Evaluation of Experimental Studies in the U-Rich Region of the U-Pu Phase Diagram
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TION
ALTHOUGH it is recognized the advantage that modeling efforts, such as CALPHAD, provide to the nuclear industry in terms of economic and laboratory constraints related to research on radioactive materials, empirical investigations with actinide materials are still relevant not only for verification of the modeling efforts but also as a source of additional data.[1] It is fitting that past research efforts that were originally pursued based on a comprehensive understanding of the chemistry of nuclear materials, are also capable of providing fundamental thermophysical data upon further analyses. Two such experimental studies have recently been re-examined with respect to the U-rich region of the U-Pu phase diagram. Following the melting of uranium alloys,[2] cooling curve analyses have been performed during the solidification portion of the process to yield phase equilibria data. Multiple melts, consisting of a binary alloy with plutonium that is rich in uranium, have been evaluated to derive transitional phase boundaries. Specifically, the liquidus and body-centered cubic (e-Pu, c-U) phase transitions have been investigated in the U-rich region of the U-Pu phase diagram and are compared against the most recent uranium-plutonium phase diagram.[3]
B.R. WESTPHAL and S.X. LI are with the Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 83415. Contact e-mail: [email protected] Manuscript submitted August 13, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
The recent U-Pu phase diagram is a culmination of over 50 years of actinide research and incorporates data from several other studies.[4–8] During a vaporization study of plutonium from a uranium alloy,[9] the activity of plutonium in uranium at various concentrations and temperatures can be calculated from the solid-vapor distribution data. Given the dilute concentration of plutonium in uranium, a Henrian activity coefficient can be derived along with other pertinent thermodynamic properties. The high temperature data confirm the non-ideality of the binary uranium-plutonium system and compares reasonably well with recent studies.[3]
II.
COOLING CURVE ANALYSES
The melting of uranium alloys containing plutonium is the final step during the pyroprocessing of used nuclear fuel.[2] The melting step is performed in an induction-heated casting furnace capable of alloy homogenization as well as molten sampling. A basic schematic of the pertinent components to the casting furnace is shown in Figure 1. The graphite crucible, heated by the induction coils, contains the molten U-Pu alloy and includes two thermocouples (Type C) on the bottom of the crucible. The thermocouples are embedded in the graphite and monitor temperatures during a casting operation. Molten samples of the alloy are taken for chemical analyses by injection-casting molten metal into quartz molds via an evacuation/pressurization cycle. A more thorough description of the casting furnace operation and equipment has been given elsewhere.[10]
Following a casting, the cast pins are removed from the qu
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