Structural lifetime assessment for the DEMO divertor targets: Design-by-analysis approach and outstanding issues

•Critical modelling issues arising in the practice of the design-by-analysis approach are presented.•The impact of these issues on the structural lifetime assessment against major failure modes is studied.•The factors affecting the lifetime assessment and the resulting uncertainties are discussed.•The predictive capability of FEM-based simulation tools is demonstrated. Divertor targets of a nuclear fusion reactor are exposed to harsh loading environments comprising high heat flux, varying stresses and neutron irradiation. For a robust design, the structural integrity must be assured against complex failure modes for the envisaged operation scenarios in order to ensure the required lifetime. To this end, the lifetime against structural failure needs to be assessed by means of either experimental tests (design-by-experiment) or computational analyses (design-by-analysis). In the latter approach, the validity of the lifetime assessment is limited by the knowledge gaps on materials properties, assumptions made for analysis and the predictive capability of the simulation tools to capture potential failure features. Given that a full proper experimental verification under fusion-relevant loading conditions is virtually not possible, the design-by-analysis approach needs to be considered a complementary design methodology albeit open issues and limitations. In this article, selected critical issues arising in the practice of the design-by-analysis approach are discussed. Particularly, those modelling issues are highlighted which may have a substantial impact on the assessment of structural integrity and lifetime against major failure modes. The potential predictive capability of FEM-based simulation tools is demonstrated. The uncertainties being implicit in the lifetime estimation are illustrated in diverse case studies and the factors affecting estimates are discussed. Finally, outstanding modelling issues needing further clarification are addressed.