Aging Effects on Curium‐Doped Titanate Ceramic Containing Sodium‐Bearing High‐Level Nuclear Waste

Abstract

Curium-doped titanate ceramic containing sodium-rich high-level nuclear waste showed a gradual decrease in density up to a dose of 8.5 X 1017 a decays ng-'. After that, the rate of density change increased apparently because of crack formation. Optical microscopy showed cracks >0.1 mm long and > I pm wide after a dose of 7.9 X lo" a decays-g-'.Leach tests suggested that the dissolution-control phases for sodium and cesium changed from freudenbergite and hollandite, respectively, to intergranular phases after significant cracking. Aging also enhanced strontium losses, relative to calcium, indicating that strontium may also be partitioned to the intergranular phases. After the fresh surfaces produced by cracking were exposed to leachant, and the dissolution of soluble intergranular surfaces was complete, the leaching of nonradioactive elements from the samples having a dose of 12.3 X lOI7 a decays. g-' was limited by the following dissolution-control phases: freudenbergite (Na), hollandite (Cs and Ba), perovskite and/or zirconolite (Sr and Ca), and alloys (Mo). The leaching behavior of the nonradioactive indicator elements revealed that chemical durability was reduced by two main factors: (1) increasing the effective surface area by crack formation and (2) decreasing the stability of the actinide-host phases by n-recoil damage. In combination these factors increased longer-term (>7 days) leach rates of sodium and cesium, and strontium and calcium by 1 and 2 orders of magnitude, respectively. In spite of deterioration of the actinide-host phases, the curium leach rate after a dose of 12.3 X 10" a decaysg -' decreased by 2 orders of magnitude, possibly as a result of precipitation of curium hydrolysis products. [

An X‐Ray Photoelectron Spectroscopy Investigation of Highly Soluble Grain‐Boundary Impurity Films in Hollandite

“…The surface Cs/Ti and Ba/Ti concentration ratios of the leached surface, as listed in Table I, are essentially the same as those of the polished surface (i.e., significantly lower than the fractured surface), demonstrating unequivocally that the grain‐boundary film is highly soluble. These results agree well with those reported previously for various titanate ceramic waste forms 10,15–17 …”

“…The enrichment of Cs (and Ba in some cases) has been reported extensively in the past for various titanate ceramic waste-form materials. 10,11,[13][14][15][16][17] In most instances, these enriched species were found to react with Si impurities, forming a silicate-like glassy phase at the grain boundary. The hollandite samples in the current study, however, do not contain any Si impurities.…”

“…These results agree well with those reported previously for various titanate ceramic waste forms. 10,[15][16][17] Figure 6 shows the Ba 4d XPS spectra of the polished surface (a), and the fractured surface before (b), and after (c) reaction with deionized water at RT for 1 h. There is only one doublet, Ba 4d 5/2 and Ba 4d 3/2 , observed from the polished (Fig. 6(a)), and leached ( Fig.…”

Radiation effects in crystalline ceramics for the immobilization of high-level nuclear waste and plutonium

“…Japanese researchers have investigated self-radiation effects in a 244 Cm-doped Synroc-related titanate ceramic consisting of perovskite, zirconolite, hollandite, freudenbergite, and loveringite that is intended for encapsulation of sodium-rich high-level waste. 117,118 The density of this material decreased linearly with dose to a value corresponding to a volume expansion of 1.0% after a dose of 0.7 3 10 18 a-decays͞g; above this dose, the rate of density change increased to a new constant value, and this phenomenon is attributed to microcracking. The maximum volume expansion approaches 4% at 1.8 3 10 18 a-decays͞g, with no indication of saturation.…”

Mineralization of Radioactive Wastes by Fluidized Bed Steam Reforming (Fbsr): Comparisons to Vitreous Waste Forms, and Pertinent Durability Testing

“…This includes, but is not limited to, the following waste forms: SYNROC, Pu-ceramics, High Al-tailored ceramics, and supercalcines. [113,114,115,116,117,118,119] In particular, it has been determined that waste impurities tend to stabilize in the intergranular glassy phase and result in elemental losses which are an order of magnitude greater for some radioactive waste species. [118] Therefore, if radionuclides have preferentially partitioned to one phase or to the intergranular phase at elevated temperatures, these elements must be directly monitored during a durability test.…”