Ion Irradiation Effects for Two Pyrochlore Compositions: Gd₂Ti₂O₇and Gd₂Zr₂0₇

Abstract

Pyrochlore is an important nuclear waste form phase for actinide immobilization. Two synthetic pyrochlores, Gd2Ti2O7 and Gd2Zr2O7 were irradiated at various temperatures (25 K to 1073 K) by different ion species (1.5 MeV Xe+, 1.0 MeV Kr+, and 0.6 MeV Ar+). The titanate pyrochlore amorphized at relatively low doses (0.5 ∼ 0.6 dpa). Temperature dependence of the amorphization dose for titanate pyrochlore was measured, and the critical temperatures for amorphization were 1300 K, 1100 K and 950 K by 1.5 MeV Xe+, 1.0 MeV Kr+ and 0.6 MeV Ar+, respectively. The higher critical temperature for the heavier ion irradiation is consistent with an amorphization mechanism by which the heavier ion produces a larger cascade. The zirconate pyrochlore, Gd2Zr2O7, showed a strong amorphization “resistance”. Gd2Zr2O7did not become amorphous under 1.0 MeV Kr+ and 1.5 MeV Xe+ irradiation. After prolonged irradiation (up to 7 dpa) even at a temperature of 25 K, no amorphization was observed. The irradiated zirconate pyrochlore showed abundant dislocations as observed by TEM. The pyrochlore structure of Gd2Zr2O7 transformed to the fluorite structure after irradiation. The diffraction patterns of irradiated Gd2Zr2O7 showed the existence of short-range ordering of cations. The large difference between these two pyrochlores emphasizes the strong effect of chemical composition on radiation-induced amorphization.

The Status of Radiation Damage Experiments

“…However, when a full complement of impurities is present, the resulting zirconolite becomes metamict about as rapidly as pyrochlore. This effect appears to be different than what is observed in heavy-ion radiation-damage studies in which the transformation from one pure phase to the other is clearly observed before the material becomes metamict (Meldrum et al 2001;Wang et al 1999a;Wang et al 1999b;Weber et al 1985;). In these studies, pyrochlore transformed to a crystalline titanate with the fluorite crystal structure and then became metamict.…”

“…According to the literature on pyrochlore and zirconolite, the pyrochlore specimens should be nearly amorphous, and the zirconolite specimens should be about 80% of the way to becoming fully amorphous Wang et al 1999a). …”

Nature of the chemical bond and prediction of radiation tolerance in pyrochlore and defect fluorite compounds

“…This may lead to volume expansion, cracking, and reduced chemical durability of the waste form. Radiation damage in these materials has been studied using samples doped with short-lived 238 Pu (half-life ¼ 87 years) or 244 Cm (halflife ¼ 18 years) [10][11][12], natural samples [13][14][15], and ion irradiation techniques [16][17][18][19][20][21][22][23][24]. Although actinide doping experiments are the most relevant to real waste forms, the experiments are difficult, expensive, and limited in scope.…”

Spent fuel canister for geological repository: Inner material requirements and candidates evaluation

“…It should also be noted that many of the experiments conducted to study these phenomena were oriented to performance assessment of glass matrices for HLW immobilization, involving higher temperatures than those expected inside the canister, higher doses from radiation originated within the glass matrix, and considering much shorter relevant time spans than those related to spent fuel disposal. Nevertheless, this fact points to another interesting possible candidate material specifically for plutonium immobilization, that is, the gadolinium zirconate (Gd 2 Zr 2 O 7 ) [169][170][171][172]. Gadolinium zirconate offers much better radiation resistance and long-term stability than BSG [166,169,171,[173][174][175], and the binding of gadolinium (the most effective neutron absorber known, with a thermal cross section of about 49 000 barns) to the crystal structure prevents its selective lixiviation.…”