Dismantling of a steam dryer in a

Nuclear power plant using diamond wire sawing technique

Executing company:

Norbert Braun GmbH Breitefeld 11

64839 Münster/Germany as subcontractor of:

Industriestraße 13 63755 Alzenau

1. Starting point and type of problem In the scope of dismantling mobile built-in parts of the reactor pressure vessel in a nuclear power plant the steam dryer 3 (DT3) made of special steel should be dismantled into individual parts and packed in Type IV Konrad containers. 1.1. Dimensions and dose rate of the DT3 The DT3 showed the following dimensions and weights:

Dimensions (diameter x height): 5.25 m x 3.5 m

Mass: Approx. 39 to

Surface: Approx. 2400 m²

Material: 1.4550, 1.4552 and Inconel 750 (approx. 2 mass-%)

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Due to its high dose rate (DL) the DT3 was located in a flooded sedimentation tank in a storage position; the radiological marginal conditions were the following:

DL contact: 5 to 70 mSv/h

DL 1m distance: 3 to 5 mSv/h

DL 5m distance: 1.5 mSv/h

DL 10m distance: 0.7 mSv/h

Activation: 10 to 250 Bq/g

Contamination: 5.00 E+04 Bq/cm², α-proportion approx. 1%

Admissible dose rates of 20 mSv per year for professional staff exposed to radiation show that an employee would have reached the annually admissible dose rate after less than 20 minutes of contact already.

Figure 1: steam dryer 3 (DT3) in flooded service pool 1.2. Dismantling concept For reasons of radiation protection a procedure for dismantling the DT3 was chosen which should ensure minimization of the dose rate. In this case, the DT3 should be placed in a shutter tank, should be completely encased with concrete and afterwards dismantled applying the diamond wire sawing method. The wire sawing process should be remote-controlled and monitored via monitors located outside the radiation area. When dismantling DT1, which has a clearly lower dose rate than DT3, wire sawing proved to be a suitable technique for dismantling complex steel components. It provides remote-controlled and automatic working to a large extent so that the staff's period of stay in the radiation area can be kept to a minimum. The periods of stay in the radiation area are

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relatively short especially when invention measures e.g. for changing the wire lock are carried out. For shielding reasons DT3 should be embedded in a layer of at least 0.2 m thick concrete on all sides and in one of at least 0.4 m on the lower side. The concrete had to be suitable for penetrating the perforated plates existing in the DT3 having a hole diameter of 6.5 mm and for safe filling of the existing dryer packages having a lamella distance of 15 mm. For shielding reasons the concrete had to have a density of at least 1.8 g/cm³.

Figure 2: DT3 in shutter tank 1.3. Packaging concept The cut parts should be dimensioned such that they could be packed in Type IV Konrad containers that are suitable for final disposal. This resulted in the maximal dimensions of 1400 mm x 1400 mm x 1100 mm if two segment pieces are loaded per container and in maximal dimensions of 2800 mm x 1400 mm x 1100 mm if one segment piece is loaded per container. The segment pieces should be placed in Type IV Konrad containers in a way that the same distance is kept between cut pieces and walls within a container. The hollow spaces should be cast with (qualified) concrete being suitable for final disposal. Furthermore, dismantling of the DT 3 was to be conceived in a way that the lowest possible number of Konrad containers would be necessary. This should be illustrated in a detailed cutting sequence and packaging plan. Dismantling and packaging should be carried out in room 6.202 of the solid waste store (FSL). 1.4. Radiation protection and estimated collective dose Room 6.202 was closed by an α-tent. To prevent aerosols from escaping a sub-atmospheric pressure holding system was created with separate ventilation. The estimated values of the average local dose rate ranged from 10 to 6000 µSv/h in the working area. Figure 3: Layout plan and facilities (overview)

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Upon issuing the radiation protection plan for the selected procedure, a collective dose in the amount of 70.6 mSv was estimated for dismounting, dismantling and conditioning of the steam dryer DT3. The above mentioned procedure was presented to the expert and to the approving authority. 2. Pretest at 1:1 scale (cold test) A pretest should prove that concreting of the DT 3 as provided and diamond wire sawing are appropriate for carrying out vertical cuts on the DT 3 later on under the narrow room conditions specified. The distance between room walls and the outer formwork was partially only 30 cm. 2.1. Test structure A test structure was chosen which ought to demonstrate the situation of the most difficult wire saw cut referring to the cut surface and to the steel cut surface to be expected at KWW. For this reason a model was built with the same materials and dimensions as they had been used with DT 3 which illustrated the middle part of the DT 3 as 1:1 copy. The model was installed on the floor of a hall. For simulating the later position of the diamond wire saw two 7 m high room scaffoldings were installed on the right and left of the model and a 10 cm thick prefabricated reinforced concrete slab, where the wire saw was erected and fastened with dowels, was placed onto them. As later done in the original steam dryer, a steel formwork with walls of 8 mm thickness was installed in the round area of the DT3 and in the straight area of DT3 another one with walls of 15 mm thickness which surrounded the replica of the middle part of the DT3 true to the original. After that, the replica was encased with concrete in the formwork.

Figure 4: Model at a 1:1 scale

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Figure 5: Replica of the steam dryer built-in parts 2.2. Concrete mixture In cooperation with the company CEMEX Deutschland AG a special concrete mixture was produced. It was suited to flow through a perforated plate having a hole diameter of 6.5 mm and to safely fill the hollow spaces of the dryer packages having a lamella distance of 15 mm. This had already been proved with test pieces in tests prior to the cold test. Moreover all concrete types were subjected to a drilling test in order to make sure that the concrete mixture would not cause smearing or glazing of the diamond segments and thus "polishing" of the diamond tools. Furthermore it was proved that the chosen concrete was capable of sharpening the diamond tools sufficiently during the cutting process.

Figure 6: Proof of the required properties of concrete

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2.3. Remote-controlled adjustable cable guide rolls After complete installation, shuttering and encasing with concrete, two 8 m long steel girders IP 200 were mounted and welded above the model; they were supported by the scaffolding towers and doweled. Conventional sawing rails were screwed on the steel girders; they were equipped with saw carriages where double swivel caster blocks made by Hilti AG were mounted. For each roller a counter-roller was installed as pressure safety roller so that the diamond wire was not able to jump off the roller. One of the saw carriages which carried the traction wire could be hydraulically moved during the cutting process, the other one could only be moved using a crank handle. This was changed after the pretest so that both saw carriages were hydraulically movable and remote controlled during hot test. The swivel caster blocks could be moved out of the radiation area on the 8 m long sawing rails so that the sawing wire could be threaded in the dead angle of radiation for new cuts. Due to any mobility of the saw carriage it was possible to change the position of the rollers for each situation and to adjust an optimal sawing geometry during the cutting process. In particular oscillating of the diamond wire could be avoided at any time as it was possible to build up a counter-amplitude towards the arising oscillation which neutralized it when slightly displacing the saw bucks and minimally changing the wire speed. 2.4. Selection of the diamond wire saw For the pretest the same wire saw was used as the one employed for dismantling later on. The wire saw chosen was a special design by Hilti AG, a WS 30 E type. Wire tensioning is effected through pneumatic cylinders with a pressure of up to 6 bars. The wire speed can be progressively adjusted between 0 and 35 m/s. From pneumatic wire tensioning we hoped for absorbing stress concentration at the wire e.g. when the diamond wire would be pulled along a sharp steel edge and thus for minimizing the risk of wire breaks. Technical data of the big wire saw: Wire storage: 25 m Power: 30 KW Voltage: 400 V, 50 Hz, 3 PH/N/PE Total weight: 560 kg Pneumatic connection: 6 bar Motor speed: 150 – 2200 rpm 2.5. Preparation of the diamond wires and of the wire guidance at the cut In order to enable optimal packaging of the cut pieces in Konrad containers it was necessary to keep slight cut tolerances of maximally 3 cm on the cut surface of 5.90 m x 3.64 m = 21.5 m² of the longest cut in concreted and shuttered condition. This required precise cutting. This was ensured if the girders lying above the steam dryer were accurately aligned with the sawing rail and the double swivel roller blocks for the diamond wire in precise horizontal direction and vertically above the planned vertical cut so that the horizontal and vertical traction forces were exactly in the requested cutting line. In addition it was necessary to avoid untrue sawing of the wire. At the same time, preparatory measures had to be taken to insert the replacement wires safely in the existing kerf if necessary. This happened when forming, exactly positioning, and welding two ducts each lying vertically above each other beneath the steel formwork using metal profiles of 1 mm thickness. In this case the diamond wire of 10.5 mm diameter, which was inserted in all

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ducts, was running in the upper duct. Two 6-mm diameter steel wires each were inserted in the lower duct in order to be able to insert replacement wires if needed. Should a replacement wire be required it had been planned to first try insertion of a new wire using the used wire before applying a steel wire. In case that a steel wire was needed, it had been planned to first insert two other steel wires in the wire duct before inserting a diamond wire so that a new steel wire would be available over and over again. Due to the fact that a long period of performance had been planned the upper wire guide ducts including the rubber-lined diamond wire were sealed with a special mounting foam while the steel wires in the lower duct were led in a plastic hose so that penetration of slurry and thus possible blocking of the sawing wires as well as possible corrosion were avoided. In order to keep friction and early wear of the diamond wires at the lower edges of the steel formwork to a minimum, these had been executed in beveled design. In addition all the wires and consequently all the wire ducts had to be applied on different levels according to the cutting sequence so that the separating cuts partially running vertically towards each other would not cause that other wires already inserted and their ducts were cut through. The diamond and steel wires including the hoses around them had been safely fastened at the edge of the cutting pit and outside the radiation area as later it was not possible any more to enter the cutting pit due to the high dose rate after placing the DT3. 2.6. Remote-controlled cutting process During the pretest, the cutting process was controlled visually and in parallel via installed monitors. Pneumatic pretensioning of the wire, wire speed, power consumption, and the use of cooling water were adjusted and controlled at the control unit of the wire saw. The position of the double swivel roller blocks was controlled via hydraulic feed motion of the saw carriage and monitored via monitors. Furthermore a hydrophone (= subaqueous microphone) was inserted in the wire saw cut to transmit the sawing noise into the room via loudspeakers. Changes in the structures cut led to immediate change of the sound and intensified the monitoring personnel's attention. Besides the tone signal was graphically illustrated on a laptop so that changes in the cut were demonstrated visually as well. 2.7. Cooling water Special attention was drawn to the supply of cooling water for the diamond saw. Cooling water was supplied on the slack-side via water nozzles just above the cutting start. The cutting geometry was planned to be adjusted in a way to get an image of a drop shape cut through which the cooling water was pulled farthest possible into the cut. As it had been expected that the water would not be capable of reaching the complete contact area of the wire and the wire could excessively heat up, another water nozzle was applied at the wire exit so that rapid cooling of the wire was ensured. At the same time, the specially designed water nozzle located at the wire exit was used to spray off the highly contaminated slurry conveyed and to keep it in the sawpit. Additionally the double swivel roller blocks were encased on the mobile saw carriages in order to avoid splashing at deflection which would lead to contamination entrainment. The water for cooling the diamond wire was delivered via pumps from the collection tank below the DT3 model and led in the circuit so that a minimum of residues was produced.

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2.8. Selection of the diamond wire 40 m of galvanic diamond wire of 10.5 mm diameter were used for each of the 3 cuts in the pretest. A wire made by Tyrolit was used for the first cut; for the second cut a wire from another manufacturer was used. While it was possible to carry out the first cut with the Tyrolit wire through the steam dryer model embedded in concrete including steel formwork even though the cutting rate dramatically dropped at the end of cutting, it was not possible to finish the second cut when using the other manufacturer's diamond wire. It was necessary to insert another thinner diamond wire of 8.9 mm diameter. The third cut was carried out successfully again when using the same Tyrolit wire as in the first cut. In this case as well the wire was worn out and used up at the end of the cut, but the wire length of 40 m employed was sufficient to carry out the cut. For this reason the diamond wire BSG-1 made by Tyrolit, which had been used successfully in the pretest, was selected as first wire for the late works on the DT3. It includes conical pearls of 10.5 mm diameter with 40 pearls per running meter and is rubber-lined. A main wire was provided for each cut and there was the possibility to insert 2 galvanic replacement wires. The second wire should have a diameter of 8.3 to 9.5 mm, the third wire one of 7.5 mm. As the manufacturer Tyrolit was not able to deliver these thinner wire diameters at that time, a decision was made to use replacement wires made by the manufacturer Hilti AG. The pretest showed that a wire speed of 10.0 to 15.0 m/s led to good cutting results in regard of service life and cutting speed of the wire. An average cutting rate of 250 cm2/ h cut surface was achieved at high steel proportion. Based on the results of the pretests it was assumed that it was possible to carry out one cut safely on the component with a maximal cut surface of approx. 21 m2 using one or maximally two wires of 40.0 m length. Furthermore it was successfully demonstrated various times in the pretest that in the described procedure a second wire can be inserted in an existing cut starting from the upper scaffolding platform and outside the radiation field. Due to the wire breaks occurred in the pretest it was specified to exclusively use hinge joints as wire connectors. Because of the high steel proportion in the cut and the high load of the connectors involved here it was specified to change the joints every 4 hours. 2.9. Result of the pretest The pretest was successfully carried out in the presence of TÜV (Technical Inspection Authority), authorities, and quality assurance (QS). The procedures chosen proved to be appropriate to carry out the specifications required for dismantling the DT3. 3. Hot test 3.1. Manufacture of the collection tank and shutter tank A sheet metal tank was placed on the floor of room 6.003, covering the floor, in order to collect the cooling water arising when sawing. As described in the pretest, the ducts for diamond sawing wires and ducts for taking up the pallet fork were placed on this tank. After that the sawing wires were installed and fastened on the edge of the pit. One diamond wire and 2 steel wires were prepared for each cut.

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Figure 7: Installation of the wire ducts and fork ducts After that, the shutter tank made of steel was manufactured; it should take up the concrete required for radiation shielding. A 40 cm high pedestal was made of concrete to cover the ducts where two fastening bolts had been installed. They were used to specify the position of the DT3 when placing it on the concrete pedestal.

Figure 8: Installation of the shutter tank with concrete pedestal 3.2. Delivery of the DT3 Using a specially manufactured traverse the DT3 was transported from the flooded service pool into the solid waste store room 6.003. Transport was ensured without any problems after checking the entire transport route with a dummy beforehand. The DT3 was placed turned by 2.8° towards the west-east building axis. This was to make sure that during predismantling no sawing cut had to be led through the load application points and/or

pressure rings. The exact position of the DT3 in the shutter tank was ensured by means of locating pins serving as positioning aid. A grating with 30 cm long pins was installed above the DT3. The pins were positioned in a way that they were located exactly above the centre of gravity of each of the segments to be cut. When lifting the segments with the pallet fork these pins were used for fastening during transport.

Figure 9: DT3 with applied pin grating in the shutter tank

Figure 10: Alignment of DT3 turned by 2.8°

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3.3. Installation of the cameras and monitors 7 cameras, which were capable of monitoring the entire working area, had been installed for telecontrol of the dismantling device. The recordings could be called on three monitors at the control unit outside the α-tent. The cameras could be panned via remote control and provided multiple zooming.

Figure 11: Control unit with monitors

Figure 12: Control unit wire saw and monitoring of radiation protection in the side room 3.4. Installation of the postdismantling device A pallet fork had been developed for lifting the segments; it was possible to insert it in the respective duct of each segment via remote control. Together with the locating pins encased with concrete it was ensured that the unreinforced concrete was not loaded with traction forces during the lifting process.

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A tilting station was installed at + 2 m in room 6.202; it was used to put the vertically standing segments in a horizontal position for postdismantling. The segments could be taken into sawing position on an integrated roller track.

Figure 13: Tilting device

Figure 14: Building section Room 6.202 + 2m and Room 6.003 – 5m 3.5. Concreting of the DT3 Due to the high dose rate originating from the DT3 it was necessary to provide efficient shielding of the component. For this reason the inner volume as well as the outer jacket zone and the upper side of the steam dryer were cast in concrete. The entire DT3 should be encased with concrete in one process. The concrete should flow downwards between the shutter tank and the DT3 and from there from the bottom to the top on the steam way. High flowability of the concrete chosen should ensure that all the hollow spaces would be filled out to the largest extent.

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Figure 15: Encasing the steam dryer with concrete 3.6. Dismantling of the DT3 Dismantling and packaging of DT3 cut pieces were specified in a detailed cutting sequence and packaging plan. The cutting sequence was chosen in a way that the number of required Konrad containers was limited to a minimum. In particular it was necessary to pay attention to the DL values for a Konrad container (contact value: 2 mSv/h, 2m-value: 0.1 mSv/h) that these were not exceeded.

Figure 16: DT3 concreted with sawing wires positioned in front

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Figure 17: Cutting sequence plan

Figure 18: Cut through the steam dryer

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3.7. Installation of the sawing equipment After concreting the steam dryer, room 6.202 was closed using an α-tent. The saw was installed on the edge of room 6.003 and the prepared wires were aligned above the cutting course by means of deflection pulleys.

Figure 19: Installation of the wire saws and the girders with double swivel rollers 3.8. Performance of dismantling (wire saws) In room 6.003 eight cuts were carried out for predismantling using the wire saw and thus the steam dryer was divided into 24 segments.

Figure 20: Cut 1

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Dismantling was carried out in accordance with the cutting sequence plan using the wire saw described above following the afore-mentioned procedure. The wires had already been positioned in the wire ducts for each cut before installing the DT3. For checking whether separation of the segments had been completed all segments were moved by hydraulic punches. Otherwise incomplete separation could have resulted in dangerous complications or even in dropping of a part when taking out a segment.

Figure 21: DT3 separated with 8 cuts

Figure 22: Proof of separation when moving the segments using hydraulic punches 3.9. Postdismantling and packaging The postdismantling device consisted of a tilting module, roller track, and saw, load distribution frame with pressure cells, where KC were placed for loading, and different shielding walls.

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The separated segments of the DT3 were picked up by the pallet fork and placed on the tilting device via remote control. During transport on the pallet fork, the segment was secured against tilting by fixing the pin embedded in concrete with a stop plate. Upon placing the DT3 segment on the tilting station, the plate was lifted before tilting and the pin was fixed by a hook at the tilting station. It was possible to tilt the segment now hydraulically and, after removing the hook, it was moved into the provided sawing position with the help of the roller track via remote control. The segment was then dismantled again at the ratio 1/3 to 2/3 using the wire saw.

Figure 23: Lifting of the segment with the pallet fork

Figure 24: Tilting of the segment on the tilting station

Figure 25: Postdismantling with the wire saw 3.10. Dose rate measurement at a distance of 2m For assessment whether or not it was possible to keep the admissible DL values for the Konrad containers, DL measurements were performed at a distance of 2 m before placing the segment piece in the KC. The measurements were carried out via remote control while the segment piece was moved to markings in front of fixed DL measuring probes using a crane. The measuring values were transmitted to a screen at the control unit outside the tent. If the admissible values were exceeded, shielding plates had to be additionally inserted in the Konrad container.

Figure 26: Segment piece during DL measurement

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Figure 27: Placement of the segment piece in KC 3.11. Admissible container load The Type IV Konrad containers must not exceed a load of 20 Mg. In order to make sure that the limit is not exceeded even after concreting, the load of each segment piece was calculated. The KC was placed on four pressure cells so that the weight and position of the centre of gravity could be checked steadily. 3.12. Packaging concept Dismantling of the DT3 was planned in a way that a minimum number of Konrad containers was necessary. A detailed packaging plan described the specifications for each container which part and/or parts is/are to be put in. The Konrad containers were loaded, concreted, and closed with a cover in accordance with this packaging plan. The entire flow was documented for each container. 4. Problems occurred 4.1. Concreting of the DT3 It had been planned to encase DT3 and the entire shutter tank with concrete in one process from the bottom to the top just as in the pretest. As however fears were expressed that the shutter tank could not stand the pressure of approx. 90 m³ of fresh concrete, a decision was made at short term to concrete the tank in two steps.

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In the first step concrete was placed from the bottom going upwards. The concrete was distributed evenly up to the lower third of the dryer packages. In the second step the concrete was filled in from the top after bonding of the lower filling. It filled the chambers and had to flow through the perforated plates having a hole diameter of 6.5 mm afterwards in order to get into the dryer packages. From there it would have been necessary to fill the hollow spaces between baffle plate and dryer packages. However upon dismantling it was found out that the dryer package was not filled completely and the hollow spaces not at all which caused problems with wire sawing later on.

Figure 28: Hollow spaces between baffle plate and dryer package

Figure 29: Position of the hollow spaces

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4.2. Wire saws Already during the first cut problems occurred when the sawing wire entered the area of the hollow spaces. The wire ran unsteadily and got hooked so that the wire got stuck several times. The cut was completed after 64 h when just using the main wire of 30 m length. The sawing period was 64 hours. During cut 2 the problems increased. The wire was torn out of the joint several times. After a cutting period of 118 h the sawing works were stopped and the cut was examined with an endoscope. The result showed hollow spaces of up to approx. 1.20 m depth starting from the upper edge of the DT3. This corresponded to the height of the first section of concrete filling. On examination it was supposed that there was some coating on the dryer packages and perforated plates which had presumably extracted so much water from the concrete from the top during the second concrete filling process that flowability of the concrete was not ensured any more.

Figure 30: Examination of the hollow spaces with an endoscope The saw was therefore modified and adjusted to cut 3. It could be completed on schedule after 110 hours of cutting. Cuts 4 and 5 caused the same problems as cut 2 and had to be stopped after 92 respectively 135 hours of cutting. Cuts 6, 7, and 8 were carried out on schedule and completed. 4.3. Remedial action for solving the problems in cut 2, 4, and 5 Investigations showed that insufficient concreting during wire sawing caused loosening of internal structures such as e.g. baffle plates and drying lamella or vibrations leading to wire clamping or wire breaks. It was also found out that the turn of the DT3 by 2.8° had led to a considerable enlargement of the cut surface in pure special steel. The subsequent assessment showed that with cut 2

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for example a cut which should originally go in parallel to a 20 mm thick bulkhead wall suddenly pulled itself through the bulkhead wall at an angle of 2.8° due to the turn so that only in one place a coherent steel cut surface of 1.70 m x 1.20 m = 2.04 m² was created. Other places were not assessed. In connection with the missing concrete embedding this had the effect that considerable untrue sawing was caused which eventually led to wire clamping too. Partly clamped wires could not be retrieved from the kerf. As all stopped sawing cuts were checked it was possible to find out that all disturbing edges resulting in clamping of the sawing wires and thus stopping wire sawing were located in the area between the DT3 upper edge and the space 1.20 m underneath that is in the area of the hollow spaces. Therefore it was possible to use different alternative separation processes in order to accomplish separation of the segments. 4.3.1. Application of downstroke wire sawing The wire saw was adjusted to cut 2. As it was not possible to continue cut 2 from below, the wire saw was equipped with downstroke rollers in order to saw from the top to the bottom applying the downstroke wire sawing technique. After a total sawing time of 140 h, including 22 h of downstroke wire sawing, the cut was stopped again due to frequently arising wire clamps and downstroke wire sawing technique was rejected for completing cuts 2, 4, and 5. 4.3.2. Filling the hollow spaces with concrete We started thinking to cast the hollow spaces occurred in the DT3 subsequently. As however it was not possible to clearly locate the hollow spaces and to ensure complete concrete filling of all hollow spaces and/or due to the fact that it could not be excluded that cuts which had already been performed would be closed again, this method was refrained from. Together with the TÜV we found out that despite the hollow spaces the shielding effect of the concrete was sufficient for segment handling. The hollow spaces were not important for transporting the segments either. As other consequences were not recognizable at that time, dismantling was continued. 4.3.3. Use of a wall saw During cuts 4 and 5, especially metal structures of 30 to 80 cm depths from the upper edge of the DT3 were not completely separated. In spite of loose metal parts or vibrating special steel structures the remaining steel webs needed to be cut. For fastening the wall saw and for changing the saw blades the DT3 had to be entered shortly. The cuts themselves were then carried out via remote control. A hydraulic wall saw, make Hydrostress, was used here together with the 40 kW power drive unit RD-S RC and the sawing head FZ 2S. The sequence of saw blades of 800 mm, 1000 mm, 1200 mm, 1500 mm, 1800 mm, and 2000 mm enabled cuts of up to 90 cm depths due to an extremely careful cutting method; and final separation of the segments could be achieved with cut 4 and 5. As to cut 2 the cutting depths of 90 cm was however not sufficient, as the remaining metal webs were located at a depth of 100 cm which was visible in an endoscopic check.

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Figure 31: Wall sawing cuts in cut 2, 4, and 5 4.3.4. Use of a core drilling device With cut 2 the last metal structures were tried to be separated by core drillings. As done with the wall saw it was necessary to enter the DT3 shortly for fastening the drilling stand. Drilling was however carried out via remote control where the feed motion was ensured via propeller shaft. In spite of 14 drillings in total it was not possible to do it as the concrete parts fell off on the left and right of the metal webs and only vertical stand-alone and thin metal structures were left for separation. It was hardly possible to drill them vertically from the top. On the other hand the remaining webs were uncovered so that they were accessible for thermal separation.

Figure 32: Core drillings

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Figure 33: Position of the core drillings cut 2 4.3.5. Application of an oxygen lance Pretests carried out in the presence of TÜV and the competent authorities showed that using the oxygen lance was appropriate for rapid separation of the remaining webs. An aspiration system was installed for avoiding aerosols being released during the separation process. Afterwards the DT3 was entered with the oxygen lance and the last remaining webs were separated in cut 2.

Figure 34: Use of oxygen lance

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5. Summary Basically diamond wire sawing is appropriate for dismantling big and complex steel components via remote control. The cut surface of the 8 separation cuts was approx. 156 m² and the one of postdismantling was approx. 28 m². In total there were approx. 184 m² of steel and concrete cut. In total 620 m of galvanic wire were used here. Due to the high steel proportion it was eventually possible to reach a service life of 0.30 m² only per diamond wire m. The cutting speed with large cut surfaces of approx. 17 to 21 m² was low and only reached 0.16 to 0.21 m²/h. After solving the problems caused by incomplete concreting, the DT3 could be exactly dismantled in the specified individual parts through accurate placement of the cuts. As a result the provided minimum number of Konrad containers required could be achieved. The specified cutting precision of 3-cm deviation per cut could be kept and was even lower. Only 2-cm deviation was measured. Concreting of the component proved to be sensible for radiological reasons too. The dose load the staff involved were exposed to could thus be minimized. The staff's periods of stay in the radiation area was also minimized. The collective dose amounting to 70.6 mSv which had been estimated during planning was clearly stayed below. Actually 32.0 mSv arose, resulting from 17.5 mSv through interventions and dismounting works and 14.5 mSv from retreatment (decontamination and postdismantling). The alpha zone could be removed upon completed decontamination.

Figure 35: Handover of the Konrad container conditioned and suitable for final disposal/storage The fact that the collective dose really absorbed – despite unforeseeable problems during dismantling – is below the estimated dose value by approx. 55 % can be attributed to the well trained local staff. The instructions for radiation protection referring to works directly carried

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out on the contaminated components were excellently translated into action. Consequently dismantling and packaging of the DT3 were successfully completed; Type IV Konrad containers were conditioned to be suitable for final disposal and/or storage. 30.10.2008 / Georg Rachor