Target Station 1 cold box seal repair
08 Sep 2026
A major repair carried out during the summer shutdown has resolved a vacuum leak on the Target Station 1 cold boxes, helping to ensure the continued reliable operation of the moderator systems.
When protons strike the neutron targets at ISIS, they produce fast neutrons that must be slowed down before they can be used for experiments. On Target Station 1 (TS1) this is done by hydrogen and methane moderators. Since the moderators are constantly heated by radiation from the target, they must be kept at extremely low temperatures. The cold boxes provide the cooling that makes this possible, with one serving the hydrogen moderator and the other the methane moderator.
During the last cycle, the ISIS Target Operations team noticed a drop in the insulation vacuum on the cold boxes. Working with the ISIS Vacuum team, they traced the leak to the large O-ring seals between the cold box lids and the main body of the units.
Replacing the seals would be a significant task, requiring a complete strip-down of the cold boxes, including the removal of pneumatic lines, control and power cables, and cryogenic transfer lines. Being located close to the target in a shielded services area, the equipment also sits in a challenging radiation environment, requiring careful planning.
The root cause was traced to the way the original O-rings had been manufactured. The ends of the O-rings had been joined using adhesive, which can degrade over time in a radiation environment and cause the seal to fail. The hydrogen cold box was relatively straightforward to repair, but the methane cold box required additional design work because a welded drain line prevented the new O-ring from being fitted directly. The solution involved a new sealing arrangement, pipework modifications and welding work.
When the cold boxes were opened, the team encountered an unexpected challenge: the existing O-rings did not match the dimensions shown on the drawings, requiring replacement seals to be sourced at short notice. While waiting for the new parts, work continued on the methane cold box, including the installation of a new flange and changes to the drain pipework.
Once the new O-rings arrived, they were installed and the cold boxes reassembled before vacuum testing and the reinstatement of pipework, electrical systems and control connections. The successful repair removes a potential long-term reliability issue and reflects the teamwork between the Target Operations and Vacuum teams to deliver a complex repair in a challenging environment.
