Fabrication routes for advanced first wall design alternatives

M Rieth,J Henry,G Pintsuk,J Rey,E Simondon,B.-E Ghidersa,S Antusch,S Bonk,Y De Carlan,H Neuberger,G Aiello,N De Wispelaere,M Dürrschnabel,J Hoffmann,C Zeile

doi:10.1088/1741-4326/ac2523

Abstract

In future nuclear fusion reactors, plasma facing components have to sustain specific neutron damage. While the majority of irradiation data provides a relatively clear picture of the displacement damage, the effect of helium transmutation is not yet explored in detail. Nevertheless, available results from simulation experiments indicate that 9%-chromium steels will reach their operating limit as soon as the growing helium bubbles extent a critical size. At that point, the material would most probably fail due to grain boundary embrittlement. In this contribution, we present a strategy for the mitigation of the before-mentioned problem using the following facts. (1) The neutron dose and related transmutation rate decreases quickly inside the first wall of the breeding blankets, that is, only a plasma-near area is extremely loaded. (2) Nanostructured oxide dispersion strengthened (ODS) steels may have an enormous trapping effect on helium, which would suppress the formation of large helium bubbles for a much longer period. (3) Compared to conventional steels, ODS steels also provide improved irradiation tensile ductility and creep strength. Therefore, a design, based on the fabrication of the plasma facing and highly neutron and heat loaded parts of blankets by an ODS steel, while using EUROFER97 for everything else, would extend the operating time and enable a higher heat flux. Consequently, we (i) developed and produced 14%Cr ferritic ODS steel plates and (ii) optimized and demonstrated a scalable industrial production route. (iii) We fabricated a mock-up with five cooling channels and a plated first wall of ODS steel, using the same production processes as for a real component. (iv) Finally, we performed high heat flux tests in the Helium Loop Karlsruhe, applying a few hundred short and a few 2 h long pulses, in which the operating temperature limit for EUROFER97 (i.e. 550 °C) was finally exceeded by 100 K. (v) Thereafter, microstructure and defect analyses did not reveal critical defects or recognizable damage. Only a heat affected zone in the EUROFER/ODS steel interface could be detected. However, a solution to prohibit the formation of such heat affected zones is given. These research contributions demonstrate that the use of ODS steel is not only feasible and affordable but could make a decisive difference in the future design and performance of breeding blankets.

Highlights

In the present pre-conceptual design phase, the operating limit of the starter blanket for a European demonstration fusion reactor (DEMO) is predefined by a neutron dose of 20 dpa
At least three main damage modes have to be taken into account: (1) thermal fatigue due to the pulsed plasma operation with some ten-thousand cycles, (2) neutron irradiation hardening and embrittlement as a consequence of displacement damage, and (3) material damage due to transmutation products, i.e. in steels this mainly leads to the formation of helium bubbles
A first step in this direction, which is not so ambitious but might still be interesting, can yield valuable information: if we focus just on the superior thermo-mechanical properties, an oxide dispersion strengthened (ODS) steel layer on top of the blanket first wall could in principle allow higher coolant temperatures and longer component operating-times

Summary

Introduction

In the present pre-conceptual design phase, the operating limit of the starter blanket for a European demonstration fusion reactor (DEMO) is predefined by a neutron dose of 20 dpa. The coolant for the second (advanced) blanket of the European DEMO reactor should/will be helium gas [1,2,3,4]. At least three main damage modes have to be taken into account: (1) thermal fatigue due to the pulsed plasma operation with some ten-thousand cycles, (2) neutron irradiation hardening and embrittlement as a consequence of displacement damage, and (3) material damage due to transmutation products, i.e. in steels this mainly leads to the formation of helium bubbles (and hydrogen is produced). Available results indicate that EUROFER-type steels (the selected base and reference material within the European fusion project) will reach their operating limit when the formation of helium bubbles reaches a critical amount or size, which leads to brittle fracturing [5,6,7]

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