ORNL-TM-7207 - the Molten Salt Energy Technologies Web Site

56
to bring that understanding to the level that exists for LiP-BeF2 solu-
tions.
Oxide-fluoride behavior. The behavior of molten flu~ridie systems
such as the DMSB fuel mixture can be affected markedly by addition of
significant concentrations sf oxide ispa, For example, we know that crys-
tals of UO2 precipitate when welts of EiF2-UF4 are treated with a reac-
tfve oxide such as water v a ~ o r ~ ~ 9 ~ ~ ~ 9 3 3
The solubilities of the actinide dioxides in L~F-B~F~-T~F~-.UFL+ mix-
tures are low, and they decrease in the order Tho2, Pa02, U0z9 and
~~02.29 16-24
oreo over^ these dioxides all psssess the same (fluorite)
crystal structure and can all form solid solutions with one another.
Sokmbility products and their temperature dependence have been mea-
sured; 34-43 their behavior is generally we11 understood.
Trivalent plutonium shows little or no tendency to precipitate as
oxide from LiF-BeF2-ThPq-UFq mixtures 27
bility seems to be general for trivalent oxides, it is highly lfkeky
that precipitation of h203 and other trivalent actinides would be dif-
ficult to achieve.
Because relatively large so%u-
If Pa4+ is oxfdized to Pa5+ (which can be do~e readily in LiF-BeF2-
ThF4-UF4 by treatment with anhydrous and hydrogen-free KF gas), then Pa205
(QP an addition compound of it> can be precipitated selectively.4l3 44
such oxidation to pas* can be avoided by maintaining a small fraction of
the uranium as UF3 in the fuel mixture.
he relaiively ~ s oxide w tolerance of DMSR fuel will require reason-
able care to avoid inadvertent precipitation of actinide oxides within the
reactor system.
substantially diluted with ii2> serves to Lower the oxide concentration
to tolerable levels. 18,45
However, treatment of melts with anhydrous I-iF (even when
Compatibility of fuel with reactor materials, Molten fluorides are
excellent fluxes for many materials. Though some oxides are relatively
insokuble, most are readily dissolved, and d l are rapidly recrystal-
lized; consequently, protective coatings are n ~ useful, t and the bare
clean metal must withstand C Q P ~ Q S ~ attack. V ~ The reactor metal (Hastel-
loy-N, described Prt detail in Sect, 3.4) was chosen and tailored to be
thegm~dp~i~ieally stable to the fuel components, as much as possible,
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