Engineerlive.com 22nd May 2018
Case study showing how advanced technology is speeding up MRO at the world’s largest nuclear fusion facility
As the world’s largest nuclear fusion power experiment, the JET (Joint European Torus) nuclear fusion tokamak is designed to harness energy with the intention of furthering the development of fusion power generation. Fusion is based on the same principle that powers our sun and stars and is a key stepping stone towards a carbon-free world in energy production.
Operated by the UK Atomic Energy Authority’s (UKAEA) RACE (Remote Applications in Challenging Environments) department at Culham Science Centre near Oxford for the EuroFusion consortium of European fusion scientists, the project started in 1983 and is at the cutting edge of scientific development.
Third Dimension supports quality control and maintenance, repair and overhaul (MRO) of JET with its profile measurement systems. When the company originally started working with UKAEA, a third generation GapGun was installed onsite and used for inspection, in and out of the fusion rig, for many years. However, the GapGun, designed for handheld measurements, was upgraded in 2017 to the recently launched Vectro system.
Vectro is not only based on the fifth generation GapGun Pro technology, but was also designed to be installed as an integrated robotic inspection tool. Already, it has proven to deliver improvements to the speed and productivity of maintenance procedures for UKAEA. This has been achieved with the authority’s MASCOT robot.
During operation, the reactor runs for 30 seconds every half hour, with scientists from around the world eagerly awaiting their data and test results. The tokamak has a heating capacity of around 40MW. To achieve power generation through nuclear fusion, plasma temperatures of over 100 million °C are required. This makes the reactor the hottest temperature in the solar system, hotter than the sun.
Inside the reactor specially designed tiles are densely packed to cover the inner area of the tokamak core and protect it from the extreme temperatures and hostile environment generated by the process. These castellated, beryllium-coated Inconel tiles are made in the USA at a cost of over US$700 per 100g.
Tiles are around 1cm square and are slightly offset at an angle to encourage
the hot gas – known as plasma – in the core to circulate in a controlled manner. This presents a challenge to ensure that the tiles are within a strictly controlled tolerance band.
Any excessive amount of step or gap between tiles increases the danger that plasma could cause tiles to detach or get damaged. This would then mean expensive replacements and so inspection and the correct positioning carried out on these tiles is critical to the project.
However, inevitably tiles do get damaged occasionally – often by plasma escaping from JET’s powerful magnetic fields. Therefore, as part of regularly scheduled maintenance procedures, the JET facility is closed for six months every two years for an overhaul, and this is when the Vectro comes into play.
Due to the hostile environment of the reactor, it is left to cool for a couple of months before the MRO begins; though even then it is still unsafe for humans to enter the tokamak safely without the use of protective suits. However, Vectro thrives and is effective under these conditions and so is operated remotely with a robotic device called MASCOT.
MASCOT is mounted onto an in-vessel transporter system to enable Vectro to check for damage to the surface of every single tile lining the reactor. Each tile is inspected to see if it needs to be replaced and if so, to ensure that the replacements are re-positioned in exactly the right place and orientation.
MASCOT is a highly dextrous haptic force-feedback master-slave telemanipulator, with each kinematically similar master or slave unit consisting of two seven-degrees-of-freedom arms. The MASCOT master station is driven by experienced remote handling operators and can be positioned around the vessel by a transporter system; a 12m-long articulated robot.
James Kent, remote handling development engineer at RACE, says: “Every tile we replace, we check with the Vectro system. This speeds up our overhaul time, which means JET can be up and running again sooner, delivering the results that will help to make fusion a dependable source of energy in the future. GapGun and Vectro are very precise. We couldn’t position or check the tiles as accurately or efficiently without them.”
During a regularly scheduled shutdown, around 600 tiles are removed and replaced during the six-month period including many sample divertor tiles for chemical and physical examination.
The divertor is a device within the JET tokamak that allows removal of waste material from the plasma while the reactor is operating. This allows control over the build-up of fusion products in the fuel and removes impurities in the plasma that have entered from the vessel lining. A diverter is made up of the following components costing between £60,000 – £100,000:
* 48 Gasbox Inner Carrier (tiles)
* 48 Gasbox Outer Carrier (tiles)
* 48 Bulk Tungsten LBSRP (tiles)
* 48 Base Carriers (tiles)
Vectro is used to check any tile that is replaced and to quality check new tiles before they are installed into the JET wall. During the 2010-2011 shutdown, every single tile was replaced, which was in the region of 4-5,000 tiles.
The GapGun technology is the only way to check that the tiles are in the right position, within 10 microns, which ensures minimal damage when the reactor is operational. It is impossible for the human eye to detect to such accuracy.
Before GapGun and Vectro, operators had to check quality by eye, using a standard gap flush test, compare against a checklist and manually input all data into a spreadsheet. This was slow and prone to error. GapGun, and now Vectro, deliver the results immediately and electronically, so operators have a record of what has been done. Saving the project time and money, they deliver repeatable results time and time again.
Using this inspection method means that not only is it possible to reduce the number of tiles that fail during experiments (allowing for a higher chance of successful tests), but that in the future it could also be possible to meet even tighter tolerances.
Looking ahead to the next phase of fusion power development, the ITER reactor in the south of France is currently being built and is due for completion in 2035. JET is carrying out technical preparations for ITER to ensure it is a success – playing a powerful role in the development of fusion energy.
UKAEA’s RACE is now developing the next phase of the robot, MASCOT 6, to be launched in 2018. Robert Howell, Mechatronics Engineer at RACE, explains: “MASCOT 6 addresses the obsolescence issues present in the older MASCOT 4.5 system but also introduces performance improvements and new features. This includes: new actuator designs for both the master and slave units; a modern control system; and improvements to the system software and the operator’s GUI.”