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 followup 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.

Towards reliable design-by-analysis for divertor plasma facing components—Guidelines for inelastic assessment (part II: irradiated)

“…The presence of ratcheting is detected by a method where multiple load cycles are simulated explicitly, and the resulting cyclic strain/deformation produced are inspected for signs of incremental growth. Note, ratcheting itself is not "damage", but its presence indicates the potential for excessive strain or deformation being created leading to material/component failure or loss of function (see [2] for more detail).…”

“…Hence, in the case of ductility usage (for example): if 50% ductility usage (Ud-pre) occurs before irradiation, then this results in a 50% reduction in the ductility usage available after irradiation (ΔUd-post). This is captured in the IAPs definition of the exhaustion of ductility rule (equation 4 in the IAP [2]) as reiterated here:…”

“…This inelastic analysis Procedure (IAP) as it is known, provides details of analysis methodologies and preferred analysis design rules overcoming some of the difficulties identified in the current assessment methods. A full description the IAP methodologies and rules can be found in part I of this paper [2].…”

Application of Inelastic Method and Its Comparison with Elastic Method for the Assessment of In-Box LOCA Event on EU DEMO HCPB Breeding Blanket Cap Region

“…For irradiated structures subjected to monotonic loading, the nuclear components design codes RCC-MRx [23] and SDC-IC [24] define two potential failure modes due to reduced ductility: Immediate plastic flow localization (IPFL) and Immediate local fracture due to exhaustion of ductility (ILFED). For DEMO divertor plasma-facing components, the IPFL is not considered as a potential failure mechanism for inelastic assessment because it is argued that the flow localization helps in strain redistribution and the ultimate failure will be captured by the ILFED criteria [21]. However, since the irradiated Eurofer retains considerable durability after necking, IPFL is considered the main concern for Eurofer instead of ILFED [23,25].…”

“…The ductile material failure mechanism is essentially inelastic and nonlinear, while the elastic design codes are evolved from a linear, single-strength concept that does not account for the plastic deformation or its growth history [20]. It is reasoned in [21,22] that elastoplastic methods may be especially significant for the fusion community where non-standard components and construction are prevalent; in which the actual stresses may bear little resemblance to that predicted by the elastic rules. However, within the European fusion community, specific design criteria for the DEMO in-vessel components are under development and in early stages.…”