A Study of Radiation Effects in Curium-Doped Gd2Ti2O7 (Pyrochlore) and CaZrTi2O7 (Zirconolite)

Wald, J.W.; Offemann, P.

doi:10.1557/proc-11-369

Cited by 9 publications

(18 citation statements)

References 10 publications

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“…1984;. When doped with 238pu or 244Cm these phases become metamict at 0.5-1.0 x 1016 alphas/mg, equivalent to 0.5-1.0 dpa (Wald and Offermann 1982;Clinard et al 1984a, b;Foltyn etal. 1985;Weber etal.…”

Section: Variation Of Dose and Dpa With Agementioning

confidence: 98%

“…In particular, k values of 2.3-3.8 x 10-16 mg/alpha-event were determined for natural zircons based on variation of 20112 (Hurley and Fairbairn 1953), unit cell parameters, refractive indices, and density (Holland and Gottfried 1955). Bulk swelling has been used to measure a k value of approximately 5 x 10-16 mg/alpha-event for synthetic pyrochlore and zirconolite doped with 2++Cm (Wald and Offermann 1982;Weber et al 1985. Bulk swelling data determined by Clinard et al 1984a, b for synthetic zirconolites having either partial or complete substitution of 238Pu for Zr were used to calculate k values of 4 x 10-16 rag/alpha-event.…”

Section: X-ray Diffraction Analysismentioning

confidence: 99%

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Alpha-decay damage in minerals of the pyrochlore group

1988

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Abstract. X-ray diffraction analysis and transmission electron microscopy have been used to study the effects of alpha-decay damage in pyrochlore group minerals, characterized by the general formula A1 ,~BzO6(O,OH,F)I -n'pH20.As defined by the XRD intensity ratio I/Io, both the saturation dose (for which I/Io =0.1-0.0) and the dose which signifies the initial loss of crystallinity (for which I/Io = 1.0-0.8) increase as a function of geologic age. The increase is attributed to annealing of isolated alpha-recoil tracks back to the original crystalline structure. The tracks have calculated mean lives, za, on the order of 108 years. In contrast, minerals which remain crystalline (e.g., uraninite, UO2) despite doses of up to 1018 alpha-events/mg have mean alpha-recoil track lives ~ 104 years (Eyal and Fleischer 1985).After correcting the calculated dose for annealing of alpha-recoil damage, I/Io is observed to decrease exponentially to zero over the dose range 0.02-1.0 x 1016 alphaevents/rag. The relationship between I/Io and "corrected" dose was used to calculate an average alpha-recoil track diameter of 4.6 nm, in which < 2600 atoms are displaced. XRD line broadening due to strain dominates the first half of the crystalline-to-metamict transition, reaching a maximum of 0.003, then decreasing to < 0.001. Line broadening due to decreasing crystallite size dominates the latter half of the transition. Estimated crystallite dimensions decrease from 450 nm to < 15 nm prior to reaching the fully metamict state. With increasing dose HRTEM images of microlites from the Harding pegmatite sequentially exhibit: 1) mottled diffraction contrast, 2) isolated 1-5 nm aperiodic areas, 3) coexisting aperiodic and crystalline areas, 4) relict" islands" of crystalline material in an aperiodic matrix, and 5) complete loss of lattice fringe periodicity. With no consideration given to alpha-recoil track fading, the transition covers a dose range of 0.04-1.7x 1017 alpha-events/mg. Using a value of za=108 years, this dose range is corrected down to 0.02-1.2 x 10 ~6 alpha-events/mg.The metamict state is characterized by a range of M-M and M-O distances which give rise to bands of diffuse scattering centered at 0.30 nm and 0.18 nm, respectively, in x-ray and electron diffraction patterns. Random image contrast shown by HRTEM is consistent with a random network type structure, an interpretation supported by EX-AFS/XANES studies (Greegor et al. 1985a(Greegor et al. , b, 1987. The structure of metamict pyrochlore consists of an aperiodic framework of corner-sharing B--O polyhedra. Compared to the crystalline precursor, the metamict state displays a reduced M-O coordination number and mean bond length, increased distortion of the B-site, and a slight increase in the average M-M distance.

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Section: Variation Of Dose and Dpa With Agementioning

confidence: 98%

Section: X-ray Diffraction Analysismentioning

confidence: 99%

Alpha-decay damage in minerals of the pyrochlore group

1988

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“…Recently, they have attracted attention in the scientific world as geometrically frustrated magnetic systems, exhibiting 'spin ice' behaviour [1][2][3]. During the last decades they have found many interesting industrial applications: they are studied for thermal barrier coating [4][5][6][7], sub-catalysts for automotive exhaust gases (Ce 2 Zr 2 O 7 þ y ) [8][9][10][11] as well as for nuclear applications [12][13][14][15][16][17][18][19][20][21]. For the latter, which is the topic of our research, it is important that the chemical configuration of the lanthanide zirconate pyrochlores enables incorporation of actinides in the crystal lattice.…”

Section: Introductionmentioning

confidence: 99%

The low-temperature heat capacity of some lanthanide zirconates

Lutique

Javorský

Konings

et al. 2004

The Journal of Chemical Thermodynamics

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The heat capacities of the lanthanide zirconates Gd 2 Zr 2 O 7 and Eu 2 Zr 2 O 7 with pyrochlore structure were measured by adiabatic calorimetry and the hybrid adiabatic relaxation method in the temperature range (0.3 to 400) K. A thermal anomaly was observed for Gd 2 Zr 2 O 7 below T ¼ 15 K. From the measurements the thermodynamic functions of Gd 2 Zr 2 O 7 and Eu 2 Zr 2 O 7 were determined. By combining our results with the heat capacity data of La 2 Zr 2 O 7 , the lattice component of the heat capacity and the entropy were derived for the lanthanide series Ln 2 Zr 2 O 7 (Ln ¼ La to Gd). The Schottky contributions were derived for Ce 2 Zr 2 O 7 , Nd 2 Zr 2 O 7 and Eu 2 Zr 2 O 7 by subtracting the lattice component from the experimental data, and compared to the values calculated from the crystal field energy levels.

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“…Pyrochlore Wald, Weber et al (WALD, 1982;1984;WEBER, 1985;) studied the effects of internal alpha decay on 3 wt. % 244 Cm-doped Gd 2 Ti 2 O 7 pyrochlore.…”

Section: Consideration Of Individual Mineral Phasesmentioning

confidence: 99%

“…Wald et al (WALD, 1982) performed radiation damage studies on 3 wt. % 244 Cm-doped CaZrTi 2 O 7 zirconolite.…”

Section: Zirconolitementioning

confidence: 99%

Expected radiation effects in plutonium immobilization ceramic

Konynenburg¹

1997

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The current formulation of the candidate ceramic for plutonium immobilization consists primarily of pyrochlore, with smaller amounts of hafnium-zirconolite, rutile, and brannerite or perovskite. At a plutonium loading of 10.5 weight %, this ceramic would be made metamict (amorphous) by radiation damage resulting from alpha decay in a time much less than 10,000 years, the actual time depending on the repository temperature as a function of time.Based on previous experimental radiation damage work by others, it seems clear that this process would also result in a bulk volume increase (swelling) of about 6% for ceramic that was mechanically unconfined. For the candidate ceramic, which is made by cold pressing and sintering and has porosity amounting to somewhat more than this amount, it seems likely that this swelling would be accommodated by filling in the porosity, if the material were tightly confined mechanically by the waste package.Some ceramics have been observed to undergo microcracking as a result of radiation-induced anisotropic or differential swelling. It is unlikely that the candidate ceramic will microcrack extensively, for three reasons: (1) its phase composition is dominated by a single matrix mineral phase, pyrochlore, which has a cubic crystal structure and is thus not subject to anisotropic swelling; (2) the proportion of minor phases is small, minimizing potential cracking due to differential swelling; and (3) there is some flexibility in sintering process parameters that will allow limitation of the grain size, which can further limit stresses resulting from either cause.The metamictization process will convert the original, ordered titanium-oxygen polyhedral framework of the ceramic to an aperiodic, random network of corner-sharing polyhedra in which the titanium has a slightly lower average coordination number. When water in the repository eventually contacts the metamict ceramic, there will likely be leaching of a small amount of non-network ions (Ca, U, Gd, and Pu, in the order of decreasing expected release) from a thin layer at the surface, based on 2 studies by others on natural actinide-containing mineral analogs. The titanium-oxygen network in this thin surface layer will be transformed to anatase, and the long-term release behavior of the ceramic will likely then be governed by the behavior of this layer. In the mid-pH range, titanium oxide is known to be very insoluble in water. For example, the rutile form of titanium oxide has a measured solubility in water at room temperature between pH9 and pH10 of 1 0 -9 molal .Very limited experimental data by others suggests that the process of metamictization may increase the release rate of plutonium from the ceramic in contact with water by a factor of one to one and one-half orders of magnitude.Another reference point for long-term behavior is the observed condition of samples of uranium-and thorium-containing, high-titanium natural pyrochlores, which have been rendered metamict and have achieved measured ages up to a billion years. These ...

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A Study of Radiation Effects in Curium-Doped Gd₂Ti₂O₇ (Pyrochlore) and CaZrTi₂O₇ (Zirconolite)

Cited by 9 publications

References 10 publications

Alpha-decay damage in minerals of the pyrochlore group

Alpha-decay damage in minerals of the pyrochlore group

The low-temperature heat capacity of some lanthanide zirconates

Expected radiation effects in plutonium immobilization ceramic

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