Performance of molybdenum with UF 4 at high temperatures as a wall material for space reactors
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I.
INTRODUCTION
NEW nuclear reactor concepts for the space mission to Mars have been considered during the last decade. Gas core reactors in which uranium tetrafluoride (UF4) is envisioned as the circulating fuel could be a major alternative in nuclear energy generation. These types of reactors require very high temperature resistant core materials. The calculated peak temperature of the fissioning gas and the inner wall temperature of the reactor core can reach up to 4000 K and 1600 to 2000 K, respectively.[1] Therefore, structural materials must necessarily possess high melting points and chemical stability to survive in this severe environment. Many oxide ceramics were not able to withstand the very corrosive nature of UF4.[2–5] Uranium tetrafluoride is an intermediate product in the conversion of uranium ore to UF6 and is also used in the manufacture of UO2 and uranium metal fuels. It is a greencolored powder with a monoclinic structure. It is nonvolatile, insoluble in water, and relatively stable in air. It has a melting point of 1036 K and a boiling point of 1715 K under 1 atmosphere.[6,7] Thermodynamical data for chemical reactions between UF4 and different materials have been obtained using the computer code FACT (Facility for the Analysis of Chemical Thermodynamics).[8] Among many materials, molybdenum (Mo) showed good compatibility under the operating conditions; in addition, it has a high melting point (2890 K) and low neutron absorption cross section (0.20 barn), which is essential for neutron economy. Figure 1 presents the thermodynamical stability range of uranium and some metal fluorides as a function of temperature. For all temperatures, MoF6 possesses higher free energy values than uranium fluoride, indicating that the latter is thermodynamically more stable than the former. Therefore, Mo has been thought to be one of the candidate materials for space power and propulsion applications. Z.E. ERKMEN, Assistant Professor, is with the Department of Metallurgical Engineering, Istanbul University, Avcilar Campus, 34850, Istanbul, Turkey. S. ANGHAIE, Professor, is with the Department of Nuclear Engineering, University of Florida, Gainesville, FL 32611-8300. Manuscript submitted December 11, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
However, Lundberg[9] and Kuznietz et al.[10,11] reported that some diffusion of uranium and compound formation in Mo matrix did occur and progressive dissolution of Mo grains was also observed in molten uranium at high temperatures. The Mo-based alloys were tested in molten sodium at 923 K for periods of 1000 hours. The corrosion rate was estimated by measuring weight loss or gain of the test specimens. The Mo alloys showed much higher resistance than Ni-based alloys.[12] Data on the diffusion of uranium in the Mo matrix is also provided by Federov and Smirnov.[13] In order to understand the effect of UF4 on Mo, a set of experiments has been designed and performed in both gas and liquid phase UF4. II.
EXPERIMENTAL PROCEDURE
Samples of high-purity Mo foils (99.9 pct)
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