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Appendix D
Plutonium
Because both plutonium and uranium are fissionable by slow neutrons and are
used in nuclear weapons, there is ~ tendency to think that they have similar
physical and chemical properties, but this is not the case. Both are silvery metals,
with freshly exposed surfaces resembling iron or nickel in appearance, and both
have densities approximately 50 percent greater than lead. Beyond these
similarities, the two elements differ widely in their properties. Plutonium is
harder and more brittle than uranium, and has a melting point some 500°C
(900°F) lower. Although both are relatively easily oxidized, plutonium is much
more reactive chemically, and in air it is readily oxidized to plutonium dioxide,
PuO2, the most common fond of plutonium in the environment Two useful
reference works covering the properties and chemist of plutonium are Cleveland
(1976) and Comar et al. (1976).
Uranium is less reactive, but it, too, is oxidized, producing a variety of oxides.
In contrast to plutonium, which does not exist in nature to a significant degree,
uranium occurs naturally in a number of chemical and mineral forms.
Plutonium dissolves more readily in acids, and once dissolved particularly in
nitric acid its chemistry is so different from that of uranium that the two
elements can be chemically separated from each other. Simply stated, the chemical
differences between the two elements in solution result primarily from the
differences in electrical charges on their ions. Because their ions behave differently,
they may be separated from each other by a process known as solvent extraction.
Plutonium is produced in nuclear reactors by the irradiation of uranium-238
with neurons emitted in the fission of uranium-235. (Natural uranium contains
99.3 percent uranium-238, O.7 percent uranium-235.) After discharge from the
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APPENDIX D
119
reactor and storage for several months to allow the short half-life fission products
(produced by the fission of uranium-235) to decay, the uranium must tee processed
to remove the few hundred parts per million of plutonium product. The irradiated
uranium is dissolved in concentrated nitric acid After suitable adjustments, this
nitric acid solution, containing uranium, plutonium, and fission products, is
contacted with an immiscible organic solution of ~ibutyl phosphate (TBP) in a
diluent that is essentially a highly purified kerosene fraction. The uranium and
plutonium are extracted into the organic phase, leaving the fission products in the
nitric acid solution. After additional extraction to remove residual uranium and
plutonium, this solution is sent to waste treatment and storage; it is the primary
source of high-level waste in the DOE weapons complex.
The organic solution containing the plutonium and uranium is first contacted
with a more dilute nitric acid solution containing a"reducing agent" to decrease
the electric charge on the plutonium ions, so that they are extracted, leaving only
the uranium in the organic phase. The uranium is then extracted into a very dilute
nitric acid solution. The nitric acid solution of plutonium is further purified and
concentrated by ion exchange, a process in which the plutonium is selectively
sorbed onto beds of organic resin while impurities remain in solution and pass
through the bed. The plutonium is then removed from the resin (eluted) with
dilute nitric acid.
This solvent extraction procedure is known as the PUREX process, and it is
used with minor modifications at Hanford, SRS, and ~EL. The process can
achieve separation factors of uranium from plutonium of greater than 10 million,
and of plutonium from uranium of 1 million. Decontamination of fission products
from plutonium exceeds 100 million. Recovery of both plutonium and uranium is
about 99.9 percent An additional advantage of the PUREX process is the solid
waste minimization: because the primary chemical used in the process is nitric
acid, a volatile liquid, it can be removed by evaporation, leaving only a small
volume of solid waste. The organic solution of TBP in kerosene, after a simple
cleanup step, can be reused.
Preparation of plutonium metal from the nitric acid solution is accomplished
by one of several conversion processes which are based on similar chemistry. All
three involve precipitation reactions and all require the use of hydrogen fluoride
(HF), either as a gas or in aqueous solution. Plutonium is precipitated from the
nitric acid solution as the oxalate, peroxide, or trifluoride (the latter only at SRS,
using an aqueous solution of HF). After drying, the oxalate or peroxide is
converted to PuO2 by heating in a stream of air. [The trifluoride is converted into
a mixture of PuO2 and plutonium te~fluoride (E>uF) by heating in air.] The PuO2
is then heard in a stream of gaseous HE to convert it to PUF4, which can be
reduced to plutonium metal by reaction with metallic calcium in a pressure vessel.
(The PuF4-PuO2 mixture produced from the tnfluoride precipitate is reduced
directly to metal without reaction with gaseous HF.) Reduction yields average 97
to 98 percent for PuF and about 95 Dercent for the PuF.-PuO. The calcium
—4
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APPENDIX D
fluoride reduction slags are dissolved and reprocessed to recover the residual
plutonium.
Plutonium metal scrap, calcium fluoride induction slags, reduction crucibles,
and plutonium-contain~ng incinerator ash are dissolved in concentrated nitric acid
and purified usually by ion exchange. The purified solution is then treated by one
of the conversion processes described above to produce the metal.
The choice of PuF,, as the plutonium compound for reduction is based on
several favorable factors. The large amount of heat released in the reaction of
PUF4 with calcium, combined with the relatively low melting point of the resulting
calcium fluoride slag, results in a low viscosity medium that allows plutonium
aggregation and thus enhanced yield. In addition, Puff, unlike plutonium
mchloride, another possible reduction candidate, does not absorb appreciable
moisture from the air. (Reduction of compounds with a high moisture content
results in excessive PuO2 formation and lower yield of metallic plutonium.) The
principal disadvantages of using PllF4 are the high neutron fluxes it produces as a
result of alpha reactions with fluoride ions, the corrosiveness and toxicity of the
aqueous or gaseous HF used to produce it, and the need to use an aluminum salt
(typically the nitrate) in dissolving the calcium fluoride slag, thus increasing the
volume of solid wastes.
Alternative conversion processes have been studied with varying degrees of
success. The nitric acid solution of plutonium may be evaporated and the solid
plutonium nitrate converted directly to PuO2 by heating in air. This procedure,
known as direct denigration, is not promising: it tends to produce gummy
residues, and the product PuO2 is inert toward either reaction with HE or direct
reduction with calcium. It appears likely that the existing processes involving
precipitation and calculation to produce PuO2 as an intermediate will be retained
for the foreseeable future.
It is in the subsequent trea~nent of PuO2 that viable alternatives exist. Calcium
can reduce PuO2 directly to metal, but there are problems because the heat
evolved is lower than for PUF4 reduction and the calcium oxide has a higher
melting point. The slag is not melted by the heat of reaction, and as a result finely
dispersed metal is produced. This problem has been overcome, however, by the
use of a molten calcium chloride flux to dissolve the calcium oxide slag and allow
the product plutonium to coalesce. The process has found production application
at LANL.
Impurities can sometimes be removed from plutonium metal without resort to
aqueous processing. Often americium~he impurity of most concern can be
removed froth plutonium in recycled weapons by molten-salt extraction using, for
example, a stadium chlonde-potassium chloride salt containing a few percent
plutonium bichloride: the americium, being more reactive, goes into the salt
phase and is replaced in the metal phase by more plutonium. Impure metal also
may be purified by molten-salt elec~oref~ning procedures using similar salt
mixtures. Judicious use of these nonaqueous procedures can, In many cases,
simplify processes and increase efficiency, and safety.
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APPENDIX D
121
Because plutonium reacts with the air win the evolution of heat and because it
is a poor conductor of heat, it can be pyrophoric, that is, it can spontaneously
ignite in air, particularly when in the fonn of lathe turnings, which have relatively
high surface area and poor contact between individual turnings. Such conditions
can promote the build-up of a "hot spot" in a small area that can exceed the
ignition temperate of the metal. Several serious fires in the weapons complex
have started in this manner. To prevent their recurrence, current practice calls for
handling potentially ignitable plutonium in enclosures with a low-oxygen
atmosphere.
Since plutonium reacts so readily with the air, it is rarely, if ever, found in the
metallic form in the environment. Thus the properties of PuO2, the common
environmental form, are most relevant when attempting to assess the behavior of
plutonium. Plutonium dioxide can vary in color from tan to olive green to black,
depending on purity and conditions of formation; it should be noted, however,
that it is not observed in the environment in quantities anywhere near large
enough for its color to be perceived by the eye. Typically, when it is present in
soils, for example, it is in the form of a relatively small number of microscopic
particles. The density of PuO2 is high compared to that of most chemical
compounds, but only slightly more than half that of the metal.
Nevertheless, individual particles, depending on how they were formed, can
vary considerably in density and in aerodynamic properties. Panicles are frequently
very small and can be subject to short-range atmospheric dispersion under suitable
climatic conditions. The dispersion will be spatially nonunifonn, but even a small
isolated panicle can emit appreciable radiation. These factors combine to cause
high variability in soil contamination analyses: whether a given soil sample
contains high radioactivity or no detectable activity whatever may depend on
whether it contains a single "hot particle."
Plutonium dioxide is normally quite insoluble in water and in body fluids (with
a few exceptions as noted below); it is even less soluble when formed at high
temperature, as in a fire. Hence its dispersion in soil is primarily by mechanical
means. It can also be blown along the surface by the wind ("saltation"~. It can be
washed downward into the soil column by natural factors, and it can be spread
both horizontally and vertically by plants and animals. Some limited dissolution
of PuO2 can occur in ocean water and in ground-waters with chemical compositions
that enhance plutonium volubility, but this does not generally occur in domestic
groundwaters because of Heir low chemical contents.
The low solubility of PuO2 in body fluids has several rarniD~caiions. Uptake
through the gastrointestinal system is small, since PuO2 is poorly absorbed through
the intestinal walls. The most serious modes of entry are inhalation and the
contamination of wounds. Once in the body, plutonium can be difficult to
remove. Inhaled PuO2 can be lodged in the lungs for considerable periods of time,
and ultimately it works its way into the lymph nodes. Plutonium entering the
blood stream through a contaminated wound ultimately deposits in the liver or the
bone marrow: in the latter site it can be especially harmful to the blood-forming
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APPENDIX D
process. Some success has been achieved in the removal of plutonium Mom body
systems by flee use of chemicals lmown as chelating agents that can dissolve it and
allow it to be excreted from the body. Such treatments are more effective when
administered soon afte, contamination, before the plutonium has been `'fixed" in
the body.
The comparable uranium compound, UO2 is similar in density to PuO2, but it
is considerably more soluble. Because the common fonn~uranium-23X and
uranium-23Ware much less radioactive than plutonium, the radiotoxicity of
uranium is lower. In fact, the primary hazard of uranium ingestion it tends to
concentrate in We kidneys~s chemical ("heavy metal poisoning') rather than
radiological.
Representative terms from entire chapter:
acid solution
