PNNL

Abstract

Three samples of a model nuclear waste glass, DRG-P1, P2, and P3, were prepared at PNNL in 1982 with identical chemical compositions but were respectively batched with 0.0, 0.1, and 0.9 wt% of 238PuO2 (half life 87.8 years) partially replacing the 1.0 wt% 239PuO2 present in DRG-P1. In 1999, samples of these glasses were sent to the Naval Research Laboratory for electron spin resonance (ESR) measurements. No radiation-induced point defects were observed. Profound alpha 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 mole% Fe3+ and 2.79 mole% Mn2+, assuming all of those ions to behave as paramagnetic S=5/2 states at room temperature. Separate experiments and calculations ruled out the possibility of precipitated magnetite-like precipitates comprising even so much as 0.01% of the total iron. A relatively weak ESR spectral feature observed at g=4.3 is the known signature of dilute Fe3+ in glasses. However, the strongest ESR signal was found to be characterized by a first-derivative zero crossing at g=2.06 and a peak-to-peak derivative linewidth of ~150 mT, which is virtually invariant in shape with both measurement temperature and alpha-decay dose. 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 Fe3+, Fe2+, Mn2+, and Ni2+ ions characterized by extremely short-range magnetic order. The result is a speromagnetic system rather than exhibiting a distinct Néel temperature. 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 concomitant increases in the amplitude of the g=4.3 feature. It was additionally observed that cooling these glasses to successively lower temperatures 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 non-interacting paramagnets. The truly remarkable observation that the broad lines could be simulated as weighted sums of pure Lorentzian functions of differing widths fortuitously opened the way for high precision measurements of the ESR intensities of experimental spectra that are far broader than the magnetic field range of the available laboratory electromagnets. The areas under the simulated absorption curves fitted to the experimental spectra in the manner described provided an empirical measure of the degrees to which the present model nuclear waste glasses had been affected by alpha-decay self irradiation. Specifically, the broad-line ESR integrated-intensity data as a function of 238Pu alphadecay dose (proportional to the 238Pu doping level in these fixed-time experiments) proved to be accurately fitted by a simple saturating exponential function asymptotic to zero for infinite-time self irradiation. This result promises a precise means of extrapolating thousands of years into the future the process of "super-vitrification" that results from the creation and rapid quenching of thermal spikes due to alpha decay in glasses immobilizing 239Pu and 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 Fe2O3 are also shown here to be decomposable into sums of pure Lorentzians, the analytical method we describe should be applicable to these and many other HLW glasses containing both iron-group oxides and radionuclides.