Problems with the current state of the art with respect to the requirements of ITER have been outlined in the introduction of this work and are explored in the publication [Shuff 2009]. The issues can be summarised as follows:
Alignment Single pass autogenous welding has the lowest risk of failure compared to other forms of welding for UHV pipe work. In order to ensure good reliability for this method the tolerances found to be effective at JET were: abutment + 0.1 mm, co-axial alignment +/- 0.2 mm and angular axial alignment +/- 0.08 . These tolerances were possible at JET due to the use of thin walled compliant pipes and thin walled bellows.
The same degree of tolerance will prove difficult to achieve on the ITER machine given the use of thick pipes (5-9 mm thickness) and the anticipated prohibition of bellows. Multiple pass welding with filler material can tolerate greater misalignment - up to 0.5 mm abutment - but carries the risk of lack of fusion and voids in the weld seam and as such was found to have a higher failure rate at JET.
Risk of distortion during cutting Various unexpected anomalies were encountered during pipe maintenance operations at JET. These were somewhat mitigated by using thin walled pipes (<1.5 mm). This for example allowed defects in circularity discovered after cutting to be redressed by simple jacking tools. Thicker pipes such as those anticipated for ITER will prove more difficult to manipulate using such tools. Cut edges should ideally be inspected prior to re-welding. Unacceptable finish on the pipe ends will require additional tooling for edge profiling, further complicating the task.
Lost material following cutting Material lost during cutting must somehow be made up in the subsequent re-join. Typically this is solved by utilising some adjustment in bellows. However the use of bellows has been ruled out at ITER for in-vessel systems therefore replacement components must be designed with additional material to bridge the resultant gap. Cut-off material also represents a contamination risk to the VV and must be carefully collected.
Limitation in the number of re-joins Re-welding must be performed outside the Heat Affected Zone (HAZ) of the previous weld, therefore only a limited number of re-welds can be made for a given length of connecting pipe-work.
Large tool inventory Many tools are required for RH pipe maintenance using cutting and welding. The tools covered in the state of the art are the bare minimum necessary for pipe maintenance, careful study of the literature and the return of experience form JET shows additional tools are necessary for tasks such as refurbishment, metrology and bellows manipulation:
- Orbital cutting tool
- Cutting debris extraction device
- Refurbishment tools: de-burring, circularity and bevelling
- Metrology tools: laser/tactile tools to measure in VV stub, to assess pipe fit up; tactile sensors, visual feedback
- Alignment tools
- Bellows manipulation tooling
- NDT tooling: X-ray, ultrasonic, He leak detection.
- Orbital welding tool.
Complex mechanical tools The tools described in the above are relatively complex mechanical systems with many moving parts. Such tools have an appreciable risk of failure compared to solid state devices.
The issues cited at above were found to be problematic at JET; the more stringent demands of ITER in terms of tool access, radiation tolerance and structural integrity will serve to magnify these concerns.
3 REVIEW OF ALTERNATIVE PIPE JOINTING TECHNOLOGIES
In view of the shortcomings in the available RH for fusion type pipe jointing technology explored in chapter 2 state of the art, in this chapter a broad literature search of applicable alternative pipe jointing technologies not based on cutting and welding was performed, with joint reversibility/disassembly as the primary search criterion.
Material joining is a ubiquitous phenomenon, present in most aspects of our daily lives. Brazing, soldering, riveting, swaging, bolting, bonding to give a few examples and their countless incarnations present the Remote Handling engineer with a seemingly broad choice of alternative solutions to welding and cutting.
To focus the search for an alternative joining method it is worth revisiting the engineering requirements set out in table 2, summarised here:
- High strength (comparable to welded joint) - High durability (comparable to welded joint) - Metallic (Stainless Steel) construction
- Operating temperature 240 °C - Max Leak rate 10-9Pa•m3•sec-1 - Ease of disassembly
As part of the search for an alternative joining method to welding and cutting, diverse industries were considered such as oil & gas and chemical processing.
It quickly became clear that the available solutions would be limited to a relatively small domain due to one requirement in particular, the leak rate. As we have seen Tokomaks require an ultra-high vacuum to operate effectively, this narrows the available joining technologies considerably. The degree of leak tightness required, 10-9Pa•m3•sec-1, is the equivalent of approximately no more than one droplet of water every 50 years. For most pipe jointing
applications this level of stringency is clearly not necessary. To take an example, for swaged type pipe joints encountered in the oil and gas industry as part of this research, half the leak rate stated above would be considered more than necessary.
Much of the material cited in this chapter is therefore drawn from the relatively narrow domain where vacuum systems are to be found such as high energy physics and some industrial applications.