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ITER
In-Vessel Viewing System Analysis
 
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Abstract

Having recently developed a harmonious working relationship with ITER, CADFEM UK CAE Ltd. were contracted to carried out an extensive simulation analysis of the In-Vessel Viewing System (IVVS).

The In-Vessel Viewing System (IVVS) will be one of the essential machine diagnostic systems at ITER to provide information about the status of in-vessel and plasma facing components and to evaluate the dust inside the Vacuum Vessel. The current design consists of six scanning probes and their deployment systems, which are placed in dedicated ports at the divertor level. These units are located in resident guiding tubes 10 meters long, which allow the IVVS probes to go from their storage location to the scanning position by means of a simple straight translation. Moreover, each resident tube is supported inside the corresponding Vacuum Vessel and Cryostat port extensions, which are part of the primary confinement barrier. As the Vacuum Vessel and the Cryostat will move with respect to each other during operation (especially during baking) and during incidents and accidents (disruptions, vertical displacement events, seismic events), the structural integrity of the resident tube and the surrounding vacuum boundaries would be compromised if the required flexibility and supports are not appropriately assured. This paper focuses on the integration of the present design of the IVVS into the Vacuum-Vessel and Cryostat environment. It presents the adopted strategy to withstand all the main interfacing loads without damaging the confinement barriers and the corresponding analysis supporting it.

Sketch of the in vessel viewing system
Sketch of the in vessel viewing system
Description for the IVVS  
  • 6 units are installed in ITER
  • The probe can be inserted between the divertor outer target baffle and the lower outer blanket modules.
  • It can be operated between discharges or during maintenance
  • It can be operated in vacuum or atmospheric pressure
  • It can be operated with or without magnetic fields
  • Resident tube:
    • Support at the cryostat: hinge with two rotational degrees of freedom
    • Support in the VV: allows to slide radially
  • Cartrige, which is inserted into the resident tube
  • Mobile assembly carrying the optical probe
  • Shield block, protecting the optical probe when IVVS is retracted
  • Push chain, transporting the optical probe into the VV
Thermal analysis Map of temperature during nuclear heating

Nuclear heating: 450 s pulse length and 1800 s repetition time.

Map of temperature during nuclear heating. A max temperature of 123℃ was reached at the head after 15 cycles.

Baking: a temperature of 200℃ was reached after 14 h.

Load

  • Weight: The total weight of the IVVS is 2615 kg
  • For the IVVS, preliminary calculations indicate the total forces acting on each component due to these events are quite small because it is retracted. Forces above 1kN are present only on the ports (vessel side and cryostat side) and on the stub.
  • Seismic loads

SL-2 Floor Response Spectra

  • Thermal loads:
    • Nuclear heating: 0.04 W/cm3

Map of nuclear heating (W/m3) in the IVVS resident tube structure
Map of nuclear heating (W/m3) in the IVVS resident tube structure.

  • Interface loads: displacement during baking: 40 mm radially, 20 mm vertically.

Structural analysis

Only considering the self-weight of the resident tube and its internal components simulated as point masses, clashes with the ports have been found at the joint (bellows) of VV with cryostat port where the minimum clearance is about 10mm→ The implementation of a third support was considered. Results:

SL-2 event during baking

SL-2 event during baking SL-2 event during baking
Equivalent stress

In section 1, plastic collapse under SL-2 event is guaranteed following the RCC-MR 2007 Service D allowable stress at 240°C. Even under the consideration of this event as SMVH (corresponding to RCC-MR 2007 Service Level C) no plastic collapse is expected.

In section 2, the protection against plastic collapse under SL-2 event is guaranteed following the RCC-MR 2007 Service Level D. However, stress values do not meet the Service Level C criterion and dedicated analysis is required to analyze the SMHV event. Considering the enveloping factor of 0.73 to SL-2 results, it can be followed that Service Level C criterion is also fulfilled although the safety margin would need to be enlarged. In any case, this section has been identified as a hotspot to optimise in further design stages.

Conclusion

The conceptual designs suggests that

  • Seismic events and baking will be dominate load conditions
  • Cooling not required
  • Additional support for guide tube needed
  • Some sections of the guide tube require reinforcement
  • The tip of the guide tube needs optimization. Nevertheless, this can be easily solved with a tray type resident tube
Relative Displacements during baking
The VV will experience displacements of 20mm upwards and 40mm radially. The tube structure will deform during the vertical movement and some points will suffer displacements in the range of 25 mm. At this position, the clearance between tube and the VV is only 5mm. A modified geometry towards a tray shape, opening the upper part of the tube, was find the proper solution which is fully compatible with the deployment of the arm. Relative Displacements during baking

 

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