The high-power laser energy research (HiPER) project was a European project for demonstrating the feasibility of inertial fusion energy based on using direct-drive targets in a shock ignition scheme using a drywall evacuated chamber. HiPER was intended to drive the transition from a scientific proof of principle to a demonstration power plant in Europe. The project was divided into three realistic scenarios (Experimental, Prototype, and Demo) to help identify open problems and select appropriate technologies to solve them. One of the problems identified was the lack of appropriate plasma-facing materials (PFMs) for the reaction chamber. Therefore, a major challenge was to develop radiation-resistant materials able to withstand the large thermal loads and radiation in these reactors. In this paper, we describe the main threats that coarse-grained W would face in the diverse HiPER scenarios. Based on purely thermomechanical considerations, the W lifetimes for the HiPER Prototype and Demo scenarios are limited by fatigue to 14 000 h and 28 h, respectively. The combined effects of thermal load and atomistic damage significantly reduce these lifetimes to just ∼1000 shots for the Experimental scenario and a few minutes and seconds for the Prototype and Demo scenarios, respectively. Thus, coarse-grained W is not an appropriate PFM for the Prototype or Demo scenarios. Therefore, alternatives to this material need to be identified. Here, we review some of the different approaches that are being investigated, highlight the work done to characterize these new materials, and suggest further experiments.

In this paper, we report on an enhanced hydrogen permeation effect along grain boundaries in tungsten. Sputtered nanocolumnar tungsten layers (column lateral dimensions 100–150 nm and layer thickness 2 μm) were analysed by hydrogen permeation measurements in the temperature range 520–705 K. The experiments constitute a direct observation of this effect, previously postulated by means of a combination of indirect experiments and simulations and considered controversial due to the lack of direct measurements. DFT results support this observation since (i) the hydrogen binding energy to the grain boundary is 1.05 eV and (ii) the migration energies along the grain boundary and along the bulk are 0.12 eV and 0.20 eV, respectively. OKMC simulations, parametrized by DFT data, were used as a supporting tool to attain a better understanding of the involved phenomena. The OKMC results are also compatible with the observations. Indeed, they show that the fraction of hydrogen flux along grain boundaries in the steady-state permeation regime increases when decreasing the ratio of lateral dimensions to length of the nanocolumns, rapidly approaching unity when this ratio is < 2. Therefore, grain boundaries act as preferential migration pathways for H atoms at the studied temperature range in the studied samples. This behaviour has interesting implications to reduce the retention of hydrogen in several applications, in particular, fusion materials exposed to plasma discharges.