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This paper provides an overview of analysis methodologies that can be employed to overcome many of the issues associated with the structural integrity assessment of plasma facing components. These issues arise from the multiple materials construction of the plasma facing components (tungsten armour, CuCrZr heat sink and copper interlayer) making direct application of standard (elastic) structural integrity assessment methods problematic. Example analysis results are used to illustrate these issues, from which it is concluded that PFC structural integrity assessment requires the use of elastoplastic analysis methods, and that this should include: a manufacturing simulation cycle to account for residual stress (and manufacturing strains); pre and post irradiation simulation cycles (with appropriate material models) to correctly assess accumulated ductility usage, and the study of plastic strain in tungsten recrystalised layers to anticipate deep cracking. The issues raised by stress/strain singularities (caused by dissimilar material joints) and multiple ratcheting mechanisms (global, local, material and bi-material ratcheting) are also discussed and proposals made.

In the framework of the European DEMO divertor project, several novel design concepts of the plasma-facing components of vertical targets are being developed. One of those concepts is the tungsten monoblock design (similar to the ITER divertor) but with a very thin interlayer (roughly 25 mm thick only) at the armor/tube bond interface instead of a thick (1 mm) copper interlayer as has been the case in the conventional tungsten monoblock design developed for ITER divertor. The thin interlayer serves as bonding agent, but not as structural constituent. The reasoning for this novel design concept to omit the thick soft copper interlayer, which has been used as stress-relaxing buffer between the stiff armor block and tube, is to prevent plastic fatigue damage under cyclic high heat flux loads and irradiation embrittlement which the soft copper interlayer is predicted to undergo. On the other hand, the desirable stress relaxation effect on a global scale is abandoned. In this study, such trade-off effects are computationally investigated in a comparative assessment of structural impact, which the presence (or absence) of the thick copper interlayer is expected to bring forth, in terms of the fracture and fatigue behaviour of the armour block and cooling tube in two representative cases of tungsten monoblock plasma facing component design, namely, with and without a thick copper interlayer. Quantitative results of cyclic plastic strain history and crack tip fracture energy are presented for the armour surface, bond interface and tube of the respective plasma facing component models. The positive and negative implications of these impacts on the structural integrity are discussed.

The present European standard DEMO divertor target technology is based on a water-cooled tungsten mono-block with a copper alloy heat sink. This paper presents the assessment of the operational space of this technology under static and transient heat loads. A transient thermo-hydraulic analysis was performed using the code RACLETTE, which allowed a broad parametric scan of the target geometry and coolant conditions. The limiting factors considered were the coolant critical heat flux (CHF), and the temperature limits of the materials. The second part of the work is devoted to the study of the plasma strike point sweeping as a mitigation technique for the divertor power exhaust. The RACLETTE code was used to evaluate the impact of a large range of sweeping frequencies and amplitudes. A reduced subset of cases, which complied with the constraints, was benchmarked with a 3D FEM model. A reduction of the heat flux to the coolant, up to a factor ∼4, and lower material temperatures were found for an incident heat flux in the range (15–30) MW/m2. Finally, preliminary assessments were performed on the installed power required for the sweeping, the AC losses in the superconductors and thermal fatigue analysis. No evident show stoppers were found.

The inherent brittleness of tungsten at low temperature and the embrittlement by neutron irradiation are its most critical weaknesses for fusion applications. In the current design of the ITER and DEMO divertor, the high heat flux loads during the operation impose a strong constraint on the structure–mechanical performance of the divertor. Thus, the combination of brittleness and the thermally induced stress fields due to the high heat flux loads raises a serious reliability issue in terms of the structural integrity of tungsten armor. In this study, quantitative estimates of the vulnerability of the tungsten monoblock armor cracking under stationary high heat flux loads are presented. A comparative fracture mechanical investigation has been carried out by means of two different types of computational approaches, namely, the extended finite element method (XFEM) and the finite element method (FEM)-based virtual crack tip extension (VCE) method. The fracture analysis indicates that the most probable pattern of crack formation is radial cracking in the tungsten armor starting from the interface to tube and the most probable site of cracking is the upper interfacial region of the tungsten armor adjacent to the top position of the copper interlayer. The strength threshold for crack initiation and the high heat flux load threshold for crack propagation are evaluated based on XFEM simulations and computations of stress intensity factors and J -integrals.

One possibility to mitigate the maximum high heat flux (HHF) load on the target is to sweep the position of the strike-point back and forth periodically in order to spread the peak thermal load over a wider width. The aim of this work is to investigate the thermal and structure-mechanical responses of a water-cooled tungsten mono-block target under cyclic HHF loads which are applied in sweeping modes. The study was performed by means of finite element analysis using an ITER-like target geometry. Extensive parametric simulations were carried out for a wide range of HHF loads and for selected sweeping amplitudes and frequencies, respectively. The simulation shows that the maximum temperature and the maximum heat flux to the coolant can be significantly reduced by sweeping. For the parameters studied in this work (0.5–4 Hz, 5–20 cm), higher sweeping frequency or smaller sweeping amplitude offers advantages in terms of fatigue lifetime of interlayer. Sweeping is suitable for the stationary loading if the sweeping frequency is high enough (e.g. 4 Hz) based on the fatigue lifetime prediction of interlayer.