Nuclear analysis and assessment of irradiation effects on the divertor plasma facing components of the DEMO fusion reactor

The Plasma Facing Components (PFCs) of the divertor target contribute to the crucial functions of the divertor such as heat removal and particle exhaust during fusion operation. The development of a well designed divertor target, able to fulfill this fundamental function, represents a crucial step in the realization of the first demonstrative prototype of a fusion reactor (DEMO) and hence for the feasibility of nuclear fusion as energy source. 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. In particular way this study is fully devoted to the comprehension of the neutron irradiation impact on the DEMO divertor PFCs performance (lifetime, potential failures phenomena, main critical issues and concerncs), which represents one of the main issues still with many pending questions. The achievement of this purpose has required the performing of a detailed neutronics and activation analyses carried out for the first time so thoroughly, followed by a thermo-structural integrity assessment based on the Inelastic Analysis Procedure (IAP) methodology, under development by the DEMO divertor project group of the EUROfusion consortium. A first element of novelty introduced by this study, therefore, lies in the adopted approach of the detailed neutronics analysis, combined with a thermo-structural integrity verification, which takes into account the effects of irradiation on the physical and thermo-mechanical properties of the principal plasma facing materials (tungsten, copper and copper alloys). Generally, these steps are performed separately, not always considering the irradiation impact on the materials, due to the not easy availability of the experimental test’s data. Three-dimensional neutronics analyses have been performed with the MCNP5 Monte Carlo code and the recommended JEFF 3.3 nuclear data libraries, activation analyses have been performed by means of Fispact II inventory code and the suggested TENDL 2015 nuclear data libraries. Several DEMO MCNP models have been generated, starting from the reference baseline, varying the blanket typology (Water Cooled Lithium Lead and Helium Cooled Pebble Bed), the divertor cassette body configuration (reference 2018 and 2019 designs) and the PFCs concepts (ITER-like which is the baseline design and the Low Activation Chromium based PFCs, representing an alternative concept developed in parallel). In particular, the MCNP representation of the divertor and the targets is extremely faithful to the original geometry, requiring a significant effort in the modelling approach. This allowed an extensive assessment of the nuclear loads (radiation damage, Helium production, nuclear heating density) distributions and of the main activation quantities (activity, contact dose rate and decay heat, transmutation products) over the entire arrangement of both the PFCs configurations, considered for this study. The calculation of nuclear and activation loads plays a crucial role to accomplish the aim of this work, due to the secondary effects triggered in the plasma facing materials. Atomic displacements and gas production (primary neutronics effect) can cause the following secondary responses: swelling, change in electrical resistivity, dimensional instabilities, change in yield strength, loss of ductility, change in creep rate, change in fatigue life, loss of fracture toughness. Transmutations can lead to other secondary effects as induced radioactivity and decay heat, change in thermal conductivity, composition change. Nuclear heating can lead to an overheating of the materials, but this effect for PFC can be neglected compared to the external high heat flux. In order to estimate the variation of these properties due to irradiation, an extensive analysis and collection of the literature data has been performed, highlighting important considerations on these topics, as well as some gaps that should be filled with future focused experimental tests. Thermo-structural integrity assessment has been performed by means of ANSYS 2019 R1 workbench, and considering the reference ITER-like design. It has been highlighted the following main critical issues related to the PFCs performance under irradiation: the exceeding of the maximum temperature for CuCrZr with the risk that thermal creep on pipe may occur, loss of ductility for CuCrZr and copper indicating severe risk of at least crack initiation which could cause thermal isolation of tungsten from the heat sink pipe leading to overheating and early failure, the exceeding of the admissible ultimate strength of the tungsten which may lead to cracking of the armour which, combined with the copper failure, potentially causes complete armour separation, decrease of fatigue life for copper and CuCrZr implying a potential halving of the PFCs lifetime with neutron irradiation. The latter phenomenon is the most limiting for the operating lifetime of the PFCs, entailing the necessity of frequent replacements or a breakthrough in the R&D program on first wall materials. Finally, neutronics and activation analyses made it possible to calculate the maximum expected loads on chromium and the other components of the alternative PFCs concept, which could be used for further thermo-structural verification on this design. The chromium confirms to be advantageous compared to tungsten in terms of activation, especially for short-medium cooling times, while, for longer times, the differences are less significant due to the impurities. The comparison between WCLL and HCPB blanket shows the impact of the blanket typology on the main nuclear quantities of ITER-like PFCs: with the Helium cooled blanket, generally, there is an average reduction of 15% on all the quantities taken into consideration, except for the Helium production which does not appear to be affected by the type of blanket. Most of the computing resources and the related technical support used for this work have been provided by CRESCO/ENEAGRID High Performance Computing infrastructure and its staff*. CRESCO/ENEAGRID High Performance Computing infrastructure is funded by ENEA, the Italian National Agency for New Technologies, Energy and Sustainable Economic Development and by Italian and European research programmes, see http://www.cresco.enea.it/english for information.