Final design review for ITER divertor

Starting 2 December 2008, the final design review for the ITER divertor will take place in Cadarache. The three-day meeting will be attended by about 50 experts from the ITER Organization, the concerned Domestic Agencies (Europe, Japan, Russia) plus additional experts.The panel will be chaired by Rene Raffray from the US. Prior to this meeting we talked to Mario Merola, Divertor Section Leader, about the goal of this review and the roadmap for the next months to come.

Newsline: Last year we went through a very comprehensive design review of the ITER machine as a whole. Why is this specific design review focusing on the divertor necessary?

Merola: Design reviews are an integral part of the systems engineering process. They are conducted to assess whether the proposed solution meets the design input requirements and whether the proposed solution is the most robust, efficient and effective solution to achieve the product requirements. With this specific design review we also want to assess the status of the design in terms of the completeness of the drawings and specifications. In particular, this final design review is mandatory for Quality Class 1 components like the divertor at the completion of the detailed design phase and before the signature of the Procurement Arrangement. It thus forms an important part of the ITER quality assurance program.

What are the key issues of this design review? Are we talking about choice of material or design?

All the features that may have an impact on the divertor design will be reviewed. After an overview of the ITER divertor requirements and physics basis, the divertor design will be presented, including all the physical interfaces concerned and a description of how the divertor system is integrated into the machine. The design supporting analysis will also be assessed as well as the manufacturing issues. These are of particular importance since the divertor is one of the most technically-challenging components of the ITER machine. The remote handling, the assembly procedure as well as the proposed schedule are also key features and form part of the review.

There has been a proposal from Europe to implement an all-tungsten divertor right from the start of ITER operations? What is the status of this proposal?

As a result of the review by the Science and Technology Advisory Committee (STAC) of the 2007 ITER Baseline design, the Topical Working Group (TWG) 07 "Divertor Armour Strategy" was established in November 2007. This Group included experts from the ITER Organization, and from the European, Russian, Japanese, US, Chinese and Korean Domestic Agencies. The conclusions were reported at the STAC meeting of May 2008. It was identified that operation with a carbon-fibre-reinforced carbon-composite (CFC) target has advantages for the start of ITER operation given its proven range of compatibility with a number of plasma conditions in present devices, particularly at low densities with significant additional heating. CFC also promises to make the development of techniques for ELM control and disruption mitigation easier by taking advantage of the larger tolerance of the plasma to carbon as opposed to tungsten and its absence of melting. A decision taken now to implement a full-tungsten divertor at start of operation would lead to a significant delay in the project, mainly due to the need to develop and qualify the high heat flux technologies required. Furthermore it would necessarily assume that the related physics R&D, planned for the period until 2013, is successful.

As a consequence, the TWG-07 recommended not to implement a divertor with full-tungsten armour at the start of operations but proposed to support the physics and technology R&D required to qualify the use of tungsten in the divertor starting from the second set of plasma-facing components and almost certainly in advance of deuterium-tritium operation. STAC agreed with the ITER recommendation.

Which parties are participating in the divertor design and manufacturing?

The procurement sharing foresees that the inner and outer vertical targets will be procured by Europe and Japan respectively. However, the related performance tests during construction will be carried out by the Russian Domestic Agency, which will also procure the divertor dome. All three divertor plasma-facing components have to be shipped to Europe to be assembled onto the cassette body, which is also procured by Europe, prior to final shipment to the ITER site. This complex sharing will bring additional challenges to the technical ones, but I am confident that we will cope with them.

When is the Procurement Arrangement foreseen to be signed?

The Procurement Arrangementss for the divertor plasma-facing components will be ready to be signed in February 2009, and for the cassette body in July 2009. We have already had several iterations with the Domestic Agencies concerned and I cannot identify any showstoppers. As a consequence, I hope that the Domestic Agencies will sign the Procurement Arrangements soon after we do.