Preventive failure assessment of divertor armor blocks with a postulated flaw under slow transient conditions

Abstract

As part of ITER research to devise suitable facilities withstanding extreme plasma, divertor modules have been designed in a way equipping tungsten armor blocks with optimized shapes at target regions. However, in spite of comprehensive considerations for the material and geometry, thermally damaged cracks were observed from recent limited mock-up tests. Objectives of the present numerical study are to address preventive failure assessment of mono- and multiple-armor blocks with a postulated flaw under slow transient conditions and to discuss engineering application. Firstly, systematic heat transfer and stress analyses were conducted without any defect according to typical heat fluxes and internal pressure. Subsequently, stress re-distributions due to a specific surface crack were taken into account and fracture mechanics analyses were carried out by changing their locations, orientations and sizes. As results, crack driving forces were quantified in terms of J-integral values and compared with each other to find out plausible cases threatening structural integrity of the armor block. Critical crack sizes were also estimated on the basis of a representative fracture toughness for future enhanced design and/or safe operation of the divertor.