introduction to sodium technology – heat transport …nptel.ac.in/courses/103106101/module -...

NPTEL – Chemical Engineering – Nuclear Reactor Technology

Joint Initiative of IITs and IISc – Funded by MHRD of 9

Introduction to sodium technology – Heat transport system (secondary circuit)

K.S. Rajan

Professor, School of Chemical & Biotechnology

SASTRA University



Table of Contents 1 SECONDARY CIRCUIT .................................................................................................................. 3 1.1 STEAM GENERATOR ................................................................................................................................... 3 1.2 SAFETY GRADE DECAY HEAT SYSTEM (SGDHS) ................................................................................ 4

2 STEAM-‐WATER CIRCUIT ............................................................................................................ 6 3 ELECTROMAGNETIC PUMPS ..................................................................................................... 7 2 REFERENCE/ADDITIONAL READING ..................................................................................... 9



This lecture will focus on the description of secondary circuit of heat transport system in a fast reactor At the end of this lecture, learners will be able to

(i) list the components of secondary circuit (ii) understand the geometry and working of steam generator (iii) list the material used for the manufacture of steam generator (iv) constraints in the design of steam generator for PFBR

1 Secondary circuit Let us discuss secondary circuit taking the example of 500 MWe PFBR. The important components of secondary circuit of heat transport system in a sodium cooled fast reactor are steam generators, secondary sodium pumps, intermediate heat exchangers (common with primary circuit) and surge tank. Two steam generators are used for each intermediate heat exchanger and hence in total, eight steam generators are required for PFBR. In other words, four steam generators are used per primary sodium loop or pump. This number for FBTR was arrived based on cost optimization after taking into account of cost for capital, operation and breakdown in case of a water leak in one of the steam generators in the loop. Under these conditions, the other three steam generators can be used for power generation.

1.1 Steam generator Steam generator is a vertical shell and tube type, with sodium occupying the shell side (outside the tubes) while water flows through the tubes. The tubes can be either straight tubes or helical coils or serpentine type. One of the key constraints is the need to prevent sodium-water contact, which may be caused by leakage of water into the sodium side. Hence, considering the ease of defect-free manufacture, straight tubes are used in PFBR. To account for thermal expansion, each tube is provided with an expansion bend. The steam generator is a once-through evaporator with water passing through the tube only once during their passage in steam generator. With the once-through water flow in steam generator, the quantity of water in the steam generator is less. This serves to minimize the consequences of sodium-water reaction. Secondary sodium enters the steam generator at 798 K and leaves at 528 K. Water enters the steam generator at 508 K and leaves at 763 K and at about 165 atm. A schematic diagram of steam generator used in PFBR is shown in Figure 1. Each steam generator has 547 stainless steel tubes with outer diameter of 17.2 mm and thickness of 2.3 mm resulting in inner diameter of 12.6 mm. The tubes are 23 m



long and are welded to the tubesheets at the top and bottom. The material used for the construction of steam generator is the modified ‘9Cr-1Mo ferritic steel’. The steam generator is designed to meet the service life of 40 years.

Fig 1. Schematic diagram of Steam Generator used in PFBR (Ref. [1])

1.2 Safety Grade Decay Heat System (SGDHS) Apart from ensuring the generation of steam for power production from the nuclear energy, steam generators and intermediate heat exchangers serve to remove the decay heat after a normal shutdown. The maximum capacity of this operational grade decay heat removal system is 20 MWt. If the plant encounters a scenario where there is a power failure either on-site or off-site, a dedicated system called safety grade decay



heat system (SGDHS) is used to remove the decay heat. The SGDHS (shown in Figure 2) consists of four independent loops, each rated for 8 MWt.

Fig 2. Schematic diagram of the Safety Grade Heat Removal System (Ref. [3])

SGDHS contains a decay heat exchanger (sodium-sodium heat exchanger) immersed in the pool of the inner vessel (hot sodium pool) and a sodium/air heat exchanger. Sodium-sodium heat exchanger is a vertical, countercurrent, shell & tube heat exchanger with primary sodium from the pool entering the shell side and intermediate sodium passing through the tubes. The heat exchanger consists of an inner and outer shell with the annular region consisting of tube bundle. Cold sodium flows in the



downward direction in the hollow inner shell. Upon reaching the annular region, sodium flows in the upward direction inside the tubes. When the SGDHS is activated, cold sodium enters the decay heat exchanger immersed in the pool and extracts decay heat transferred from fuel to the primary sodium. As the density of sodium decreases with temperature, the cold sodium from sodium/air heat exchanger replaces the lighter, hot sodium. To facilitate the natural convection in SGDHS of PFBR, the sodium-sodium heat exchanger and air-sodium heat exchanger are separated by 42 m in the vertical direction. The hot sodium is cooled in sodium/air heat exchanger and returns to sodium/sodium heat exchanger. Air-sodium heat exchanger is a finned tube heat exchanger with cross flow of air and sodium. Dampers (two at the inlet and two at the outlet) present in the sodium/air heat exchanger are motor-operated with provisions for manual operation as well. Other parts of SGDHS operate under natural circulation and hence represent a passive system. The dampers are controlled to ensure that sodium is not cooled below its freezing point. As a measure of precaution, dampers are closed when the sodium temperature falls to 433 K (160 °C), well above its freezing point. In case of fire due to sodium leak in air-sodium heat exchanger, nitrogen is supplied to the heat exchanger casing from a dedicated nitrogen source. In such a case, the sodium is drained to the storage tank. It may be recalled that in the primary circuit, the sodium pumps (primary sodium pumps) are located inside the pool, beneath the inner vessel and in the cold primary sodium. But, the secondary sodium pump is located outside the reactor vessel. The secondary sodium pumps are located in the cold leg of the secondary sodium i.e. between the secondary sodium outlet in the steam generator and the secondary sodium inlet to the intermediate heat exchanger. The secondary sodium pumps are vertical, single-stage centrifugal pumps.

2 Steam-water circuit A schematic diagram of steam-water circuit is shown in Figure 3. Steam-water circuit consists of three turbines: high-pressure, intermediate pressure and low-pressure. High-pressure turbine is run by steam at 165 atm, while the intermediate pressure turbine is run by 30 atm steam. The spent steam from the low-pressure turbine is close to atmospheric pressure and enters the condenser. To ensure that there is a continuous supply of coolant to condense the spent steam and reuse the condensate, the condenser must be supplied with a perennial water source. In PFBR, sea water around 303 K is pumped to the condenser. A 10° change in the temperature between the sea water outlet from the condenser and the sea water inlet to the condenser is chosen for this purpose. The condensate is heated in a low-pressure heater, before being passed through a deaerator. Deaerator servers to remove the dissolved gases present in condensate water. The presence of dissolved gases in water reduces the heat transfer coefficient, due to the poor thermal conductivity of gases. This would result in the reduction in heat transfer rate and underutilization of heat generated. Water is then



pumped to high-pressure heaters through boiler feed pump. The temperature of water is increased to 508 K and is ready to be supplied to steam generator for extraction of heat from secondary sodium.

Fig 3. Schematic layout of the steam-water circuit (Ref. [2])

The steam reheat cycle is chosen owing to the advantages of simplified design, reduced capital costs and use of turbine with proven performance and reliability. Most of the fast breeder reactors operating in the world use steam reheat cycle for this purpose. If one observers the steam pressure and temperature (165 atm & 763 K), it is evident that the steam is superheated. (Note: Saturated steam is the one at a temperature equal to the saturation temperature corresponding to the steam pressure. Steam at a temperature above saturation temperature corresponding to the steam pressure is superheated steam). Superheated steam is chosen to harness the potential of achieving higher coolant temperatures with sodium-cooled reactors. It may also be recalled that higher steam temperature increases the thermodynamic efficiency of the plant also.

3 Electromagnetic pumps



Electromagnetic pumps may also be employed for handling liquid sodium. Sodium, apart from being a good conductor of heat, is a good electrical conductor also. The electrical resistance of liquid sodium increases with temperature as shown by the following equation:

𝜌 Ω𝑚 = 6.1405𝑋10!! + 3.5047𝑥10!!"𝑇 + 5.6885𝑋10!!"𝑇!

+ 1.66797𝑋10!!"𝑇! The low electrical resistance or good electrical conductivity of liquid sodium is utilized in designing a pump for the same. Most of us would have learnt that when a current carrying conductor is placed in a magnetic field, a force is exerted on the conductor causing it to rotate. This is the principle of working of electric motors. Similarly when a pipe carrying a conducting fluid is placed in a magnetic field orthogonal to the direction of the liquid and current passed through the liquid perpendicular to it, an electromagnetic force is created that acts on the liquid causing the liquid to move (Figure 4).

Fig 4. A schematic diagram showing the principle of operation of electromagnetic pump

This principle is used in the electromagnetic pumps, which are normally suited for liquid metals. A schematic diagram showing the direction of application of magnetic field, current flow and liquid metal flow is given in Figure 4. Electromagnetic pumps

Magnetic field

Current



were used in some fast breeder reactors in the secondary loop and in certain decay heat removal systems. Electromagnetic pumps are less-efficient when compared to the centrifugal pumps. However, electromagnetic pumps are highly reliable and require little maintenance owing to the absence of moving parts. These pumps are used in auxiliary circuits for filling and draining sodium. The impure sodium can be pumped to the cold trap using such electromagnetic pumps.

2 Reference/Additional reading 1. Laxman T. Patil, A.W. Patwardhan, G. Padmakumar, G. Vaidyanathan, Distribution of liquid sodium in the inlet plenum of steam generator in a Fast Breeder Reactor, Nuclear Engineering and Design, 240 (4) (2010) pp. 850-859) 2. S.C. Chetal, V. Balasubramaniyan, P. Chellapandi, P. Mohanakrishnan, P. Puthiyavinayagam, C.P. Pillai, S. Raghupathy, T.K. Shanmugham, C. Sivathanu Pillai, The design of the Prototype Fast Breeder Reactor, Nuclear Engineering and Design, 236 (2006) pp. 852-860. 3. Sodium Fast Reactor Design: Fuels, Neutronics, Thermal-Hydraulics, Structural Mechanics and Safety, in: Vol. 21, Handbook of Nuclear Engineering, Dan Gabriel Cacucu (Ed. In Chief), Springer. 3. http://www.igcar.ernet.in/events/anup2010/TK%20Mitra.pdf 4. http://www.igcar.ernet.in/events/anup2010/Nandakumar.pdf 5. http://www.iaea.org/NuclearPower/Downloads/Technology/meetings/2011-12-21-

22-TM-FR/6_India-IHX-for-PFBR-and-future-FBRs.pdf 6. http://www.ne.doe.gov/pdfFiles/SodiumCoolant_NRCpresentation.pdf 7.http://www.if.uidaho.edu/~gunner/Nuclear/LectureNotes/Lecture18_Reactor_Physics.pdf 8. http://www-pub.iaea.org/mtcd/meetings/PDFplus/2009/cn176/cn176_Presentations/plenary_session_3/KN-02.Sorokin.pdf

introduction to sodium technology – heat transport …nptel.ac.in/courses/103106101/module -...

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