Towards reliable design-by-analysis for divertor plasma facing components – Guidelines for inelastic assessment (part 1: Unirradiated)

Abstract

Guidelines are presented for the recommended method of assessing by analysis the structural integrity of divertor plasma facing components (PFCs) under high heat flux (HHF) loads. The objective is to enable analysis to provide greater supporting evidence for PFC qualification (which for ITER is currently achieved by test alone) and to provide reasonable estimates of irradiated PFC performance where component test data is limited or non-existent. Typically, PFCs comprise tungsten armour, CuCrZr heat-sink and copper interlayer. For more reliable assessment of this type of construction, two shortfalls in existing elastic analysis methods have been addressed. Firstly, the scope of assessment rules has been extended to cover armour failure (e.g. by deep cracking) and secondly the accuracy of analysis is increased by using inelastic analysis methods to address complexities created by PFC’s multi-material block like construction. These complexities arise mainly from the dissimilar yield strengths and thermal expansion coefficients of the component materials. This paper details the proposed “low temperature” design code rules in the guidelines, which are based on a combination of existing inelastic code rules plus a set of methodologies devised specifically for PFC structures. The failure mechanisms of exhaustion of ductility, fast-fracture, fatigue and ratcheting are addressed, and the process of assessment is demonstrated by means of an example analysis on an ITER-like monoblock component in its unirradiated condition. In two follow- up papers, creep rules and methods for estimating the HHF structural integrity of an irradiated component using sparse irradiated materials data are presented. In the unirradiated condition it is shown that fatigue and armour cracking performance dominate predicted structural integrity, but when irradiated, exhaustion of ductility is expected to be more prevalent. It is argued that with these more rigorous assessment methods, a significant step has been made towards developing reliable design-by-analysis methods for predicting the expected HHF structural integrity performance of divertor PFC designs.