AB INITIO STUDY OF ADVANCED METALLIC NUCLEAR FUELS FOR FAST BREEDER REACTORS Page: 4 of 14
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of a-U phase. In other words, each of these high-melting temperature elements plays a role of '-
stabilizer' helping to keep uranium in the metastable bcc phase upon cooling.
Zirconium metal possesses a unique capability to suppress interdiffusion between the
nuclear fuel and stainless-steel cladding and this makes Zr a good candidate as a solver to
nuclear fuels for fast breeder reactors. The Zr-based actinide alloys, particularly U-Pu-Zr, proved
to be very promising fuels for liquid metal fast breeder reactors because of their advantage in
view of superior performance, reactor safety, and fuel cycle economics . It was established 
that the U-Zr system is characterized by the complete solubility of the body centered cubic high-
temperature phases, y-U and f-Zr, that is usually referred to in phase diagrams by '-phase' solid
solutions. Below T ~ 722 0C, these solutions separate into a relatively flat miscibility gap.
The intermediate UZr2 phase with C32 (AlB2)-type structure is formed on cooling from
the y-phase with the homogeneity range from 63 to 82 at. % Zr . It is well known that the
high-temperature Zr-based solid solutions may transform into the so-called metastable ow-phase
at low temperatures, which can also be stabilized from the a (hcp) phase of Zr under
compression . Ogawa et al.  suggested that the 8-UZr2 phase could be regarded as the cw-
phase solid solution that is stabilized against the a-Zr (hcp) structure by addition of U due to
increase of Zr d-band occupancy.
Recently Kim et al.  suggested some advantages of U-TRU-Mo fuels over U-TRU-Zr
in TRU-burning advanced fast nuclear reactors: U-Pu-Mo fuels have higher thermal
conductivity, lower thermal expansion, and higher melting points than U-Pu-Zr fuels resulting in
better safety. However, the main advantages of U-TRU-Mo fuels lies in a much lower
constituent redistribution, including migration of minor actinides (MA) and lanthanides (LA)
toward the cladding due to the existence of a single y-phase over typical fuel operation
temperatures. Contrary to U-Pu-Mo fuels, in U-Pu-Zr fuels MA Am redistribution is similar to
that of Zr with tendency to precipitate to the center and near the fuel surface .
Low-enriched uranium alloys with 6 to 12 wt. % of Mo are under consideration by the
GTRI program as very high density fuels (8-9 gU/cm3 and 15-17 gU/cm3 for dispersion-type and
monolithic-type, respectively) that allow nuclear research and test reactors conversion from use
of HEU to LEU fuels . According to the U-Mo phase diagram , Mo exhibits a high
solubility (- 35 at. %) in y-U (bcc) but below 560 0C the equilibrium state corresponds to a
mixture of a-U (orthorhombic) and so-called Y-phase, which is the U2Mo compound with the
C 11b (MoSi2 prototype) structure. However, by rapid cooling from the y-phase a metastable y-
state can be retained up to room temperature.
Semi-empirical model calculations , supported by experimental observations, indicate
that the excess enthalpy of solution of the y-U-Zr phase controls the constituent redistribution
process in the U-Zr fuels. This statement encouraged us to perform ab initio calculations of the
heat of formation of the y-U-Zr solid solutions [14, 15]. We later expanded our study to the
ternary U-Pu-Zr system  as well as to the bcc alloys that plutonium forms with MA . In
our recent paper  we presented results of ab initio calculations of the heat of formation of the
y-U-Mo solid solutions. In this study we summarize results published in Ref. [14-18] and present
results of ab initio calculation of the decomposition curve for the y-U-Mo alloys. We also present
results of ab initio calculations of the heat of formation of Am with Zr and Mo and discuss a
possible mechanism of Am redistribution in the y-U-Zr and y-U-Mo.
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Landa, A; Soderlind, P; Grabowski, B; Turchi, P A; Ruban, A V & Vitos, L. AB INITIO STUDY OF ADVANCED METALLIC NUCLEAR FUELS FOR FAST BREEDER REACTORS, article, April 23, 2012; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc831967/m1/4/: accessed December 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.