TREAT reactor LEU fuel-clad chemical interaction empirical modeling analysis

The Transient Reactor Test (TREAT) facility resides at the Materials and Fuels Complex at Idaho National Laboratory. TREAT first achieved criticality in 1959 and successfully performed many transient tests on nuclear fuel until 1994, when its operations were suspended. Resumption of operations at TREAT was approved in February 2014 to meet U.S. Department of Energy Office of Nuclear Energy objectives in transient testing of nuclear fuels. In parallel, the National Nuclear Security Administration, through the Office of Material Management and Minimization, is converting TREAT from its existing highly enriched uranium (HEU) core to a low-enriched uranium (LEU) core. To maintain TREAT experimental performance capabilities, the LEU core must be able to match the test sample total energy deposition achievable in the HEU core and satisfy operational safety requirements. However, there are dimensional and material restrictions to achieving this goal. Historical information about the original TREAT HEU and upgraded core fuel assembly and a finite element analysis of the new LEU fuel assembly design has shown collapse of thin wall cladding toward the fuel block due to pressure differential between exterior atmospheric pressure and the assembly's interior high vacuum. To assess the magnitude of chemical interaction between fuel and cladding materials during physical contact under expected TREAT operational limits, planned transients tests, and reactivity accident scenarios, a combination of experimental testing and thermodynamic modeling was performed to predict the expected chemical interactions among fuel and cladding chemical constituents. Thermodynamic calculations were then validated with empirical data from experiments that emulate TREAT's expected upper operational limits. Pellet samples composed of LEU oxide powder dispersed in a graphite matrix had intimate contact with zirconium-based alloy cladding. The samples were subjected to long-term isothermal heating under high vacuum. Specimen characterization consisted of scanning electron microscopy, x-ray diffraction analysis, and x-ray tomography. ThermoCalc software and Ellingham's diagrams were used for thermodynamic calculations. This paper presents preliminary results of experimental and modeling work.