Insulated fixation system of plasma facing components to the divertor cassette in Eurofusion-DEMO

https://doi.org/10.1016/j.fusengdes.2020.111710Get rights and content

Highlights

  • The present work deal with the design activities of an insulated Plasma Facing Components-Cassette Body (PFCs-CB) support.

  • The PFC support model for European DEMO divertor has been developed and optimized in CAD environment including an electrical insulation system.

  • Structural and thermal FEM analysis have been performed with the aim to optimize the ceramic insulating system

The design activities of an insulated Plasma Facing Components-Cassette Body (PFCs-CB) support has been carried out under the pre-conceptual design phase for Eurofusion-DEMO Work Package DIV-1 "Divertor Cassette Design and Integration" - Eurofusion Power Plant Physics & Technology (PPPT) program.

The Eurofusion-DEMO divertor is a key in-vessel component with PFCs which directly interact with the plasma scrape-off layer. The PFCs have to cope with high heat loads, neutron irradiation and electromagnetic loads. The mechanical integrity of the PFCs and water cooling pipes can be jeopardized by high heat loads and by electromagnetic loads generated in a disruption event. In European-DEMO the possibility to estimate the heat load by measuring the relative thermocurrents is under investigation. In order to allow thermocurrents measurements, a divertor design option provides that PFCs are electrically insulated from CB.

In this work authors aim to analyze the opportunity that the PFC-CB fixing system incorporates an electrical insulation system, thus acquiring also an important diagnostic role in the measurement of the thermocurrents and in the management of the current flows. The possible use of ceramic material (e.g. alumina) as the insulating layer between the support components is investigated.

Introduction

Within the program activity Horizon 2020 [1,2] EUROfusion Consortium is finalizing the Pre Conceptual Design phase of a demonstration fusion power reactor (DEMO).

The DEMO-divertor (Fig. 1) is a key component [[3], [4], [5]]. It directly interacts with plasma Scrape-Off Layer and it has to withstand to complex load combinations of different nature (thermal, mechanical, electromagnetic, neutron loads, etc.).

The Plasma Facing Components-Cassette Body (PFCs-CB) fixing system (Fig. 2) has to cope with these loads ensuring the correct mechanical and thermal connection.

The heat load on PFCs must be controlled to prevent damage. It is possible to estimate the heat load by measuring the relative thermocurrents and it can be controlled introducing gas impurity in the plasma [[6], [7], [8]].

An isolated-PFCs divertor solution is a design option for reliable measurement of thermo-currents that are indirectly estimated as a voltage signal over a single resistive element (a shunt) attached to the CB [9,10]. Hence, it is necessary to develop an appropriate electrical insulation system between the PFCs and CB.

In this work authors aim to analyze the opportunity that the PFC-CB fixing system incorporates an insulation system, thus acquiring also an important diagnostic role in the measurement of the thermocurrents and in the management of the current flows (and relative Lorentz loads) in off-nominal operating conditions (e.g. Vertical Disruption Event_VDE).

The possible use of a ceramic material (e.g. alumina) as the insulating layer between the support components is investigated.

At this early stage of the study the preliminary dimensioning of the support is mainly influenced by the following requirements and design constraints:

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    Capability to withstand electromagnetic traction loads of 9 kN

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    Allow maintenance operations with relative disassembly of PFCs

  • -

    Allow thermal expansion of PFC

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    Electrically insulate the PFCs from the Cassette

Taking into account these requirements, the first CAD model of the DEMO PFC-CB support was generated using CAE system CATIA V5 (Fig. 3_a). The design was influenced by the ITER Inner Vertical Target (IVT) PFCs to CB fixing support described in [11].

A future step of this analysis foresees the realization of a support mockup on which electrical and mechanical tests will be performed. The mockup is focused only on the support not considering the tungsten tile attached to it. Therefore a CAD model of the support mockup has been designed starting from the DEMO PFC-CB support model and this mockup model was used for the study (Fig. 3_b). To allow technical testing, the top and bottom area of the support mockup model have been modified as described in the next section. These geometrical changes are present only in the areas at the interface between PFC and CB but do not alter the general support configuration.

Afterwards a design optimization was performed in terms of mechanical behaviour by means of a 3D FEM (Finite Element Method) analysis using the Design Exploration package of ANSYS Workbench.

Section snippets

Support geometry

The support components that play the role of mechanical connection between PFC and CB are the Support Plug, the Support Leg and the Pin (Fig. 3_c).

The support plug consists of two rectangular parallelepipeds. The lower one is the base and in the DEMO model it will be first inserted in a dedicated hole designed inside the cassette surface and then welded. In the mockup support model, the height of the base has been increased by 10 mm and two M6 holes have been inserted in the lower face to allow

Design optimization

After the realization of the first CAD model a structural verification was carried out by FEM with the Design Exploration package of ANSYS Workbench.

The attention has been focused mainly on the insulation system to have a first estimate of the stresses acting on the ceramic material. For this preliminary design calculation, a simplified structural design technique based on the Weibull theory was taken as a reference for the assessment of ceramic components [12]. The technique includes several

Conclusion

A CAD model concept of the PFC-CB support has been developed with an electrical insulating system based on ceramic alumina components.

A first design assessment and optimization has been carried out in order to have a first evaluation of the structural behavior of the insulating system and to reduce the volume fraction of the ceramic component with a tensile stress equal or greater than the 80% of the alumina maximum tensile stress.

Further studies are planned in order to make the design

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training program 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

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    This is purely due to perfect geometrical embedment of the PFC pipe into much stiffer and stronger tungsten monoblock. The value of 9 N/mm3, when multiplied by the volume of the PFC pipe segment (∼900 mm3), gives a total force of 8 kN, which is in good agreement with the requirement [19] that each monoblock support should be able to sustain Fc = 9 kN of tensile force.4 This amount of force can be induced, for example, when a total current of I = Fc/lB ≈ 21 kA is flowing through a PFC pipe, where l ≈ 70 mm is the distance between the two PFC supports (see Fig. 6) and B = 6 T is the expected magnetic field at the outer target position.

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