Structural lifetime assessment for the DEMO divertor targets: Design-by-analysis approach and outstanding issues
Introduction
The divertor target of a nuclear fusion reactor is a plasma-facing component (PFC) contributing to crucial functions of the divertor such as heat removal and particle exhaust during fusion operation. The targets are subjected to a very harsh and complex loading environment characterized by intense particle bombardment, high heat fluxes (HHF), fast neutron irradiation, varying stresses and impact loads [[1], [2], [3], [4], [5]]. Such combined loads may possibly cause premature damage and failure of the components [[6], [7], [8], [9]]. Unfortunately, there is only tenuous knowledge about the synergistic effects of the actual combined loads on the materials and structure of the divertor target during a fusion operation.
For a robust design, the structural integrity must be assured against complex failure modes for the envisaged operation scenarios in order to ensure the required lifetime. To this end, the lifetime against structural failure needs to be estimated by means of either experimental testing (design-by-experiment) or computational analyses (design-by-analysis, design-by-code). In the former approach, a full experimental verification of lifetime is currently not possible because no proper nuclear testing facility (fast neutron irradiation and high heat flux testing on components) is available. In the latter approach, the validity of the lifetime assessment is limited by the knowledge gaps on materials properties, assumptions made for analysis and the predictive capability of the simulation tools to capture potential failure features.
The HHF technology for the ITER divertor target was validated mainly through the design-by-experiment route supported by code-based design studies to take into account irradiation effects [[10], [11], [12], [13]]. This approach may be justified by the fact that the neutron irradiation dose expected for the ITER divertor is relatively low (≤0.5 dpa) so that the irradiation test data obtained from fission reactors could be utilized for specification of design allowables relevant for analysis [14]. This will not be the case for the DEMO divertor since an order of magnitude higher damage dose is expected whereas only few materials data are available for the relevant dose level [5,15]. At high neutron fluence, adverse effects due to transmutation are expected while the effects due to dpa damage will probably be saturated below 1 dpa [14]. The recent HHF technologies developed for the DEMO divertor were tested only under non-nuclear testing conditions [[16], [17], [18], [19]].
The design-by-code approach will not be fully applicable for the DEMO divertor targets in the near future, as the design code is still under development and not mature enough for use [20]. It surely will take many years until missing materials data are populated and design rules with valid failure criteria are established.
Therefore, the design-by-analysis approach may need to be considered a primary design methodology complementary to the other two approaches. The virtue of the design-by-analysis approach is its capability to deliver a quantitative assessment of lifetime as well as information on failure features for a wide spectrum of operational scenarios. This is particularly useful when an experimental verification becomes impractical.
On the other hand, there are also implicit limitations in the design-by-analysis approach because its practice is subject to modelling assumptions, predictive capability of simulation tools and uncertainties in materials data [[21], [22], [23]]. As the validity of predicted failure features and assessed lifetime is affected by such limitations, the dependence of predicted results on the individual factors affecting the validity needs to be understood. If the limitations should pose a significant consequence in lifetime assessment for a specific failure mode, the case shall be clarified in terms of credibility and caveats.
In this article, a number of practical case studies are presented where the outstanding issues arising in the design-by-analysis practice are discussed and the uncertainties implicit in lifetime assessment are examined. Focus is placed on major failure modes (fracture, fatigue, ratchetting) expected under typical HHF loads and neutron irradiation. The predictive capability of employed FEM-based simulation tools is demonstrated.
Section snippets
Geometry, thermal loads and boundary conditions
The preconceptual design of the European DEMO divertor target has been performed in the framework of the dedicated project (WPDIV) of the EUROfusion Consortium [24,25].
The CAD model of the outboard divertor target of the European DEMO reactor is illustrated Fig. 1(a) [26]. The underlying cassette body is not displayed for brevity. The cooling pipe of the individual target elements are connected to the feeding pipe (blue) and the outlet pipe (dark red) via the coolant distributor and collector
Thermal response
Fig. 5 shows the temperature distribution in the target monoblock unit in the thermal equilibrium under the heat flux load at 20 MW/m². The steep temperature gradient with the extremely large temperature range building up in the armor and the large mismatch of thermo-elastic properties between the armor and the cooling pipe suggest that substantial thermal stresses are produced. The maximum temperature on the top face reaches 2400 °C (homologous temperature: >0.7) whereas the block region
The cooling pipe
The cooling pipe is classified as structural member as it carries primary loads (coolant pressure and impact force upon disruption). Together with the attachment units for target fixation, the cooling pipe contributes to the structural stability of the entire target system. As such, the cooling pipe design should fulfill structural design rules. The initiation and evolution of a structural failure is governed by the stress states, temperature, strain history and the actual state of the
Copper interlayer
Although the copper interlayer is not classified as structural member, its mechanical stability is an essential prerequisite for maintaining the global integrity and functionality of the entire target. In this context, the structural failure features of the interlayer should be handled on the same footing as the cooling pipe. Owing to the very low yield stress of soft copper at elevated temperatures, the interlayer undergoes cyclic plastic deformation. As a result, it is prone to LCF and
Impact of irradiation
Although not being a structural member per se, the intactness of the tungsten blocks as plasma-facing armor is important for keeping the essential functionality as sacrificial armor. Thus, a holistic approach is required for a global lifetime assessment also taking the failure of the armor blocks into account.
Fig. 26 shows the hoop stress states in a target monoblock unit. Plotted are the stress states after fabrication, under HHF load at 20 MW/m² (1st and 5th cycle) and during cooling at
Summary and conclusions
In this article, we discussed some modelling issues arising in the practice of the design-by-analysis approach for DEMO divertor target. Particularly, those issues were highlighted which may have a substantial impact on the assessment of structural integrity and lifetime. These issues were inherently related to the peculiar features of the component structure (multi-materials joint incl. soft constituent) and the nature of the applied loads (e.g. cyclic high heat flux, neutron irradiation) [54
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
CRediT authorship contribution statement
Jeong-Ha You: Conceptualization, Methodology, Data curation, Funding acquisition, Project administration, Supervision, Writing - original draft, Writing - review & editing. Muyuan Li: Investigation, Formal analysis. Kuo Zhang: Investigation, Formal analysis.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training program 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. The authors are particularly grateful to Dr. Mike Fursdon (CCFE) for his highly qualified and inspiring discussions during the seven years of the WPDIV project period.
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Currently at OSRAM Opto Semiconductors GmbH.