Development and qualification of irradiation tolerant tungsten and novel toughness-enhanced composites for fusion applications

Yin Chao

Promoter

Pardoen Thomas, (UCL), thomas.pardoen@uclouvain.be

SCK•CEN Mentor

Terentyev Dmitry
dmitry.terentyev@sckcen.be
+32 14 33 31 97

SCK•CEN Co-mentor

Goussarov Andrei
andrei.goussarov@sckcen.be
+32 14 33 85 06

Expert group

Structural Materials Modelling and Microstructure

PhD started

2017-01-15

Short project description

Development and qualification of materials for plasma facing and structural applications for DEMO is one the heaviest tasks in the EUROfusion programme (HORIZONT2020) with the total budget exceeding 100M€ [1]. Tungsten and tungsten-based composites are presently considered as main candidates for first wall and diverter (including structural function) armor in DEMO [2]. An armor material needs high crack resistance under extreme thermal operation conditions as well as compatibility with plasma-wall interaction phenomena, while a structural material has to be ductile within the operation temperature range [3]. Both material types have also to be stable with respect to high neutron irradiation doses and helium production rates.

The plasma facing materials envisaged for ITER application should receive only limited amounts of dpa during their lifetime (~0.1-0.5 dpa), while Tungsten selected as the first wall armour and Tungsten-based composites for structural applications in DEMO are expected to receive doses up to 20 dpa (for the EARLY DEMO) or even higher (full power DEMO) [1]. Under these conditions, the mechanical properties of the materials are known to degrade radically due to (i) neutron irradiation, (ii) heat transients, (iii) plasma gas uptake and (iv) nuclear transmutation. Consequently, the combination of these phenomena will define structural integrity and operational limit of the plasma facing components (PFC).

The physical origin of the above mentioned effects has been investigated thanks to recent developments of physically-based modelling techniques operating at different time and space scales thus capturing variety of phenomena, such as: evolution of radiation induced microstructure, plastic flow in the presence of radiation defects, permeation and retention of plasma components, etc [4]. SCK-CEN (structural materials modelling, SMM, unit)  has significantly contributed to the investigation of the microscopic level processes  and by now owns well established expertise ready to apply for the neutron damage accumulation in tungsten. The engineering computational model (based on finite elements tool) to account for the structural integrity under simultaneous neutron and plasma heat exposure is currently under development at SMM. From that view point, SCK-CEN can offer a unique opportunity to combine neutron irradiation facility, expertise in post-irradiation testing & characterization and theoretical expertise to validate the advanced fusion materials production routes and application of those materials in novel composites (such as fiber- or laminate-reinforced ones).

On the side of materials, the technology to produce ultra-fine grain (UFG) tungsten and chemically-tailored tungsten-based alloys has made a significant progress recently, by application of spray plasma sintering (SPS) [5]. Tungsten with micro-scale grain size is expected to exhibit improved radiation resistance due to the sinking of radiation defects on the grain boundary interfaces, on the one hand. On the other hand, the recrystallization process may also be delay/suppressed in UFG tungsten if an appropriate chemical tailoring is applied to stabilize grain-boundary. Already now, the SPS production route delivers tungsten in amount suitable for the mechanical testing. It is expected that SPS-UFG W should have an advantage as compared to the baseline tungsten grades produced by conventional powder metallurgy, rolling/forging/hammering and/or hot isotactic pressure. It is therefore necessary to demonstrate experimentally that UFG W grades (i) exhibit acceptable mechanical and thermal performance before irradiation; and (ii) provide an obvious advantage after neutron irradiation in terms of radiation-induced microstructure and hardening.

This proposal aims at investigation of the radiation damage and post-irradiation mechanical-thermal behaviour of UFG tungsten. This will include the experimental study of the novel and baseline grades with respect to tolerance to the neutron damage, and complementary computational assessment. The irradiation-induced hardening, embrittlement and microstructure will be the major phenomena under investigation. The work will be executed in tight collaboration with currently running PhD students engaged in experimental and computational study of tungsten for fusion, and will be also incorporated as a part of the task force of TEC (trilateral Euregio Cluster) so as to maximize the support from TEC partners, and part of FOD-supported programme. The proposed work will have a high impact on currently available knowledge on the the evolution of microstructure and thermo-mechanical properties of baseline ITER tungsten grades and candidate tungsten grades (developed within EFDA/EUROfusion projects) for applications in DEMO.

 

References:

[1] The Road to Fusion Electricity | EUROfusion, F. Romanelli, https://www.euro-fusion.org/wpcms/wp-content/uploads/2013/01/JG12.356-web.pdf

[2] M. Rieth et.al. Recent progress in research on tungsten materials for nuclear fusion applications in Europe, J. Nucl. Mater. 432 (2013) 482-500.

[3] M. Wirtz et.al.  Thermal shock behaviour of tungsten after high flux H-plasma loading, J. Nucl. Mater. 433 (2013) 497-501.

[4] D. Stork et.al. Developing structural, high-heat flux and plasma facing materials for a near-term DEMO fusion power plant: The EU assessment, J. Nucl. Mater. 455 (2014) 277-291.

[5] J. Matijicek et.al. Nukleonika 60 (2015) 267-273 doi: 10.1515/nuka-2015-0049.

Objective

The objective of this project is to investigate theoretically and experimentally the impact of the grain size on accumulation of neutron damage in microstructure and its subsequent yield to hardening and embrittlement. This investigation is called by the need to validate the performance of novel tungsten grades with a potential to tolerate irradiation damage and if positive to offer matrix material for enhanced toughness composites. The Spray Plasma Sintering (SPS) production route will be applied to deliver micro-grain Tungsten (and Tungsten-based alloys) with dedicated microstructure promising attractive thermal, mechanical and radiation-tolerant properties.

The validation of the UFG-tungsten will require the candidate to follow-up pre-arranged neutron irradiation campaign and perform post-irradiation testing to assess the radiation-induced microstructure, hardening and transmutation effects. The expected results will comprise of microstructural investigation including: transmission electron microscopy (SCK-CEN/NMS), positron annihilation spectroscopy (SCK-CEN/NMS), mechanical testing including: tensile and impact tests (SCK-CEN/NMS); high heat flux thermal testing and plasma-exposure testing at the site of Julich Forschungszentrum (our TEC collaborator, using Judith 1 and Jule-PSI setups), and atom probe measurements in collaboration with Oxford University/CCFE (our EUROfusion partner). The appropriate guidance will be provided by colleagues from SCK-CEN, University Promoter(s) and EUROfusion partners of this project. Complementary modelling, on the basis of tools and with guidance available at NMS/SMM unit, will be performed to rationalize the major mechanisms behind the evolution of microstructure and mechanical properties.

The ultimate goals is thus to envisage up to what extent the neutron irradiation damage degrades the mechanical properties of UFG-tungsten as compared to the conventional baseline grades for ITER/DEMO application and whether the improved grades (with enhanced fracture toughness and radiation tolerant microstructure) retain their attractive properties also after neutron irradiation. Two specific novel grades will be investigated: (i) ultra-fine grain pure tungsten produced by plasma spraying; and (ii) tungsten fiber-reinforced composite (with conventional matrix material). Additional grades such as W-Cu laminates might also be considered depending on the level of the technological readiness and interest of EUROfusion.