Week 9: ‘Nibbling’ with a difference
The Neutral Beam Injection ‘additional heating’ system for JET was mentioned in week 4, along with the plans to enhance the performance of the system. Part of this enhancement project is to improve the cooling of the walls of the ducts between the neutral injection boxes (NIBs) and the torus, and this will be described in future instalments of the ‘Shutdown Weekly’. Access to these ducts is gained by removing two large highly specialised vacuum valves which are used to isolate the NIBs from the torus.
These valves are called the Rotary High Vacuum Valves (RHVV). They are welded into place using special thin sheet components called ‘weld lips’. The weld lips allow a small amount of movement between the components but ensure that a high-vacuum seal is maintained.
You may think that it would be difficult to break these welds, but it is done by a technique known as ‘nibbling’. A special pneumatically operated tool bites pieces of metal away from the edge of the weld lip, producing small regularly shaped sharp fragments. After this it is necessary to prise the weld lips apart as shown in the photograph.
Vacuum Technician Joe Banks can hardly be recognised because he is wearing protective clothing. This is necessary to protect him from any possibility of inhaling beryllium dust from the assembly and to ensure that the dust is not spread outside the ‘controlled area’ where he is working.
JET is currently the only tokamak in the World capable of operating with beryllium components inside the vacuum vessel. Beryllium is expected to be the material of many plasma-facing components in ITER.
The Shutdown Weekly team, Nick Balshaw and Petra Nieckchen, wish Merry Christmas!