Laser Dismantling of PHWR Spent Fuel Bundles and De-cladding of Fuel Pins in the Highly Radioactive...

8/14/2019 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 Bundlesand Decladding of Fuel Pins in the

Highly Radioactive Hot CellsG. 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: gosw ami@apsara .barc.ernet .in

Abstract

For reprocessing of PHWR fuel, fuel bun dles are at p resent chopp ed m echanically 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

bun dles into pins. In ad dition to pin sepa ration, the pins are to be chopp ed into small pieces

using a sma ll mechanical chopp er. Since the spent fuel is highly rad ioactive, all these operations

are p erformed rem otely in hot cells. Post irrad iation examination also requ ires 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 an d tested. This paper d escribes the experience gained d uring the d evelopment

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 d iameter and 495mm long. For the pu rpose 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 prob lems in mechan ical 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 individu al pins und er highly radioactive environm ent of the

hot cell. It wa s prop osed to carry ou t the disassembly of PHWR fuel bund les by laser cutting

of both the tie plates. Accordingly, the experiments were cond ucted at A FD, BARC to stud y

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 N d-YAG laser-cutting system has been assembled and tested for disassemblyof 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 moun ted on a 3-axis CNC tab le. The fuel bund le with tie end p late

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 tothe opposite end plate. In the proposed scheme, the bundle disassembly will be performed

in tw o stages in hot cells. In th e hot cell, both the end plates are cut into p ieces by laser to

separate fuel pins. It involves tools for han dling of bun dle, CNC station for laser movements

and beam d elivery system. A 150W Nd -YAG laser is used for cutting. All these operations

are performed remotely and automatically.

2. Feasibility Studies & Concept Design

Experiments w ere condu cted to stud y the feasibility of the scheme. The experiments w ere

condu cted inside a simu lated 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 w orkstation. The total path covered by the laser head w as 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 w as given helical and circum ferential 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 abou t 20 metres (0.5 m per linear cutie length of the tube X 2 cuts p er 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 PH WR fuel pins are mad e 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 p rocessing tim e and also observationof 100 mm cuts on the pallets due to laser, the option of cutting single pin by laser was not

considered ap propriate and mu ltiple shear device (hydrau lic press) for chopp ing was studied.

To see the performance of fuel chopping, fuel pins were cut into pieces in a hydraulic

press. The cut section wa s found to be clean w ith no generation of Zr fines[3]. No p inching


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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 nu clear rad iation on various op tical componen ts and Optical fiber used for beamdelivery inside the hot cell were studied and systematic experiments have been carried out

for various dose rates an d exposure times. The chan ge in optical properties like transm ission,

coloration and refractive index were recorded before and exposu re [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 prop er protection. This data w as used for design of laser beam d elivery 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 rad ioactive area before going for the d esign asp ect of the m achine. Accordingly

following aspects were considered in designing the machine [5].

l Remote operation of laser with fibre optics beam delivery system. Main parts of themachine are outside the radioactive compartment.

l Modular system design to allow easy removal of defective parts.

l Easy plug in type electrical conn ectivity to facilitate easy disconnection/ connection

through m anipulators.

l Special quartz lenses to sustain the radioactive environment.

4. The Sys tem

The system consists of a bu nd le lifting system to lift the fuel bu nd le 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 p redetermined path autom atically 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 d elivered by

CAT, Indore was interfaced with the CN C system and a nu mber of dum my su bassemblies

were cut u sing the laser to test the functioning of the machine. Two such systems are being

mad e. One of them will be integrated w ith other mechanical shear system for fuel reprocessing

at N RG wh ile the oth er system sha ll be used in RLG for p ost irradiation examination activities.

(a) For Fuel Reprocessing Division

A fully automatic remotely operable system has been designed and fabricated to carry

out N d-YAG laser cutting of the tie-plates followed by m echanical shear of individu al pins

(15 cuts at a time). The mechanical shear of each pin, carried out in AFD, BARC indicated

that arou nd 100-150 Kg load is need ed to chop the individu al pin. Add itionally it has a conveyer

system to carry the disman tled pins to the chop ping bed an d th en the single stroke chop pingaction is done by the system and then fuels are taken out for dissolution works.

(b) For Post Irradiation Examination D ivisi on

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

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


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examination of the ind ividual pins. For this purp ose add itionally a motorised bun dle 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 a nd 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 p arty w hile the laser system was su pp lied 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 p erform ed u sing the system au tom atically

and remotely.

6. Co ncl us ion

The tie plate cutting operation by laser was a very clean operation. It was found that

after cutting both the tie plates pins separa te easily as soon a s the gripp er is released. Laser

cutting also avoids opening up of the pins from the welds as no pressure is exerted on thepins d uring the cutting process. It shou ld also be mentioned that the dissolution time w ould

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 th e feasibility studies, a design for totally autom atic and rem otely operable

system has been made & the machine has been fabricated and tested for its satisfactory

performance.

7. References

[1] Laser cutting for d ismantling of PHWR fuel bund le- Dilip Kr, B.P. Badgu jar, G.L Goswam i-

National welding Seminar, IIW- CAT Indore Jan-94 (1994)

[2] Autom ated laser cutting system for disassembly of PHW R fuel bun dle - A report on laser

cutting trial - Sanjiv K. Jha AFD, BARC, Oct (1999)

[3] Single Pin shear for developm ent of laser based d isassembly system- A report - G.L.

Goswami & Munish Chandra, (2000)

[4] A Technical report on rad iation effect on optical compon ents and optical fibres. Life of

such system un der su stained rad iation. 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. Goswam i et al Proceedings sym posium on Thoria Technology, Ind ian N uclear

Society, Mumbai, (2000)


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Fig. 1. Gripper assembly with Fuel bundle

Fig. 2. Laser head & sensor head

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