Plasma-wall interactions in a commercial-scale fusion power station may exert high transient thermal loads on plasma-facing surfaces, repeatedly subjecting underlying structural materials to high temperatures for short durations. Specimens of the reduced activation ferritic-martensitic steel Eurofer-97 were continuously aged at constant temperature in the range of 550 °C to 950 °C for up to 168 hours in a furnace to investigate the microstructural effects of short-term high temperature exposure. A CO 2 laser was also used to repeatedly heat another specimen from 400 °C to 850 °C a total of 1,480 times over a period of 41 hours to explore transient heating effects. Microstructural changes were studied via scanning electron and focused ion beam microscopy and include (i) the coarsening of Cr-rich secondary phase precipitates when continuously heated above 750 °C, (ii) an increase in average grain size above 800 °C and (iii) the evolution of a new lath martensite microstructure above 850 °C. Conversely, transient heating via a laser was found to result in the decomposition of the as-received lath martensite structure into ferrite and Cr-rich carbide precipitates, accompanied by a significant increase in average grain size from 0.1-2 μm to 5-40 μm. Experimental analysis was supported by thermodynamic simulation of the equilibrium phase behaviour of Eurofer-97 in MatCalc and thermal finite element modelling of plasma-wall interaction heating on the water-cooled lithium-lead tritium breeding blanket concept in Comsol Multiphysics. Simulated thermal transients were found to significantly alter the microstructure of Eurofer-97 and the implications of this are discussed.

Resonant inelastic x-ray scattering (RIXS) using an incident energy tuned to the uranium N 4,5 absorption edges is reported from epitaxial films of α-U3O8 and UN. Theory shows that for U3O8 the multiplets associated with a 5f 1 configuration with a ground state of 2 F 5/2 and the excited state of 2 F 7/2 are observed. However, the strong transition predicted at a transfer energy of 1.67 eV is not observed. We assume this is a consequence of the intermediate state lifetime broadening due to interaction with continuum states when the transferred energy exceeds the onset of the continuum in the presence of the core hole. This hypothesis is supported by the results obtained for the 5f-itinerant system UN, where no sharp transitions have been observed, although the broad scattering response centered at ∼ 1 eV is considered a signature of a predominantly 5f 3 configuration in this band-like semi-metallic system. These experiments and theory add important information on these materials, both of which have been investigated since the 1960s, as well as whether RIXS at the uranium N edge can become a valuable tool for actinide research.

No abstract

In a prototype tokamak fusion reactor, a hydrogen plasma will be raised to extreme temperatures of 150 -200 million °C. Despite confinement within strong magnetic fields and a plethora of mitigation measures, undesirable plasma instabilities and disruptions may occasionally cause this hot fusion plasma to touch the plasma-facing surfaces of the reactor [1]. The short duration but high intensity transient thermal loads that arise (<0.1 sec, ~7.5 MW m -2 ) may briefly subject the underlying structural materials of the reactors tungsten-armoured interior wall (the first wall) to temperatures of up to 850 °C [2].