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Design and full scale test of the fuel handling system
Nuclear Engineering and Design 218 (2002) 169 /178 www.elsevier.com/locate/ned
Design and full scale test of the fuel handling system J.G. Liu *, H.L. Xiao, C.P. Li Institute of Nuclear Energy Technology, Tsinghua University, Beijing 100084, PR China
Abstract In the 10 MW High Temperature Gas-cooled Reactor-Test Module (HTR-10) fuel elements move through the core driven by gravity. To reach their design burn-up the fuel elements are re-shuttled five times. This transportation outside the core is mainly achieved pneumatically. Although, adopting the international experience at design and operation of similar systems some key components were improved so that the fuel handling system (FHS) becomes simpler and more reliable. The improved components were tested in full-scale testing facilities. The debugging test and the first loading operation for the FHS indicate that the FHS meets the demands of the HTR-10. In this paper, the functions, design parameters, technological processes, main components and design characteristics of the FHS are described in detail. The flow schemes, design parameters of the full-scale testing facilities and the experimental results are briefly introduced. # 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The fuel handling system (FHS), one of the key systems of the pebble bed reactor, must ensure that the reactor operates continuously. Most of the components in the FHS are moving, and work at high temperatures and pressure, in helium atmosphere and under intense radiation. These severe conditions pore upon high demands on the design of functions and transmission components in the FHS. Large attention was paid to some important problems, for example, the helium seal, the bearing * Corresponding author. Tel.: /86-10-62770238x375; fax: / 86-10-69771464 E-mail address: [email protected] (J.G. Liu).
lubrication, the radioactivity shield, as well as the reliability and the maintainability of components. If a main component in the FHS would fail, normal operation of the reactor would be affected, and the availability of the reactor would be decreased. According to the actual features of the small experimental reactor, some key components were improved, based on the experience in design and operation of the similar components in the AVR so that the FHS became simpler, safer and easier to maintain (Hantke and Buelling, 1990; Hennings, 1969; Ziermann, 1975). The improved components were fully tested on full-scale testing facilities. The debugging test of the FHS and the operation of the first loading indicate that the FHS is reliable.
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2. Functions and technological processes of the FHS 2.1. Functions
Table 1 Principal parameters of the FHS No. Designation 1
The HTR-10 is designed to use spherical fuel elements. Its FHS is different from the refueling machines of reactors using rod shaped or block shaped fuel elements. The main feature of the FHS is to charge, recirculate and discharge fuel elements in the course of the reactor operation. The further functions are as follows: 1) To load the initial core. 2) To remove fuel elements from the fuel discharge tube of the lower part of the core. 3) To return the fuel elements to the core which have not reached the maximum burn-up. 4) To sort fuel element fragments and defective fuel elements, and drop them into the failed fuel cask. 5) Discharge of the spent fuel elements and transfer to the shipping casks. 6) To charge the new fuel elements into the fuel cycle. 2.2. Technological processes Table 1 summarizes the principal parameters of the FHS. The schematic diagram of fuel circulation of the FHS is shown in Fig. 1 (Liu, 2001). Three subsystems are especially note worthy. 2.2.1. Feed subsystem for the new fuel elements Before new fuel elements (10 or five balls every bag) are fed in the core, they are stored temporarily in two tanks in the charging room. When they are needed to be fed into the core, new they are taken out from a tank and put into the glove box with a sealing lock. The fuel elements roll gradually to the elevator by gravity. The glove box is kept at negative pressure while being fed with fuel elements. There are three isolation valves, one counter and two electromagnetic sliding valves in the feed line. Every two adjacent ones of the three isolation valves is in interlocking. They could only be closed synchronously, and could not be opened synchronously for atmosphere exchange in order
2 3
4 5 6 7 8 9 10 11 12
Parameter
Fast circulation
Approximately 350 balls per hour Conventional circulation Approximately 50 balls per hour The number of fuel elements fed Approximately 125 per equivalent full-power day balls (EFPD) New fuel elements fed per EFPD Approximately 25 balls Average burn-up measuring time Approximately 60 s per fuel element Operating pressure Approximately 3.0 MPa Operating pressure after the first Approximately 0.2 charging isolation valve MPa Operating pressure ahead of the Approximately 0.2 third discharging isolation valve MPa Operating temperature 150 /180 8C Fuel element capacity of failed Approximately 1000 fuel cask balls Fuel element capacity of buffer Approximately 30 line balls Spent fuel element capacity of Approximately 2000 shipping cask balls
to isolate the primary coolant from the atmosphere. Twenty-five new fuel elements in the feed tube of the elevator are transported individually to the core by means of purified helium through the electromagnetic valve. 2.2.2. Circulation subsystem for the fuel elements The spherical fuel elements move by gravity from top to bottom of the reactor core, and put in the discharge tube with an inside diameter of 500 mm. The discharge tube is connected to a reducer at the bottom head of the reactor pressure vessel. The fuel elements form a ‘sphere bridge’ at the bottom outlet of the lower tank body of the reducer. Pulsed airflow puts intermittent impact force on the fuel elements to remove the ‘sphere bridge’ when discharge is demanded, so that the fuel elements are discharged one by one through a removal tube. Then the fuel elements arrive at a cam with an opening in the failed fuel separator. Whenever the failed fuel separator turns once one fuel element drops on the roller of the separator
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Fig. 1. The schematic diagram of the fuel circulation.
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from the opening. Therefore, single transportation of the fuel elements can be sustained. The separator for failed fuel sorts out fuel element fragments and defective elements, which drop into the failed fuel cask. Whole fuel elements roll to the elevator in which the burn-up measurement is performed. Depending on the result of the burn-up measurement, the control system gives instruction to rotate the distributive tray of the elevator towards transporting tubes for its object. By means of gas pressure, partially depleted fuel elements are transferred through the elevation tube into the core, while spent fuel elements are transferred through the discharge tube of spent fuel element into the discharge buffer line. 2.2.3. Discharge subsystem for the spent fuel elements The discharge operation will start when the number of the spent fuel elements in the discharge buffer line reaches 25 balls. Firstly, the first isolation valve is closed, then the atmosphere in the tubes between every two neighboring isolation valves is exchanged. After the atmosphere exchange is completed, the elements arrive at the distributor and wait for distributing. At the beginning of the reactor operation, the graphite elements that have been charged into the initial core need to be discharged. The control system gives the distributor instructions to transport the elements according to the result of the burn-up measurement system. The spent fuel elements are discharged into the spent fuel shipping casks while the graphite elements are discharged into the graphite element containers. The distributor will complete its mission when all graphite elements are discharged from the reactor core.
reliability become very important. Their functions, structure and principle are described as in the following. 3.1. Pulsed airflow discharge components The lower part of the core, whose shape is like a funnel, is connected with the fuel element discharge tube. The reducer is located at its end. The structure of the reducer is shown in Fig. 2. It consists of the upper tank body, the lower tank body, a large funnel, a small funnel, pulsed airflow tubes and a removal tube. At the bottom head of the reactor pressure vessel, the lower end of the steel tube with an inside diameter of 500 mm in the upper tank body changes its direction and forms a fuel tank. Its bottom has an inclination angle of 158. Owing to the action of the friction, the height of the elements is maintained within a fixed range in the large funnel, thus, they cannot spill over. The inlet of the small funnel is jointed on the outlet of the large one. The outlets of the pulsed airflow tubes are located on the wall of the small funnel. The outlet diameter of the large funnel is so small that the fuel elements will naturally form a ‘sphere bridge’. Compressed gas in the pulsed airflow generator is suddenly released at a high speed
3. Description of main components The main components of the FHS include the pulsed airflow discharge components, failed fuel separator, elevator, isolation valve, counter, distributor, spend fuel shipping cask, seal mechanism of cover, control system, etc. They are the most important components of the FHS in order to perform its functions. Hence their safety and
Fig. 2. The structure of the reducer.
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when the fast-acting valve in the pulsed airflow generator operates. An impulsive force acts on the elements of the ‘sphere bridge’. The gravity and the static friction force between the element surfaces are exceeded and the elements begin to move towards the removal tube. If the impulsion is repeated many times, the elements in the fuel tank fall continuously one by one into the entrance tube and on the annular cam of the failed fuel separator. Due to the use of the pulsed airflow discharge reducer, there are no moving machinery components, which need to be disassembled and maintained in the fuel tank and the pressure vessel. Only the pulsed airflow tubes and the removal tube lead into the vessel, other corresponding components are installed outside the vessel.
3.2. Failed fuel separator The main function of the failed fuel separator is to sort out the scrap fragments and defective elements, which might hinder the transport of fuel elements. Another function is to allow only single transportation of a string of elements in the feed tube. The separator is composed of a shell, a roller, a measuring track board, a failed fuel funnel, a magnetic driver, a decelerator, a motor and others. The measuring track board and the roller are assembled horizontally with a gap of 56.5 mm. The failed fuel funnel is assembled below the roller in the shell. There is an annular cam with an opening at the initial end of the roller. The breach is used as a fuel element single-exit gate. Only one element falls on the surface of the roller and goes into the measuring area when the separator makes a turn. Between the surfaces of the measuring track board and the roller, the element moves and rotates continually randomly toward the end of the roller by the use of the inclined teeth. The defective elements and the fragments fall into the funnel from the gap of the measuring area and roll down into the scrap container. Whole elements move to the end of the roller, then into the inclined tube and finally into the elevator.
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3.3. Elevator The elevator has the following functions: 1) To elevate new, partially depleted and spent fuel elements individually. 2) To locate the fuel elements which need burnup measurement. The elevator consists of a shell, a distributive tray, a magnetic driver, a decelerator, a stepping motor, a rotating encoder, etc. On the distributive tray, one end of the loading channel is aimed at the gas line, another might be aimed at one of loading fuel line, the elevating line to the core and the buffer line of the spent fuel elements. The receivedball cup on the end of the loading channel is also a localizer that accommodates only one element. The distributive tray rotates from measuring position to receiving position at the beginning of operation, it rotates 458 counter clockwise after receiving a fuel element and goes back to the measuring position. After measurement, the fuel element will be reloaded into the core if it has not reached the target burn-up, otherwise the distributive tray will rotate 908 clockwise and the fuel element will be elevated to the buffer line of the spent fuel element. The tray rotates 908 in reverse to its measuring position after elevating. The tray swings only between the receiving position and the measuring position when new fuel elements are elevated. Burn-up measurement is not needed during loading of new fuel elements. 3.4. Isolation valve The isolation valves are installed on the pressure boundary of the primary loop, this ensures its integrity. An atmosphere exchange could happen gradually when the valves are installed in the lines of atmosphere exchange, therefore gas in the lines can be isolated stage by stage, to prevent the air from entering and the helium gas from flowing out. The valves are a kind of straight passage ball valve with a full sealing structure. A mechanical limit block is assembled on the valve head so as to ensure a rotation of 908. The design torque of the
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magnetic driver and preset torque of the electric actuator are 400 Nm. The fuel elements might pass through it successfully when the valve is opened for 10 s. 3.5. Counter Counters are installed on important positions of the FHS (Muncke, 1976). Their functions are to record the number of fuel elements that pass through various measuring stations and to monitor the filling level of the lines for purposes of control and recording. The counters are composed of sensors and secondary instrumentation. Each sensor consists of two coils, which are wound around a ceramic tube. The ceramic tube is assembled in a steel pressure shell. The leading wires of the sensor coils are led to the outside of the shell via electrical penetrations. An inductive bridge is used in the counter. When an electrically conducting fuel element passes through the magnetic field of the coils it induces eddy currents which alter the impedance of the coils and balance of the bridge. An inductive voltage signal is generated in the coils due to this change of magnetic field, and the signal is amplified and demodulated. Besides the use of counting, it can be used to find out the moving status and direction of the fuel element.
single-inlet gate in the piston rod carrying one ball is aimed at one of two removal tubes. The graphite elements and the spent fuel elements will be discharged into their respective containers. The processes described above take place in an air atmosphere and at approximately 0.4 MPa. 3.7. Spent fuel shipping cask The function of the spent fuel-shipping cask is to safely store spent fuel elements. It consists of a steel liner, an outside shell, a lead shield, a cask plug (or cask cover), a cask neck, a board cover, etc. There are many steel support bars between the steel liner and the outside shell to obtain a reasonable gap to fill lead. One cask can hold approximately 2000 fuel elements. When filled, the cask is sealed with a board cover and stored in the spent fuel storehouse. The spent fuel elements in the cask may be removed by pump feed method. 3.8. Seal mechanism for the cover of the spent fuel shipping cask A seal mechanism for the cover of the spent fuelshipping cask is installed in a small cavity that is located in the spent fuel storehouse. Its structure is shown in Fig. 3. It fulfils the following functions:
3.6. Distributor
1) To take out the cask plug of the cask neck. 2) To put the cask plugs into the cask neck.
The distributor is installed after the third discharging isolation valve. Its function is to distribute graphite elements and spent fuel elements into correspondent storage containers. It includes an air cylinder, two locating pistons, two direction-changing pistons, a piston rod, some change-valves, etc. The direction-changing piston with a piston rod is assembled within the locating piston. The piston rod has a single-inlet gate, which can hold only one ball. The single-inlet gate is on the receiving position when compressed air enters two cells of the air cylinder simultaneously. The change-valve acts are controlled by the control system, which allows only admission of one cell and exhaust of another cell. The directionchanging pistons are pushed to one side. The
It consists of a main cylinder, two assistant cylinders, a claw mechanism, a removal tube, a sliding socket, some pilot valves, etc. It is actually a pneumatic manipulator using compressed air. The control unit of the compressed air is fixed into a control box, which mainly includes electromagnetic valves, pressure-reducing valves, pressure switch, pressure meters, one-wag valves, etc. The piston in the main cylinder is joined to the claw mechanism (includes a pair of claw, a claw cylinder, a piston and a cam) through a piston rod, and may move to and back vertically. When the piston in the main cylinder moves to its upper limit, it is locked. Normally, the claw is opened by the spring action. When the cask plug needs to be taken out, the compressed air enters from the
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Fig. 3. The structure of the seal mechanism.
lower end of the claw cylinder, to make the claw fold. Then the piston in the main cylinder moves to its lower limit, in the meantime the claw goes into the inverse ‘T’ shaped hole of the cask plug. Subsequently the lower end of the claw cylinder bleeds air, so that the claw opens and catches the plug. Finally, the cask plug moves towards its upper limit with the piston elevating in the main cylinder up to the exit of the removal tube. The piston in the main cylinder moves towards its lower limit after the shipping cask is filled, and the cask plug is put into the cask neck. The task of the assistant cylinder is to seal the cask neck using the sliding socket in order to prevent active dust being released.
(IPC) was designed. The PLC has high precision, reliability and anti-interference. The IPC has welladapted person /computer interface and abundant software resources, therefore it is convenient to implement management and inspection of the system, monitor and analysis of device state. In accordance with the technical specification of the FHS, the control system controls the movement of the fuel elements in the pipeline via opening and closing sequences of the various valves. The system also controls operation of various mechanical devices e.g. sorting out of defective elements, elevating of fuel elements and controlling the distribution. To ensure qualified operation under technical requirement, the pressure sensors monitoring the He pressure, the temperature sensors measuring the He temperature in the FHS, the counters recording the number of the fuel elements that pass through various measuring station and monitoring the motion state of the fuel elements were installed. The control system collects the real-time data above mentioned, and analyses, estimates and carries out the corresponding signal to control the operation of various devices. The state monitoring, data analyses, effective software management and failure diagnosis to give the alarm before failure have been also implemented to avoid misoperation. The structural diagram of the control system designed is shown in Fig. 4. This shows the control system consisting of PLC, IPC and the display device.
3.9. Control system In order to ensure the FHS to work reliably, the control system based on the programmable logic controller (PLC) and industrial personal computer
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Fig. 4. Structural diagram of the control system.
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4. Full-scale test Three full-scale testing facilities have been operated since 1986, in which the following research and development were finished: 1) The flowing behavior of the pebble bed in the HTR-10 was studied on. The results of the flowing residence spectra and the relative speed distributions of the pebble bed are gained, and obtained data were used in the construction design of the FSH and the physical calculation of the core. 2) Examinations of some new components were carried out and correct design parameters were determined, design and operation experiences were gained also.
This paper describes only one of the three testing facilities. The testing facility for the fuel circulating system is shown in Fig. 5. Its main components are designed and manufactured on the basis of the prototype size. Fuel transportation was performed by gravitational and pneumatic means. In addition the component that uses the pulsed airflow to break bridging to discharge the balls in a single-line way from the discharge tube is first applied to a pebble bed reactor. Table 2 summarizes the main parameters of the testing facility. 4.1. Circulation system for the graphite balls The graphite balls in the discharge tube move from top to bottom by gravity, then they roll into
Fig. 5. Flow scheme of the full scale testing facility for the fuel circulating system.
J.G. Liu et al. / Nuclear Engineering and Design 218 (2002) 169 /178 Table 2 Main parameters of the testing facility No. Designation
Parameter
1 2 3 4 5
0.5 m 4.2 m 4.0 m 0.5 m 60 balls per hour
6 7 8 9 10 11
Discharge tube inside diameter Discharge tube height Maximal loading height Ball tank width Feed speed of circulating graphite balls Operating pressure Operating temperature Working pressure of pulsed airflow Operating times of pulse Rotation speed of failed ball separator Diameter of graphite ball
0.1 MPa 150 /180 8C 0.3 MPa 30 times per hour 1 rpm 60 mm
the ball tank through the outlet in the discharge tube bottom, and accumulate in the large funnel of the ball tank bottom. Due to the friction, their height is maintained within a fixed range in the large funnel. Here a ‘sphere bridge’ on the exit is formed and stops the motion. The pulsed airflow impacts the ‘sphere bridge’ intermittently, so that the graphite balls are discharged one by one. The counter records the number of the discharged balls. The discharged balls drop in the failed ball separator through a vertical removal tube, where fragments and defective balls are sorted out, and fall in the scrap container through the funnel of the damaged balls. The counter records the number of damaged balls. Undamaged balls roll to the elevator by inclination. Two transportation modes were designed. They are modes 1 (main circulation) and 2 (discharge circulation). For transportation mode 1, the elevator distributive tray with the ball rotates to the elevating position one counter clockwise, the transportation gas brings the ball back to the discharge tube and the counter records the number of the elevated ball. For transportation mode 2, only two balls are circulated repeatedly so that the discharging process is simulated. The distributive tray with one ball rotates to the elevating position one clockwise, two balls are elevated to the single-exit device. One ball rolls back to the elevator, the
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other still stays in front of the single-exit device. The circulating testing of two balls between the elevator and the single-exit device may be carried out repeatedly, under this circulation mode. In front of each bend of the elevating tube a buffer was installed to reduce the impact speed of the balls. 4.2. The auxiliary helium system The auxiliary helium system offers helium to the pulsed airflow discharge reducer, the elevator and the single-exit device, and sustains helium flow in the circulating system. The operating pressure of approximately 0.1 MPa is sustained in the ballcirculating loop. The pressure in the loop will continuously increase when the pulsed airflow discharge reducer, the elevator and the single-exit device are working, so the helium compressor must draw helium from the loop. The drawn helium is filtered to remove the graphite dust and other impurities. Then it is cooled to 50 8C by an air cooler. After dehydration by the drier, the helium flows into the lower pressure container, is compressed again, and then enters the high-pressure container. The pressure of helium from the highpressure container is decreased to the preset pressure via the pressure-reducing valve. Part of the helium is transported to the elevator through the regulator valve and electromagnetic valve, and the rest to the pulsed airflow generator. Helium from lower pressure container may directly be used as the control gas resource of the single-exit device. A vacuum pump is used to obtain the needed vacuum for the circulating system, and a watercooled system is applied to the cooling of the compressor. 4.3. Electric heating system The electric heaters were installed outside the discharge tube, failed fuel separator and elevator. All tubes and functional components in the graphite ball circulating system were wrapped in the thermal insulation materials. The temperature in the circulating system is measured and controlled by the thermocouples and automatic con-
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trol system of temperature, and is kept in suitable range.
About 160,000 graphite balls have been treated in the pulsed airflow discharge reducer and about 5000 balls in the failed ball separator.
4.4. Control system The control system makes use of the PLC, computer and simulation screen. PLC has the advantage of the multifunction, agilely to operate, easy to maintain, and excellent stability, etc. The computer can monitor the operation status of the PLC. A large-scale mosaic simulation screen displays the operating status of all components, the temperature and the pressure in main components, and ball information through each measurement point. The moving direction and position of the ball can be displayed. The control system consists of two independent ball circulating systems and a temperature control system. Both manual and automatic modes are used. The manual mode implements the start and stop of all electric devices, without being affected by the signals from the pressure, temperature, counter, etc. 4.5. Test This full scale testing facility had been successfully operated for over 500 h. The main components of the FHS have fully been tested in the factory and the laboratory after manufactured.
5. Conclusions It has been proved by practice during the full scale test, a debugging test and the first loading that the fuel handling system described above meets all demands of the HTR-10.
References Hantke, H.J., Buelling, H., 1990. Fuel Feed System, Design and Experiment, AVR-Experimental High-temperature Reactor, Duesseldorf, pp. 187 /202. Hennings, U., 1969. Fuel handling system for core elements of a pebble-bed reactor. Nuclear Applications and Technology 7, 334 /341. Liu, J.G., 2001. Development of Fuel Handling System in 10 MW High Temperature Gas-Cooled Reactor. Proceedings of the Seminar on HTGR Application and Development, Beijing, China. Muncke, A., 1976. Graphite ball detectors for the fuel handling machine of a helium-cooled pebble bed reactor. Kerntechnik 18, 201 /206. Ziermann, E.L., 1975. Operating Experience with the AVR Experimental Power Station. Proceedings of a Symposium Jointly Organized by the IAEA and the OECD Nuclear Energy Agency and Held in Juelich, Germany.
Design and full scale test of the fuel handling system J.G. Liu *, H.L. Xiao, C.P. Li Institute of Nuclear Energy Technology, Tsinghua University, Beijing 100084, PR China
Abstract In the 10 MW High Temperature Gas-cooled Reactor-Test Module (HTR-10) fuel elements move through the core driven by gravity. To reach their design burn-up the fuel elements are re-shuttled five times. This transportation outside the core is mainly achieved pneumatically. Although, adopting the international experience at design and operation of similar systems some key components were improved so that the fuel handling system (FHS) becomes simpler and more reliable. The improved components were tested in full-scale testing facilities. The debugging test and the first loading operation for the FHS indicate that the FHS meets the demands of the HTR-10. In this paper, the functions, design parameters, technological processes, main components and design characteristics of the FHS are described in detail. The flow schemes, design parameters of the full-scale testing facilities and the experimental results are briefly introduced. # 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The fuel handling system (FHS), one of the key systems of the pebble bed reactor, must ensure that the reactor operates continuously. Most of the components in the FHS are moving, and work at high temperatures and pressure, in helium atmosphere and under intense radiation. These severe conditions pore upon high demands on the design of functions and transmission components in the FHS. Large attention was paid to some important problems, for example, the helium seal, the bearing * Corresponding author. Tel.: /86-10-62770238x375; fax: / 86-10-69771464 E-mail address: [email protected] (J.G. Liu).
lubrication, the radioactivity shield, as well as the reliability and the maintainability of components. If a main component in the FHS would fail, normal operation of the reactor would be affected, and the availability of the reactor would be decreased. According to the actual features of the small experimental reactor, some key components were improved, based on the experience in design and operation of the similar components in the AVR so that the FHS became simpler, safer and easier to maintain (Hantke and Buelling, 1990; Hennings, 1969; Ziermann, 1975). The improved components were fully tested on full-scale testing facilities. The debugging test of the FHS and the operation of the first loading indicate that the FHS is reliable.
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2. Functions and technological processes of the FHS 2.1. Functions
Table 1 Principal parameters of the FHS No. Designation 1
The HTR-10 is designed to use spherical fuel elements. Its FHS is different from the refueling machines of reactors using rod shaped or block shaped fuel elements. The main feature of the FHS is to charge, recirculate and discharge fuel elements in the course of the reactor operation. The further functions are as follows: 1) To load the initial core. 2) To remove fuel elements from the fuel discharge tube of the lower part of the core. 3) To return the fuel elements to the core which have not reached the maximum burn-up. 4) To sort fuel element fragments and defective fuel elements, and drop them into the failed fuel cask. 5) Discharge of the spent fuel elements and transfer to the shipping casks. 6) To charge the new fuel elements into the fuel cycle. 2.2. Technological processes Table 1 summarizes the principal parameters of the FHS. The schematic diagram of fuel circulation of the FHS is shown in Fig. 1 (Liu, 2001). Three subsystems are especially note worthy. 2.2.1. Feed subsystem for the new fuel elements Before new fuel elements (10 or five balls every bag) are fed in the core, they are stored temporarily in two tanks in the charging room. When they are needed to be fed into the core, new they are taken out from a tank and put into the glove box with a sealing lock. The fuel elements roll gradually to the elevator by gravity. The glove box is kept at negative pressure while being fed with fuel elements. There are three isolation valves, one counter and two electromagnetic sliding valves in the feed line. Every two adjacent ones of the three isolation valves is in interlocking. They could only be closed synchronously, and could not be opened synchronously for atmosphere exchange in order
2 3
4 5 6 7 8 9 10 11 12
Parameter
Fast circulation
Approximately 350 balls per hour Conventional circulation Approximately 50 balls per hour The number of fuel elements fed Approximately 125 per equivalent full-power day balls (EFPD) New fuel elements fed per EFPD Approximately 25 balls Average burn-up measuring time Approximately 60 s per fuel element Operating pressure Approximately 3.0 MPa Operating pressure after the first Approximately 0.2 charging isolation valve MPa Operating pressure ahead of the Approximately 0.2 third discharging isolation valve MPa Operating temperature 150 /180 8C Fuel element capacity of failed Approximately 1000 fuel cask balls Fuel element capacity of buffer Approximately 30 line balls Spent fuel element capacity of Approximately 2000 shipping cask balls
to isolate the primary coolant from the atmosphere. Twenty-five new fuel elements in the feed tube of the elevator are transported individually to the core by means of purified helium through the electromagnetic valve. 2.2.2. Circulation subsystem for the fuel elements The spherical fuel elements move by gravity from top to bottom of the reactor core, and put in the discharge tube with an inside diameter of 500 mm. The discharge tube is connected to a reducer at the bottom head of the reactor pressure vessel. The fuel elements form a ‘sphere bridge’ at the bottom outlet of the lower tank body of the reducer. Pulsed airflow puts intermittent impact force on the fuel elements to remove the ‘sphere bridge’ when discharge is demanded, so that the fuel elements are discharged one by one through a removal tube. Then the fuel elements arrive at a cam with an opening in the failed fuel separator. Whenever the failed fuel separator turns once one fuel element drops on the roller of the separator
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Fig. 1. The schematic diagram of the fuel circulation.
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from the opening. Therefore, single transportation of the fuel elements can be sustained. The separator for failed fuel sorts out fuel element fragments and defective elements, which drop into the failed fuel cask. Whole fuel elements roll to the elevator in which the burn-up measurement is performed. Depending on the result of the burn-up measurement, the control system gives instruction to rotate the distributive tray of the elevator towards transporting tubes for its object. By means of gas pressure, partially depleted fuel elements are transferred through the elevation tube into the core, while spent fuel elements are transferred through the discharge tube of spent fuel element into the discharge buffer line. 2.2.3. Discharge subsystem for the spent fuel elements The discharge operation will start when the number of the spent fuel elements in the discharge buffer line reaches 25 balls. Firstly, the first isolation valve is closed, then the atmosphere in the tubes between every two neighboring isolation valves is exchanged. After the atmosphere exchange is completed, the elements arrive at the distributor and wait for distributing. At the beginning of the reactor operation, the graphite elements that have been charged into the initial core need to be discharged. The control system gives the distributor instructions to transport the elements according to the result of the burn-up measurement system. The spent fuel elements are discharged into the spent fuel shipping casks while the graphite elements are discharged into the graphite element containers. The distributor will complete its mission when all graphite elements are discharged from the reactor core.
reliability become very important. Their functions, structure and principle are described as in the following. 3.1. Pulsed airflow discharge components The lower part of the core, whose shape is like a funnel, is connected with the fuel element discharge tube. The reducer is located at its end. The structure of the reducer is shown in Fig. 2. It consists of the upper tank body, the lower tank body, a large funnel, a small funnel, pulsed airflow tubes and a removal tube. At the bottom head of the reactor pressure vessel, the lower end of the steel tube with an inside diameter of 500 mm in the upper tank body changes its direction and forms a fuel tank. Its bottom has an inclination angle of 158. Owing to the action of the friction, the height of the elements is maintained within a fixed range in the large funnel, thus, they cannot spill over. The inlet of the small funnel is jointed on the outlet of the large one. The outlets of the pulsed airflow tubes are located on the wall of the small funnel. The outlet diameter of the large funnel is so small that the fuel elements will naturally form a ‘sphere bridge’. Compressed gas in the pulsed airflow generator is suddenly released at a high speed
3. Description of main components The main components of the FHS include the pulsed airflow discharge components, failed fuel separator, elevator, isolation valve, counter, distributor, spend fuel shipping cask, seal mechanism of cover, control system, etc. They are the most important components of the FHS in order to perform its functions. Hence their safety and
Fig. 2. The structure of the reducer.
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when the fast-acting valve in the pulsed airflow generator operates. An impulsive force acts on the elements of the ‘sphere bridge’. The gravity and the static friction force between the element surfaces are exceeded and the elements begin to move towards the removal tube. If the impulsion is repeated many times, the elements in the fuel tank fall continuously one by one into the entrance tube and on the annular cam of the failed fuel separator. Due to the use of the pulsed airflow discharge reducer, there are no moving machinery components, which need to be disassembled and maintained in the fuel tank and the pressure vessel. Only the pulsed airflow tubes and the removal tube lead into the vessel, other corresponding components are installed outside the vessel.
3.2. Failed fuel separator The main function of the failed fuel separator is to sort out the scrap fragments and defective elements, which might hinder the transport of fuel elements. Another function is to allow only single transportation of a string of elements in the feed tube. The separator is composed of a shell, a roller, a measuring track board, a failed fuel funnel, a magnetic driver, a decelerator, a motor and others. The measuring track board and the roller are assembled horizontally with a gap of 56.5 mm. The failed fuel funnel is assembled below the roller in the shell. There is an annular cam with an opening at the initial end of the roller. The breach is used as a fuel element single-exit gate. Only one element falls on the surface of the roller and goes into the measuring area when the separator makes a turn. Between the surfaces of the measuring track board and the roller, the element moves and rotates continually randomly toward the end of the roller by the use of the inclined teeth. The defective elements and the fragments fall into the funnel from the gap of the measuring area and roll down into the scrap container. Whole elements move to the end of the roller, then into the inclined tube and finally into the elevator.
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3.3. Elevator The elevator has the following functions: 1) To elevate new, partially depleted and spent fuel elements individually. 2) To locate the fuel elements which need burnup measurement. The elevator consists of a shell, a distributive tray, a magnetic driver, a decelerator, a stepping motor, a rotating encoder, etc. On the distributive tray, one end of the loading channel is aimed at the gas line, another might be aimed at one of loading fuel line, the elevating line to the core and the buffer line of the spent fuel elements. The receivedball cup on the end of the loading channel is also a localizer that accommodates only one element. The distributive tray rotates from measuring position to receiving position at the beginning of operation, it rotates 458 counter clockwise after receiving a fuel element and goes back to the measuring position. After measurement, the fuel element will be reloaded into the core if it has not reached the target burn-up, otherwise the distributive tray will rotate 908 clockwise and the fuel element will be elevated to the buffer line of the spent fuel element. The tray rotates 908 in reverse to its measuring position after elevating. The tray swings only between the receiving position and the measuring position when new fuel elements are elevated. Burn-up measurement is not needed during loading of new fuel elements. 3.4. Isolation valve The isolation valves are installed on the pressure boundary of the primary loop, this ensures its integrity. An atmosphere exchange could happen gradually when the valves are installed in the lines of atmosphere exchange, therefore gas in the lines can be isolated stage by stage, to prevent the air from entering and the helium gas from flowing out. The valves are a kind of straight passage ball valve with a full sealing structure. A mechanical limit block is assembled on the valve head so as to ensure a rotation of 908. The design torque of the
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magnetic driver and preset torque of the electric actuator are 400 Nm. The fuel elements might pass through it successfully when the valve is opened for 10 s. 3.5. Counter Counters are installed on important positions of the FHS (Muncke, 1976). Their functions are to record the number of fuel elements that pass through various measuring stations and to monitor the filling level of the lines for purposes of control and recording. The counters are composed of sensors and secondary instrumentation. Each sensor consists of two coils, which are wound around a ceramic tube. The ceramic tube is assembled in a steel pressure shell. The leading wires of the sensor coils are led to the outside of the shell via electrical penetrations. An inductive bridge is used in the counter. When an electrically conducting fuel element passes through the magnetic field of the coils it induces eddy currents which alter the impedance of the coils and balance of the bridge. An inductive voltage signal is generated in the coils due to this change of magnetic field, and the signal is amplified and demodulated. Besides the use of counting, it can be used to find out the moving status and direction of the fuel element.
single-inlet gate in the piston rod carrying one ball is aimed at one of two removal tubes. The graphite elements and the spent fuel elements will be discharged into their respective containers. The processes described above take place in an air atmosphere and at approximately 0.4 MPa. 3.7. Spent fuel shipping cask The function of the spent fuel-shipping cask is to safely store spent fuel elements. It consists of a steel liner, an outside shell, a lead shield, a cask plug (or cask cover), a cask neck, a board cover, etc. There are many steel support bars between the steel liner and the outside shell to obtain a reasonable gap to fill lead. One cask can hold approximately 2000 fuel elements. When filled, the cask is sealed with a board cover and stored in the spent fuel storehouse. The spent fuel elements in the cask may be removed by pump feed method. 3.8. Seal mechanism for the cover of the spent fuel shipping cask A seal mechanism for the cover of the spent fuelshipping cask is installed in a small cavity that is located in the spent fuel storehouse. Its structure is shown in Fig. 3. It fulfils the following functions:
3.6. Distributor
1) To take out the cask plug of the cask neck. 2) To put the cask plugs into the cask neck.
The distributor is installed after the third discharging isolation valve. Its function is to distribute graphite elements and spent fuel elements into correspondent storage containers. It includes an air cylinder, two locating pistons, two direction-changing pistons, a piston rod, some change-valves, etc. The direction-changing piston with a piston rod is assembled within the locating piston. The piston rod has a single-inlet gate, which can hold only one ball. The single-inlet gate is on the receiving position when compressed air enters two cells of the air cylinder simultaneously. The change-valve acts are controlled by the control system, which allows only admission of one cell and exhaust of another cell. The directionchanging pistons are pushed to one side. The
It consists of a main cylinder, two assistant cylinders, a claw mechanism, a removal tube, a sliding socket, some pilot valves, etc. It is actually a pneumatic manipulator using compressed air. The control unit of the compressed air is fixed into a control box, which mainly includes electromagnetic valves, pressure-reducing valves, pressure switch, pressure meters, one-wag valves, etc. The piston in the main cylinder is joined to the claw mechanism (includes a pair of claw, a claw cylinder, a piston and a cam) through a piston rod, and may move to and back vertically. When the piston in the main cylinder moves to its upper limit, it is locked. Normally, the claw is opened by the spring action. When the cask plug needs to be taken out, the compressed air enters from the
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Fig. 3. The structure of the seal mechanism.
lower end of the claw cylinder, to make the claw fold. Then the piston in the main cylinder moves to its lower limit, in the meantime the claw goes into the inverse ‘T’ shaped hole of the cask plug. Subsequently the lower end of the claw cylinder bleeds air, so that the claw opens and catches the plug. Finally, the cask plug moves towards its upper limit with the piston elevating in the main cylinder up to the exit of the removal tube. The piston in the main cylinder moves towards its lower limit after the shipping cask is filled, and the cask plug is put into the cask neck. The task of the assistant cylinder is to seal the cask neck using the sliding socket in order to prevent active dust being released.
(IPC) was designed. The PLC has high precision, reliability and anti-interference. The IPC has welladapted person /computer interface and abundant software resources, therefore it is convenient to implement management and inspection of the system, monitor and analysis of device state. In accordance with the technical specification of the FHS, the control system controls the movement of the fuel elements in the pipeline via opening and closing sequences of the various valves. The system also controls operation of various mechanical devices e.g. sorting out of defective elements, elevating of fuel elements and controlling the distribution. To ensure qualified operation under technical requirement, the pressure sensors monitoring the He pressure, the temperature sensors measuring the He temperature in the FHS, the counters recording the number of the fuel elements that pass through various measuring station and monitoring the motion state of the fuel elements were installed. The control system collects the real-time data above mentioned, and analyses, estimates and carries out the corresponding signal to control the operation of various devices. The state monitoring, data analyses, effective software management and failure diagnosis to give the alarm before failure have been also implemented to avoid misoperation. The structural diagram of the control system designed is shown in Fig. 4. This shows the control system consisting of PLC, IPC and the display device.
3.9. Control system In order to ensure the FHS to work reliably, the control system based on the programmable logic controller (PLC) and industrial personal computer
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Fig. 4. Structural diagram of the control system.
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4. Full-scale test Three full-scale testing facilities have been operated since 1986, in which the following research and development were finished: 1) The flowing behavior of the pebble bed in the HTR-10 was studied on. The results of the flowing residence spectra and the relative speed distributions of the pebble bed are gained, and obtained data were used in the construction design of the FSH and the physical calculation of the core. 2) Examinations of some new components were carried out and correct design parameters were determined, design and operation experiences were gained also.
This paper describes only one of the three testing facilities. The testing facility for the fuel circulating system is shown in Fig. 5. Its main components are designed and manufactured on the basis of the prototype size. Fuel transportation was performed by gravitational and pneumatic means. In addition the component that uses the pulsed airflow to break bridging to discharge the balls in a single-line way from the discharge tube is first applied to a pebble bed reactor. Table 2 summarizes the main parameters of the testing facility. 4.1. Circulation system for the graphite balls The graphite balls in the discharge tube move from top to bottom by gravity, then they roll into
Fig. 5. Flow scheme of the full scale testing facility for the fuel circulating system.
J.G. Liu et al. / Nuclear Engineering and Design 218 (2002) 169 /178 Table 2 Main parameters of the testing facility No. Designation
Parameter
1 2 3 4 5
0.5 m 4.2 m 4.0 m 0.5 m 60 balls per hour
6 7 8 9 10 11
Discharge tube inside diameter Discharge tube height Maximal loading height Ball tank width Feed speed of circulating graphite balls Operating pressure Operating temperature Working pressure of pulsed airflow Operating times of pulse Rotation speed of failed ball separator Diameter of graphite ball
0.1 MPa 150 /180 8C 0.3 MPa 30 times per hour 1 rpm 60 mm
the ball tank through the outlet in the discharge tube bottom, and accumulate in the large funnel of the ball tank bottom. Due to the friction, their height is maintained within a fixed range in the large funnel. Here a ‘sphere bridge’ on the exit is formed and stops the motion. The pulsed airflow impacts the ‘sphere bridge’ intermittently, so that the graphite balls are discharged one by one. The counter records the number of the discharged balls. The discharged balls drop in the failed ball separator through a vertical removal tube, where fragments and defective balls are sorted out, and fall in the scrap container through the funnel of the damaged balls. The counter records the number of damaged balls. Undamaged balls roll to the elevator by inclination. Two transportation modes were designed. They are modes 1 (main circulation) and 2 (discharge circulation). For transportation mode 1, the elevator distributive tray with the ball rotates to the elevating position one counter clockwise, the transportation gas brings the ball back to the discharge tube and the counter records the number of the elevated ball. For transportation mode 2, only two balls are circulated repeatedly so that the discharging process is simulated. The distributive tray with one ball rotates to the elevating position one clockwise, two balls are elevated to the single-exit device. One ball rolls back to the elevator, the
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other still stays in front of the single-exit device. The circulating testing of two balls between the elevator and the single-exit device may be carried out repeatedly, under this circulation mode. In front of each bend of the elevating tube a buffer was installed to reduce the impact speed of the balls. 4.2. The auxiliary helium system The auxiliary helium system offers helium to the pulsed airflow discharge reducer, the elevator and the single-exit device, and sustains helium flow in the circulating system. The operating pressure of approximately 0.1 MPa is sustained in the ballcirculating loop. The pressure in the loop will continuously increase when the pulsed airflow discharge reducer, the elevator and the single-exit device are working, so the helium compressor must draw helium from the loop. The drawn helium is filtered to remove the graphite dust and other impurities. Then it is cooled to 50 8C by an air cooler. After dehydration by the drier, the helium flows into the lower pressure container, is compressed again, and then enters the high-pressure container. The pressure of helium from the highpressure container is decreased to the preset pressure via the pressure-reducing valve. Part of the helium is transported to the elevator through the regulator valve and electromagnetic valve, and the rest to the pulsed airflow generator. Helium from lower pressure container may directly be used as the control gas resource of the single-exit device. A vacuum pump is used to obtain the needed vacuum for the circulating system, and a watercooled system is applied to the cooling of the compressor. 4.3. Electric heating system The electric heaters were installed outside the discharge tube, failed fuel separator and elevator. All tubes and functional components in the graphite ball circulating system were wrapped in the thermal insulation materials. The temperature in the circulating system is measured and controlled by the thermocouples and automatic con-
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trol system of temperature, and is kept in suitable range.
About 160,000 graphite balls have been treated in the pulsed airflow discharge reducer and about 5000 balls in the failed ball separator.
4.4. Control system The control system makes use of the PLC, computer and simulation screen. PLC has the advantage of the multifunction, agilely to operate, easy to maintain, and excellent stability, etc. The computer can monitor the operation status of the PLC. A large-scale mosaic simulation screen displays the operating status of all components, the temperature and the pressure in main components, and ball information through each measurement point. The moving direction and position of the ball can be displayed. The control system consists of two independent ball circulating systems and a temperature control system. Both manual and automatic modes are used. The manual mode implements the start and stop of all electric devices, without being affected by the signals from the pressure, temperature, counter, etc. 4.5. Test This full scale testing facility had been successfully operated for over 500 h. The main components of the FHS have fully been tested in the factory and the laboratory after manufactured.
5. Conclusions It has been proved by practice during the full scale test, a debugging test and the first loading that the fuel handling system described above meets all demands of the HTR-10.
References Hantke, H.J., Buelling, H., 1990. Fuel Feed System, Design and Experiment, AVR-Experimental High-temperature Reactor, Duesseldorf, pp. 187 /202. Hennings, U., 1969. Fuel handling system for core elements of a pebble-bed reactor. Nuclear Applications and Technology 7, 334 /341. Liu, J.G., 2001. Development of Fuel Handling System in 10 MW High Temperature Gas-Cooled Reactor. Proceedings of the Seminar on HTGR Application and Development, Beijing, China. Muncke, A., 1976. Graphite ball detectors for the fuel handling machine of a helium-cooled pebble bed reactor. Kerntechnik 18, 201 /206. Ziermann, E.L., 1975. Operating Experience with the AVR Experimental Power Station. Proceedings of a Symposium Jointly Organized by the IAEA and the OECD Nuclear Energy Agency and Held in Juelich, Germany.