Status and key issues of reduced activation ferritic/martensitic steels as the structural material for a DEMO blanket

Critical Assessment 12: Prospects for reduced activation steel for fusion plant

“…(i) an acceptably low neutron capture cross-section to ensure a sufficient tritium breeding ratio; 19 this implies limits on the quantity of material that can be used, and in particular limits on elements (such as W) with a high neutron capture cross-section (ii) compatibility with remote handling: critically this will require compatibility with welding techniques, presently designed around laser welding 20 (iii) stability under cyclic operation, with .1?5610 4 cycles anticipated for the blanket in the current EU demonstrator reactor 13 (iv) retention of mechanical properties within engineering design criteria under irradiation (the anticipated peak fusion neutron flux is of the order y1610 19 neutrons m 22 s 21 for steels at the front of the blanket and expected component lifetimes are .1?33 full power years for DEMO and 5 full power years for an operating power plant) 12,21,22 (v) sufficient tolerance to He and H embrittlement to ensure a brittle to ductile transition temperature (BDTT) .20uC during operation, with anticipated levels of .100 at.-ppm of He and H produced per full power year in steels at the front of the blanket due to (n, a) and (n, p) reactions 9,10,23,24 (vi) chemical compatibility with coolant (such as water or He) to ensure negligible corrosion (vii) compatibility with tritium removal systems, to ensure negligible tritium retention in the material and to meet safety regulatory requirements for the total tritium inventory (tritium inventory limits set at y3 kg for ITER) (viii) dimensional (,1% total swelling) and structural integrity at the operational temperature. 12 The operating temperature of the blanket plays an important role in the thermodynamic efficiency and hence the anticipated cost of electricity generation from fusion reactors.…”

9-12Cr Heat-Resistant Steels

“…Currently, reduced activation ferritic/martensitic (RAFM) steels are the leading candidate structural materials for fusion reactors due to the good thermal properties and superior swelling resistance, compared with austenitic stainless steel. The typical microstructure of the RAFM steels is the tempered martensite with a large number of precipitates (Tanigawa et al 2011). There are two main types of precipitates in RAFM steels, M 23 C 6 (M is for Cr, Fe, W, etc.)…”

Friction Stir Welding of ODS and RAFM Steels

“…[17,22] TIG and EB welding of RAFM F82H (developed in Japan) led to a highly inhomogeneous microstructure in the weld. [22,23] It consisted of several distinctive regions including the fusion zone (FZ), with cast-like microstructures, a coarse-grained heat-affected zone (CG-HAZ), a finegrained HAZ (FG-HAZ), and an over-tempered HAZ (OT-HAZ). The OT-HAZ near the ferrite/austenite phase transformation temperature is the weakest region due to softening.…”