Heavy Ion Irradiation of Brannerite-type Ceramics
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Heavy Ion Irradiation of Brannerite-type Ceramics Jie Lian a, Lumin Wang a, Gregory R. Lumpkin b and Rodney C. Ewing a a
Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109-2104, USA b Materials Division, Australian Nuclear Science and Technology Organisation, Menai, NSW 2234, Australia
ABSTRACT Brannerite, UTi2O6, occurs in polyphase Ti-based, crystalline ceramics that are under development for plutonium immobilization. In order to investigate radiation effects caused by αdecay events of Pu, several brannerite compositions were synthesized: UTi2O6, ThTi2O6, CeTi2O6 and a more complex material, which is composed of Ca-containing brannerite and pyrochlore. An 1 MeV Kr+ irradiation was performed over a temperature range of 25 K to 1020 K with in-situ TEM. The ion irradiation-induced crystalline-to-amorphous transformation was observed in all brannerite samples. At room temperature, the critical amorphization dose, Dc, resulting from Kr+ ion irradiation increases in the order: Dc (CeTi2O6) < Dc (ThTi2O6) < Dc (Cacontaining brannerite) < Dc (UTi2O6) < Dc (Ca-containing pyrochlore). The critical amorphization dose for stoichoimetric brannerite increases at elevated temperature due to the effect of thermal annealing. The critical amorphization temperatures of the different brannerite compositions are: 970 K, UTi2O6; 990 K, ThTi2O6; 1020 K, CeTi2O6. The effects of structure and chemical composition on radiation resistance of brannerite-type and pyrochlore-type ceramics are discussed. INTRODUCTION Brannerite, UTi2O6, is an accessory phase in titanate-based crystalline ceramics of synroc, presently under development for the immobilization of Pu from the dismantling of nuclear weapons [1]. Up to 30 wt % brannerite, containing neutron absorbers Gd and Hf, occurs in the final pyrochlore-based waste form for Pu immobilization [2]. Recently, there has been extensive interest in the crystal chemistry, radiation damage effects and chemical durability of brannerite. The high U-content of brannerite (up to 62.8 wt%) and its potential as a nuclear waste form for the immobilization of actinides have emphasized the importance of radiation damage effects [3, 4]. The ideal formula of natural brannerite is (U, Th)1-xTi2+xO6 with a deficiency in uranium and excess titanium. Many cation substitutions have been identified for both uranium (Pb, Ca, Th, Y and Ce) and titanium (Si, Al, Fe) in natural brannerite. Natural brannerite is completely metamict (amorphous) as a result of the alpha-decay damage from the constituent U and Th [5]. The studies of composition and geochemical alteration indicate that alteration occurs in the natural brannerite samples, and 40~90% of the original amount of U could loss due to alteration. However, the overall durability of the titanate matrix remains high [6, 7]. Synthetic brannerite has been produced with various cations substituting into structure, and brannerite-bearing ceramics have been synthesized with Ca and Gd that are charge balanced by U5+ ions [8
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