Physico‐chemical properties of international simple glass (ISG) nuclear waste simulants: Luminescence baseline study

Abstract

Borosilicate glass has been adopted internationally for the treatment of nuclear waste with the object of long‐term stabilization through vitrification. Aiming to facilitate experimental comparisons across laboratories, the six‐component international simple glass (ISG) was developed as benchmark. The original ISG produced by Mo‐SCI Corporation (Rolla, MO) was distributed and characterized for different physical properties thus serving as reference for further studies. Still, photoluminescence (PL) properties, which may be useful for scrutinizing radiation‐induced damage, have not been reported. Further, the original ISG contained significant iron impurities, which may present interference in various studies (e.g., optical). Consequently, two new ISG analogs lacking iron impurities (labeled ISG‐1 [closest composition to original ISG] and ISG‐2 [some MgO added at expense of CaO]) were synthesized by Corning Inc. (Corning, NY) (also doped with lanthanum for subsequent corrosion tests involving atom probe tomography), which remain to be fully characterized. Accordingly, this work was undertaken to perform a comprehensive study comparing the three ISG specimens. Various characterizations were then performed on the pristine glasses: X‐ray diffraction and vibrational spectroscopy (structural properties); static‐leach product consistency test (PCT, dissolution behavior); dilatometry and calorimetry measurements (thermal properties); and optical absorption with Tauc and Urbach plots analysis, followed by PL spectroscopy with decay kinetics assessment (optical properties). While the general structural features appeared similar among the glasses, the deconvolution of Raman spectra suggested a lower degree of connectivity in the silica network for ISG‐2. In addition, some differences were indicated from the PCT results and thermal properties assessed, which were discussed based on compositional variations. Further on, the optical properties were shown to be the most distinct. The optical absorption of the original ISG was characterized by Fe2+/Fe3+ impurities not detected for ISG‐1 and ISG‐2. The glasses all exhibited two main PL features (∼1.8 and 2.5 eV) with multiexponential decay behavior apparently of intrinsic origin. In addition, ISG‐1 and ISG‐2 showed a third high‐energy (∼3.4 eV) fast‐decaying contribution tentatively credited to tin impurities, which appeared most significantly for ISG‐1. The results are expected to serve as baseline for future studies simulating the effects of radioactive element decay.

In Situ Optical Microspectroscopy Study of Isothermal Bleaching of γ-Irradiated International Simple Glass

“…Since glasses are structurally heterogeneous (amorphous), electron energies are not truly discrete but form an exponential tail of semicontinuous levels within the band gap. The parameter of Urbach energy is utilized for assessment of the width of these exponential tails as commonly carried out in the literature, , as well as our previous works on the ISG. , The analysis follows the relation of the absorption coefficient as α = α 0 exp ( ℏω E U ) where α 0 is a constant and ℏ ω is the photon energy. , The Urbach energy is then evaluated from the ln(α) vs ℏ ω plot, taking the reciprocal of the slopes of the linear portion of the plots. − ,, For consistency, the numerical procedure follows an algorithm for finding a tangential line of the steepest portion of the plot. The steepest section located via the maximum of the first derivative of the ln(α) versus ℏ ω plot.…”

“…E U is the characteristic parameter of choice since it allows understanding the microscopic physical processes in connection with the macroscopically measured spectra. [13][14][15]18,19 In the glass being an insulator, strongly bound electrons reside in the valence band. This is a result of the localized electrons of the negatively charged non-bridging oxygens (NBOs) of the network former.…”

“…The parameter of Urbach energy is utilized for assessment of the width of these exponential tails as commonly carried out in the literature, 13,14 as well as our previous works on the ISG. 15,18 The analysis follows the relation of the absorption coefficient as…”

“…Details regarding gamma radiation experiments as well as the composition and properties of the pristine ISG and the γirradiated counterparts can be found elsewhere. 15,17,18 The γirradiated ISG samples used in the present study were ∼1/16″ thick glass slabs that were polished for optical measurements. 15 An analytical methodology was developed for assessing the activation energy of thermal bleaching of γ-irradiated glasses.…”

“…16,17 The ISG originally produced by Mo-SCI Corporation (Rolla, MO, USA) was distributed and subsequently characterized by various means such as X-ray diffraction, thermal analysis techniques, chemical durability through the product consistency test, vibrational spectroscopy (e.g., Raman scattering), optical absorption, and photoluminescence (PL) spectroscopy. 17,18 In ref 15, the authors reported on the influence of γ-ray irradiation on the original ISG available at the Savannah River National Lab (SRNL, Aiken, SC, USA). The glass was subjected to γ-ray irradiation with a Co-60 source up to an accumulated dose of ∼2 MGy, and the structural, thermal, corrosion, and optical properties were studied.…”