2, Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne, , Switzerland
3, TRIUMF (Present Address), Vancouver, British Columbia, Canada
The Isotope Separator On-Line DEvice ISOLDE is a facility dedicated to the production of radioactive ion beams located at CERN, the European Organization for Nuclear Research. With over 50 years of experience, the ISOLDE facility is able to deliver more than 1000 different isotopes of 74 chemical elements for different applications in the field of nuclear and atomic physics, material science and nuclear medicine.
Radioactive nuclides are produced by irradiating thick targets of 20 cm length and 2 cm diameter, with a 1.4 GeV proton beam. In more than 65% of the beam time, the targets are made of refractory materials such as highly porous depleted uranium or thorium carbide with excess graphite (UCx or ThCx). In this contribution, the life cycle of actinide target materials at the ISOLDE facility will be discussed, from the raw materials and carbide production, to operation and waste disposal.
Actinide targets are used as oxides or as carbides by mixing an oxide powder with graphite, pressed into thin pellets and heat treated to produce carbides. Oxides are normally in the form of micrometric powders and occasionally of fibers in the ThO2 case. The targets are operated for 2 weeks while they are kept under vacuum at temperatures higher than 2000 °C to promote isotope release. During irradiation the material undergoes structural changes and trap some of the produced isotopes, while others are released, which makes the material activated and the use of a hot cell required.
To increase isotope release efficiency, a new uranium based material was engineered. By making green compacts of carbon nanotubes with nanometric uranium oxide, and subsequent carbothermal reduction, a novel porous nanometric uranium carbide was produced. This target material had a stable microstructure, however the material was extremely pyrophoric due to its large surface area and required extreme care in all handling procedures. This target was successfully operated at ISOLDE to bring increased isotope beam intensities.
While both UCx and ThCx are pyrophoric and cannot be kept in this form for long-term storage, a safe process for the conversion into oxide is investigated. However, UCx oxidation in air is a highly exothermic reaction that is associated with the risk of thermal runaway, which in turns depends highly on the starting microstructure. Systematic investigations are underway in order to develop a safe and controlled stabilization process. The work developed could be transferred to other facilities worldwide, as a new waste disposal channel.
N.-T. Vuong and T. Stora acknowledge that the UCx oxidation research project has been supported by a Marie Sklodowska-Curie Innovative Training Network Fellowship of the European Commission's Horizon 2020 Programme under contract number 642889 MEDICIS-PROMED.