Chemical Composition, Geochemical Alteration, and Radiation Damage Effects in Natural Perovskite
- PDF / 1,585,061 Bytes
- 8 Pages / 417.6 x 639 pts Page_size
- 81 Downloads / 166 Views
Preliminary analytical and transmission electron microscopy (AEM and TEM) results for a small suite of natural perovskites are reported in this paper and discussed in relation to previous work. We show that perovskite compositions in Synroc and tailored ceramics plot within the known fields of natural perovskite compositions. AEM analyses and electron diffraction work on selected samples indicate that they are predominantly stoichiometric variants of the cubic perovskite structure. Geochemical alteration was observed in one sample of loparite from Bratthagen, Norway. The primary result of this alteration was leaching of Na from the A-site. Although sufficient alpha-decay dose levels for complete amorphization are not realized in this suite of samples, the available data bracket the beginning of the crystalline-amorphous transformation at doses that are - 2-4 times greater than those of zirconolite of similar age. These results may be due to fundamental differences in the damage annealing rates of perovskite and zirconolite. INTRODUCTION
Perovskite, ideally CaTiO 3 , is a major constituent of ceramic nuclear waste forms including Synroc-C [I] and tailored ceramics [2]. The composition of perovskite is highly flexible, but it is the least durable of the major phases in Synroc when exposed to aqueous solutions at low temperatures (70-150 °C) [3, 4]. Studies of element partitioning indicate that tetravalent actinides will partition preferentially into zirconolite and that trivalent actinides will preferentially enter perovskite [5, 6]. As a result, both phases will suffer alpha-decay damage but the accumulated dose for each phase may be quite different at a given point in time. Furthermore, the effects of radiation damage on the structure of perovskite are not completely understood. The rhombohedral perovskite CmAIO 3 becomes X-ray amorphous after a dose of - 3 x 1015 oX/mg, a value close to
that of zirconolite doped with 238Pu or 244 Cm [7-9]. However, recent in situ HVEM ion irradiation experiments on thin crystals indicate that the saturation dose of perovskite (CaTiO 3 ) is approximately 2-5 times higher than that of zirconolite [10, 11]. In this paper, we present preliminary results for natural perovskite describing the extent of solid-solution, geochemical alteration effects, and radiation damage as a function of alpha-decay dose and geologic age. EXPERIMENTAL PROCEDURES
AEM and TEM techniques were used to correlate alpha-decay dose with damage microstructures using a JEOL 2000FX TEM equipped with a hybrid Tracor Northern Be window Si(Li) detector and Link ISIS microanalyzer. The method employs well established procedures [12] and provides detection limits on the order of 0. 1-0.2 wt% for Th and U, allowing the dose and displacements per atom (dpa) to be calculated if the age of the sample is known [13]. RESULTS Chemical Composition
Mitchell [14] has shown that the compositions of most natural perovskites can be described in the quaternary system defined by the end-members CaTiO 3 (perovskite), (Na 0 .5REEo.
Data Loading...
