The effects of fusion reactor thermal transients on the microstructure of Eurofer-97 steel

D Kumar,A Bharj,R Burrows,H Dawson,A.D Warren,J Hargreaves,L.M Harding,T.L Martin,R Holmes,H Dominguez-Andrade,P.E.J Flewitt,A Scorror

doi:10.1016/j.jnucmat.2021.153084

Abstract

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 CO2 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.

Highlights

High temperature plasmas are generated within magnetic confinement fusion devices - the fusion plasma within the International Thermonuclear Experimental Reactor (ITER) will reach temperatures of 150,000,000°C, and the plasma-facing first wall will be exposed to temperatures of 350°C - 550°C during normal operation [1]
This paper explores the effects of short-term high temperature exposure on the microstructure of Eurofer-97 and the effects of both continuous and transient heating
No significant microstructural change was observed for Eurofer97 thermally aged at 550°C and 650°C for 168 hrs compared with the as-received condition

Summary

Introduction

High temperature plasmas are generated within magnetic confinement fusion devices - the fusion plasma within the International Thermonuclear Experimental Reactor (ITER) will reach temperatures of 150,000,000°C, and the plasma-facing first wall will be exposed to temperatures of 350°C - 550°C during normal operation [1]. Turbulent plasma phenomena such as edge localised modes (ELMs), blobs, or disruptions may result in plasma-wall interactions which temporarily raise the temperature of first wall materials beyond their design limits [2,3,4]. These events may occur thousands of times during the design life of a prototype fusion power station, and repeated strikes to the same surface may result in the accumulation of localised thermal damage and the degra-. The degradation mechanisms of first wall materials are likely to be complex

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Discussion

Conclusion