Microstructure of n-irradiated tungsten: experimental investigation and modelling

Dwivedi Prashant

SCK•CEN Mentor

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

SCK•CEN Co-mentor

Konstantinovic Milan
milan.konstantinovic@sckcen.be
+32 14 33 31 92

Expert group

Structural Materials

PhD started

2018-02-01

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 within HORIZONT2020 with the total budget exceeding 100ME. Tungsten and tungsten-based composites are presently considered as main candidates for armor and divertor (including structural function) in DEMO. 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. Both material types have also to be stable with respect to high neutron irradiation doses and helium production rates. Design of DEMO divertor remains one of the most challenging tasks in the current Fusion Roadmap.

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. Under these conditions, the impact of n-irradiation on mechanical and thermal properties is currently unknown ! Four physical degrading process will take place simultaneously: (i) neutron irradiation damage, (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).

Although fission irradiation is currently the only option to proceed with the design material database, fission is not fully equivalent to fusion, because of: softer neutron spectra (lack of 14 MeV), high thermal to fast ratio (enhanced W transmutation), lower dose rate (enhanced defect annealing). SCK•CEN launches a large-scale neutron irradiation campaign in support of EUROfusion needs for the baseline and advanced tungsten grades. It is call TUNER (Tungsten Neutron Radiation) project and it involves more than 500 samples to be delivered for Post Irradiation Investigation (PIE) within 2017-2018. Among many others, one of the goal of this programme is to clarify the impact of the neutron irradiation on the microstructure of commercially available tungsten grades and investigate the neutron induced features in a wide range of doses and temperatures. This action will be done by means of (a) post-irradiation campaign involving usage of positron annihilation lifetime spectroscopy (PALS) and transmission electron microscopy (TEM), as well as (b) physically-based computational tool to rationalize microstructure obtained in BR2 conditions. 

Objective

The scientific objective of the project is to validate the performance of baseline and novel tungsten grades with a potential to tolerate neutron irradiation damage and to offer enhanced toughness and low swelling as matrix material for tungsten-based composites. The project will be based on:

(a) usage of experimental facilities available at LHMA for microstructural characterization, such as PALS, TEM and SEM-EBSD.

(b) exploitation/development of the computational tool on the basis of kinetic Monte-Carlo techniques to simulate microstructural damage induced by fast and thermal neutrons in BR2 conditions and specifically applied to the TUNER project.

 

The main technical goal is to describe the irradiation-induced microstructure evolving as a result of neutron damage in a wide temperature range (400-1200 C) as a function of initial material's microstructure and accumulated irradiation dose (0.05, 0.1, 0.5 and 1 dpa). One of the major technological concerns is the formation of voids which results in swelling, loss of thermal conductivity and raise of Ductile to Brittle Transition Temperature (DBTT). Therefore, the primary experimental information is to obtained using PALS coupled with annealing runs to clarify thermal stability of the neutron damage. Transmission electron microscopy will be used as supplementary tool.

In support of PALS/TEM measurements, the rationalization of the experimental results will be performed using physically-based modelling, for which NMS/SMM unit has long experience in applying for similar BCC metal: Iron and Fe-Cr-Carbon alloys. Main computational tool will be kinetic Monte Carlo (KMC) techniques based on the most recent developments done by Dr. Castin (who is also co-mentor for this project) to enable hybridization of object and atomistic methods. This will enable to treat simultaneously both radiation induced objects (i.e. point defects, loops, voids, etc) and transmutation induced Re/Os solutes. Treatment of the transmutation products is absolutely necessary because the high fraction of thermal neutrons in BR2 spectrum as compared to the expected fusion spectrum.

Given the success of the model, the prediction of the impact of neutron damage on the microstructure of baseline and advanced tungsten grades is to be made for expected fusion conditions. This can be done by introducing the relevant fusion spectrum and damage rate, according to available conceptual studies, in the KMC tool and modelling the microstructure in the diverter strike zone, dome and first wall armor parts. This project will therefore help to clarify up to which extend and what is the best way to use the fission irradiation to characterize impact of neutron irradiation in expected fusion conditions.