## 1 Introduction  
The EU DEMO denotes a crucial step between the International Thermonuclear Experimental Reactor (ITER) and the commercial generation of fusion power in Europe [1]. The HCPB breeding blanket is one of the two \"driver\" blanket concepts being considered for the EU DEMO [2]. The reference structural material for EU DEMO blankets is a Reduced Activation Ferritic-Martenstic (RAFM) steel: Eurofer97, which is capable of withstanding high neutron irradiation with low-activation. While for ITER, due to its experimental nature, an austenitic stainless steel type 316L(N)-IG is used as the structural material for the blankets is [3]. Figure 1 shows a brief overview of the present HCPB BB concept within the DEMO tokamak. Each BB segment consists of a breeder zone volume formed between a _first wall_ and a _backplate_, which is supported by a _back supporting structure_ (BSS). The breeder zone is filled with a hexagonal arrangement of _pressure tubes_ connecting the _first wall_ and the _backplate_ and acting as radial stiffeners. The ceramic breeder material (LiSiO\({}_{4}\) + 35% mol Li\({}_{2}\)TiO\({}_{3}\)) in the form of a pebble bed is filled inside _breeder pins_ and inserted into each of the _pressure tubes_. Further, the _pressure tubes_ are surrounded by hexagonally shaped (prismatic) _neutron multiplier blocks_ (NMB) made up of Beryllium alloy (Be\({}_{12}\)Ti). More details of the present HCPB BB architecture can be found in [4]. Several HCPB BB concepts were extensively investigated using linear design-by-analysis methods with respect to codes and standards over the last decade [5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. Additionally, a few studies employed non-linear methods to conduct the failure analysis under normal operation (NO) conditions [15; 16; 17]. In-box LOCA is an identified design basis accident scenario [13; 14; 18] for the BB, in which the blanket box has to withstand the pressure exerted by the leakage of high-pressure coolant. The structural assessments of the HCPB breeding blanket under the in-box LOCA conditions are carried out in [5; 9; 14]. In contrast to the breeding zone at other regions, the cap region of the BB is considered predominantly a structural element, which does not contribute largely to tritium breeding. Therefore, the design of this region is oriented towards its structural strength and stiffness; while the breeding zone structural material has to be minimized to reduce the neutronic parasitic absorption. Consequently, the LOCA instead of the NO condition is considered as the design driver of the cap region. In this paper, the analysis is focused on the structural integrity of the BB cap region under the thermo-mechanical loading from an in-box LOCA event. The elastic design-by-analysis codes and standards with the relatively large safety factors are thought of as a conservative criteria. However, it has been reported in [19] that the elastic rules failed to predict the failure due to non-standard loading on a pressure vessel, which could only be successfully predicted by the nonlinear elastoplastic analysis. 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. 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).