(Ref. BAP-2015-19)

Occupation : Full-time

Period : Fixed-term contract

Place : Leuven

Apply no later than : March 23, 2015

MAX phases are compounds, the chemical composition of which is given by the general formula Mn+1AXn, where n = 1, 2, or 3. The letter “M” in the formula is an early transition metal (Ti, Nb, V, Cr, Zr, etc), “A” is an A-group element (Si, Al, Sn, etc) and “X” is C or N. MAX phases are a fascinating family of layered solids characterized by a unique combination of properties that can be associated with their atomic bonding, nano-laminated crystal structure and the fact that the multiplication and mobility of basal dislocations is possible even at room temperature. The nano-laminated nature of MAX phases is reflected in the fact that near-closed-packed M6X layers are interleaved with pure group-A element layers with the X atoms filling the octahedral sites between the group-A element layers. The properties of MAX phases are on one hand close to the properties of their corresponding binary carbides and nitrides, i.e. they are elastically stiff, have good thermal and electrical conductivity, are resistant to chemical attack, and have relatively low thermal expansion coefficients. On the other hand, MAX phases can also behave like metals, i.e. they are relatively soft (2-8 GPa) and readily machineable, thermal shock resistant, and damage tolerant; moreover, some are fatigue, creep, and oxidation resistant. At room temperature, they can be compressed to stresses as high as 1 GPa and fully recover upon load removal, while dissipating 25% of the mechanical energy. At higher temperatures, however, they undergo a brittle-to-plastic transition, and their mechanical behavior depends strongly on the imposed deformation rate.The compatibility of MAX phases with heavy liquid metals, such as the envisaged lead-bismuth eutectic (LBE) primary coolant of the MYRRHA system, is another appealing property of these materials, as reported in literature and experimentally verified at SCK-CEN by means of dedicated liquid metal corrosion tests on a broad selection of MAX phases. The superb liquid metal corrosion resistance of MAX phases makes them promising candidate materials for various applications in Gen-IV lead fast reactors (LFRs). SCK-CEN started exploring the potential of MAX phases for Pb/LBE-cooled fast reactors in 2013 in close collaboration with the Dept. of Materials Engineering (MTM) of KU Leuven. Another promising potential application of MAX phases for Gen-IV LFRs is the fabrication of cladding materials, in view of the promising reported first results on the radiation resistance of this special class of materials. One of the key material properties that must be optimized before MAX phases can be used as candidate cladding materials is their fracture toughness, which has not been reported to exceed 15-18 MPam1/2 for selected MAX phases with an optimized (i.e. large-grained and textured) microstructure. Even though such fracture toughness values are already very high for carbides, fracture toughness must be further improved. Since the targeted fracture toughness of these novel clads must be at least comparable with the reported fracture toughness of commercial and already qualified cladding materials, such as zircalloys (beginning-of-life KIC = 75 MPam1/2; end-of-life KIC = 25 MPam1/2), the envisaged approach is to produce novel clads made of MAX phase-based cermets (i.e. ceramic composites with metallic ‘matrix’). Embedding MAX grains in a metallic matrix is expected to provide an additional mechanism for dislocation motion apart from the already-identified mobility of basal dislocations and the formation of kink bands in MAX phases.MAX phases are currently also being considered as cladding materials, either monolithic or as coatings deposited on commercial clads, for Gen-III+ light water reactors (LWRs). In fact, one of the high-priority goals of the post-Fukushima era is the development of accident-tolerant fuels (ATFs), as the combination of uranium-based fuels with zircalloy clads is characterized by some inherent weaknesses of this fuel/clad system under severe accident conditions. These include potential exothermic reactions between zircalloy clad and water coolant that can raise the core temperature to such levels that the cladding, fuel and core structures are at risk of melting, with the associated release of highly-radioactive fission products. The novel ATF cladding materials must ideally tolerate the loss of active cooling in the reactor core for a considerably longer time period during design-basis and beyond design-basis accidents: in fact, they must tolerate temperatures >>1200°C, while fission products must be contained in the fuel rod for a significantly longer time than the approx. 3 hours associated with a conventional zircalloy cladding.

Towards the development of novel cladding materials based on nanolaminated ternary carbides (MAX phases) for different nuclear systems