Conference Agenda

Tungsten is considered as the material of choice for the divertor application of fusion power plants due to its intrinsic thermo-physical properties. However, one main drawback is the recrystallization induced reduction of its mechanical properties at elevated temperatures. Therefore, the aim of the conducted research was to improve the material properties to be able to resist especially the high thermal loads imposed on the divertor during operation.

We will show that the particle reinforcement of fusion-relevant tungsten through the incorporation of tungsten sub-carbide W₂C particles at the grain boundaries is demonstrated to be an effective way of eliminating the oxygen present in the starting powder without subjecting it to the hydrogen atmosphere at elevated temperatures [1]. At the same time the densification is being promoted, composite’s microstructure is being strengthened and flexural strength at room and high temperatures is increased when compared to the pure tungsten [2]. While higher concentration of W₂C particles lead to the refined grain sizes and higher hardness, low concentration (2-3 wt%) of W₂C particles dispersed in isotropic W matrix displayed the DBTT between 200 and 400 °C, which is lower or comparable to the isotropic and IGP [3] tungsten. Additionally, the prolonged thermal treatment (ageing) at temperatures above 1250 °C with retention times from 1 to 7 days confirmed that the presence of W₂C particles at W grain boundaries prevents the abnormal growth of W grains. The examination of D retention revealed that the composite in which W was reinforced by 4 wt% of W₂C, and it was (78.7 ± 2.9) 10¹⁹ D/m² and (148.4 ± 1.5) 10¹⁹ D/m² for a D exposure temperature of 370 K and 523 K, respectively [4].

High thermal stability coupled with good mechanical properties, comparable or even better thermal shock behaviour than W, reasonable thermal transport properties (λ > 100 W/mK @ 1000°C) make the W₂C composites a very interesting and competitive material for the DEMO divertor application.

[1] A. Šestan et al., “Tungsten carbide as a deoxidation agent for plasma-facing tungsten-based materials,” J. Nucl. Mater., vol. 524, pp. 135–140, Oct. 2019.

[2] S. Novak et al., “Beneficial effects of a WC addition in FAST-densified tungsten,” Mater. Sci. Eng. A, vol. 772, no. September 2019, p. 138666, Jan. 2020.

[3] C. Yin, D. Terentyev, T. Pardoen, R. Petrov, and Z. Tong, “Ductile to brittle transition in ITER specification tungsten assessed by combined fracture toughness and bending tests analysis,” Mater. Sci. Eng. A, vol. 750, no. February, pp. 20–30, 2019.

[4] P. Jenuš et al., “Deuterium retention in tungsten, tungsten carbide and tungsten-ditungsten carbide composites,” J. Nucl. Mater., vol. 581, no. April, pp. 0–6, 2023.

Accurate prediction of boiling phenomena in actively cooled divertor targets remains an important challenge for fusion reactor design, primarily due to the extreme operating conditions characterized by heat fluxes exceeding 10 MW/m², liquid subcooling around 150 K, and coolant flow velocities approaching 10 m/s. Commonly used Eulerian two-fluid Computational Fluid Dynamics (CFD) simulations employ the classical three-heat flux partitioning Rensselaer Polytechnic Institute (RPI) model, which can significantly overestimates wall temperatures in these severe conditions.

This study explores capability of a new heat flux partitioning CFD model, which integrates enhanced mechanistic boiling parameters proposed by researchers at Massachusetts Institute of Technology (MIT), specifically accounting for improved correlations of nucleation site density, bubble detachment diameter, frequency, and incorporating bubble sliding effects. The model's predictive capabilities are evaluated by comparison against experimental data from the High heAt loaD tESt (HADES) at CEA, where conditions closely match those expected in fusion divertor cooling channels, including heat fluxes above 11 MW/m² and fluid velocities over 10 m/s.

In this work simulations using the advanced CFD boiling model are compared with results obtained using the conventional RPI model, with particular attention given to the predicted wall temperature and boiling parameters. Present findings provide valuable insights into the applicability of advanced mechanistic boiling models under fusion-relevant conditions.

The integration of systems into a tokamak concept requires extensive neutronics analyses to ensure that nuclear loads such as peak nuclear heating in superconducting coil winding packs, material damage (displacements per atom), and He production in steels for re-weldability are within design limits. The European Volumetric Neutron Source (VNS) is a tokamak-based machine currently being developed by EUROfusion to help test, qualify and reduce risks associated with components such as the tritium breeding blanket, which are crucial for the operation of future fusion power plants like the European DEMO.

The initial design concept of the VNS Electron Cyclotron (EC) system integrated into the VNS neutronics model provides a good initial point for estimating nuclear loads through Monte Carlo simulations of neutron and gamma ray transport (MCNP) to quantify various potentially limiting nuclear loads in the system and its surroundings. The aim of the present analyses is to guide further developments of the design. Simulations performed with this MCNP model showed good performance and areas where improvements are needed were identified.