Heavy-ion irradiation eects on structures and acid dissolution of pyrochlores B.D. Begg * , N.J. Hess, W.J. Weber, R. Devanathan, J.P. Icenhower, S. Thevuthasan, B.P. McGrail Paci®c Northwest National Laboratory, Richland, WA 99352, USA Received 22 March 2000; accepted 10 October 2000 Abstract The temperature dependence of the critical dose for amorphization, using 0.6 MeV Bi ions, for A 2 Ti 2 O 7 py- rochlores, in which A Y, Sm, Gd and Lu, exhibits no signi®cant eect of A-site ion mass or size. The room tem- perature dose for amorphization was found to be 0.18 dpa in each case. After irradiation with 2 MeV Au 2 ions glancing-incidence X-ray diraction XRD) revealed that each pyrochlore underwent an irradiation-induced structural transformation to ¯uorite in conjunction with amorphization. The eect of amorphization on the dissolution rates of fully dense pyrochlores, at 90°C and pH 2 nitric acid) varied from a factor of 10 to 15 increase for Gd 2 Ti 2 O 7 to none for Y 2 Ti 2 O 7 . Signi®cant dierences were observed in the A-site dissolution rates from the crystalline pyrochlores, in- dicating dierences in the manner in which the A-site cations are incorporated into the pyrochlore structure. These indications were supported by Raman spectroscopy. Crown Copyright Ó 2001 Published by Elsevier Science B.V. All rights reserved. PACS: 61.10.Eq; 61.43.Er; 61.80.Jh; 61.82.Ms; 64.60.Cn; 81.05.Je 1. Introduction Pyrochlore is one of a number of candidate materials proposed for the immobilization of actinide-rich wastes, and has recently been selected as a key component in the synroc-based pyrochlore-rich ceramics for the geological immobilization of excess weapons Pu. The pyrochlores under consideration typically exhibit A 2 B 2 O 7 stoichi- ometry, where actinides and lanthanides are incorpo- rated on the A-site and either Ti or Zr occupy the B-site. One of the principal factors that will determine the suitability of a candidate wasteform is its aqueous du- rability. Given that actinide-bearing crystalline waste- forms will be subject to considerable a-recoil radiation damage leading to amorphization, it is important to understand the eect of the irradiation-induced crystal- line-to-amorphous transformation on the aqueous du- rability of relevant pyrochlore compounds. Irradiation-induced amorphization due to alpha de- cay has been previously studied in 244 Cm-doped Gd 2 Ti 2 O 7 [1±3]. The alpha-decay-induced amorphiza- tion process occurs heterogeneously at ambient tem- perature and results in 6.5% swelling, a decrease in hardness and elastic modulus, and a 20±50% increase in the 244 Cm and 240 Pu dissolution rates, respectively, at 90°C and pH 6. The heterogeneous amorphization either occurs directly in the cascade of the 240 Pu alpha- recoil particle or from the overlapping of cascades, al- though the exact mechanism for amorphization has yet to be determined. Isochronal 12 h) annealing studies indicated that thermal recrystallization of the amor- phous state in Gd 2 Ti 2 O 7 begins to occur at about 975 K [2]. Heavy ion-beam irradiation studies have also been used to study damage accumulation and amorphization Journal of Nuclear Materials 288 2001) 208±216 www.elsevier.nl/locate/jnucmat * Corresponding author. Present address: ANSTO, Materi- als Division, PMB 1, Menai, NSW 2234, Australia. Tel.: +61-2 9717 3747; fax: +61-2 9543 7179. E-mail address: bruce.begg@ansto.gov.au B.D. Begg). 0022-3115/01/$ - see front matter Crown Copyright Ó 2001 Published by Elsevier Science B.V. All rights reserved. PII:S0022-311500)00708-X