity The opening and closing of AIPV is dependent on its differential actuator pressure, which is the difference between accumulator and ECSS header pressure. From in situ calibration results, it is found that at 55 bar accumulator pressure, a non-linearity of 9% exists for “actuator’s differential pressure” of −15 to 45 bars. Based upon the process designer’s inputs, linearity or opening characteristics of the valve can further be fine-tuned by suitable trim design. 8.5.3. Repeatability As per in situ calibration results shown in Fig. 17 for accumulator pressure of 55, 45 and 40 bar, the maximum non-repeatability in valve opening is observed as 1.1 mm. Considering the inaccuracy in measurement of stem movement by general purpose DC LVDT, voltage to current converter and chart recorder, the deviation of 1.1 mm is acceptable. Better quality of bellows and springs used in the valve can improve the aforesaid linearity and repeatability. 8.5.4. AIPV opening characteristics The flow through AIPV depends on its opening, which in turn, depends on differential actuator pressure and AIPV spring setting. Again with less accumulator pressure the amplitude of differential actuator pressure reduces and thus valve opening also reduces. The same is also confirmed by above in situ calibration results. 8.5.5. Simulating cold LOCA This experiment was conducted with process water in both accumulator and ECCS header at ambient temperature. The initial ECCS header and accumulator pressure were maintained respectively at 68 bars and 51 bar. The LOCA simulation was initiated by actuating a quick opening valve (QOV) connected to ECCS header. The QOV, a special purpose valve has fast opening time of 3 ms. On pressure reversal at 51 bar, the AIPV starts opening as desired and proves its functionality as intended. The experiment is terminated with closing of QOV. The process parameter trend recorded during the experimentation is shown in Fig. 18. AIPV opens when header pressure equals accumulator pressure and thereafter pressure reversal occurs. The ECCS header pressure reduces from 51 bars to 10 bars in approximately 130 s. The reason for such large time is limited break size simulation orifice used in this particular experiment, which did not allow pressure to fall instantly. Due to this reason, the differential pressure valve actuator is providing a small valve opening of 3.9 mm and flow of 51.6 LPM as measured by a V-cone flow meter.
8.5.6. Simulating high temperature LOCA After successful results of the cold LOCA experiments at ambient temperature, AIPV testing was carried out at rated conditions of Advanced Heavy Water Reactor (AHWR). For this purpose, the ITL was operated at rated conditions and the loop was maintained at 70 bar pressure and 285 ◦ C temperature. The quick opening valve (QOV) was opened to initiate LOCA experiment at ITL. The process parameter trend recorded during the experiment is shown in Fig. 19. Even after QOV opening, the header pressure falls very slowly due to the fact that the process is not in liquid phase alone. The steam drum, which has steam phase too with water level of 50%, does not allow the process MHT pressure (of ITL) to fall rapidly. During the experiment, with 7% break size simulation, header pressure takes around 500 s to fall and become equal to accumulator pressure. Pressure reversal in Fig. 19 is clearly seen at 500th second. Here AIPV starts opening initiating flow through it. Since there is a very small difference initially between the two actuator pressures, opening of the AIPV is merely 0.35 mm and the maximum flow is 20.1 LPM. At 3800 s, the QOV was closed to terminate the experiment. The noise in flow signal measured by in-house developed V-cone flow meter after 1600 s; is due to delay in MOV closing; which allowed passing of mixed water and nitrogen from accumulator. Though this was not a desirable feature, it is being improved by incorporating a passive isolation mechanism which prevents inadvertent gas injection into ECC header circuit. 9. Conclusion Conventional reactors use active systems, which require signals for control and protection. The reliability of active systems cannot be reduced below a threshold and operator action is required for reactor safety. Considering the severe consequences of accidents in Nuclear Power Plants (NPPs), it is desirable to have passive safety features that do not depend upon external signals or source of power or human actions for successful performance. The inclusion of passive safety systems such as a passive core decay heat removal system, a Passive Poison Injection System and a passive Emergency Core Cooling System in the design of advanced reactors result in a plant, less vulnerable to extreme external events and malevolent acts, thereby providing adequate protection against release of radioactivity outside the plant containment. However, before implementation it is required to demonstrate functionality of these passive systems for successful design and reliability. The experimental results presented here at simulated process conditions demonstrate the successful operation of these systems, which
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are primarily using in-house developed innovative passive valves. With this progress, an important milestone is achieved toward the design and development of different types of passive valves for AHWR application. References Chang, S.H., Kim, S.H., Choi, J.Y., 2013. Design of integrated passive safety system (IPSS) for ultimate passive safety of nuclear power plants. Nucl. Eng. Des. 260 (July). Dureja, A.K., Sapra, M.K., Pandey, R.P., Chellapandi, P., Sharma, B.S.V.G., Kayal, J.N., Chetal, S.C., Sinha, R.K., 2011. Design, analysis and shape optimization of metallic bellows for nuclear valve applications. Trans. SMiRT 21 (November). IAEA TECDOC-626, 1991. Safety Related Terms for Advanced Nuclear Power Plants, September. IAEA TECDOC-1624, 2009. Passive Safety Systems and Natural Circulation in Water Cooled Nuclear Power Plant, November. Nayak, A.K., Sinha, R.K., 2007. Role of passive systems in advance reactors. Progr. Nucl. Energy 49.
Sapra, M.K., Kundu, S., Pal, A.K., Sharma, B.S.V.G., 2007. Functional and Performance Evaluation of 28 bar Hot Shutdown Passive Valve (HSPV) at Integral Test Loop (ITL) for Advanced Heavy Water Reactor (AHWR). BARC Report. Sapra, M.K., Kundu, S., Pal, A.K., Sharma, B.S.V.G., Saha, D., Sinha, R.K., 2010. Design, Development and Testing of 25 NB size Accumulator Isolation Passive Valve (AIPV) for Advanced Heavy Water Reactor. BARC Report. Sinha, R.K., Kakodkar, A., 2006. Design and development of the AHWR – the Indian thorium fuelled innovative nuclear reactor. Nucl. Eng. Des. 236, 683–700. Srivastava, A., Lele, H.G., Ghosh, A.K., Kushwaha, H.S., 2008. Uncertainty analysis of LBLOCA for advanced heavy water reactor. Ann. Nucl. Energy 35 (2), 323–334. Vijayan, P.K., Kamble, M.T., Nayak, A.K., Vaze, K.K., Sinha, R.K., 2013a. Safety features in nuclear power plants to eliminate the need of emergency planning in public domain. Sadhana J. Nucl. Power Progr. India – Past Present Future. Vijayan, P.K., Dhiman, M.K., Kulkarni, P.P., Kamble, M.T., Nayak, A.K., Vaze, K.K., Sinha, R.K., 2013b. AHWR analysis for the post-Fukushima scenarios. In: 22nd National and 11th International ISHMT ASME Heat and Mass Transfer Conference, IIT Kharagpur, India. Yonezo Tujikura, Toshihiro Oshibe, Kazuo Kijima, Kozo Tabuchi, 2000. Development of passive safety systems for next generation PWR in Japan. Nucl. Eng. Des. 201.
Contents lists available at ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Design and development of innovative passive valves for Nuclear Power Plant applications M.K. Sapra ∗ , S. Kundu, A.K. Pal, P.K. Vijayan, K.K. Vaze, R.K. Sinha Reactor Design and Development Group, Bhabha Atomic Research Centre, Department of Atomic Energy, Trombay, Mumbai 400085, India
h i g h l i g h t s • • • • •
Passive valves are self-acting valves requiring no external energy to function. These valves have been developed for Advanced Heavy Water Reactor (AHWR) of India. Passive valves are core components of passive safety systems of the reactor. Accumulator Isolation Passive Valve (AIPV) has been developed and tested for ECSS. AIPV provided passive isolation and flow regulation in ECCS of Integral Test Loop.
a r t i c l e
i n f o
Article history: Received 4 August 2014 Received in revised form 26 February 2015 Accepted 27 February 2015
a b s t r a c t The recent Fukushima accident has resulted in an increased need for passive safety systems in upcoming advanced reactors. In order to enhance the global contribution and acceptability of nuclear energy, proven evidence is required to show that it is not only green but also safe, in case of extreme natural events. To achieve and establish this fact, we need to design, demonstrate and incorporate reliable ‘passive safety systems’ in our advanced reactor designs. In Nuclear Power Plants (NPPs), the use of passive safety systems such as accumulators, condensing and evaporative heat exchangers and gravity driven cooling systems provide enhanced safety and reliability. In addition, they eliminate the huge costs associated with the installation, maintenance and operation of active safety systems that require multiple pumps with independent and redundant electric power supplies. As a result, passive safety systems are preferred for numerous advanced reactor concepts. In current NPPs, passive safety systems which are not participating in day to day operation, are kept isolated, and require a signal and external energy source to open the valve. It is proposed to replace these valves by passive components and devices such as self-acting valves, rupture disks, etc. Some of these innovative passive valves, which do not require external power, have been recently designed, developed and tested at rated conditions. These valves are proposed to be used for various passive safety systems of an upcoming Nuclear Power Plant being designed by India. For example, the Hot Shutdown Passive Valves (HSPV), developed for the decay heat removal system keep the main heat transport system under hot conditions by passively sensing and controlling the system pressure. Another crucial and important valve which has been successfully developed is the Poison Injection Passive Valve (PIPV) for the Passive Poison Injection System. It not only provides higher reliability, but also ensures safe shutdown of the reactor in case of insider threats or malevolent acts in disabling active shutdown system of the reactor. Recently, an innovative valve called the Accumulator Isolation Passive Valve (AIPV) has been developed for the Emergency Core Cooling System (ECCS), which is engineered to mitigate the consequences of Loss of Coolant Accident (LOCA). During normal operation of the reactor, the pressurized accumulators (55 bar) are kept isolated from the reactor core (70 bar) by means of AIPVs. In case of a LOCA, these passive valves open when the main heat transport system pressure falls to a desired value. For prolonged cooling of the core, these passive valves regulate the discharge in a desired manner. These are non-standard, high pressure and high temperature valves, which are unavailable commercially and hence have to be indigenously designed and developed.
∗ Corresponding author. Tel.: +91 22 25591617; fax: +91 22 25505151. E-mail address: [email protected] (M.K. Sapra). http://dx.doi.org/10.1016/j.nucengdes.2015.02.022 0029-5493/© 2015 Elsevier B.V. All rights reserved.
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This paper primarily deals with the design, development and testing of Accumulator Isolation Passive Valves (AIPV) proposed to be used in the ECCS. A 25 NB size AIPV has been designed and successfully tested at Integral Test Loop (ITL) under simulated reactor conditions. It is a self-acting, ANSI 600 rating valve, which requires no external energy (i.e., neither air nor electrical power). It not only provides passive isolation but also passively controls high pressure liquid discharge through it. The design concept of the valve, functional performance, in situ valve testing methodology and the test results at simulated conditions are discussed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
2. Advanced Heavy Water Reactor (AHWR)
Following the Fukushima accident, which occurred on Friday, March 11, 2011, in the Fukushima Daiichi Nuclear Power Plant on the east coast of northern Japan, the safety features of Nuclear Power Plants (NPPs) are being re-examined to demonstrate their capabilities to cope with severe accidents. An increasing need is being recognized to provide systems with passive or intrinsic characteristics which would ensure the continued cooling of fuel and its containment systems (Vijayan et al., 2013a). The use of passive safety systems such as accumulators, condensation and evaporative heat exchangers, and gravity driven safety injection systems eliminate the costs associated with the installation, maintenance and operation of active safety systems that require multiple pumps with independent and redundant electric power supplies. As a result, passive safety systems are being considered for numerous reactor concepts (including in Generation III and III+ concepts) and are expected to find applications in the Generation-IV reactor concepts, as identified by the Generation IV International Forum (GIF). Another motivation for the use of passive safety systems is the potential for enhanced safety through increased safety system reliability (IAEA, 2009). Active safety systems have the advantages of easily allowing a power increase and using proven technologies, on the other hand, while passive safety systems are operated by means of natural phenomena such as gravity, natural circulation and pressure differences. Passive safety systems have the advantages of high reliability, minimal human errors, simplification and easy modularization. Based on advantages and disadvantages of active and passive safety systems, both active and passive safety systems must be installed in Nuclear Power Plants for enhanced safety (Chang et al., 2013). A large scale simplified pressurized water reactor as a candidate for the Japanese Next Generation Pressurized Water Reactor (PWR) has innovative features such as hybrid safety systems (an optimum combination of active and passive safety system) and horizontal steam generators. Here passive safety systems act as backups to prevent core damage if the active safety systems do not operate correctly due to operational or other errors (Yonezo Tujikura et al., 2000). The Advanced Heavy Water Reactor (AHWR) being designed in India incorporates several passive safety systems which will enable the plant to survive potential severe accidents without fuel damage and require no human intervention (Nayak and Sinha, 2007). This paper describes some of the important passive safety systems adopted in the AHWR design. These passive systems use innovative passive valves which are custom designed to suit process requirements and have been developed in-house. The development and testing of passive valves and experimental demonstrations of passive safety systems of the AHWR under simulated process conditions are included in this paper. The development and testing of one of the innovative passive valves called the Accumulator Isolation Passive Valve (AIPV) is discussed in detail.
Advanced Heavy Water Reactor (AHWR) shown schematically in Fig. 1 is a 300 MWe, vertical, pressure tube type, heavy watermoderated, boiling light-water-cooled reactor fueled by dual mixed oxide (MOX) consisting of (PUTh)O2 and (233 U-Th)O2 with a 100 year lifetime. The fuel cluster is designed to generate maximum energy from thorium, to maintain self-sufficiency in 233 U and to achieve a slightly negative void coefficient of reactivity. Natural circulation mode of coolant circulation is adopted during normal operation, transient and accident conditions, which eliminates all accident scenarios resulting from pump failure in addition to reducing capital and operating costs (Sinha and Kakodkar, 2006). 3. Passive safety systems The AHWR incorporates passive systems for core heat removal during normal operation, shutdown conditions and postulated accident conditions such as a Loss of Coolant Accident (LOCA). During normal operation, the core heat is removed by the natural circulation of coolant through hot fuel channels. In Station Black Out (SBO) conditions, the decay heat is removed in a passive mode by Isolation Condensers (ICs) immersed in a large pool of water in a Gravity Driven Water Pool (GDWP), located near the top of the reactor building. During a postulated accident such as a LOCA, high-pressure coolant is directly injected into fuel clusters in a passive mode initially from accumulators and later from GDWP for removal of decay heat. Passive containment cooling and isolation, core submergence, passive poison injection in the moderator by usage of system steam pressure during high pressure transients, and passive cooling of concrete structure in high temperature zones are the additional passive features of AHWR. As per IAEA-TECDOC – 626, the passive safety systems are defined as “Either a system which is composed entirely of passive components and structures or a system which uses active components in a very limited way to initiate subsequent passive operation” (IAEA, 1991). There are four categories to distinguish the different degrees of passivity, namely: Category A, B, C and D as described below;
Fig. 1. Schematic of AHWR.
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Fig. 2. Passive systems of AHWR.
Category A is characterized by: • • • •
No signal inputs of “intelligence” No external power sources or forces No moving mechanical parts, and No moving working fluid. Category B is characterized by:
• • • •
No signal inputs of “intelligence” No external power sources or forces No moving mechanical parts; but Moving working fluids. Category C is characterized by:
• No signal inputs of intelligence. • No external power sources or forces; but • Moving mechanical parts, whether or not moving working fluids are also present.
3.1. Passive core decay heat removal system The main objective is to provide removal of decay heat to an ultimate heat sink after safe shutdown of the reactor. The conventional active decay heat removal systems use water as a cooling medium whose availability cannot be ensured in the event of a Fukushima type long term SBO condition (Vijayan et al., 2013b). The passive decay heat removal system assures core decay heat removal for extended periods of SBO ensuring safety of the reactor. 3.2. Passive Poison Injection System Safe shut down of the reactor is an important safety function which must be ensured to avoid and mitigate accidents of a severe nature. Conventional wired shutdown system depends on active signals (electricity/air). Therefore, this system is vulnerable to the failure of the active signals, inability of plant operators to manage the events, or malevolent acts by an insider. The Passive Poison Injection System passively shuts down the reactor even with failure of wired shutdown systems. 3.3. Passive Emergency Core Cooling System
The examples of Category D safety systems are as follows: • Emergency core cooling/injection systems, based on gravitydriven or compressed nitrogen driven fluid circulation, initiated by fail-safe logic actuating battery-powered electric or electropneumatic valves; • Emergency Core Cooling Systems, based on gravity-driven flow of water, activated by valves which break open on demand (if a suitable qualification process of the actuators can be identified); and • Emergency reactor shutdown systems based on gravity-driven, or static pressure driven control rods, activated by fail-safe trip logic. The passive valves and devices developed for the AHWR fall under Category C (IAEA, 1991). Some of the important passive safety systems of the AHWR that use innovative passive valves are shown in Fig. 2 and are described as follows:
To mitigate the consequences of a LOCA, a passive Emergency Core Cooling System (ECCS) is provided in AHWR. This system provides direct injection of water into fuel cluster in a passive mode during LOCA and removes heat from the fuel. The ECCS water is injected initially from accumulators and later from the GDWP. 4. Passive valves The operation of the above mentioned three passive safety systems is made possible by innovative passive valves called the Hot Shutdown Passive Valves (HSPV), Poison Injection Passive Valve (PIPV) and Accumulator Isolation Passive Valve (AIPV). Since the valves used in these safety systems do not require active power supply (i.e. electrical or pneumatic) for operation, they are known as passive valves. These passive valves are indigenously designed and developed to suit different process requirements of the individual passive safety systems. The salient features of passive valves are as follows:
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• • • • •
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No external power – electrical or pneumatic. Uses process fluid pressure energy for actuation. Provides regulation or isolation as per the process requirement. Bellows sealed and bellows actuated. Double sealed pressure boundary.
Some of the passive valves required for AHWR application, which have been designed, developed and successfully tested and demonstrated at scaled experimental facilities, are listed below. • Hot Shutdown Passive Valve (HSPV) for passive core decay heat removal system, Poison Injection Passive Valve (PIPV) for Passive Poison Injection System, and • Accumulator Isolation Passive Valve (AIPV) for passive Emergency Core Cooling System. Presently, these valves are manufactured for experimental facility use and therefore the test conditions are based upon the facility requirement and not for operating Nuclear Power Plant. Series of tests conducted using these valves have given satisfactory performance and have met all the functional and process requirement of the reactor safety systems. This has given enough confidence to manufacture a new set of passive valves as per ASME Section III NB-3500 to meet in general the test requirement of ASME – QME-1-2012 and our technical specifications. These valves will be nuclear grade valves and will undergo stringent functional testing and qualification program including thermal cycling, pressure cycling, vibration and seismic testing. Since there are no rubber or non-metallic components; radiation aging is not envisaged at this stage.
5. Hot Shutdown Passive Valve (HSPV) AHWR has a large pool of water called Gravity Driven Water Pool (GDWP) at top of the reactor primary containment. The isolation condensers are immersed in the GDWP and connected to the Main Heat Transport System (MHTS), forming a natural circulation loop through a HSPV and active valve in parallel. During hot shutdown, the steam drum pressure rises due to decay heat. The HSPV opens on high steam drum pressure to establish communication between the steam drum and the IC, rejecting decay heat to the GDWP water. Once the steam drum pressure lowers to a normal value, the HSPV closes. Thus, hot shutdown condition can be maintained for prolonged periods of SBO. The schematic of the passive core decay heat removal system with HSPV is shown in Fig. 3.
Fig. 3. Decay heat removal system and Passive Poison Injection System of AHWR.
5.1.1. In situ calibration of HSPV The in situ calibration (i.e. online field calibration) of the HSPV was carried out by pressurizing its actuator from an external pump. Actuator pressure was increased and the valve movement was recorded with an LVDT output to generate valve opening and closing characteristics as shown in Fig. 5. The experimental data shows the linearity, controllability and hysteresis of the developed passive valve which was found to be within acceptable limits. 5.1.2. Simulation of hot shutdown and decay heat removal AHWR station blackout (i.e. class III and class IV power supply failure) will lead to the plant shutdown. Due to decay heat generation after reactor shutdown, the steam drum (SD) pressure rises. Under these conditions, decay heat will be removed using IC through the HSPV. This capability was verified during testing by simulating hot shutdown at 14 bar pressure. The HSPV was set at
5.1. HSPV testing and results In the process of developing a Hot Shutdown Passive Valve (HSPV) for AHWR, a prototype 2 in. size ANSI 600 passive valve, suitable for 28 bar pressure has been designed and successfully tested at 220 ◦ C in Integral Test Loop (ITL) at the institute shown in Fig. 4 (Sapra et al., 2007). Following tests were conducted: • • • • • • •
Station Black Out simulation. Functional testing of HSPV at rated conditions. Decay heat removal function. Controllability, linearity, hysteresis and repeatability of HSPV SD pressure control using HSPV Valve performance at different pressure and flow conditions. In situ calibration of HSPV.
Fig. 4. HSPV installed at ITL.
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110 100 valve opening valve closing
90
Full opening 21.6 bar
80
Start closing 21 bar
HSPV open (%)
70 60 50 40 30 20
Full reclosing 16.8 bar
10
Start opening 18 bar
0 -10 13
14
15
16
17
18
19
20
21
22
23
24
25
Steam drum pressure (bar) Fig. 5. In situ calibration of HSPV
Fig. 7. Bellows Test Facility (BTF) at IGCAR.
14 bar opening pressure and the operating power of the ITL was reduced to 50 kW. As shown in Fig. 6, HSPV opening occurred for 5 min duration. This maintained the primary main heat transport (MHT) of the ITL in the HOT condition and the plant in shutdown condition with SD pressure at 14 ± 0.5 bar. After 6 h, the pressure again rises above set point of 14 bars and the HSPV opens momentarily, which brings down the system pressure to acceptable levels. This simulation test successfully demonstrated the functional performance requirement for the HSPV and decay heat removal system of the AHWR.
Fig. 6. Experimental results for decay heat removal.
6. Qualification and testing of valve bellows Metallic bellows are widely used in the valve industry in lieu of dynamic seals or packing to permit stem travel and sealing with zero leakage. The passive valves described here also use bellows in their actuator for gland sealing. These bellows are generally made out of thin metallic sheets manufactured using the hydro-forming process. They can perform very well under high pressure and temperature conditions. Structural integrity of the bellows is required for overall system reliability. Bellows are generally designed based on guidelines given by Expansion Joint Manufacturer Association (EJMA). However, EJMA only supports standard U-shaped/toroidal bellows. The nuclear industry faces stringent requirements of high pressure and large displacement to match reactor operating conditions and required valve flow conditions. High pressure requires large thickness, but it introduces high axial stiffness, which is not desirable. For this diverse requirement, shape of the bellows needs to be optimized and multiple layer construction is favored to reduce the resulting alternating stress levels which govern the fatigue life of the bellows. Stainless steel grade 304L, if used as the material of construction, needs six plies of 0.5 mm thickness. But this introduces very high axial stiffness and cannot meet the required design criteria. The material of construction was changed to INCONEL-600 and was found to be satisfying both the requirements of stresses as well as axial stiffness with two plies of 0.5 mm thickness. The bellows design was evaluated using finite element method. A parametric finite element model was developed to quickly incorporate any dimensional or shape changes during the design stage. A procedure was developed to determine membrane and bending stresses from the component stresses computed using the finite element model. This procedure was validated against published literature values (Dureja et al., 2011). The shape of the bellows plays a major role in the componentdeveloped membrane and bending stresses. Various shapes were modeled and analyzed and it was found that an omega shaped bellows with different lobe radii outperform the conventional U shaped bellows. Based on the above studies, omega shaped bellows were procured from reputable suppliers and subjected to cyclic test to determine the fatigue life. A dedicated Bellows Test Facility (BTF) as shown in Fig. 7 has been designed, developed and commissioned to test 19 bellows at a time. A given test set of bellows is externally pressurized and heated to the design temperature. Axial
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Fig. 9. Experimental results for response time. Fig. 8. Test facility for PIPV
displacement of ±10.0 mm was applied for cyclic testing. The first bellows assembly consisting of three bellows has successfully passed the required number of 25,000 cycles. Further tests are planned to meet requirement of ASME Section III Appendix II for testing of 5 bellows assemblies subjected to 65,000 cycles. 7. Poison Injection Passive Valve (PIPV) The Passive Poison Injection System (PPIS) provides the passive means of reactor shutdown through a passive valve called the Poison Injection Passive Valve (PIPV). AHWR has two independent active shutdown systems, one comprising the mechanical shut off rods and the other employing injection of a liquid poison in the low pressure moderator, both requiring active signals for reactor shutdown. As shown in Fig. 3, during normal reactor operation, the PIPV isolates high pressure helium gas tank and poison tank. Failure of two active (wired) shutdown systems results in increased main heat transport (MHT) system pressure beyond a predetermined value that bursts rupture disks RD-1 and 2, which in-turn actuates the PIPV. The opening of the PIPV initiates an inrush of high pressure helium gas into the poison tank to push the poison into the moderator, thereby shutting down the reactor.
pressure upstream of the rupture discs was raised up to 86 bars of nitrogen pressure. As soon as the actuator pressure reached 84 bar, the two rupture disks in series prior to the PIPV actuator burst at 84 bar (RD-1) and 82 bar (RD-2). This action pressurized the valve actuator to open the valve instantaneously. A series of such tests were conducted to obtain the performance objectives mentioned above. One of the test profiles with upstream pressure, downstream pressure, actuator pressure and valve opening time is shown in Fig. 10. 8. Accumulator Isolation Passive Valve (AIPV) In the AHWR, the Emergency Core Cooling System (ECCS) provides direct injection of coolant into the fuel bundle to mitigate the consequences of a Loss of Coolant Accident (LOCA) in the Main Heat Transport (MHT) circuit. To achieve complete passive operation, a passive valve called the Accumulator Isolation Passive Valve (AIPV) is proposed to be incorporated in the High Pressure Injection System (HPIS) of the ECCS. The HPIS consists of four accumulators. There are four Emergency Core Cooling (ECC) Headers and each accumulator is connected through an injection pipe to one ECC Header. The ECC headers are connected to each fuel channel through ECC feeders. A typical sketch of the ECCS HPIS for the AHWR is shown in Fig. 11. The injection pipes between each accumulator and ECC header have isolation valves, check valves and an AIPV. The
7.1. PIPV testing and results
• • • •
Valve functional performance at rated conditions. Valve set pressure actuation. Opening time of the valve. Seat leak test, isolation capability and repeatability.
7.1.1. Opening time of PIPV The opening time of a normally closed (NC) PIPV is measured by LVDT movement attached to the stem of the valve. The high speed Oscilloscope records the LVDT output from valve full close to full open position. The opening time observed was 400 ms as shown in Fig. 9. 7.1.2. PIPV testing at simulated conditions The prototype PIPV is a bellows-actuated and normally closed (NC) valve. At the PPIS experimental test facility, the inlet of the PIPV from the gas tank was pressurized to 30 bar and then the
RD-1 bursting
RD-2 bursting
90
Pressure(bar), Valve opening(mm)
Toward the development of a PIPV for the AHWR, a prototype 2.5 in. size valve has been designed and successfully tested at AHWR simulated conditions at the passive valve test facility (Fig. 8) with the following important objectives;
80 70
Gas tank pr. PIPV position PIPV actuator pr. RD-1 inlet pr. RD-2 inlet pr.
60 50 40 30 20 10
PIPV opening 0 1080
1085
1090
1095
1100
Time (x125 ms) Fig. 10. Test profile at 30 bar gas tank pressure.
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Fig. 12. Variation of ECCH pressure.
8.2. AIPV design
Fig. 11. ECCS of AHWR.
ECCS HPIS is designed to provide coolant injection from the accumulators directly into the core for 15 min after a LOCA to ensure adequate core cooling during all phases of the accident.
8.1. Role of AIPV in ECCS During normal reactor operation the MHT/Emergency Core Cooling Header (ECCH) pressure will be 70 bars and accumulator pressure will be 55 bars. Motorized valves will be in an open condition (Fig. 11) and the AIPV will be in a closed condition. When a LOCA occurs, the MHT system pressure starts falling. At 50 bars, the AIPV located between the accumulator and the ECC Header will open. On AIPV opening, due to high nitrogen pressure in the accumulators, cold water enters into the ECC headers and directly into the core. During normal reactor operation, the AIPV serves as a passive isolation device between the accumulator and MHT system. The RELAP analysis for post LOCA scenario with 200% inlet header break is shown in Fig. 12 in terms of variation in ECCH pressure, accumulator pressure and accumulator flow. The analysis was carried out with actual AHWR process line size of 200 NB between the accumulator and ECCH (Srivastava et al., 2008). Initially large flow is required and subsequently the flow should reduce to cool the core for longer duration. The opening of AIPV is governed by resultant pressure difference between accumulator and ECC header acting across the actuator piston. Initially, it opens more as the difference between accumulator and header pressure is at a maximum. As the differential pressure reduces, it starts closing, thereby reducing the discharge flow rate. The discharge flow rate is further controlled through its non-linear plug profile as per process requirement. In this manner, apart from passive isolation of the accumulator and MHT system, the AIPV also serves as a passive flow control device to regulate the accumulator discharge to the MHT in a desired manner to provide longer cooling at reduced flow rate.
The required size of the AIPV for AHWR will be 200 NB. However, a prototype 25 NB size Accumulator Isolation Passive Valve (AIPV) has been designed and developed for qualification testing. It is ANSI 600 class rating, stainless steel bellows-sealed and bellows-actuated globe valve with spring loaded differential pressure actuator. The AIPV is a self-acting type of valve requiring no external energy. The energy in the process fluid pressure is utilized for valve operation thereby achieving its passive design features. Two feedback lines each from upstream and downstream of AIPV will be connected to its differential pressure actuator as schematically shown in Fig. 13. The force balance analysis carried out for design of the valve takes due care of all the pressure forces within valve body and actuator and also due to spring force, bellows force, and packing friction force. 8.3. AIPV testing The developed AIPV was installed at the Integral Test Loop (ITL), as shown in Fig. 14 between the existing accumulator and ECCS header to demonstrate its closed loop performance and operational capabilities under simulated process conditions (Sapra et al., 2010). Following are the important online testing objectives of AIPV at ITL. • Functional performance of the valve at rated simulated conditions.
Fig. 13. Schematic view of the AIPV.
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PI
PI
AIPV
TO ECCS HEADER
FROM ACCUMULATOR
CHECK VALVE
V-2
V-1 PI
V-3 HAND PUMP-2
HAND PUMP-1
Fig. 16. In situ calibration setup for AIPV.
are carbon steel material, ANSI 600 class rating. Upstream of the AIPV is connected to the Accumulator through a NRV. Downstream of the AIPV is connected to the ECCS header through a motor operated valve (MOV). The pressure difference between accumulator and ECCS header is communicated to differential pressure actuator of the AIPV. On ECCS header pressure falling below the accumulator pressure (55 bar), AIPV will start opening. This pressure difference is adjusted to 5 bar maximum using spring adjuster on the valve. The AIPV will continue to open as this differential pressure rises. The AIPV will close, once the pressure on two sides i.e. accumulator and ECCS header gets equalized. Fig. 14. AIPV test setup at Integral Test Loop.
• Verify set pressure opening and closing characteristics such as set point repeatability, accumulation, and blow down. • Demonstrate successful ECCS operation with AIPV. • Verify valve dynamic performance (i.e. travel vs. time measurement at rated conditions). • Characterize the flow regulation with differential pressure change across the valve and its actuator. • Prove in situ calibration capability of the AIPV 8.4. Test setup The piping layout of the AIPV test setup is shown in Fig. 15. The piping used for installing the AIPV in between the accumulator and ECCS header is carbon steel material, 25 NB Sch. 80 pipe. Flanges
8.5. Experiments and results 8.5.1. In situ calibration of AIPV The in situ calibration of installed AIPV was carried out to obtain a change in valve opening with variation in differential actuator pressure. As shown in Fig. 16, the AIPV was isolated from the accumulator and ECCS header by closing respective manual valves. For external pressurization, two portable hand pumps were used. The pressure by hand pump-2 simulates the accumulator pressure and hand pump-1 simulates the ECCS header pressure. By maintaining a constant accumulator pressure, process and ECCS header pressure were reduced from 70 bar to zero to observe the opening of the valve. This demonstrated the regular testing and calibration of the AIPV in the plant without disconnecting or removing the same. The extracted data from calibration results are graphically shown in Fig. 17. The experimental results provide the linearity, repeatability and valve opening characteristics of AIPV.
100 90 80
AIPV opening (%)
70 60
Accumulator pressure 55 bar Accumulator pressure 45 bar Accumulator pressure 40 bar Accumulator pressure 20 bar Accumulator pressure 5 bar Accumulator pressure 0 bar
50 40 30 20 10 0 -70 -60 -50 -40 -30 -20 -10
0
10
20
30
40
Differential actuator pressure (bar) Fig. 15. AIPV test setup piping layout at ITL.
Fig. 17. In situ calibration results of AIPV.
50
60
70
203 2.0
QOV opens 70
QOV opens 60
50
40 QOV closed 30
20
1.5
Accumulator pressure ECCS header pressure Flow Valve travel
60
50
1.0
40
0.5
30 0.0 20 AIPV opens
-0.5
10
10
Valve opening (mm)
Accumulator pressure ECCS header pressure Valve opening Flow
Pressure (bar), Flow (lpm)
Pressure(bar), Flow(lpm), Valve opening(mm)
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AIPV opens -1.0
0 0
0 0
200
400
200
400
600
800
1000
1200
1400
1600
1800
2000
Time (second)
600
Time (second) Fig. 19. High temperature LOCA experimental results. Fig. 18. Cold LOCA experimental results.
8.5.2. Linearity The opening and closing of AIPV is dependent on its differential actuator pressure, which is the difference between accumulator and ECSS header pressure. From in situ calibration results, it is found that at 55 bar accumulator pressure, a non-linearity of 9% exists for “actuator’s differential pressure” of −15 to 45 bars. Based upon the process designer’s inputs, linearity or opening characteristics of the valve can further be fine-tuned by suitable trim design. 8.5.3. Repeatability As per in situ calibration results shown in Fig. 17 for accumulator pressure of 55, 45 and 40 bar, the maximum non-repeatability in valve opening is observed as 1.1 mm. Considering the inaccuracy in measurement of stem movement by general purpose DC LVDT, voltage to current converter and chart recorder, the deviation of 1.1 mm is acceptable. Better quality of bellows and springs used in the valve can improve the aforesaid linearity and repeatability. 8.5.4. AIPV opening characteristics The flow through AIPV depends on its opening, which in turn, depends on differential actuator pressure and AIPV spring setting. Again with less accumulator pressure the amplitude of differential actuator pressure reduces and thus valve opening also reduces. The same is also confirmed by above in situ calibration results. 8.5.5. Simulating cold LOCA This experiment was conducted with process water in both accumulator and ECCS header at ambient temperature. The initial ECCS header and accumulator pressure were maintained respectively at 68 bars and 51 bar. The LOCA simulation was initiated by actuating a quick opening valve (QOV) connected to ECCS header. The QOV, a special purpose valve has fast opening time of 3 ms. On pressure reversal at 51 bar, the AIPV starts opening as desired and proves its functionality as intended. The experiment is terminated with closing of QOV. The process parameter trend recorded during the experimentation is shown in Fig. 18. AIPV opens when header pressure equals accumulator pressure and thereafter pressure reversal occurs. The ECCS header pressure reduces from 51 bars to 10 bars in approximately 130 s. The reason for such large time is limited break size simulation orifice used in this particular experiment, which did not allow pressure to fall instantly. Due to this reason, the differential pressure valve actuator is providing a small valve opening of 3.9 mm and flow of 51.6 LPM as measured by a V-cone flow meter.
8.5.6. Simulating high temperature LOCA After successful results of the cold LOCA experiments at ambient temperature, AIPV testing was carried out at rated conditions of Advanced Heavy Water Reactor (AHWR). For this purpose, the ITL was operated at rated conditions and the loop was maintained at 70 bar pressure and 285 ◦ C temperature. The quick opening valve (QOV) was opened to initiate LOCA experiment at ITL. The process parameter trend recorded during the experiment is shown in Fig. 19. Even after QOV opening, the header pressure falls very slowly due to the fact that the process is not in liquid phase alone. The steam drum, which has steam phase too with water level of 50%, does not allow the process MHT pressure (of ITL) to fall rapidly. During the experiment, with 7% break size simulation, header pressure takes around 500 s to fall and become equal to accumulator pressure. Pressure reversal in Fig. 19 is clearly seen at 500th second. Here AIPV starts opening initiating flow through it. Since there is a very small difference initially between the two actuator pressures, opening of the AIPV is merely 0.35 mm and the maximum flow is 20.1 LPM. At 3800 s, the QOV was closed to terminate the experiment. The noise in flow signal measured by in-house developed V-cone flow meter after 1600 s; is due to delay in MOV closing; which allowed passing of mixed water and nitrogen from accumulator. Though this was not a desirable feature, it is being improved by incorporating a passive isolation mechanism which prevents inadvertent gas injection into ECC header circuit. 9. Conclusion Conventional reactors use active systems, which require signals for control and protection. The reliability of active systems cannot be reduced below a threshold and operator action is required for reactor safety. Considering the severe consequences of accidents in Nuclear Power Plants (NPPs), it is desirable to have passive safety features that do not depend upon external signals or source of power or human actions for successful performance. The inclusion of passive safety systems such as a passive core decay heat removal system, a Passive Poison Injection System and a passive Emergency Core Cooling System in the design of advanced reactors result in a plant, less vulnerable to extreme external events and malevolent acts, thereby providing adequate protection against release of radioactivity outside the plant containment. However, before implementation it is required to demonstrate functionality of these passive systems for successful design and reliability. The experimental results presented here at simulated process conditions demonstrate the successful operation of these systems, which
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are primarily using in-house developed innovative passive valves. With this progress, an important milestone is achieved toward the design and development of different types of passive valves for AHWR application. References Chang, S.H., Kim, S.H., Choi, J.Y., 2013. Design of integrated passive safety system (IPSS) for ultimate passive safety of nuclear power plants. Nucl. Eng. Des. 260 (July). Dureja, A.K., Sapra, M.K., Pandey, R.P., Chellapandi, P., Sharma, B.S.V.G., Kayal, J.N., Chetal, S.C., Sinha, R.K., 2011. Design, analysis and shape optimization of metallic bellows for nuclear valve applications. Trans. SMiRT 21 (November). IAEA TECDOC-626, 1991. Safety Related Terms for Advanced Nuclear Power Plants, September. IAEA TECDOC-1624, 2009. Passive Safety Systems and Natural Circulation in Water Cooled Nuclear Power Plant, November. Nayak, A.K., Sinha, R.K., 2007. Role of passive systems in advance reactors. Progr. Nucl. Energy 49.
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