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ScienceDirect
Energy Procedia 7 (2011) 609-615
Asian Nuclear Prospects 2010
Operating Experience of High Temperature Sodium Loops for Material Testing
M. Shanmugavel, S.Vijayaraghavan, P.Rajasundaram, T. Chandran, M.Shanmugasundaram, K.K.Rajan and P. Kalyanasundaram
Fast Reactor Technology Group, Indira Gandhi Center for Atomic Research, Kalpakkam 603 102, India
Abstract
Two independent sodium loops under common name INSOT facilities were constructed in Fast Reactor Technology Group, IGCAR for conducting material testing of PFBR components in dynamic sodium. One loop is utilized for in-sodium Low Cycle Fatigue (LCF) and Creep - Fatigue Interaction (CFI) studies and the second loop for in sodium creep studies. The loop components and piping were constructed using AISI type 316LN/316L material. The sodium loops were constructed by adopting stringent quality control measures stipulated in relevant international codes. Suitable creep and fatigue testing chambers were designed and developed indigenously. The test chambers were qualified for sodium service before commencement of experiments by conducting trial runs. The sodium purity was achieved by on-line purification method prior to admitting into the test chambers and the purity was maintained during testing. Characterization of sodium in the loops during test run were carried out by foil equilibration technique, electro-chemical carbon meter and chemical analysis of sodium sample drawn from the loop using state of art sampling techniques. A versatile PLC based instrumentation system has been successfully implemented for monitoring and controlling the process parameters. Safety interlocks were provided by hardware logic to ensure reliability.
© 2011 Published by El sevier Ltd. Selection and/or peer-review under responsibility of Indra Gandhi Centre of Atomic Research
Key words: Sodium; material; operation; purification; PLC; safety; high temperature
1. Introduction
In the Prototype Fast Breeder Reactor (PFBR) under construction at Kalpakkam, liquid sodium is used as a heat transfer medium in primary and secondary circuits. The Facility for Materials Testing InSodium (INSOT) was constructed to study the mechanical properties of PFBR component materials under the influence of flowing sodium. INSOT facility comprises creep and fatigue testing facilities.
^Corresponding author. Tel.: +91 44 27480311; fax: +91 44 27480311 E-mail address: _msv@igcar.gov.in
1876-6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Indra Gandhi Centre of Atomic Research doi:10.1016/j.egypro.2011.06.081
These experiments are essential to generate indigenous data on mechanical properties in-sodium like creep, Low Cycle Fatigue (LCF), Creep-Fatigue Interaction (CFI) and tribology studies for the PFBR. Both the loops were operated at high temperature of 873K and experiments have been conducted simulating the reactor conditions. The creep and fatigue loops have logged around 17500 hours and 13500 hours respectively. This paper gives the salient features of the facilities and enumerates the experience gained in operating them for conducting the PFBR material testing at high temperature in dynamic liquid sodium environment.
1.1. Salient features
The sodium loop consists of dump tank, expansion tank, Electro-Magnetic Pump (EMP), sodium to sodium heat exchanger, air cooler, test sections, bellows sealed globe valves, PM flow meters, level probes, thermocouples, leak detectors and instrumentation control systems. The loop has been broadly classified into hot and cold leg region from the functional point of view. The cold leg region (below 673K) comprises of sodium purification and impurity monitoring systems. The hot leg region (above 673K) comprises of creep test sections in the case of creep loop and fatigue test sections in the case of fatigue loop.
2. Sodium System piping and Components
The design of the dump tank, expansion tank etc have been followed as per the pressure vessel code ASME section VIII Division I for internal and external pressure. The design of the SS piping has been carried out as per the ASME B31.1 Power piping code[6]. The piping and components for the sodium system have been fabricated, tested and erected as per standards.
The sodium loops are constructed using type SS316LN/316L grade 15NB, 25NB and 40NB sch40 size pipes. SS304L materials have also been used in few piping locations of cold leg region. All the horizontal running pipes are provided with a slope of 3% to ensure sodium drain by gravity [ 1].
All welding joints have been carried out by GTAW process. The welding procedure and performance qualification for the welders were carried out as per ASME Section IX. All weld joints were visually inspected at all stages to ensure defect free weld. Liquid Penetrant Inspection (LPI) was carried out for root and final passes of the weld joints as per ASTM - E -165 and ASME Section-III. The acceptance criteria was followed as per ASME Section-III (Para NB 5350). All sodium vessels/ components and piping butt weld joints have been subjected to 100% radiography. The argon cover gas carbon steel piping joints were subjected to 10°% radiography. The X-ray radiography were carried out as per techniques recommended in ASTM - E-94 and ASTM - E -142. The acceptance criteria was followed as per ASME Section III (Para NB 5320). A pneumatic proof test was carried out at a maximum test pressure of 5 kg/cm2 (g) for the CS piping and 12.5 kg/cm2 (g) for the SS piping systems respectively in accordance with UG-99 of ASME Section -VIII Division-I using argon gas. The cleaning, passivation, rinsing and drying of the SS materials were carefully followed as per ASTM standards[7]. Sodium components/ piping were then subjected to helium leak test under pressure to ensure the leak tightness of the entire system. The HLT was carried out as per ASME Section-V. The acceptance criteria for helium leak test was a leak rate less than 10-8 Std .cc.atm/s ec.
2.1. Sodium SS Piping Flexibility analysis
The piping flexibility analysis of the INSOT sodium piping loops was carried out for the as-built condition. The loops have been analyzed for flexibility and sustained load as per ASME Power Piping Code, B31.1-1998[6] and CAESER-II®, Version- 4.3 piping software was used for the analysis. The stresses in the loop were found to be within allowable limits.
2.2. Argon cover gas system
Purified argon is used as cover gas to provide an inert gas blanket over the free sodium surface, secondary containment of test chambers and to provide the operational functions i.e., maintaining the required pressure in the system, sodium filling and dumping function. An argon gas purification system with BASF® and molecular sieve arrangement ensures that the commercial grade argon gas has moisture and oxygen levels within acceptable limits. An equalized argon cover gas pressure of 0.3 kg cm-2 (g) was maintained in the dump tank and expansion tank. Carbon steel piping as per ASTM A106
Gr.B has been used for the argon cover gas circuit. Pressure relief valve has been provided in the argon manifold to limit the argon gas pressure in the cover gas system within 1 kg/cm2 (g). The complete draining of the sodium loop piping, test chambers and components after dumping of the loop is ensured by conducting systematic argon gas purging and gas communication checking in all flow paths.
2.3. Sodium loop components
The dump tank is located in a dump pit lined with carbon steel. It serves as a sodium storage tank with a total hold up of around 500 liters of liquid sodium. Dump tank is provided with low, medium, high and very high level indications to monitor the sodium levels. Heat exchangers, heater vessel, degassing chamber, fatigue and creep test chambers, carbon meter and foil equilibration chamber have been installed in hot leg region where as cold trap, plugging indicator, air cooler, a sampler device and oxygen meter have been installed in cold leg region. The expansion tank is located at an elevation of eleven meters. It provides net positive suction head to the EM pump and accommodates the expansion of liquid sodium in the loop. Two auto transformer controlled ALIP type EM pumps of capacity of 5 m3/hr of sodium at 5 bar each have been installed in series for creep loop where as in fatigue loop one pump is available to circulate the sodium in the loop.
2.4. Safety measures
Leak collection trays have been positioned at the bottom of each component and loop area. Spark plug type leak detectors have been provided for leak collection trays, bellow sealed valves, secondary containment on the test chambers. The SS pipelines and equipments are provided with wire type leak detectors on the surface. Smoke detectors and SID are provided to detect the sodium fire, if any in the system. Adequate quantity of Dry Chemical Powder (DCP) fire extinguishers have been made available in the loop area in case of sodium fire if any. The loop area is provided with exhaust fans to ensure proper ventilation to maintain the ambient temperature rise of the loop area below 5oC. The loop has been operated by qualified and trained personnel . Fast dump switch is provided in control room /loop area for fast dumping of the sodium in the loop in case of an emergency. The dumping is by gravity and the dumping time is less than 10 minutes.
2.5. Instrumentation system
The loop operation has been simplified by using state of the art electronics and instrumentation system. Programmable Logic Controllers (PLC's), Personal Computers (PC's) and Field Access Units (FAU's) are the major constituents of data acquisition and control system of the test facility. PLC's have been programmed to suit the operational requirements. Sensors are connected to FAU's which are mounted close to the measuring points. FAU's as input devices reduce cabling required between field sensors and PLC's. The real time values of the leak, temperature, flow, events, alarms, equipment status were monitored, controlled for both loops and refreshed in the computer screen for the ease of operation. The signals are monitored both at field by FAU display and at the control room by PLC, PC based SCADA system. The healthiness of all the sensors, control logics were ensured by conducting the periodic surveillance as per technical specifications. Class-I, Class-II, Class-III and Class-IV power supply systems are provided to meet safety requirements. K-type chromel-alumel thermocouples are used to measure the temperatures in the pipelines, valves, components, test chambers etc. and monitored, controlled by PLC. The surface heaters are grouped into sections based on operational requirement. This average temperature value is compared with the set point of the respective group. Based on the feedback, PLC turns "ON" or "OFF" the heaters in that group. The dump and expansion tanks are provided with resistance type discontinuous level probes for measurement of sodium level. The level appearance or disappearance is indicated in the control room by mimic panel lamp indication and hooter annunciation. The Sodium Ionization Detector (SID) which was indigenously developed at IGCAR, has been installed in the loop area to detect sodium leak and fire if any.
2.6. Challenges in testing chambers
The test chambers have been indigenously designed and developed for low cycle fatigue and creep testing in sodium. The test chambers comprises of inner bellow encapsulated with test specimen, outer bellow assembly, provision for LVDT assembly [Fig.1]. The design of test chambers to suit the testing
Fig .1.Fatigue test chamber
machine, suitability of bellows for high temperature sodium service and correct measurement of strain values were some of the challenges successfully overcome. The performance of the test chambers are excellent.
3. Commissioning Experience
The fatigue loop was commissioned in 2002 and creep loop was commissioned in 2003. The loop was subjected to pressure hold test at 3 bar for the duration of 48 hours. The piping supports were checked for the desired setting and location as per flexibility requirements. The regeneration of the argon purification system was carried out, the purity of argon from the purification chambers was checked and the oxygen and moisture levels were found within acceptable limits. Cold purging operation was conducted number of times to reduce the oxygen level to acceptable limits in the system. The hot purging operation was conducted by using argon at 523K in order to achieve the oxygen and moisture levels within acceptable limits. Leak detection, level detection and data acquisition systems were made operational and checked for their performance. 500 liters of liquid sodium was transferred from Large Component Test Rig transportation tank to the dump tanks of fatigue and creep loops. All equipments and systems were checked for their performance as per design intent. The loop was filled as per operation procedure [2], sodium was purified to reactor grade and further program of testing were taken up. The loop was operated in isothermal run at 573K and subsequently hot run of 823K or 873K operation were commenced. The temperature was raised in steps of 25 oC using immersion heaters .
3.1. High temperature operation experience
The sodium loop has been operated continuously to conduct the fatigue and creep experiments at 823 K and 873 K. The schematic block diagram for creep and fatigue loops are shown in [Fig. 2] and [Fig.3] respectively. On line sodium purification has been carried out during loop operation to keep the oxygen level in sodium to below 2 ppm by continuous operation of the cold trap. The cold traps which were getting plugged at the outlet nozzle have been replaced with a modified version. The performance of the present cold trap is excellent and there is no plugging problem. The sodium flow rate through the cold trap was maintained at 200 liters per hour. The sodium was cooled by air blowers and the air flow was controlled by dampers. The cold point temperature was maintained at 120oC during experiment. The impurity level in the sodium was confirmed by taking the plugging runs periodically during loop operation. The periodic sampling of loop sodium is carried out periodically to analyze for the various metallic and non-metallic impurities. The impurity levels of oxygen, carbon and trace elements are determined by chemical analysis. In-sodium carbon activity measurement has been carried out by foil equilibration method.
Fig. 2 .Schematic block diagram for Creep loop
The fatigue and creep loops were operated in different campaigns and the performance of the major components have been assessed. The U-tube shaped shell and tube type sodium to sodium heat exchangers has been used in the test sections of both the loops. It separates the hot and cold leg regions of the sodium system. The residual heat of the sodium returning from the test
Fig. 3 . Schematic block diagram for Fatigue loop
chamber is transferred from the shell side to the sodium entering the immersion heater vessel on the tube side. The temperature rise on the tube side in this counter flow heat exchanger is nearly 523K. Due to very good performance of this regenerative exchanger; less heat input is required in the heater vessel to raise the temperature of the sodium to test condition. The Annular Linear Induction Pump (ALIP) type EM Pump is used for the circulation of liquid sodium. The winding temperature was maintained below 70oC using centrifugal blowers and controlled by variable frequency drives. The preheating of pump was ensured before sodium filling and the forbidden working zone was avoided by logic control. The maximum voltage of the EM pump to achieve the required flow in the test sections was around 300V. There were a few minor sodium leak incidents in INSOT and they are efficiently detected and the corrective measures were taken to restore loop operation with less downtime.
The SCADA software "powerscope" was developed with vendor support and further refined to suit the operational requirement. Individual screens for leak, temperature, flow signals, equipment status, PLC online status, plugging run graph, alarms, events, shift log status were created for providing clear and precise data in a convenient graphical interface for the operation personnel.
3.2. Experiments in the facility
The experiments in creep and fatigue have been conducted with direct involvement of the scientists of the Material and Metallurgy Group (MMG) of IGCAR. The procedure for installation, connecting to sodium loop and removal of the test chambers has been streamlined. There are four test sections in creep loop and two test sections in fatigue loop for conducting experiments. Sodium purity was maintained at reactor grade by continuous operation of the cold trap. The temperature in test section; either at 823K or 873K was precisely maintained by PID controllers within ±1°C and sodium velocity was maintained at 2.5 m/s around the specimens in the test chambers. The test specimen is
encapsulated with primary SS bellow and sodium is circulated through annular space around the test specimen.
3.3. Fatigue experiments
Dynamic LCF tests were conducted at 823K or 873K sodium temperatures at different stress levels on indigenous materials such as SS316L(N), modified 9Cr-1Mo base metal and their weld joints. The application of test load cycles is done using servo-hydraulic machines. The first few test campaigns were conducted to check the performance of the testing chamber developed in house [Fig.3]. The LCF tests conducted on indigenous materials in flowing sodium environment at 823K and 873K have shown that the LCF life in sodium environment are significantly improved when compared to the data obtained in air environment [3]' In fatigue loop all the planned fatigue experiments have been successfully completed.
3.4. Creep experiments
Creep tests were conducted at 873K sodium temperature on indigenously developed SS316L (N) material to determine the creep rupture life. The tests were carried out on test specimens positioned in test chambers [Fig. 4] developed indigenously. The initial creep tests in sodium indicate that the material life in sodium at higher stress level is almost equal to life in air, whereas the life in sodium at low stress level is substantially high [4].
Fig. 4.Creep test chamber
4. Conclusions
The sodium loops were operated at high temperature for material testing of PFBR components. The loop has been logged around 17500 hours and 13500 hours for creep and fatigue loop respectively. In fatigue loop all the planned fatigue experiments were successfully completed at 823K and 873 K with different strain amplitudes. In creep loop, few experiments were completed and experiments are being carried out at different stress levels. Overall experience in operating the sodium loops at high temperature for the above material testing are excellent and both the sodium loops are performing well.
Very good experience has been gained in operating the sodium loops at high temperature for testing on indigenous materials [5].
Acknowledgement
The author's wish to thank Director, IGCAR and Director, MMG for providing the constant encouragement and support. The role of all the members of INSOT facility (IFS), Fast Reactor Technology Group (FRTG), IGCAR in operating and maintaining the sodium loop and related systems, Mechanical Metallurgy Division (MMD) scientists and testing personnel in conducting in-sodium experiments is gratefully acknowledged.
References
[1] M.Shanmugavel, M.Shanmugasundaram, P.Shanmugam, M.P.Mishra and S.L.Mannan, Safety report on In-sodium test facility.
[2] M.Shanmugavel, M.Shanmugasundaram, T. Chandran S.Vijayaraghavan, P.Rajasundaram, K.K.Rajan and P.Kalyansundaram, Operation manual for creep and fatigue loops of INSOT Facility.
[3] R.Sandhya, R.Kannan, V.Ganesan, M.Valsan, K.Bhanu Sankara Rao, S.L.Mannan, Evaluation of the Effect of Dynamic Sodium on the Low Cycle Fatigue Properties of 316L(N) Austenitic Stainless Steel, Proceedings of the International Conference & Exhibition on Pressure Vessels and Piping, "OPE 2006", Chennai, February 2006
[4] S.Ravi, K.Laha, M.D.Mathew, S.Venugopal, K.Bhanu Sankara Rao, K.K.Rajan, M.Rajan, S.L.Mannan, Experimental facility for Creep rupture testing in flowing sodium, Proceedings of the International conference & Exhibition on Pressure Vessels and Piping, "OPE 2006", Chennai, February 2006.
[5] S.Vijayaraghavan, M.Shanmugasundaram, M.Shanmugavel, K.K.Rajan, S.Venugopal, K.Bhanusankara Rao and M. Rajan,, "Operating experience of In-Sodium Test facility", Proceedings of the National conference on "Operating Experience of Nuclear Reactors and Power Plants' -"OPENUPP-2006" November, 2006, Mumbai.
[6] ASME B 31.1 - Power Piping code, 1998 edition.
[7] ASTM standards, 1998 edition.
