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Technology Factsheet

Belgian Reactor 3 (BR3) Dismantling System

Category: Robotics > Dismantling and Retrieval > Bespoke System
Reference # : Model No :

Two main mechanical cutting techniques were selected: the circular saw and the band saw in association with a turntable. The goal was to cut the highly active internals into segments compatible with the final disposal waste package (400 l waste drum). All cutting operations were carried out underwater in the refuelling pool.

The decommissioning project for the BR3 pressurized water reactor was also important as a test bench for different dismantling tools and decontamination techniques. After the dismantling of the reactor pressure vessel and its internals, using different mechanical and thermal cutting tools, a new tool, water jet cutting, was used for the dismantling of some large contaminated components inside the reactor building. The equipment was supplied by the consortium CYBERNETIX – AQUARESE.

Site:Other
Industry:Nuclear
Size:Very Large (>100kg/200lb, >120cm/48in)
TRL:Operational (9)
TRL2:Operational (9)
Tether: *
Waterproof: *
Payload: *
Reach: *
Manipulator: *

Benefits

Limitations

Comments

The BR3 plant at Mol in Belgium was the first PWR (Pressurized Water Reactor) plant outside the USA (United States of America), and was built at the end of the fifties. It is from the same generation of reactors as those in Trino Vercellese (Italy) and Zorita (Spain). The reactor had a small net power output (10 MWe) but comprised all the loops and features of a commercial size PWR plant. In 1964, after 2 cycles, the original Westinghouse internals were exchanged (except for the thermal shield) with different internals for a project called “Vulcain”. The Westinghouse internals were stored in a shielded chamber situated in a corner of the refuelling pool. Although the intention had been to reload the original internals when the Vulcain experiment was completed, this was never done, and the Vulcain internals remained in the reactor until the final shutdown.

Three different cutting techniques were examined for dismantling the highly radioactive reactor internals: plasma arc torch cutting, electric discharge machining (EDM) and mechanical cutting using a milling cutter.

The results obtained led to the selection of underwater mechanical cutting as the preferred technique. The main advantages of mechanical cutting can be summarised as:
- well known technique, used in workshops and only requiring adaptation to work under water;
- secondary waste (chips or swarf) easily trapped;
- low amount of waste if the tool and the kerf are thin;
- no emission of smoke, gas or dissolved ions;
- overall operation duration comparable to other cutting processes.

Both techniques (circular saw and band saw) were shown to be reliable, usable and efficient. The circular saw produced more volume of secondary waste (metal swarfs) due to a greater kerf width. The required volume of metal (swarfs) to be removed was three times higher than with the band saw. The average overall cutting speed was 1.25 times higher with the band saw. During the project both types of cutting tools were used in a complementary way, but where possible (depending on the height, the shape and the existing access on both sides of the piece), use of the band saw was maximised.

It was originally planned to collect the swarf during the cut by means of a suction frame surrounding the saw blade. Swarf was also collected in a funnel with a collecting basket placed under and inside the work piece. On completion of the horizontal cutting of the Vulcain internals, due to frequent blocking of the suction system, the swarf collection was stopped during the cut, but was pushed into the funnel by a water jet after each cut. The remaining swarf located on the turntable was then sucked off at the end of each cutting campaign using a straight suction hose. The total calculated weight of swarf generated for the whole cutting campaign was 133 kg, of which 104 kg were collected by the two methods described above. The remaining 29 kg were located at the bottom of the pool and in the reactor pressure vessel, and were removed by suction afterwards.

The main experience gained on the BR3 project has shown:
- The benefit of full system decontamination in reducing operator doses, and lowering some waste categorisation. This operation should be carried out as soon as possible after shutdown if existing plant equipment such as the primary pumps, heat exchangers, valves, instrumentation, etc. is to be used during decommissioning. This is not only important for aging, maintenance and repair considerations, but also if the existing knowledge of operators in running that equipment is to be utilised.
- The importance of using full-scale mock-ups to trial the remote dismantling of highly radioactive components. The main advantages are:
(a) avoidance of having to solve equipment “teething problems” in the controlled zone and on contaminated pieces;
(b)optimisation of the cutting parameters to produce as little waste as possible and to work as fast as possible for a specific cutting tool and task;
(c) testing of the various parts of the dismantling procedure, including the handling of cut components, dismantling equipment and maintenance and tool exchange;
(d) operator training on the actual dismantling equipment in an environment similar to the real one, thus leading to shorter operational time and improved understanding of the functioning of the dismantling machines. This also improves the operators’ radiation protection.
- The benefit of using proven industrial cutting techniques for the dismantling of highly radioactive components. This avoids the R&D (Research & Development) or “teething problems” of new technologies and enables better estimation of the amount of waste generated and the actual time to set up and to operate the equipment.
- The use of robotics was more demanding than expected:
(a) the combination of the process with the deployment system proved to be very complicated. The very low speed and the high stability and rigidity required from the cutting process were very difficult to attain with a hydraulically driven telerobotic arm.
(b) much attention should be given to the “man-machine” interface. BR3 never tested a master arm in combination with a "virtual" model. Probably this could facilitate some interventions
(c) working with only indirect viewing by cameras is indispensable (hostile environment) but difficult if the working area is rather limited. The cameras used should be small, have a good resolution and be flexible. The biggest problems encountered were bad positioning, bad light and shadows from the environment.
- That research and development of new techniques, processes and procedures should be made in research centres and trialled in realistic environments to minimise problems when carrying out the actual dismantling on site.
- The benefit of using under water cutting for the remote dismantling of highly radioactive components. The use of water as radiation shielding is a very effective way of working. The water provides several advantages which are important for the dismantling operation:
(a) a good shielding capacity: e.g. for typical components from LWR (Light-Water Reactor) reactors, about 2 metres of water depth are enough for shielding;
(b) full visibility for the operators: as a lot of operations are one-off operations, and as one cannot avoid some surprises in dismantling, direct viewing (i.e. not through a television system) is a definitive advantage;
(c) a trapping medium for aerosols and dust produced by various cutting processes. It can also decrease the (toxic and dangerous) gas production from thermal cutting processes
(d) with adequate filtering and purification, it can even limit the production of effluents and waste from the operations.
- The underwater operations carried out on different work pieces (and for instance on the two sets of internals, having undergone different decay storage periods) have shown that the different specific activity of the components had no significant influence on the dose uptake of the operator. This can play an important role in the selection of a decommissioning strategy. The importance of setting up the waste routes for contaminated materials. The volume of materials to be sorted, handled and consigned is a huge undertaking in decommissioning logistics. The dismantling of a power plant or nuclear facility produces a very large amount of material (waste, contaminated materials, effluent) which has to be managed efficiently to avoid any bottlenecks in the process. The benefit of good planning for the installation of handling equipment (e.g. decontamination area, size reduction workshop, sorting area, measurement and characterization areas, truck loading area, etc), so that future dismantling work does not interfere with existing material transfer routes.

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