Electron spin resonance study of Fe 3+ and Mn 2+ ions in 17-year-old nuclear-waste-glass simulants containing PuO 2 with different degrees of 238Pu substitution

Abstract

In 1982, three samples of a model nuclear waste glass, DRG-P1, P2, and P3, were prepared at Pacific Northwest National Laboratory with identical chemical compositions but respectively batched with 0.0, 0.1, and 0.9 wt.% of 238PuO 2 (half life 87.8 years) partially replacing the 1.0 wt.% 239PuO 2 (half life 2410 years) present in DRG-P1. In 1999, sub-samples of these three glasses were sent to the Naval Research Laboratory, where electron spin resonance (ESR) was to be used to search for self-irradiation effects due to 238Pu α decay. However, no radiation-induced point defects associated with the aluminoborosilicate network were observed. Rather, profound α-decay-induced changes in the ESR spectra of the batched iron-group ions were found. The spectra recorded for DRG-P1 were shown by absolute spin counts to have ESR intensities equivalent to ~ 85% of the sum of the batched 8.28 mol% Fe 3+ and 2.79 mol% Mn 2+, assuming that all of those ions behave as paramagnetic S = 5/2 states at room temperature. (Only 1.7 mol% Ni 2+ was batched, and ion-for-ion this S = 1 specie is calculated to contribute only ~ 1/3 of the ESR intensity of an S = 5/2 ion.) Separate experiments and calculations ruled out the possibility of small-particle magnetite-like precipitates comprising even so much as 0.01% of the total iron. A relatively weak ESR spectral feature observed in all three of the DRG-P n samples at g = 4.3 is the known signature of dilute Fe 3+ in glasses. By far the strongest ESR signal was found to be a broad line characterized by a first-derivative zero crossing at g = 2.06 and a peak-to-peak derivative linewidth of ~ 150 mT, both of which are shown to be virtually insensitive to temperature variations in the range 4.2 to 500 K and α-decay doses in the range provided by the 17-year aging of the three samples with differing 238Pu contents. It was discovered that these broad line shapes could be accurately simulated as weighted sums of Lorentzian shape functions of differing widths but having the same g value. The absence of any measurable anisotropy in the broad line, coupled with the temperature invariance of its width, imply the existence of extremely strong exchange interactions within clusters of Fe 3+, Fe 2+, Mn 2+, and Ni 2+ ions. The result is a speromagnetic system (amorphous antiferromagnet) characterized by progressive freezing out of like-ion pairs as the temperature is lowered, as opposed to exhibiting a distinct Néel temperature. Calculations that confirm this inference hinge on use of an equation previously derived by one of the authors [D.L. Griscom, V. Beltrán-López, C.I. Merzbacher, and E. Bolden, J. Non-Cryst. Solids 253 (1999) 1–22] that expresses the ESR intensity of ions behaving as non-interacting paramagnets as a function of their spin S, the spectrometer frequency ν, and the temperature T. The most evident ESR effect of 17 years of 238Pu decay is the (irreversible) lowering of the intensity of the broad line in rough proportion to the amount of 238Pu in the sample, with associated increases in the amplitude of the narrow g = 4.3 feature. It was additionally observed that cooling these glasses gives rise to reversible lowering of the broad-line intensity and increasing of the strength of the g = 4.3 feature when compared with theoretical expectation for temperature dependence of non-interacting S = 5/2 paramagnets . The ESR integrated intensity of the broad line as a function of 238Pu α-decay dose proved to be accurately fitted by a simple saturating exponential function asymptotic to zero for infinite-time self irradiation. This result thus promises a precise means of extrapolating thousands of years into the future the process of “super vitrification” resulting from the creation and rapid quenching of “thermal spikes” due to α decay in glasses immobilizing 239Pu or other actinide elements. In addition, because the ESR spectra of several very different candidate high-level nuclear waste (HLW) glass compositions containing even higher amounts of Fe 2O 3 are also shown here to be decomposable into sums of pure Lorentzians, the analytical method we have devised should be applicable to these and many other HLW glasses containing both iron-group oxides and radionuclides.