The ITER Pre-compression Rings – A First in Cryogenic Composite Technology

1

Hannu Rajainmaki, Arnaud Foussat, Jesus Rodriguez, David Evans, John Fanthome, Marcello Losasso, and Victor Diaz

Fusion for Energy

ITER Organisation

EADS CASA Espacio S.L.

Advanced Cryogenic Materials Ltd

ITER Design

Central Solenoid

Outer Intercoil Structure (OIS)

Toroidal Field Coil

Poloidal Field Coil

Machine Gravity Support

Cryostat

Feeders Inner Intercoil Structure (IIS)

Pre-compression

system

The interaction of the 9.1 MA current and the magnetic fields during operation create:

• In-plane forces:

• 403 MN centripetal force reacted by the cylindrical vault that form the 20° wedge concept of the 18 coils

• Outward forces in the outboard leg

• Out-of-plane forces:

• Overturning moments

• 300 T, but slender…

TF Coil forces

TF Coil forces are reacted by

• Friction between coils • Cylindrical closed vaulted shape that forms the 18 coils (20˚ wedge shape)

• IIS

• 13 dowels shared by adjacent TF coils

• OIS

• Upper and lower OIS

• 4 bands of Intermediate OIS

… and

• The Pre-compression system

Pre-compression rings

(X6 in total)

Floating flanges

(x4 per coil)

The pre-compression system

• The pre-compression system is the keystone of ITER. Their replacement during the machine life time is possible, but very difficult (expensive and time consuming).

• Pre-compression rings will apply a centripetal force of ~30 MN on top and bottom of each TF coil mitigating the breathing effect caused by the bursting forces occurring during plasma operation.

• Two main positive impacts:

• It reduces cyclic stresses in IIS keys allowing fatigue life within design criteria

• It reduces the toroidal stresses in the 4 bands per coil of the IOIS

Design criteria

An efficient design of the pre-compression system would require:

• centripetal radial force applied as close as possible to the IIS

• to minimize the load to keep together the facing keyways during the pulse,

• thermal expansion close to or bigger than stainless steel

• not to relax the assembly pre-load at operating conditions,

• Young’s modulus lower than that of the case of the TF coils

• to provide a higher elongation during the pre-tensioning to minimize the influence of TF coils assembly tolerances and/or settlement effects

• eddy currents hampering • to eliminate heat dissipation during

operation

‘Unidirectional’ glass fibre/epoxy composite rings were found as the most suitable option.

J. Knaster et al., “Design issues of the pre-compression rings of ITER”, Adv. Cryog. Eng. Mater. 56, pp. 145-154 (2010)

Rings

Operational requirements

The pre-compression rings present unique operational conditions

• Loaded at around 400 MPa in hoop tension and around 50 MPa in compression;

• Highest load at RT due to the combined expansion coefficients of composite, 316LN and Inconel (Superbolts)

• Cyclic loads of around +/-5% in hoop, radial and shear stress;

• Over 10 thermal cycles expected from RT to 4.5K;

• Ionizing radiation over 100 kGy;

• Continuously loaded for 20 years.

This requires 5 m diameter and ~900 cm2 cross section rings weighing >3,000 kg.

Composite ring manufacturing processes

Three processes were envisaged during the R&D phase: • Filament winding

• Resin wetted glass fibre tow is wound in tension on a mandrel mould.

• VARTM (VPI) • Dry glass fibre tow is wound in

tension on a mandrel mould and vacuum pressure impregnated.

• Prepreg tape winding • Pre-impregnated glass tape (tow)

is wound in tension on a mandrel mould.

Common challenges: • Only radial build is possible. How to

avoid wrinkles during the winding and compaction?

• How to NDT inspect 337mm thick section?

Ring center

AFT Automated Filament Placement

Composite lay-up process commonly used in the aerospace industries

• Automated, highly reliable Consistent quality;

• Tapes (tows) compacted during lay-up Allows curves surfaces;

• Each tow speed controlled separately Allows curved contours;

• Controlled heat applied Bonds effectively and minimizes trapped air.

AFP Advantages

WRINKLES:

The three envisaged processes Only radial build is possible

Inner layer Compression during winding wrinkles into inner layers

Compression during compaction 5-10% wrinkles into outer layers

No easy solution.

AFP build on any surface axial build on flat annular tool

Compression homogeneous No wrinkles

NDT:

The three envisaged processes Radial build

337 mm section to inspect

Radial sectorization possible, but difficult and risky

No solution

AFP axial build with thinner slices

testing each slice 100 % NDT

bonding slices and testing each layer 100 % NDT Ring center

Manufacturing Plan

• AFP on a flat annular tool

• Manufacturing of slices

• Each slice NDT tested separately

• Slices bonded together

• Each bond interface NDT tested separately

• Final overall machining

FE analysis

1. FE analysis: micromechanical and macro-mechanical calculations and environmental effects

• Expected ring load characteristics of the glass composite;

• Allowable defects number & sizes.

2. The studies to consider:

• Thermal cycles (x100) with a allowable ∆T

1. Material selection and qualification with coupons and mock-ups

• Mechanical, functional, logistic, etc.

2. Qualification with manufacturing and testing the sub-scale rings

• 1/5 scale in diameter, 1/160 in cross section.

3. Qualification with manufacturing and testing the prototype ring

• Process, tooling, NDT, material properties.

Process qualification

Model rings tested

Pre-compression Ring

X 800 heavier

Qualification Status

• Material selection in final stage; • Functional tests in final stage; • 1/5 scale ring manufacturing and testing after material selection; • Manufacturing and handling tooling in progress; • NDT assessment in progress.

Follow us on:

www.f4e.europa.eu

www.twitter.com/fusionforenergy

www.youtube.com/fusionforenergy