As the first generation of nuclear reactors come to the end of their useful lives, it is estimated that around 200 reactors will be taken out of service worldwide over the next twenty years. Most nuclear reactors are designed with thick bioshields as protection against radiation leaks, and the controlled demolition of these structures will present unique problems, many of which can be solved by diamond cutting tools.
Decommisioning with Diamonds
Within the next few years one of the most famous landmarks in Britain will have vanished to be replaced by trees and green fields.
At least that's the theory behind the proposed demolition of' the Windscale Advanced Gas-cooled Reactor (WAGR) the famous spherical nuclear power station whose shape has dominated the Cumbrian skyline at Sellafield since 1962 In many ways it is appropriate that this reactor should be the first in what will be a long line of decommissioning, since it has always been a symbol of' nuclear power to both advocates and opponents alike It was the forerunner of' Britain's present chain of' AGR's which use uranium dioxide fuel pins instead of uranium metal clad in magnesium aluminium alloy which gave the earlier less their efficient Magnox reactors their name.
With an electrical Output of only 33 MW compared to the 1000 MW+ of' which modern reactors are capable, the WAGR has served its useful life. In 1981 it was shut down after 18 years of' being used both as a pilot plant for testing the performance of' the AGR system as well as operating as at full scale nuclear power station.
It is this latter role that makes the decommissioning of WAGR so important since it will be the first such fully operational reactor in the UK to be decommisioned. Others have been much smaller experimental reactors.
The three stages of' reactor decommissioning that are recognised internationally can be simply defined as:-
Stage I The reactor is shut down the control system disconnected and the fuel removed.
Stage 2 All buildings and plant that are outside the concrete bioshield protecting the reactor core are either demolished or adapted to other uses.
Stage 3 All remaining comp-onents and structures in and around the reactor core (including the bioshield) are removed or-demolished, leaving the land available for unrestricted re-use.
In 1981 the United Kingdom Atomic Energy Authority drew up a plan to decommission WAGR to stage 3- the 'greenfield site' concept. It is this stage that will present the greatest problems to UKAEA, since much of the demolished material will be 'activated' rather than just contaminated.
Activation means that the nuclei in the atoms in the surrounding material have been bombarded by stray neutrons during the nuclear fission of the uranium.
The result is that trace elements such as cobalt, in the steel and concrete that surround the nuclear core become radioactive themselves. Cont-aminated material, on the other hand, has radioactive contamination of its surfaces but is not active itself. Generally the contamination is modest and allows it to be classified as low-level nuclear waste.
To guarantee safety of handling and storage of the active waste, the UKAEA team are constructing a waste packaging building attached to the WAGR. The waste packaging plant will receive activated material, weigh it, check it for radioactivity and seal it inside concrete boxes. These 'boxes', each a 2.0m cube with 230mm thick walls, will contain nearly 6m³ of waste and around 150 boxes will be needed to accommodate the estimated 700 tonnes of intermediate level waste or activated waste.
Creating an access from the reactor core to the waste packaging plant involved cutting through the outer steel dome and reinforced concrete bioshield. Also, two of the four heat exchangers had to be jacked up to facilitate this access, which involved cutting a hole in the reinforced concrete roof above them. As would be expected, diamond sawing and stitch-drilling were used to cut the openings in the concrete, the work being carried out by Thermic (UK) LTD of Blackpool. This is undoubtedly the first of what will be a considerable amount of diamond tooled concrete demolition since bioshield thicknesses can be as much as 2.8 m.
Remote operation
The later stages of demolition will involve the cutting up of the pressure vessel which contains the nuclear core. The top dome of this 6.5 m diameter, 16 m high vessel will be removed and a specially designed robotic arm will cut up and remove the contents and the vessel itself. A remotely operated hoist will transfer this waste directly to the packaging plant. The final act will be to demolish the reinforced concrete bioshield. Parts of this close to the core will be activated, and, at this stage, it seems likely that a hydraulic chisel will be used to 'nibble' away at this active concrete-the steel can be cut up afterwards by oxyacetylene torch. The remaining reinforced concrete material can then be demolished by more conventional methods.
Final installation and operation of the robotic machine will not take place until considerable development trials have been carried out first. Present estimates are that £50 million will be spent on decommissioning WAGR. That figure is not unrealistic since a pioneering scheme like this can take no risks.
Besides demonstrating how nuclear reactors can be effectively demolished, the UKAEA findings may influence future methods of building nuclear reactors, to allow for eventual decommissioning. One of the major problems now is that reactors were not designed to be taken apart easily, as well as being constructed in materials easily susceptible to radiation. At WAGR, less than half of the estimated 1800 tonnes of waste demolition material is steel, but this contains 97% of the total radioactivity. Stainless steel, in particular, is the worst offender-comprising 5% of the total activated material . it contains 75% of the radiation. But lessons are being learnt in all aspects of the demolition-including diamond cutting. Not long ago. Marcrist was experimenting with a large remote-controlled floor and wall saw capable of single-pass cutting to 1m depth through reinforced concrete Developed specifically for use in cutting concrete bioshields, the machine consists of a 2.5 m diameter diamond sawblade, mounted conventional wallsaw track.
Several of these systems have been successfully commissioned although more recent developments of diamond wire sawing and plunge sawing may be more versatile for this particular type of work, In fact, many different methods of diamond Cutting will be used in decommissioning. So far at Windscale, the openings in the concrete bioshield which provide access to the waste packaging plant have been formed using conventional diamond sawing, stitch drilling and hydraulic bursting.
A hole of 5m diameter was cut in the roof above each of the two heat exchangers that were to be lifted. The roof construction consisted of a 600mm reinforced concrete slab on top of a grillage of 300mm deep RSJ's, welded to a mild steel plate which formed a soffit. The top reinforced concrete slab was removed by diamond sawing and hydraulic bursting.
Grid pattern
A remote - controlled Marcrist wall saw was used to make 12 cuts for each opening in a Grid pattern. Forty 200 mm diameter holes were then cored and Marcrist hydraulic bursters broke the slab up into blocks of approximately 600mm square A 900 mm blade made the initial cut followed by a 1400 mm diameter blade to the full 600 mm depth. Over 100 m² of concrete was sawn using these two Marcrist diamond blades. The infill concrete between the RSJ's was then broken out with a lightweight impact hammer mounted On a walk-behind remote controlled excavator and the steel was cut up by oxyacetylene torch.
Access openings in the much thicker parts of the bioshield (up to 2 m thick in parts) were formed by stitch drilling, usually with Marcrist diamond core bits of 100 mm diameter.
In future other techniques may become more appropriate In particular, diamond wire-sawing should prove its worth in cutting through the thickest of concrete bioshields. Marcrist have also developed special Dry cutting saws and drill bits which will also play their part, since the presence of' cutting fluid should be minimised in the nuclear environment because of possible contamination problems.
All in all, the lessons being learnt at Windscale should prove invaluable in the next twenty years. By this time, around 200 nuclear power stations around the world could be ready for decommissioning.