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

Abstract

This paper gives guidelines for assessing the structural integrity of plasma facing components (PFC)when irradiated to the levels expected in DEMO after two full power years. The paper is part II of a 3-part paper describing the EuroFusion DEMO Divertor group (WPDIV) Inelastic Analysis procedure (IAP), created to improve the assessment of PFCs, and specifically those constructed from tungsten armour cooled by CuCrZr heat sink (with Copper interlayer). The paper first provides a recap of the IAP methodology detailed in the part 1 paper and then presents a detailed review of the limited relevant irradiated materials data on material properties (thermal conductivity, swelling and stress-strain curves) and materials limit data (rupture-strain, fracture-toughness and fatigue strength). The data is used in an example structural integrity assessment estimate of an ITER-like divertor “monoblock” PFC (tungsten block with through CuCrZr pipe) when irradiated to ∼14dpa (CuCrZr) & ∼ 4dp (tungsten). The assessment uses IAP methodologies to determine the susceptibility of the design to failure by exhaustion-of-ductility, fast-fracture, fatigue and ratcheting in the CuCrZr pipe, exhaustion of ductility in the copper interlayer and brittle fracture in the tungsten armour. These methodologies ensure that contributions from changes in both material limit-levels and material properties are included. The paper documents the extrapolations required to extend the existing irradiated materials data to the expected dpa and temperature range. The assessment exposes significant shortfalls in the monoblock type design in coping with the drastic reduction in copper ductility and tungsten strength caused by irradiation. This illustrates that maintaining structural integrity when irradiated poses a far more stringent constraint on a PFC design than the un-irradiated condition, and as such should be given priority in future design studies. Although the prime aim of the paper is to present assessment methodologies, it also helps identify the key gaps in irradiated materials property data (and emphasise the severe need for a fully populating irradiated materials database).