Speaker
Description
Aluminum has long served as the primary choice of filler material for the production of critical radioisotopes such as actinium-227, barium-133, californium-252, thorium-228, thorium-229, and plutonium-238 in the High Flux Iso-tope Reactor at the US Department of Energy’s (DOE’s) Oak Ridge National Laboratory (ORNL). Aluminum’s high thermal conductivity and ease of use in fabricating ceramic–metallic (cermet) targets make it an ideal material. How-ever, the high charge density of the aluminum cation and the thermal instability of its nitrate salt can be major hin-drances during ion exchange purification and matrix exchange operations, respectively. Carbon powder is proposed as an alternative due to its comparable thermal conductivity and desirable chemical properties. The relative chemical inertness of carbon means it will not interfere with the ion exchange and heating techniques commonly employed in radioisotope production. Furthermore, by eliminating the necessity of solvating aluminum, process solution vol-umes and ion exchange resin volumes could be significantly reduced, resulting in a reduction in waste and overall process complexity.
The recovery of radium-226 and its irradiation products, actinium-227 and thorium-228, serves as a useful example of the difficulties that could be solved by this proposed substitution. Currently, a small quantity of irradiated radi-um-226 is unrecoverable by ion exchange due to the large quantity of cosolvated aluminum. Additionally, the purifi-cation process incurs a risk of trapping actinium-227 and thorium-228 within the chemically inert thermal degrada-tion products of aluminum nitrate. The substitution of aluminum with carbon would eliminate these issues, allow-ing for a closed loop in the recovery of these isotopes and a reduction in waste, process volumes, and overall com-plexity.
The benefits allowed by this substitution could be generalized to any process that uses aluminum as a filler material for irradiation. This work supports the DOE Office of Isotope R&D and Production and ORNL Radioisotope Pro-gram mission to strengthen the domestic isotope supply chain by improving target performance and downstream chemical processing. We evaluate replacing aluminum powder in carbonate cermet targets with graphite to reduce ion-exchange resin demand and elution volumes, decrease acid/base consumption and waste generation, prevent radi-oisotope trapping in inert alumina residues during heat-to-dryness steps, and improve thermal conductivity by avoid-ing aluminum oxidation. Because handling radium directly is challenging, BaCO3 was used as a surrogate for RaCO3. Graphite/BaCO3 composite pellets (10–30 vol % BaCO3) were fabricated via uniaxial pressing, vacuum hot pressing, graphite matrix overcoating, and spark plasma sintering, followed by heat treatments from 750°C to 1650°C guided by thermodynamic expectations for carbonate decomposition and potential carbide formation. Pellets were characterized by laser flash analysis, scanning electron microscopy/energy-dispersive X-ray spectroscopy, and X-ray diffraction to assess microstructure, phase evolution, and barium distribution. Initial results showed good barium dispersion in graphite and the highest thermal diffusivity for uniaxial-pressed and vacuum hot-pressed pellets. Time-of-flight secondary ion mass spectrometry indicated surface BaO and subsurface BaC2 in some cases. Preliminary barium recovery tests using 1 M HCl demonstrated strong dependence on thermal history: pellets heat-treated above 1600°C showed low recovery (≤33%), but a graphite matrix overcoat pellet heat-treated at 750°C achieved essentially complete recovery (~103%). Ongoing work will tune heat treatment conditions (e.g., ~1500°C) to balance thermal performance with chemical recoverability and enable scalable graphite/RaCO3 target fabrication and closed-loop iso-tope recovery.