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EU remote

Thermal-Hydraulics Engineer

thorizonLyonPosted 1 Sept 2026

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About this role

You will work at the heart of our reactor development, using modelling, simulation and experiments to understand how our systems behave and to guide the engineering choices we make. Your work will cover a broad range of questions, from the behaviour of individual components to the interaction of thermal-hydraulics with the wider reactor system. You will work closely with colleagues in system engineering, nuclear safety, neutronics, chemistry, mechanics and other areas of expertise.

Together, you will translate complex physical behaviour into practical design decisions. Your expertise will matter not only because of the analyses you perform, but because of the judgement you bring to the project. We are looking for someone who knows when a question requires sophisticated modelling, when a pragmatic engineering approach is sufficient, and when an assumption needs to be challenged. You will also help us close the loop between simulation and reality .

You will contribute to defining experiments, tests and prototypes that allow us to validate our models, challenge our assumptions and demonstrate that our innovations work outside the computer. Depending on the stage of the technology, your work may support our non-nuclear Pilot, the Pioneer nuclear demonstrator or the future FOAK and NOAK reactor designs. This is an opportunity to have a real impact on the development of a novel nuclear technology while helping build the thermal-hydraulics capability of a growing deep-tech company.

This role is based in Amsterdam, Netherlands or Lyon, France . Own and deliver thermal-hydraulics analyses that support important reactor and system design decisions. Translate engineering questions and system requirements into appropriate thermal-hydraulic models, simulations and analyses. Select the right modelling approach and computational tool for each problem, balancing accuracy, complexity, available data and project needs.

Perform high-quality CFD and system-level thermal-hydraulic studies and critically assess the results. Analyse thermal-hydraulic behaviour across different levels of the reactor design, from components and systems to integrated reactor behaviour. Investigate relevant reactor transients and the interaction between thermal-hydraulics and other physical phenomena, including neutronics. Work with system engineers to assess and improve the design of thermal-hydraulic systems and components, including heat exchangers, pumps and related equipment.

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Develop and improve modelling methodologies and tools that strengthen Thorizon's engineering capabilities, building computational workflows involving data preparation, simulation, post-processing and, where appropriate, coupling between different tools and physical models. Contribute to the verification and validation of computational tools and models used to support design and safety decisions following high standards like ASNR guide 28 and the execution of the thermal-hydraulic validation programme, including work performed with external research and engineering partners.

Help identify where experiments, tests or prototypes are needed to increase confidence in the technology. Review technical work produced internally and externally and challenge assumptions, methods and conclusions where appropriate. Support the technical specification and management of external specialists, engineering contractors and suppliers. Provide technical guidance to colleagues and share your knowledge and good practices with the wider engineering team.

Participate in technical exchanges with regulators and external experts when thermal-hydraulics is relevant to the project. Contribute to international initiatives and technical networks in thermal-hydraulics and advanced nuclear technology, including relevant IAEA and Euratom activities. Keep up with developments in modelling techniques, computational tools, experimental methods and thermal-hydraulics applied to advanced nuclear systems.

Work across different project teams and contribute to building a strong and collaborative thermal-hydraulics capability at Thorizon. Master’s degree in nuclear engineering, mechanical engineering, fluid mechanics, heat transfer, applied mathematics, or a related technical discipline. PhD in nuclear engineering, fluid mechanics, heat transfer, numerical methods, or a related field is an advantage. Around 8 years or more of relevant professional experience in thermal-hydraulics, CFD, reactor engineering, or a closely related field.

Strong hands-on experience with CFD methods, including RANS, LES and/or DES. Professional experience with CFD tools such as OpenFOAM, STAR-CCM+ or Fluent. Experience with thermal-hydraulic system codes such as SPECTRA, CATHARE, TRACE or ATHLET. Strong command of numerical methods and modelling approaches, with the ability to select an appropriate level of modelling for the engineering question at hand. Experience designing, developing or using complex computational workflows, including multiscale and/or multiphysics approaches.

Programming skills enabling the automation of engineering tasks, data preparation and processing, results post-processing, and integration of different computational tools. Sound understanding of the thermal-hydraulic challenges associated with molten salt reactor technology, or a strong interest in developing expertise in this area. Solid knowledge of nuclear reactor thermal-hydraulics, including key reactor transients and the interaction between thermal-hydraulics and neutronics.

Experience in the analysis or design of thermal-hydraulic systems and components, such as heat exchangers, pumps or similar equipment. Familiarity with verification and validation methodologies for scientific and engineering computational tools. Knowledge of relevant IAEA, NRC, ASNR or other national/international practices is an advantage. Experience in systems engineering, requirements management or safety-related engineering is an advantage.

But technical expertise is only part of what we are looking for. Take ownership. See what needs to be done and make it happen. Use your judgement and don’t wait for perfect instructions. Work as one team. Seek input from other disciplines and recognise that good solutions rarely come from one perspective. Speak up. Challenge assumptions, raise risks and be clear about what is known, assumed or still needs to be proven.

Learn and adapt. Stand by your position, but change it when the facts change. When things go wrong, learn from them rather than look for someone to blame. Keep things moving. Bring structure to complexity, focus on what matters and keep solutions as simple as possible without compromising quality. Build trust and raise the bar. Be open, direct and reliable. Care about your work and help others do their best. Enjoy the challenge.

The problems are difficult and the path is not always predictable. You find that energising. We value people who combine high standards with low ego : people who want to make an impact, challenge each other constructively and understand that ultimately, we are all working towards the same goal. You communicate fluently in English , both written and spoken. French or Dutch is an advantage. The opportunity to help realize the most promising reactor technology and advance Europe's energy future Being part of an exceptional team that combines engineering rigor, scientific excellence, and entrepreneurial drive Working in multidisciplinary teams on ambitious missions, with responsibility and meaningful impact from day one A fast-moving deep-tech environment where initiative is valued and people are trusted to make decisions Complex, multidisciplinary challenges that will stretch you and accelerate your growth Competitive compensation and participation in our employee profit-sharing plan An international workplace with offices in the heart of Amsterdam and Lyon

Source listing: personio_thorizon

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