Virtual simulation in tokamaks

13 April 2018

Simulation can be used to measure the temperature of the walls of experimental fusion reactors such as ITER. Jacques Delacour gives an overview of some of the work underway.

In nuclear fusion experiments, plasma-facing components are exposed to high heat fluxes, and infrared (IR) imaging diagnostics are routinely used for surveying surface temperatures to prevent damage. However, metallic components in the ITER tokamak will complicate the temperature estimation, as the metal walls create a highly reflective environment. ITER, currently under construction in Saint-Paul-lez-Durance, France, should lead to a prototype nuclear fusion facility that can provide electricity.

As René Magritte said in his famous painting, The Treachery of Images, a picture is not the reality. The same is true for nuclear fusion engineers measuring the temperatures of the walls of fusion reactors. The tungsten walls of the machine are highly reflective, making the interpretation of temperature measurement by IR thermography difficult. The bright colours of an IR image may be the result of reflections, instead of being associated with a real hot spot. While measuring these hotspots is an essential safety tool for the machine, the integrity and correct operation of the machine depends on the interpretation of the IR measurements.

Simulation is a complementary tool that models the transport of photons in the environment and thus is able to differentiate reflections from true hot spots.

OPTIS, a company that has worked with CEA, the French Atomic Energy and Alternative Energies Commission since 2010, uses its simulation software to create an accurate simulation of these infrared images. The collaboration aims to measure wall temperature of the ITER tokamak using simulation technology.

Spectral simulation software, for UV, IR and visible simulation, can simulate and measure the temperature of the walls of fusion machines by determining the difference between the proper temperature of the walls and the temperature resulting from the reflection of the infrared radiation on the wall’s materials. Using the complex vessel geometry and the thermal and optical properties of the surfaces, the simulation recreates the complex interactions between photons and materials to predict the global response of the complete infrared surveillance system.

The CEA has chosen to use the OPTIS Virtual BSDF Bench VBB to precisely define the materials to be used for tokamak projects like ITER. The virtual laboratory for measuring materials makes it possible to simulate the use of materials under realistic conditions.

It provides the CEA with precise and informative images, modelling complex physical phenomena involved in the interaction of photons with matter. The CEA can have virtual control of the quality of the materials and anticipate the influence of their ageing on the reactors, especially degradation of their surface state and the consequences on the interpretation of the infrared measurement. This tool contributes to a better control of the performance of IR measurements, which is essential to optimise the operation of future reactors and ensure their safe operation.

In addition, the Institute for Magnetic Fusion Research (IRFM) teams at the CEA centre in Cadarache can now, for the first time, precisely model the plasma radiation and study the polarization phenomena and their possible impact on the simulation results. OPTIS’s product SPEOS allows them to review and analyse the results as a function of the polarisation of the properties of this plasma.

This uses physically realistic numerical simulation, and the CEA will compare OPTIS’s simulations with the experimental results obtained in its WEST tokamak.

These simulations are very demanding in terms of computation time, and the CEA uses OPTIS high performance computing, which guarantees a higher calculation efficiency. The CEA has reduced its simulation time from one day to less than one hour, significantly increasing the number of simulations each year. Another advantage of the SPEOS software is that it is possible to conduct all these studies in a single environment – Dassault Systèmes’ Catia V5 CAD software – and the digital model of the entire thermonuclear reactor.

IR simulation may be used for similar projects. Temperature measurements in other parts of the tokamak, e.g. the colder target require compensation for reflections to make them realistic.

Several methods are being considered to compensate for the reflected flux. An active photo-thermal method using pulse or modulated sources may be used to measure the surface temperature locally, independently of the reflected flux. Another approach may be the exploitation of possible polarization of the reflected flux to minimise its contribution on the sensor plane. Finally, the photonic modelling itself could help to get the surface temperature by inverting the signals collected by the camera, taking into account the contribution of the reflected flux.

Further applications

Building further on simulation capabilities, other solutions are being developed to support other levels of the nuclear development. Immersive virtual reality tools, based on physical simulation, now assist with handling in a range of nuclear installations.

Initially a joint project developed by AIRBUS and OPTIS and dedicated to aerospace material and installations, the solution has been refined in a version dedicated to the nuclear industry. It consists of a fully immersive virtual reality tool, allowing the user to simulate various operations in a life-size environment. In real-time, operators virtually interact with the future installations, supervising maintenance, use, assembling and disassembling operations. Teams have two different points of view to get an understanding of the human interactions with the installations: a first-person view for a fully immersive experience, and the interactive manipulation of a human mannequin within the system, for collaborative reviews.

This direct human experimentation on a digital prototype has two complementary purposes for the nuclear industry.

The first one is a pre-step, to improve the global design of the nuclear infrastructure before it is built. From the initial design phase, it gives an understanding of the space, proportions and reachability of the elements in the nuclear plant. Users can evaluate the installation’s overall accessibility and comfort, the effort required to perform specific tasks and the fields of view of the operators, in order to validate the global ergonomics and ease the operations of the teams onsite.

The second purpose is training operators. They can use systems as in real life, even though they are not available yet. Operators can simulate interventions, even in harsh environments, to be ready for actual interventions while improving their safety, both during training and operation. The system can also simulate a radiation dose to operators during maintenance or dismantlement, to determine the best strategic actions to perform in an emergency.

This accurate and predictive physical simulation software can model innovative and complex design features to help qualify new reactor designs. The objectives of the simulation are to guarantee the safety of the analysis, operation, and design. Simulation addresses the engineering aspects of both nuclear development and security. As such, it is an important option for the further development of the nuclear industry.  

Author information: Jacques Delacour is CEO and founder of OPTIS 

OPTIS Vessel sector showing possible camera view from upper port plug. Vessel illustration (Credit: ©ITER Organization)
OPTIS Divertor illustration (Credit: ©ITER Organization)

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