Development of an automated system for wire

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end plug. Crimping is done to restrain the tension in the wire before weld bead. Scope of this paper is to present development of an automated system required.
Development of an automated system for wire wrapping and spot welding of PFBR fuel pin Anupam Saraswat1, Prateek Pareek1, Madhusudan Sharma1, Farman Ali1, Rajashree Dixit1, P.S. Somayajulu1, Sibasis Chakraborty1, Vrinda Devi1 1Radio Metallurgy Division Bhabha Atomic Research Centre, Mumbai Email: {anupams, prateekp, mssharma, farman, rajashree, somaya, sibasis, kvvdevi}@barc.gov.in

Abstract In the upcoming fuel fabrication facility for Indian Prototype Fast Breeder Reactor (PFBR), automation of Fuel fabrication is required to be done. Fuel pin carries spacer wire wrapped around its surface. This Pin contains groove at bottom end plug for bead entanglement. Wire has to be welded on the bottom end plug. Crimping is done to restrain the tension in the wire before weld bead. Scope of this paper is to present development of an automated system required for remote handling of wire wrapping process. A prototype has been designed to simulate various processes of wire wrapping. This is an automated system for handling of pin and wire wrapping processes. Keywords: Automatic wire wrapping system, Spacer wire, PFBR fuel pin

1. Introduction India is pursuing a three stage nuclear program. In the second stage, fast breeder reactors are planned to generate electricity. Short cooled, fast reactor reprocessed fuel is expected to have high radiological activity [1]. High amount of dose rate in the feed material necessitates remote operability and automation in the fuel fabrication facility [2]. Fuel pins for prototype fast breeder reactor (PFBR) are required to be wrapped with spacer wire. There is a provision provided in the bottom end plug of the pin to entangle the spacer wire. This spacer wire is welded to the top end plug. Spacer wire is wound on the pin with a pitch of 200mm, which is a crucial parameter and needs to be maintained. Scope of this paper is to present development of an automated system for remote operation of complete wire wrapping process. A prototype has been designed to simulate wire wrapping of 6 to 8 pins in an hour on true scale. This paper presents the design philosophy and constraints of an automated wire wrapping system.

2. Design considerations For a system which is required to perform in a high radiation environment, some important design considerations shall be incorporated in the design. Apart from the mechanical loadings, other design parameters listed below are very important for a fool proof design.

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Static loads: it includes dead weights of components like Moving slide system, Fuel pin, Structural supports and Housing are considered for design load. The system is designed to handle one fuel pin at a time from 8 pins kept in a tray. The pick and place mechanism has to lift one pin at a time and place it on the wire wrapping machine. Pin is of 2580mm in length and 6.6mm diameter weighing close to 2 kg. This has to be rotated on the v groove platform for wire wrapping process. A wire of same length approximately is to be lifted from a similar tray placed adjacent to it. Automation is required to mechanize this complete process with minimum or negligible human intervention. Dynamic loads: All the forces experienced by the structural elements while the system is in motion i.e. forces required to rotate the pin or position them on the v grooves, turning moment experienced by the pin during rotation, force applied during crimping etc. comes under dynamic loads. System shall be modular in construction. It is important that in the case of failure of any one component operator should be able to remove/repair that component without disturbing the entire system. Material and components selection shall be carried out based on the high radiation background levels. Since this is a prototype, radiation resistant electronics and components may not be required at present. But a provision for a replacement at a later date shall be available. System shall be easy to maintain. This requires maximum use of standard and uniform size components like bolts, nuts etc. in the assembly. All the mechanisms used shall be rugged in construction.

3. System and component details The main system has various subsystems which are designed for carrying out different operations in the wire wrapping of the fuel pin. These subsystems manoeuvre various parts of the pin hence require various degrees of motion. Each direction travel has been provided with an independent drive and control units. The overall dimensions of the system are 7800 mm (along Y-axis), 2870 mm (along X-axis) and 2000 mm (along Zaxis) as shown in Fig.1.Length of the pin is 2580 mm, and various subsystems are placed around this pin. Dimension along the pin shall be identified as Y-direction for all further references. The wire feed and bend system has to travel along the complete length of the pin. Wire Feed System also provides the perpendicular feed of the wire for entanglement of the bead. This dimension shall be identified as X direction for all further references. Crimping and welding system are placed below the pin area. These subsystem move up as and when required, for performing required operation. This dimension shall be identified as Z direction for all further references.

Development of an automated system for wire wrapping and spot welding of PFBR fuel pin iNaCoMM 2017,13th to 15thDec. Mumbai, India

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Y Z

Figure 1: Overall schematic of the wire wrapping machine

Following are the main components of the system: 3.1 Fuel pin tray and Wire Tray The fuel pin tray has been designed to accommodate 8 number of fuel pins as shown in Fig.2 (a). A typical fuel pin is shown in Fig.2 (b). Fuel pin tray is made of stainless steel base for support and aluminium profile with V-grooves to hold fuel pins. Opening of each V-groove is slightly more than fuel pin diameter such that pin can be supported on more than half of the circumference. Sufficient distance is provided between V-grooves to allow the pins to be handled by a gripper individually. Design of pin tray has been standardized and the operator for various operations can use same tray interchangeably. Each pin tray is provided with hooks to enable easy locking with pick and place mechanism. Pin details are provided in the Table 1. Table 1 Details of the fuel pin

Material

Mild Steel

Outside Diameter

6.6 ± 0.02 mm

Length of finished pin tube

2580 ± 0.5 mm

The wire tray is also similar to the Pin Tray in design. It is designed to hold wires required in wire wrapping process. Wire details are provided in Table 2.

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Table 2 Details of wire to be wrapped around the pin

Material

Titanium modified Stainless steel

Outside Diameter

1.65 ± 0.02 mm

Pitch on the pin

200 ± 1 mm

Figure 2(a): Fuel Pin tray drawing,

Figure 2(b):A typical fuel pin of PFBR

Development of an automated system for wire wrapping and spot welding of PFBR fuel pin iNaCoMM 2017,13th to 15thDec. Mumbai, India

3.2 Pin Handling System Once pin trolley is in home position for loading of the pin on wire wrapping fixtures, the Pin Handling system will pick the pin from tray and place it on V shaped wrapping fixtures. Pin handling mechanism is shown in Fig.3. Pick and a place mechanism have a specially designed groove to hold the pin as well as the wire from the adjacent tray. Pin Handling mechanism shall move from its home position to loading position with the help of X and Z slide gantry. Then gripper shall grip the pin in Rhombus shaped Teflon coated cavity so as to prevent scratch or dent onto the surface of the pin. Pin shall now be kept on the fixtures provided for wire wrapping process. Then this system shall move back to home position for handling of wire. Wire comes in a tray which is placed near the pin tray and it has to be fed to the wire tensioning subsystem.

Figure 3: Pin handling mechanism

3.3 Pin Wire Feed System Tension of the wire is an important parameter to be controlled in the wire wrapping process. Moreover it has to be ensured that the wire is straight and does not have any permanent deformation which may lead to gap between the wire and the pin surface after the wire wrapping process. To ensure the same a wire feed subsystem has been provided to provide adequate tension and feed the wire during wire wrapping process. Grooved wheels also help in removing any localized deformation. Wire coming out from the feed subsystem gets wound around the Pin. Starting point of the wire consists of a bead which has to entangle in a groove on the pin. Wire Feed system aim at proper orientation of the wire so as to make sure the entanglement of bead in the groove. This complete system is mounted over LM Guide and ball screws and moves through the complete length of the pin. This system feeds

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the wire as it gets wrapped around the pin. Details of the system are given in Fig.4 below.

Figure 4: Wire feed system schematic drawing

3.4 Pin Positioning and Rotation Once the pin is placed in the groove on the table, its start and end position needs to be detected for proper positioning of other subsystems and calculation of the pitch. Two sensors are provided, one on each end to detect the start and end position of the pin. These sensors are mounted on the Left Hand Slide System and Right Hand slide system respectively as shown in the Fig.5. Left Hand Slide and right hand slide both have independent motors and gearbox. These are synchronized with a motion controller. Pin is gripped on left hand side and rotated so as to find the degree of rotation required to bring the crimping position on the end side while rotating from the pin groove from the front side. This helps in determining degree of rotation required to achieve the pin crimping position while starting from bead position on the front end. The sensors attached to the two slides help in calculation of exact length of the pin. Pitch of the wire is maintained by synchronous rotation of the pin and movement of the wire feed system

Development of an automated system for wire wrapping and spot welding of PFBR fuel pin iNaCoMM 2017,13th to 15thDec. Mumbai, India

Figure 5: Right hand slide (RHS) pin rotation and positioning

3.5 Pin Crimping System Once the wire reaches the bottom end plug it has to be welded. Weldment requires some time to cool and gain strength. If tension is applied on the weld pool it may lose its strength. The tension of the wire is restrained little before the welding point by crimping the wire into the pin so the wire is relaxed beyond this point. Crimping is done by putting the wire in a cavity and applying sufficient load to cause permanent deformation of the cavity so that it holds the wire firmly and limits the tension in the wire. Wire is without tension and relaxed beyond this point. Crimping force is calculated by considering tube with a mandrel. It’s also taken care that it is sufficient to cause permanent deformation of the tube but at the same time does not cause any damage to the pellets inside the pin. To avoid crowding on the wire wrapping area Crimping system has been placed under the wire wrapping table as shown in the Fig.6. It’s provided with a lift gearbox and crimping mechanism. After the wire is wrapped it is actuated and lifts up. Arm of the crimping system rotates precisely as per the position of the pin provided by the sensors and reached the crimping position on the fuel pin. Hydraulic mechanism is used to make sure that force does not exceed the maximum limiting force. This is crucial in preventing damage to the Pellets inside the pin.

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Figure 6: Pin crimping system

Crimping mechanism rotation motor & gearbox

Wire Disposal mechanism rotation motor & gearbox

Crimping and wire disposal mechanism Rotation : For changing the orientation of arms of Wire disposal and crimping mechanism From parallel to the pin to perpendicular To the pin.

Figure 7: Wire cutting and disposal system

3.6 Wire cutting and Disposal system Wire that comes in the wire tray has length little more than the required. After welding some length of the wire is extra and needs to be cut before wire wrapping process is complete. This extra wire poses problem during handling of pin in other processes. Hence, a Wire Cutting System is also placed below the table and has a gear and motor attached to it for precise movement of the system when required. After the crimping process the wire cutting system gets actuated and rises above the table. It has

Development of an automated system for wire wrapping and spot welding of PFBR fuel pin iNaCoMM 2017,13th to 15thDec. Mumbai, India

a scissor like arrangement to cut the extra wire coming out of the pin. Wire cutting and disposal system is shown above in Fig.7. 3.7 Wire Welding System Welding of the wire is done on the top end plug. This is required to hold the wire in place. Crimping is just to provide support for proper welding by eliminating tension from the wire. The aim of holding the wire on bottom plug is achieved by Welding. Welding Subsystem is placed adjacent to wire wrapping system. Pick and place Mechanism lifts the pin and places it on the welding station. Since welding is a hot process as it is being achieved by spot welding so care has been taken to isolate the welding process from other processes. Welding system welds the wire onto the pin by spot welding. Here the chuck in which the pin is held places the pin in proper orientation and recedes back to uncover the top end plug. The electrode of the spot weld has circular cross section area with diameter little more than that of wire so as to provide sufficient weld area for strong welding

4. Operational details Wire wrapping system is fully automatic in operation involving multiple drives operating in a synchronized manner to perform various inter linked operations. This requires very precise manipulation and coordination between various subsystems. To achieve this Motion controller is used instead of PLC. Process is completely automated and is controlled through Motion Controller. System automated various processes of Wire Wrapping along with retrieval of Pin Tray and wire Tray. Once pin trolley is in home position Pin Handling system moves to its position and Pick and place mechanism lifts one pin from the tray and places it on the V grooves provided on the wire wrapping machine. These V grooves are in form of floating supports and are subdued when the wire feed system reaches close to them so as to avoid any hindrance in the wire wrapping. Once the pin is placed on these supports its exact position is sensed by the sensor provided on the Positioning and rotation system. These systems are provided on either side of the pin rest area. The sensors on mounted on it provide the exact location of the pin and the length of the pin. Once the pin length is known pin is rotated to get the angular difference between the groove on bottom end plug and crimping spot on the top end plug. These two values are important for controller to maintain the required pitch of the wire to 200 mm. Pick and place mechanism meanwhile place the wire on to the Wire feed mechanism which provides the adequate tension in the wire which is 5-7 kg. This system first helps in entanglement of bead of the wire in grove on lower end plug and then it moves along the complete length of the pin. Pin is rotated in controlled manner with the help of servo motor so as to get the required pitch of 200 mm. As the wire feed system reaches the end of pin the wire is held on top ofthe slot provided on the top end plug. The crimping mechanism applies force on the crimping spot so as to make plastic deformation and hold the wire tightly. A cutting and dispose mechanism cuts the wire leaving behind a sufficient for welding. The cut out part is dropped into a bin attached to the system. Wire beyond the crimp spot does not have any tension. This is important to make sure proper welding and cooling of the weld bead. Welding is done by principle of spot welding on to the

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welding area adjacent to wire wrapping area in Y direction. A welding arm comes up and the wire is spot welded onto the top end plug of the pin. After the welding this wire wrapping process is complete. Pick and place mechanism lifts the pin and places in another tray which takes it out for further process

5. Control and Operator Interface Automated operation of individual components and coordination of various components have to be done by the Motion controller. Necessary interlocks were provided to ensure safe and reliable operation of the system. All necessary operating parameters were made available at the control station. It was possible to set the necessary parameters from the control station. The controller was programmed to prompt the operator for necessary actions, whenever operator intervention is necessary. The controller has features for fault detection and diagnosis. Database of tension in the wire, pitch crimping force etc is to be available to the control station.

6. Conclusion A system for automatic wire wrapping of PFBR fuel pin has been developed. This system shall be able to retrieve pin from a tray and do the complete wire wrapping process without any manual intervention. The system has all the essential features required for handling radioactive materials like modular design, parts interchangeability etc.

References [1] G. Pandikumar, V. Gopalakrishnan and P. Moahanakrishnan, “Multiple recycling of fuel in prototype fast breeder reactor,” Pramana-Journal of Physics, vol. 72, no. 5, pp. 819-832, May, 2009. [2] S. Chakraborty, A. Saraswat, K. Danny, P. Somayajulu and A. Kumar, “Design of MOX fuel fabrication plant for Indian PFBR,” in Proceedings of the second international conference on advances in nuclear materials, Mumbai, 2011.