Development of Remote Handling Pipe Jointing Tools for ITER

Abstract

Welding is broadly defined as a permanent method of material joining. In the field of Remote Handling (RH) however the combined requirements for high quality joints and regular replacement of high value components and plant on magnetic confinement fusion machines have resulted in a situation where welding is customarily treated as a reversible joining method with the use of cutting tools. This atypical usage of welding requires a range of sophisticated tooling for cutting, refurbishment, metrology, alignment and re-welding operations. In the past, on projects where RH has been used extensively such the Joint European Torus (JET) the technical demands of this approach were met by adopting a conservative design rationale; joint designs were simplified to the point where autogenous single pass welds on thin walled pipes could be used. Thin walled pipes also permit the use of thin walled bellows adding compliance to the system and thus assisting joint fit up.

The RH pipe jointing requirements of future fusion machines like ITER are such that the conditions described for JET will not necessarily be possible, consequently a more sophisticated suite of RH jointing tooling is anticipated. Larger pipe wall thicknesses have been cited as part of the ITER design; this may necessitate multiple pass welding which has an associated increased risk of producing an unacceptable joint. Pipes with larger wall thicknesses have less compliance which complicates fit-up prior to re-welding. Mechanical cutting methods carry a risk of causing distortion to pipe ends during cutting and release of weld stresses, furthermore larger wall thicknesses make the correction of such deformations more difficult. Other complications associated with mechanical cutting and welding include the refurbishment and inspection of cut edges prior to re-welding and the risk of tool jamming/failure during cutting. Material lost during the cutting process must also be accommodated somehow in subsequent joining operations. This in addition to the need to reweld outside the heat affected zone of the previous weld limits the number of re-welding operations that can be performed for a given component.

In response to the problems of applying welding and cutting principals to the pipe jointing requirements of ITER and future fusion machines, this body of research considers a diverse range of alternative pipe jointing technologies applicable to RH. Brazing was selected as the most promising research domain. Following the development of the necessary theory covering the design of a novel pipe joint system used in combination with an exotic brazing technique, this work has resulted in the successful creation of a revolutionary pipe jointing solution. With the strength of a welded joint but without the complication of using inherently risky mechanical cutting processes for disassembly, the fruits of this research offer valuable benefits to the RH engineer and a diverse range of other pipe jointing applications.