The standard method to determine the band structure of a condensed phase material is to (1) obtain a single crystal with a well defined surface and (2) map the bands with angle resolved photoelectron spectroscopy (occupied or valence bands) and inverse photoelectron spectroscopy (unoccupied or conduction bands). Unfortunately, in the case of Pu, the single crystals of Pu are either nonexistent, very small and/or having poorly defined surfaces. Furthermore, effects such as electron correlation and a large spin-orbit splitting in the 5f states have further complicated the situation. Thus, we have embarked upon the utilization of unorthodox electron spectroscopies, to circumvent the problems caused by the absence of large single crystals of Pu with well-defined surfaces. Our approach includes the techniques of resonant photoelectron spectroscopy [1], x-ray absorption spectroscopy [1,2,3,4], electron energy loss spectroscopy [2,3,4], Fano Effect measurements [5], and Bremstrahlung Isochromat Spectroscopy [6], including the utilization of micro-focused beams to probe single-crystallite regions of polycrystalline Pu samples. [2,3,6]

Plutonium, because of its radioactive nature, ages from the “inside out” by means of self-irradiation damage and thus produces Frankel-type defects (vacancies and self-interstitial atoms) and defect clusters. The self-irradiation damage in Plutonium-239 occurs mainly by α-particle decay, where most of the damage comes from the U-235 recoil nucleus. The defects resulting from the residual lattice damage and helium in-growth could result in microstructural and physical property changes. Because these self-irradiation effects would normally require decades to measure, with a fraction (7.5 wt%) of Pu-238 is added to the reference plutonium alloy thus accelerating the aging process by approximately 18 times the normal rate. By monitoring the properties of the Pu-238 spiked alloy over a period of about 3.5 years, the properties of plutonium in storage can be projected for periods up to about 60 years. This paper presents density and volume changes observed from the immersion density and dilatometry measurements equivalent to aging the reference plutonium alloys to nine years.

A calorimetric investigation of the enthalpies of formation of Gd2(Ti2-xZrx)O7, where 0≤ × ≤ 2 is underway. All samples exhibit pyrochlore (Fd3m) peaks in their XRD patterns. However, where x=2 significant local disorder is observed in the Raman spectra. Preliminary data for the enthalpies of formation from the oxides in kJ/mol are: x=0, ΔHf = -113.4±2.7; x=0.5, ΔHf = -94.0±3.0; x=1.0, ΔHf = -74.2±4.9; x=1.5, ΔHf = -64.5±2.0; x=2, ΔHf = -52.2±4.8. Two additional samples, Gd1.80Zr2.15O7.00 (pyrochlore) and Gd2.15Zr1.87O7.00 (fluorite), were also studied. Their enthalpies of formation from the oxides in kJ/mole are -50.9±3.3 and -46.4±3.4 respectively. Replacing Ti with Zr, i.e. when x=2, destabilizes the pyrochlore in enthalpy by approximately 60 kJ/mol. The ΔHmix for the Gd2(Ti2-xZrx)O7 solid-solution series is positive and can be described by a regular solution formalism with an estimated interaction parameter, ŝ = +20 kJ/mol. The results of this study suggest that the pyrochlore to fluorite transition enthalpy in Gd2Zr2O7 is small, of the order of the configurational entropy contribution due to cation disorder at the transition temperature, TΔSconf. ≍ 10 kJ/mol.

Aqueous durability has been assessed for fourteen pyrochlore- and zirconolite-rich titanate waste forms designed for the immobilisation of excess Pu. The ceramics used in this study contained about 12 wt% Pu and about 15 wt% of Hf and Gd oxides as neutron absorbers and were fabricated by cold-pressing and sintering or hot isostatic pressing.

Total release rates (i.e. unfiltered solution + vessel wall inventory) have been measured withthe MCC-1 test method at 90 °C in deionised water. For all samples, 7-day release rates of Pu are between 4 × 10−5 and about 10−3 g.m−2.d−1, reducing to between about 8 × 10−6 and 3 × 10−5 g.m−2.d−1after more than 300 days of leaching. Release rates of U from the baseline ceramic were found to decrease to values between 6 × 10−4 and 1 × 10−5 g.m−2.d−1after 200 days. Hf leach rates were generally < 5 × 10−6 g.m−2.d−l after more than 7 days. Further, the addition of several % of chemical impurities to the baseline sintered formulations was found to reduce U and Gd releases by about a factor of 10 after 200 days.

Synchrotron x-ray fluorescence microscopy has been used to study multi-metal oxide ceramics that have been designed to sequester radioactive actinide elements for long-term storage and disposal. X-ray fluorescent lines for the various elements have been used for lateral elemental mapping of the materials, and the heterogeneity of the samples is discussed with respect to the elements in the crystallographic phases that have previously been documented by other means of structural and chemical analyses.

This study reports on the use of zirconolite-rich Synroc to demonstrate the safe immobilisation of ‘high-fired’ Pu02. The zirconolite-rich Synroc used in this study was prepared by adding 13 wt% Pu with equimolar amounts of Gd and Hf, relative to Pu, as neutron absorbers. The incorporation of the Pu and neutron absorbers has been studied microstructurally as well as by longer-term leach testing. This work has shown that the sintered ceramic can immobilise 13 wt% of Pu with almost complete incorporation of the Pu (≃ 98%) into the zirconolite phase. Durability studies have shown that under a wide range of leaching conditions there is no major separation of the Pu and neutron absorbers, with the majority of these elements either remaining in the matrix or leaching at low (< 10−4 g m−2 d−1) and comparable rates from the waste form.

A titanate-based ceramic waste form, rich in phases structurally related to zirconolite (CaZrTi2O7), is being developed as a possible method for immobilizing excess plutonium from dismantled nuclear weapons. As part of this program, Lawrence Livermore National Laboratory (LLNL) produced several ceramics that were then characterized at Argonne National Laboratory (ANL). The plutonium-loaded ceramic was found to contain a Pu-Gd zirconolite phase but also contained plutonium titanates, Gd-polymignyte, and a series of other phases. In addition, much of the Pu was remained as PuO2-x. The Pu oxidation state in the zirconolite was determined to be mainly Pu4+, although some Pu3+ was believed to be present.