Laser Dismantling Of Phwr Spent Fuel Bundles And De-cladding Of Fuel Pins In The Highly Radioactive Hot Cells

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Laser Dismantling of PHWR Spent Fuel Bundles and Decladding of Fuel Pins in the Highly Radioactive Hot Cells G. L. Goswami1, K. Jayarajan3, S. Gangotra6, Munish Chandra2, Shailesh Kumar4, J.K. Mishra3, H.B. Kulkarni2, K.K. Prasad2, Anjan Chatterjee1, K.C. Sahoo6 Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085. & T. P. S. Nathan5 CAT, Indore 1. Atomic Fuels Division, 2. Nuclear Recycle Group, 3. Division of Remote Handling & Robotics, 4. Laser & Plasma Technology Division, 5. Solid State Laser Division, 6. Post Irradiation Examination Division. EMail: [email protected]

Abstract For reprocessing of PHWR fuel, fuel bundles are at present chopped mechanically into small pieces of pins using high tonnage mechanical press before dissolution. The existing method of bundle dismantling is purely mechanical using very high force for chopping. A laser based automated bundle dismantling system is developed. In the system, end-plates of bundle, which holds the fuel pins together, are cut using Nd-YAG laser to separate the bundles into pins. In addition to pin separation, the pins are to be chopped into small pieces using a small mechanical chopper. Since the spent fuel is highly radioactive, all these operations are performed remotely in hot cells. Post irradiation examination also requires dismantling of bundles into pins so that they can select the pins for the further examinations. In both these applications laser dismantling remains the most important step and this system has been developed and tested. This paper describes the experience gained during the development efforts. Key Words: laser dismantling, PHWR, fuel bundles, radioactive, hot cell, Nd-YAG

1. Introduction PHWR fuel bundle consists of 19 tubular pins, spot-welded to two end plates on each side. It is of about 14.5mm diameter and 495mm long. For the purpose of fuel reprocessing these bundles are presently being chopped by a heavy duty 20T shear machine inside the hot cell for dissolution of oxides. There are certain problems in mechanical chopping specially when the entire bundle is sheared at a very high pressure. Cutting of the Tie plate provides a better alternative where the bundle can be dismantled to individual pins and these pins can be sheared individually at lower pressure. PHWR fuel bundles are also required to be disassembled for inspection the individual pins under highly radioactive environment of the hot cell. It was proposed to carry out the disassembly of PHWR fuel bundles by laser cutting of both the tie plates. Accordingly, the experiments were conducted at AFD, BARC to study the feasibility of the scheme, which gave the encouraging results. A dummy end plate was successfully cut to separate spot weld location using existing laser on a 3-Axis CNC workstation. Based on the feasibility studies, a design for totally automatic and remotely

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operable system has been made and is proposed to be used for dismantling of PHWR fuels inside the hot cell for both reprocessing purpose and PIE Examination. A separate Nd-YAG laser-cutting system has been assembled and tested for disassembly of the PHWR fuel bundles. The system has to perform operations like moving the bundle to laser cell from charging cask, cutting both the end plates using laser to separate out 19 pins for further processing. Major sub-systems of the system are the following: l

Laser Cutting Tool

l

Bundle Handling Tools

l

Laser Source and Beam Delivery Systems

l

Controller and Operating Console

Laser focusing tool is mounted on a 3-axis CNC table. The fuel bundle with tie end plate is brought to a fixed location and the focusing tool is moved through a predetermined path to cut the end plate. After cutting one end plate, the bundle is rotated and cut is given to the opposite end plate. In the proposed scheme, the bundle disassembly will be performed in two stages in hot cells. In the hot cell, both the end plates are cut into pieces by laser to separate fuel pins. It involves tools for handling of bundle, CNC station for laser movements and beam delivery system. A 150W Nd-YAG laser is used for cutting. All these operations are performed remotely and automatically.

2. Feasibility Studies & Concept Design Experiments were conducted to study the feasibility of the scheme. The experiments were conducted inside a simulated hot Cell made by PVC material creating the actual experimental conditions. A dummy end plate was successfully cut to separate spot weld location using laser on a 3-Axis CNC workstation. The total path covered by the laser head was about 600mm, out of which about 300mm was the ON time for the laser. It was found that cutting of the tie plate is very convenient by laser. Experiments were conducted to study the feasibility of decladding of fuel pins by laser cutting. Fuel pin clad was given helical and circumferential cuts using laser to separate it from the fuel pellets inside. Helical path was taken such that it does not pass through bearing pads and spacer pads. So this method of decladding requires precise positioning of pins and also needs complex mechanical systems to be placed inside the more active cell. It was observed that about 20 metres (0.5 m per linear cutie length of the tube X 2 cuts per pin X19 pins + end plug removal) of cutting required to remove the pellets. It was observed that with the present level of cutting rate, it would need much more than an hour to achieve this. Moreover, laser cut marks of up to 100 mm on the pellets were also seen as PHWR fuel pins are made up of collapsible zircaloy tubes and pellets are in the intimate contact with the tube. These marks are difficult to avoid and would lead to evaporation of radioactive material and would result in high level of contaminations[1,2]. In view of such long processing time and also observation of 100 mm cuts on the pallets due to laser, the option of cutting single pin by laser was not considered appropriate and multiple shear device (hydraulic press) for chopping was studied. To see the performance of fuel chopping, fuel pins were cut into pieces in a hydraulic press. The cut section was found to be clean with no generation of Zr fines[3]. No pinching

Laser Dismantling of PHWR Spent Fuel Bundles and Decladding of Fuel Pins...

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or closing of chopping face was observed. Also, the chopping force per pin was found to be about 100kg, which is very low compared to the present method of chopping. Effect of nuclear radiation on various optical components and Optical fiber used for beam delivery inside the hot cell were studied and systematic experiments have been carried out for various dose rates and exposure times. The change in optical properties like transmission, coloration and refractive index were recorded before and exposure [4]. It is concluded that low impurity pure crystalline quartz optics is usable in harsh radiation environment up to one mrad exposure. Likewise low impurity silica fiber like TECS-393M is also usable inside Hot cell with proper protection. This data was used for design of laser beam delivery system for the machine.

3. Special Features for Radioactive Operation The special care was taken to find out the suitability of different parts of the machine to be used in the radioactive area before going for the design aspect of the machine. Accordingly following aspects were considered in designing the machine [5]. l Remote operation of laser with fibre optics beam delivery system. Main parts of the machine are outside the radioactive compartment. l Modular system design to allow easy removal of defective parts. l Easy plug in type electrical connectivity to facilitate easy disconnection/connection through manipulators. l Special quartz lenses to sustain the radioactive environment.

4. The System The system consists of a bundle lifting system to lift the fuel bundle to the height of the CNC stage to enable the manual pushing to the gripper stage. The gripper assembly then grips the bundle in the proper alignment and then the CNC system starts the movement of the proximity sensor to find out the alignment of the centre and the other locations of the tie plate. Accordingly, laser nozzle takes the predetermined path automatically for cutting. After cutting the first tie plate the bundle rotates automatically by 180° and completes the cutting of the tie plate on the other side, after sensing again. The laser system delivered by CAT, Indore was interfaced with the CNC system and a number of dummy subassemblies were cut using the laser to test the functioning of the machine. Two such systems are being made. One of them will be integrated with other mechanical shear system for fuel reprocessing at NRG while the other system shall be used in RLG for post irradiation examination activities. (a) For Fuel Reprocessing Division A fully automatic remotely operable system has been designed and fabricated to carry out Nd-YAG laser cutting of the tie-plates followed by mechanical shear of individual pins (15 cuts at a time). The mechanical shear of each pin, carried out in AFD, BARC indicated that around 100-150 Kg load is needed to chop the individual pin. Additionally it has a conveyer system to carry the dismantled pins to the chopping bed and then the single stroke chopping action is done by the system and then fuels are taken out for dissolution works. (b) For Post Irradiation Examination Division The requirement of the post irradiation examination division is only cutting of the tie plates to dismantle the individual fuel pins inside the hot cell for the post irradiation

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examination of the individual pins. For this purpose additionally a motorised bundle lifting system is provided to lift the bundle to the height of the CNC stage to enable the manual pushing of the bundle to the gripper stage.

5. Commissioning & Testing The system has been designed fabricated assembled and tested in the laser Lab, Atomic Fuels Division, BARC. The Mechanical handling system was designed by special task force and fabricated by the local outside party while the laser system was supplied by CAT, Indore. The system have been integrated and tested in the Laser Lab, AFD; BARC as shown in fig 1, fig 2 & fig 3. Satisfactory cutting trials have been performed using the system automatically and remotely.

6. Conclusion The tie plate cutting operation by laser was a very clean operation. It was found that after cutting both the tie plates pins separate easily as soon as the gripper is released. Laser cutting also avoids opening up of the pins from the welds as no pressure is exerted on the pins during the cutting process. It should also be mentioned that the dissolution time would be low in the proposed scheme, because of the clean edge of cut and no cut piece will be blind. The generation of Zr fines will be low in single pin chopper. Since the cutting force is very low in the proposed method of chopping, maintenance and downtime is expected to be low. Based on the feasibility studies, a design for totally automatic and remotely operable system has been made & the machine has been fabricated and tested for its satisfactory performance.

7. References [1] Laser cutting for dismantling of PHWR fuel bundle- Dilip Kr, B.P. Badgujar, G.L GoswamiNational welding Seminar, IIW- CAT Indore Jan-94 (1994) [2] Automated laser cutting system for disassembly of PHWR fuel bundle - A report on laser cutting trial - Sanjiv K. Jha AFD, BARC, Oct (1999) [3] Single Pin shear for development of laser based disassembly system- A report - G.L. Goswami & Munish Chandra, (2000) [4] A Technical report on radiation effect on optical components and optical fibres. Life of such system under sustained radiation. Results of the experiments and literature survey - G.L. Goswami et al [5] Development of laser based disassembly and single shear system for reprocessing Thoria fuels-G.L. Goswami et al Proceedings symposium on Thoria Technology, Indian Nuclear Society, Mumbai, (2000)

Laser Dismantling of PHWR Spent Fuel Bundles and Decladding of Fuel Pins...

Fig. 1. Gripper assembly with Fuel bundle

Fig. 2. Laser head & sensor head

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Fig. 3. Overall view of the machine *****