Herein we report on the effect of neutron irradiation-of up to 0.1 displacements per atom at 360(20)°C or 695(25)°C-on polycrystalline samples o f Ti 3 AlC 2 , Ti 2 AlC, Ti 3 SiC 2 and Ti 2 AlN. X-ray diffraction refinement of the irradiated samples showed irradiation-enhanced dissociation into TiC of the Ti 3 AlC 2 and Ti 3 SiC 2 phases, most prominently in the former. Ti 2 AlN also showed an increase in TiN content, as well as Ti 4 AlN 3 after irradiation. In contrast, Ti 2 AlC was quite stable under these irradiation conditions. Dislocation loops a re seen t o f orm in T i 2 AlC a nd Ti 3 AlC 2 after ir radiation at 360(20)°C. The room temperature electrical r esistivity of al l samples i ncreased by an or der of magnitude after irradiation at 360(20)°C, but onl y by 25% after 695(25)°C, providing e vidence for t he MA X pha ses' dy namic recovery at temperatures as low at 695(25)°C. Based on these preliminary results, it appears that Ti 2 AlC and Ti 3 SiC 2 are the more promising materials for high-temperature nuclear applications.

A family of ternary carbides and nitrides, known as MAX phases, combine attractive properties of both ceramics and metals, and has been suggested for potential nuclear reactor applications. The unirradiated materials properties of importance for in-core structural materials and as fuel pellet coatings for several leading MAX phase materials have been summarized from literature. The materials show high mechanical damage tolerance in terms of creep, thermal/mechanical fatigue and fracture resistance, and very good chemical compatibility with select coolants such as molten lead and sodium. Neutron activation has been calculated for commercial purity materials exposed to both idealized fast and thermal reactor neutron spectra for 10, 30, and 60 years of exposure. The specific activities of Ti 3 SiC 2 , Ti 3 AlC 2 , and Ti 2 AlC were compared to those of SiC and Alloy 617, two leading candidate materials for next generation reactor components. The specific activities of MAX phases were similar to SiC and three orders of magnitude less than Alloy 617 after 10-60 years decay for all three activation times in both the fast and thermal spectra. As with SiC, the main radioisotopes after a decay period of 10 years for all three activation times in the MAX phases are tritium and C 14. Neutron irradiation results of Ti 3 SiC 2 , Ti 3 AlC 2 , and Ti 2 AlC experimentally confirmed the neutron transmutation analysis.

Herein we report on the characterization ofdefects formed in polycrystalline Ti 3 SiC 2 and Ti 2 AlC samples exposed toneutron irradiation -up to 0.1displacements per atom (dpa) at 350±40°C or 695±25 °C, and up to 0.4 dpa at 350±40 °C.Black spots are observed in both Ti 3 SiC 2 and Ti 2 AlC after irradiation to both 0.1 and 0.4 dpa at 350 °C. After irradiation to 0.1 dpaat 710°C, small basal dislocation loops, with a Burgers vector of b = ½ [0001] are observed in both materials. At 9±3 and 10±5 nm, the loop diameters in the Ti 3 SiC 2 and Ti 2 AlC samples, respectively, were comparable. At 1x10 23 loops/m 3 , the dislocation loop density in Ti 2 AlC was ≈ 1.5 orders of magnitude greater than in Ti 3 SiC 2 , at 3x10 21 loops/m 3 .After irradiation at 350 °C, extensive microcracking was observed in Ti 2 AlC, but not in Ti 3 SiC 2 .The room temperature electrical resistivities increasedas a function of neutron dose for all samples tested, and appear to saturate in the case of Ti 3 SiC 2 . The MAX phases are unequivocally more neutron radiation tolerant thanthe impurity phases TiC and Al 2 O 3 . Based on these results,Ti 3 SiC 2 appears to bea morepromising MAX phase candidate for high temperature nuclear applications than Ti 2 AlC.

Exploratory work on gamma exposure to dried solids with chemically-bound water that are typical of those produced on aluminum-clad nuclear fuel in reactor and post-discharge storage has shown a profound production of hydrogen (as the sole gaseous species) from dry boehmite (c-AlOOH or Al 2 O 3 ÁH 2 O) powders and barely observable hydrogen from dry gibbsite (c-Al(OH) 3 or Al 2 O 3 Á3H 2 O) under gamma irradiation from cobalt-60. A computational investigation of hydrogen bond energies was performed to provide insights to explain the experimental observations. A gas phase or single-molecule model, and a solid phase model were used. For all model reactions, computations indicate hydrogen loss is more favorable for boehmite than for gibbsite, in qualitative agreement with the experimental findings.

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