Wendelstein 7-X, the world’s largest superconducting stellarator in Greifswald (Germany), started plasma experiments with a water-cooled plasma-facing wall in 2022, allowing for long pulse operation. In parallel, a project was launched in 2021 to develop a W based divertor, replacing the current CFC divertor, to demonstrate plasma performance of a stellarator with a reactor relevant plasma facing materials with low tritium retention. The project consists of two tasks: Based on experience from the previous experimental campaigns and improved physics modelling, the geometry of the plasma-facing surface of the divertor and baffles is optimized to prevent overloads and to improve exhaust. In parallel, the manufacturing technology for a W based target module is qualified. This paper gives a status update of project. It focusses on the conceptual design of a W based target module, the manufacturing technology and its qualification, which is conducted in the framework of the EUROfusion funded WPDIV program. A flat tile design in which a target module is made of a single target element is pursued. The technology must allow for moderate curvatures of the plasma-facing surface to follow the magnetic field lines. The target element is designed for steady state heat loads of 10 MW/m2 (as for the CFC divertor). Target modules of a similar size and weight as for the CFC divertor are assumed (approx. < 0.25 m2 and < 60 kg) using the existing water cooling infrastructure providing 5 l/s and roughly maximum 15 bar pressure drop per module. The main technology under qualification is based on a CuCrZr heat sink made either by additive manufacturing using laser powder bed fusion (LPBF) or by uniaxial diffusion welding of pre-machined forged CuCrZr plates. After heat treatment, the plasma-facing side of the heat sink is covered by W or if feasible by the more ductile WNiFe, preferably by coating or alternatively by hot isostatic pressing W based tiles with a soft OFE-Cu interlayer. Last step is a final machining of the plasma-exposed surface and the interfaces to the water supply lines and supports to correct manufacturing deformations.

In recent years, simulation has become increasingly important to industrial decision-making. As a result, expectations for simulation credibility have risen significantly. To achieve this goal, appropriate Verification, Validation & Uncertainty Quantification (VVUQ) processes are essential, with validation playing a central role. However, successful implementation of validation faces several challenges. One of the primary issues is the availability of dedicated high-quality experiments that serve as validation referents, in line with recommendations of existing standards. Therefore, a broader range of validation referents needs to be considered in practice, leading to a wide spectrum of validation approaches with varying levels of rigour and credibility. Consequently, assessing the rigour of validation becomes increasingly important.

In light of these challenges, this book focuses specifically on the validation of physics-based simulation models. Our aim is to provide guidance for industry practitioners, helping them overcome these obstacles and enhance the credibility of their simulation results.

The book has been written by a team of four authors, all from different engineering simulation backgrounds, aiming at covering a broad scope of engineering simulation domains. Its content intentionally avoids focusing on any specific discipline within engineering simulation or any particular industrial application.

There is a justifiable debate over the use and definition of the word “validation” in the context of engineering simulation. NAFEMS has adopted the definition of ISO 9000, as per ISO 9001:2015, “Quality management systems, Requirements”, which is the basis of the NAFEMS “Engineering Simulation Quality Management Standard” (ESQMS). Other organizations, notably the American Society of Mechanical Engineers (ASME) and their ASME VVUQ standards, require validation to be based on empirical evidence, i.e. physical experiments. The ISO/NAFEMS definition embeds the more stringent ASME definition as a subset, but allows for a wider range of validation referents, so that the same processes can be applied on applications with varying criticality and credibility requirements.

There are two key contributions of this book. First of all, it formally introduces the concept of a “spectrum of validation methods”. The methods span the range from the strict definition of validation used in the ASME VVUQ standards, through to weaker validation approaches, including those supported by expert review. The introduction of the spectrum of validation methods is purposely high level and may need appropriate tailoring for application to specific industry applications. This tailoring is well outside of the scope of this book. The second main contribution of the book lies in the formal definition of validation rigour attributes that significantly impact the credibility of simulations. It is recommended to incorporate these rigour attributes during the specification and planning of validation activities. Furthermore, this contribution is expected to stimulate additional work, particularly in defining a validation rigour scale.