Insight into the structural integrity assessment of the European DEMO fusion reactor divertor (part I : general discussion)

Abstract

This paper gives a description of the conceptual design of the DEMO divertor highlighting its two main functions and the main loading conditions acting on it during operation. The paper stresses the importance of Design by analysis (DBA) and structural integrity assessments at the conceptual design phase. It explains why currently the predominant DBA approach taken by the DEMO divertor group within the EUROfusion consortium is the elastic approach rather than the elastoplastic approach. Background to the DBA elastic approach describing linearization of stresses through the wall thickness and to stress categorisation that allows different stress allowables to be used for a more efficient design is given. Throughout the paper, the RCC MRx code structural integrity P Type and S Type checks as applied to the DEMO divertor components are summarised and difficulties in applying the elastic design rules to the divertor model are highlighted. These difficulties range from issues of code interpretation as applied to the DEMO divertor geometry to limited availability of material properties for Eurofer97 steel (a low activation steel) which is the main material selected for the divertor. In a follow up paper (Part II: Application Examples), some of these difficulties are demonstrated by considering two stress classification lines in the model of the current DEMO divertor model. In this paper, the phenomenon of elastic follow up is explained and DEMO divertor load cases when this has to be taken into consideration are discussed. A short explanation on Neuber’s rule, which is used to calculate estimates of plastic strain and to differentiate between primary and secondary stresses for Type S checks is also given. At the end of the paper, the conclusions summarise the interpretation and issues encountered in the application of the RCC-MRx rules for a typical nuclear fusion reactor component such as the DEMO divertor design.