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OCR for page 103
Appendix B
Alternative Fluorinating Agents
Fluorinating agents other than molecular fluorine and hydrogen
fluoride (HF) are identified and discussed briefly below. The challenge of
volatilizing the uranium by fluorination in the Molten Salt Reactor
Experiment (MSRE) system, particularly in places where heating is
ineffective, may provide an opportunity to use one of these fluorinating
agents. As a general rule, the short-lived fluorinating agents listed below
would be more effective in a flow system rather than a closed system.
The list below is not exhaustive; for example, xenon fluorides, because
they are reasonably well known in fluoride chemistry work, are not
included.
.
Chlorine trifluoride (CIF3) is a liquid that boils at ~ T.7°C and
is a good oxidizer that converts uranium tetrafluoride (UF4) to uranium
hexafluoride (UFO) even at room temperature. Its main drawback is that if
oxygen-containing compounds are present, it may produce chlorine
dioxide (ClO2) or chlorine monoxide (CI2O) as a reaction by-product.
Gaseous ClO2 is condensed easily to liquid at about 9°C, which can be
achieved by fluids boiling under vacuum. Liquid ClO2 is a potent
explosive, whereas gaseous ClO2 appears to be reasonably safe.
Therefore, CIF3 should not be used in systems containing oxygen in any
form, and cold trapping (which would condense moisture) should not be
used for collection of volatile compounds (UF61.
.
Bromine pentafluoride (BrF5) is a liquid at room temperature
(meIting point, -61.3°C; boiling point, 40.5°C) and is a good oxidizing
agent capable of converting all uranium fluorides, including urany}
difluoride (UO2F2,) to uranium hexafluoride at room temperature; its
oxidizing capability improves with temperature (Jerry and SteindIer,
1967, 1968; Holmes et al., 1969~. If excess BrFs is used, the reduction
B.1
OCR for page 104
B.2
ANEVALUATION OF DOE ALTERNATIVES FOR MSRE
by-product is bromine trifluoride (BrF3), which is also a strong oxidizing
agent; if oxidizable materials are present in excess, the product is
bromine, which can be seriously corrosive to stainless steels, less so to
high-nickel alloys. The BrF3 can be refluorinated with fluorine and
recycled for reuse if desired. It does not have any known explosive
reaction products.
.
Atomic fluorine can be produced from diatomic fluorine (F2)
at low pressure, up to 1 or 2 mm (of mercury) in 20 mm of argon, by the
use of microwave heating in a sapphire tube, ultraviolet (UV) light, laser
activation, or a furnace at 300-600°C. Because of its low dissociation
energy, diatomic fluorine (F2) can be dissociated extensively into atomic
fluorine, with a reasonably long lifetime (several minutes) that is
dependent on its partial pressure in an inert gas stream. Atomic fluorine
is a strong oxidizing agent, capable of oxidizing both UF4 and PuF4
(plutonium tetrafluoride) to the hexafluoride at room temperature, and
the equipment to produce it is very simple to use and easy to place close
to the point of use. Atomic fluorine is practically noncorrosive to most
alloys and can be used with copper or stainless piping. Unused fluorine
can be redissociated easily by any of the generation mechanisms noted
above; for example, if UFO is trapped from the gas stream in carbon
dioxide (CO2) or sodium fluoride (NaF) traps, the unused fluorine can be
reactivated.
.
Dioxygen difluoride (O2F2; FOOF) is a fast-acting oxidizing
agent capable of converting either UF4 or PuF4 to its hexafluoride (UFO
or PuF6) at room temperature (Eller et al., 1988~. This compound is stable
at liquid nitrogen temperatures but has a life of only a few seconds at
room temperature. It can be generate`d at a few hundred grams per hour in
a simple coaxial reactor consisting of a liquid nitrogen-cooled, 1~/2-inch
outer-diameter stainless steel jacket with an inner car-rod-heated, 3/~-inch
nickel tube. The O2F2 condenses on the inner surface of the outer jacket
at liquid nitrogen temperatures and can be eluted for later use, or the
reactor can be operated in a continuous production mode. It can be
transported easily in stainless equipment held at liquid nitrogen
temperature.
· Dioxygen monofluoride (O2F; FOO) can be made in the
same type of reactor that is also equipped with a sapphire window for
OCR for page 105
APPENDIXB ALTERNATIVE FLUORINATING AGENTS
B.3
injecting W light from a mercury discharge lamp. It is considerably
longer lived than, but has similar oxidizing capabilities to, O2F2.
· Krypton difluoride (KrF2) is a powerful oxidizing agent
capable of producing UFO or PuF6 at room temperature from any lower
fluoride or oxide. It has a room-temperature lifetime of many hours or
even days and can be piped considerable distances in standard copper or
stainless tubing. It can be made in the same W-pumped system
described for O2F production. The production efficiency is much lower
than for O2F, but better stability makes KrF2 usable at a considerable
distance from the generation area. It was used at Los Alamos National
Laboratory (LANL) to decontaminate the prototype M-LIS (Molecular
Laser isotope Separation) plutonium isotopic enrichment equipment and
remove internal plutonium contamination from the processing equipment
at room temperature.
KrF2 may be a practical (but costly) candidate for stripping
uranium residues from MSRE off-gas pipes, traps, and valves after a
more readily available agent is used to oxidize the bulk deposits (e.g.,
BrFs). The production rates of KrF2 should be able to be scaled up from
the current LANL coaxial laboratory-type reactor systems to enable
sufficient feed for stripping lower uranium fluoride residues from the
freeboard regions in MSRE drain tanks and off-gas traps. Krypton
difluoride cannot be used to fluorinate the molten salt at 460°C because it
is not stable at elevated temperature.
OCR for page 106
Representative terms from entire chapter:
oxidizing agent
