Laser heating of polycrystalline nuclear materials

An overview of the containerless, ultra-high temperature instrumentation on the high energy x-ray beamlines at the Advanced Photon Source is described. It has been implemented for the specific goal of studying crystalline transitions in radioactive materials. The experiments employ laser heating combined with aerodynamic levitation and are designed to recreate severe accident scenarios in the laboratory to benchmark atomic interactions under meltdown and post-meltdown conditions. A hermetically sealed chamber enables control of the atmosphere chemistry surrounding a floating pellet of nuclear material, 2-3 mm in diameter. Optical pyrometery enables instantaneous feedback of the sample surface temperature, and a focused 400 W CO₂ laser beam incident on a sample can typically achieve temperatures up to 3500 K. The penetration associated with high energy x-rays (cf. 100 keV) is required to enable transmission diffraction measurements in sealed complex environments. Rapid, high flux x-ray experiments on nuclear materials provides the ability to probe phase transitions and determine reaction pathways upon cooling. Phase identification and the calculation of phase diagrams from different compositions of the model corium system UO₂:ZrO₂ show the single phase tetragonal structure exists at lower temperatures than previously reported.