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National Analytical Management Program (NAMP)
U.S. Department of Energy Carlsbad Field Office
Radiochemistry Webinars
Actinide Chemistry Series
• Uranium Chemistry –
General Properties of Uranium
In Cooperation with our
University Partners
Meet the Presenter…
Dr. Mikael Nilsson
Dr. Mikael Nilsson is an assistant professor in the Department of Chemical Engineering
and Materials Science at the University of California, Irvine. Before 2009, Dr. Nilsson
was a post-doctoral research associate at Washington State University. He graduated
with an M.S. in chemical engineering in 2000, and with a Ph.D. in nuclear chemistry in
December 2005 from Chalmers University of Technology, Gothenburg, Sweden.
During his M.S. and Ph.D. studies, Dr. Nilsson was one of the Swedish contributors to
the European projects for partitioning of spent nuclear fuel, PARTNEW and EUROPART.
He was also a member of the Swedish reference group on partitioning and transmutation research during this time . In
2004, he was a visiting scientist at the research center in Jülich, Germany under the supervision of Dr. Giuseppe Modolo.
Between September and November 2008, Dr. Nilsson was a visiting scientist at Pacific Northwest National Laboratory. In
October 2010, he became a certified senior reactor operator for the UC Irvine TRIGA nuclear reactor.
At UC Irvine, Dr. Nilsson teaches courses in Advanced Chemical Engineering Thermodynamics, Chemical Engineering Unit
Operations (laboratory course), The Nuclear Fuel Cycle, Introduction to Nuclear Chemical Engineering, and Radioisotope
Techniques, and is involved in summer programs for nuclear reactor operators. His research interests include actinide
chemistry, solvent extraction fundamental chemistry and process development, extraction and detection equipment
development, radiolysis and phase composition of organic solvents. Dr. Nilsson’s research group currently comprises six
graduate student researchers and four undergraduate students. He is active in the Industrial and Engineering Chemistry
Division of the American Chemical Society. Dr. Nilsson is author of over 20 papers in peer-reviewed journals and
proceedings, ten publicly-released reports, several papers in conference proceedings, and numerous oral conference
presentations.
Tel: 949-824-2800
E-mail: [email protected]
Uranium Chemistry –
General Properties of Uranium
Dr. Mikael Nilsson
National Analytical Management Program (NAMP)
U.S. Department of Energy Carlsbad Field Office
TRAINING AND EDUCATION SUBCOMMITTEE
4
Outline
•
•
•
•
•
•
•
History
The element uranium
The uranium ion
Natural occurrence and minerals
Complexes with uranium
Solution behavior
Recovery and purification of uranium
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Definitions
• Ions:
– Ions are charged atoms, molecules or complexes.
They are either negative (anions), UO2(CO3)34-, or
positive (cations), U4+.
• Minerals:
– Minerals are naturally occurring inorganic
materials that usually have a defined chemical
composition and structure. They are formed in
nature usually over long time periods, and the
surrounding chemical environment governs what
mineral is formed. Examples: graphite, diamond,
quartz, pyrite (fool’s gold).
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Definitions (cont.)
• Chemical Reactions :
– Chemicals (reactants) react together to form other
chemicals (products). The reaction can be irreversible,
shown by a single arrow, often for redox reactions:
X+Y
Z+W
– Or reversible, shown by a double arrow, for chemical
equilibria:
X+Y
Z+W
• Chemical complexes :
– Stable complexes formed by a chemical reaction. The
complexes can bind either covalently (strong,
permanent) or electrostatically (weaker).
– Large molecules often form ring structures when
binding to metal ions, so called chelates (from Greek
for claw).
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History
• 1781, The planet Uranus was discovered.
• 1789, Klaproth dissolves pechblende
(pitchblende) in nitric acid. After addition of
potash (K2CO3), a yellow precipitate was formed
containing a “new” element, uranium.
• 1841, Eugene-Melchior Pelgiot showed that
Klaproth’s uranium was really UO2. The same
year, Pelgiot prepared metallic uranium by
heating UCl4 with potassium metal (reducing
U(IV) to U(0)).
8
History (cont.)
• 1896, Becquerel discovers that natural uranium
emits radiation. Radioactivity is discovered.
• 1898, the Curies discover polonium by isolating
it from natural uranium.
• 1935, Dempster discovers a lighter isotope in
natural uranium; 235U is found by mass
spectroscopy.
• 1938, Hahn, Meitner and Strassmann discover
nuclear fission.
9
Bucholz, C.F.: Neues allgem. J. der Chemie, 4, (1805), 157.
Beiträge zur chemischen Kenntnis des Urans
Verhalten des Schwfeläthers zu einer gesättigten Auflösung des salpetersauren
Urans und des Wassers zu dem uranhaltigen Schwefeläther.
Acht un funfzigster und neun un funfzigster Versuch:
Zu einem Theil einer mässig concentrirten Auflösung des salpetersauren
Urans in Wasser (zwei Theile Wasser, ein Theil Salz) wurden nach und nach
sechs Theile Schwefeläthers gegossen und geschüttelt. Die Auflösung schien
bei Hinzusetzung der erstern drei Theile nach dem Umschütteln jedes Mahl
blässer zu sein, die folgenden drei Theile aber schwächten die Farbe der Unten
stehenden wässrigen Auflösung gar nicht weiter, und es schien jetztein
Gleichgewicht zwischen der Anziehung des Wassers und des Aethers zu dem
aufgelösten Salze eingetreten zu sein. Jetzt wurde ein Theil der obigen
Auflösung des Salzes in Schwefeläther nach und nach mir vier Theilen Wasser
geschüttelt, wodurch solcher immer mehr und zuletzt gänzlich entfärbt
erschien. Aus dieser Versuchen ergibt sich, dass die wechselseitige Anziehung
des Äthers und Wassers zum salpetersauren Uran von ihren relativen mengen
abhängt, und dass der Äther um so weniger davon der wässrigen Auflösung
entzieht, je verdünnter diese ist. Übringens ist es offenbar, das letzteres
überhaupt nur geschehen könne, wenn das trockne Salz in Äther auflöst.
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Contributions for the chemical knowledge of the uranium behavior in sulfur ether
in contact with a saturated solution of uranium nitrate in water.
The 58th and 59th experiment:
To one part of a moderately concentrated solution of uranium nitrate in water
(two parts water, one part salt) gradually six parts of sulfur ether were added and
the two phases contacted by shaking. The ether seemed to be more colorful with
the first three additions after contact; the following three additions, however, did
not weaken the color of the (lower) aqueous phase at all and it seemed that an
equilibrium was established for the dissolved salt between the water and the
ether. Now one part of the upper organic phase containing the salt was gradually
contacted with four parts of water, whereby the organic phase was gradually
decolorized and finally completely colorless. From these experiments the results
indicate that the affinity of uranium nitrate to water or ether depends on their
relative volumes, and that the ether extracts uranium nitrate from the aqueous
phase less the more diluted the aqueous phase is. It is also obvious that the
extraction can only take place if the dry salt is soluble in ether.
11
Contributions for the chemical knowledge of the uranium behavior in sulfur ether
in contact with a saturated solution of uranium nitrate in water.
The 58th and 59th experiment:
To one part of a moderately concentrated solution of uranium nitrate in water
(two parts water, one part salt) gradually six parts of sulfur ether were added and
the two phases contacted by shaking. The ether seemed to be more colorful with
the first three additions after contact; the following three additions, however, did
not weaken the color of the (lower) aqueous phase at all and it seemed that an
equilibrium was established for the dissolved salt between the water and the
ether. Now one part of the upper organic phase containing the salt was gradually
contacted with four parts of water, whereby the organic phase was gradually
decolorized and finally completely colorless. From these experiments the results
indicate that the affinity of uranium nitrate to water or ether depends on their
relative volumes, and that the ether extracts uranium nitrate from the aqueous
phase less the more diluted the aqueous phase is. It is also obvious that the
extraction can only take place if the dry salt is soluble in ether.
12
Contributions for the chemical knowledge of the uranium behavior in sulfur ether
in contact with a saturated solution of uranium nitrate in water.
The 58th and 59th experiment:
To one part of a moderately concentrated solution of uranium nitrate in water
(two parts water, one part salt) gradually six parts of sulfur ether were added and
the two phases contacted by shaking. The ether seemed to be more colorful with
the first three additions after contact; the following three additions, however, did
not weaken the color of the (lower) aqueous phase at all and it seemed that an
equilibrium was established for the dissolved salt between the water and the
ether. Now one part of the upper organic phase containing the salt was gradually
contacted with four parts of water, whereby the organic phase was gradually
decolorized and finally completely colorless. From these experiments the results
indicate that the affinity of uranium nitrate to water or ether depends on their
relative volumes, and that the ether extracts uranium nitrate from the aqueous
phase less the more diluted the aqueous phase is. It is also obvious that the
extraction can only take place if the dry salt is soluble in ether.
13
Contributions for the chemical knowledge of the uranium behavior in sulfur ether
in contact with a saturated solution of uranium nitrate in water.
The 58th and 59th experiment:
To one part of a moderately concentrated solution of uranium nitrate in water
(two parts water, one part salt) gradually six parts of sulfur ether were added and
the two phases contacted by shaking. The ether seemed to be more colorful with
the first three additions after contact; the following three additions, however, did
not weaken the color of the (lower) aqueous phase at all and it seemed that an
equilibrium was established for the dissolved salt between the water and the
ether. Now one part of the upper organic phase containing the salt was gradually
contacted with four parts of water, whereby the organic phase was gradually
decolorized and finally completely colorless. From these experiments the results
indicate that the affinity of uranium nitrate to water or ether depends on their
relative volumes, and that the ether extracts uranium nitrate from the aqueous
phase less the more diluted the aqueous phase is. It is also obvious that the
extraction can only take place if the dry salt is soluble in ether.
14
Contributions for the chemical knowledge of the uranium behavior in sulfur ether
in contact with a saturated solution of uranium nitrate in water.
The 58th and 59th experiment:
To one part of a moderately concentrated solution of uranium nitrate in water
(two parts water, one part salt) gradually six parts of sulfur ether were added and
the two phases contacted by shaking. The ether seemed to be more colorful with
the first three additions after contact; the following three additions, however, did
not weaken the color of the (lower) aqueous phase at all and it seemed that an
equilibrium was established for the dissolved salt between the water and the
ether. Now one part of the upper organic phase containing the salt was gradually
contacted with four parts of water, whereby the organic phase was gradually
decolorized and finally completely colorless. From these experiments the results
indicate that the affinity of uranium nitrate to water or ether depends on their
relative volumes, and that the ether extracts uranium nitrate from the aqueous
phase less the more diluted the aqueous phase is. It is also obvious that the
extraction can only take place if the dry salt is soluble in ether.
15
Significance of U-extraction
• Over 200 years ago, Bucholtz was able to show
that:
– Ether extracts uranyl nitrate by itself.
– An equilibrium will establish between uranyl
nitrate in the water (aqueous) phase and the ether
(organic) phase.
– The extraction is concentration-dependant.
– One can strip the uranium from the ether using
water.
• Almost 150 years later, this would be used again
for large production of pure U.
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The Element Uranium
•Uranium is a dense metal; its appearance
resembles that of steel.
•It is by far the most studied element in the
actinide series.
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The Element Uranium (cont.)
• Uranium is the heaviest naturally occurring
element. (Pu may be found in nature, but is from
fallout and other man-made sources.)
U
Atomic Number: 92
Atomic Weight: 238.03 u
Boiling Point: 3818 C
Melting Point: 1132 C
Density: 19.07 g/cm3
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Uses of Uranium
•
•
•
•
•
As a fuel for nuclear power reactors
As a counter weight (ballast), much like lead
As radiation shielding
In armor-piercing projectiles
For coloring glass (vaseline glass)
http://www.orau.org/ptp/collection/consumer%20products/vaseline.htm
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Important Isotopes of U
Natural Abundance (%)
Mass Number
Interval
Half-life
234
0.0059 – 0.0050
2.48*105 years
235
0.7202 – 0.7198
7.13*108 years
238
99.2752 – 99.2739
4.5*109 years
238U
is the mother in the 4n+2 decay series (238U→…→206Pb)
235U is the mother in the 4n+3 decay series (235U→…→207Pb)
234U is obtained by decay of 238U in the 4n+2 decay series
(238U →234Th + α → 234Pa + β- → 234U + β- → …→206Pb)
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Nuclear Reactions with U
• Can fission by irradiation of thermal neutrons
yielding 2*107 kWh/kg 235U.
• Irradiation of uranium results in important nuclides
(99Mo, 239Pu, 137Cs, etc.).
• Nuclear reactions:
•
239Pu
+ n →FP + energy +2.5n
238U + n → 239U
239U → 239Np → 239Pu
235U
can fission by thermal neutrons (MOX fuel).
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Isotopic Separation
• Used to produce material of higher
concentration of 235U.
• A sample deviating from natural ratios of
isotopic compositions suggests anthropogenic
sources.
– Enriched uranium, higher 235U/238U ratio
– Depleted uranium, lower 235U/238U ratio
– Samples containing 236U or 233U suggest neutron
irradiated material:
235U + n → 236U
232Th + n → 233Th → 233Pa + β- → 233U + β21
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The Uranium Ions
• U3+ is a very strong reducing agent that is oxidized
by water (reduces water to hydrogen). Hence, no
stable complexes can be maintained in aqueous
solution.
• Aqueous solutions containing U4+ are stable in the
absence of oxidation agents like dissolved oxygen.
Complexes with tetravalent uranium have some
similarity with those of zirconium and thorium,
except for being prone to oxidation.
• The only known uranium ion in the +5 oxidation
state in aqueous solution is the linear dioxouranium
(V) ion, UO2+. This will disproportionate in water.
• Most stable is +6: UO22+.
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Uranium as Water Color
• U3+ : Red (pink)
• U4+ : Green (olive)
• UO2+ : Colorless
• UO22+ : Yellow
http://www.chemie-master.de/
NOTE: Indoor lighting sometimes distorts the color of actinide and
lanthanide solutions. It is important to use incandescent lighting to
observe the correct colors.
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Redox
• The redox pairs of U4+/U3+ and UO22+/UO2+ can
be rapidly altered.
• The redox pairs of UO2+/U4+ and UO22+/U4+ are
much slower reactions and require the oxygen
bonds to be broken/formed.
• The odd oxidation number (V) is the most
difficult oxidation state to stabilize in solution.
Some stability has been observed around pH
2-2.5.
• Disproportionation will turn U(V) to U(IV) and
U(VI), may be accelerated by radiation.
2UO2+ + 4H+
UO22+ + U4+ + 2H2O
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Uranium Occurrence in Nature
• Most important oxidation states: 4+ and 6+.
• Compounds with U4+ are insoluble in mild acidic to
alkaline conditions.
• Compounds with linear uranyl (O=U=O)2+ are
highly soluble and mobile.
• In solution, UO22+ forms soluble complexes with
carbonate, oxalate, hydroxide.
• UO22+ is susceptible to adsorption by organic
matter, Fe-hydroxides or by precipitation with
hydroxide, silicate, vanadate, arsenate, phosphate.
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Uranium Occurrence in Nature (cont.)
• In groundwater systems U(VI) is reduced to U(IV) if an effective
reductant is present (H2S).
• Uranium minerals display an extraordinary wide structure and
chemical variability, resulting from the different chemical
conditions of U-mineral formation.
• There are about 200 minerals that contain uranium as an essential
component.
• The U(VI) minerals constitute the largest portion.
• Understanding of uranium mineralogy
gives insight into the possible mechanism for
uranium and radionuclide retardation following
the weathering of nuclear waste materials.
Dark uranium minerals form
a roll-front deposit in Colorado
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Uranium Minerals
• Uranium is present in nature in many different
minerals, divided into two separate classes:
– Primary minerals:
• Uraninite, pechblende, etc.
• The composition varies from UO2 to UO2.67.
• Often U(III,IV), black or dark minerals.
– Secondary minerals:
• Produced by hydration or oxidation of the primary.
The new mineral is often transported away from the
source mineral and redeposited.
• Often U(VI), light yellow-green or red.
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Uranium Minerals (cont.)
• Primary minerals, uraninite (pechblende)
Uraninite in coaly wood
http://webmineral.com
Uraninite encased in gold
http://www.mineralatlas.com
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Uranium Minerals (cont.)
• Secondary minerals, uranophane, torbernite, etc.
Uranophane (CaO*2UO3*2SiO2*6H2O)
deposited on calcite
http://www.mineralatlas.com
Torbernite (Cu(UO2)2(PO4)2*8-12H2O)
deposited on quartz
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Radiation from Uranium in Nature
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Nuclear Chemical Engineering, 2nd ed.
31
Uranium Metal
• Uranium is strongly electropositive, and uranium
and its compounds are difficult to reduce.
• At elevated temperatures, uranium reacts with most
common metals, such as iron. Any crucible used for
molten uranium must be lined with CaO or other
inert material.
• Finely divided uranium metal reacts at room
temperature with all components in air (save for
noble gases), i.e., it may combust in air.
• To make metal, halide complexes of uranium are
often used since they are easier to reduce and have a
low melting point.
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Uranium Metal (cont.)
• Uranium tetrafluoride is the
common starting material for
U-metal production. It is a
green salt stable at room
temperature. UCl4 can be
used, but is less stable and
more difficult to use.
• Magnesium metal is often
used to reduce the uranium
from U(IV) to U(0). This is
done in a so-called bomb.
• The product is often a flat
uranium slab (pancake) that
needs to be further processed.
Chemistry of the Actinide Elements, 2nd ed.
33
Uranium Metal (cont.)
• Metallic uranium has some plasticity, is malleable, and
and can be extruded.
• The electrical conductivity is similar to iron. Uranium is
weakly paramagnetic and has been shown to have
superconductive properties at low temperatures.
• The natural radiation of uranium affects the metal and
may cause phase transformations in the material.
• Uranium can form intermetallic compounds with a range
of different metals, for example: Al, As, Au, Bi, Cd, Cu,
Fe, Ge, Hg, Mn, Mi, Pb, Pd, Pt, Rh, Ru, Sn, Re, Tc.
– Some of these are of importance for pyroprocessing of metallic
spent nuclear fuel.
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Uranium Metal (cont.)
• Uranium oxidizes readily in air at room temperature. Color changes
from silver to gold/yellow to black oxide/nitride (in 3-4 days).
• For uranium oxidation in air, the following reactions are of
importance:
O2(g)
O2(absorbed)
U
U4+ + 4e-
2e- + O
O2-
2O(dissociated)
• After this we consider the following:
U+ O
UO
UO+O
UO2
3UO2 + 2O
U3O8
U3O8 + O
3UO3
http://www.chemie-master.de/
35
Uranium Metal (cont.)
• Metallic uranium will also oxidize in water
according to:
U + H2O
UO2 + 2H2
2U + 3H2
2UH3
2UH3 + 4H2O
2UO2 + 7H2
and
4H+ + U
U4+ + 2H2
• The oxidation of uranium metal in water is one of
the reasons why metallic nuclear fuel is required to
be encased in cladding when used in water-cooled
reactors.
• The last reaction indicates that uranium is dissolved
by acid.
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Uranium Metal (cont.)
Acid
Reaction Rate
Reaction products
Remarks
HF
Slow
U(IV) fluoride
HCl*
Rapid
Varying amounts of U(III),
U(IV), a black residue
HNO3
Medium
H2SO4
Slow
Acid sulfate of U(IV)
No reaction with dilute H2SO4
H3PO4
Slow
Acid phosphate of U(IV)
Reaction proceeds rapidly in hot H3PO4
HClO4
Rapid**
An insoluble film is formed
Uranyl nitrate
Insoluble in dilute HClO4
*As uranium is dissolved in hydrochloric acid, the black residue formed contains
complexes of uranium hydrides.
** Fine shavings of uranium metal react very violently with concentrated perchloric
acid.
Table reproduced from “Analytical Chemistry of Uranium”
37
Uranium Metal (cont.)
• Metallic uranium undergoes a number of phase
transformations if its temperature increases. During these
transformations it can undergo drastic density changes.
• At ~662 °C metallic uranium changes from the so called
α-phase to β-phase, and the density changes from
~19.07 kg/L to 18.37 kg/L.
• This large decrease in density results in swelling of the
metal. Uranium metal used as fuel in nuclear reactors is
canned in a non-reactive metal, such as stainless steel,
aluminum or zirconium. The drastic change in density can
cause damage to the fuel elements and therefore metallic
fuel is not used in reactors operating above ~650 °C.
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Uranium Complexes
• Uranium forms a number complexes of varying
stability with a range of different elements:
– Hydrides, UHx
– Halogens, UZx (Z is F, I, Br, Cl)
– Nitrides, UNx
– Carbides, UCx
– Oxides, UOx
– Phosphates, UPx
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REACTIONS OF URANIUM WITH NONMETALS
Reacting substance
Temperature of reaction, °C
Reaction products
compact uranium
powdered uranium
250
25
UH3
1800-2400
800-1200
UC, U2C3, UC2
-
600-1000
U3P4
700
500
UN, UN1.75, UN2
150-350
Pyrophoric
UO2, U3O8
Fluorine
25
-
UF6
Chlorine
500-600
150-180
UCl4, UCl5, UCl6
Bromine
650
210
UBr4
Iodine
350
260
UI3, UI4
Water
100
25
UO2
Hydrogen fluoride
-
200-400
UF4
Hydrogen chloride
-
250-300
UCl3
700
400
UN1.75
400-500
-
U3O8
-
900
UC
Hydrogen
Carbon
Phosphorus
Nitrogen
Oxygen
Ammonia
Nitrogen oxide
Methane
40
Halides
• The uranium halides constitute an important group
of compounds because some of them are used
extensively in the uranium industry.
• The most important compounds from a nuclear
energy perspective are the U-fluoride complexes,
UF4 (tetrafluoride) and UF6 (hexafluoride).
• Bromide, iodine and chloride also form complexes
of the general form UX4 (X= Br, Cl, I), but they are
less stable than UF4. Higher halogens (except UF6)
of uranium are quite unstable and the stability
decreases as:
UF6>>UCl6>UBr6≈UI6
41
Halides (cont.)
• UF4 is an emerald green solid salt at room
temperature. It is usually formed by reacting
uranium oxide with hydrofluoric acid:
UO2 + 4HF
UF4 + 2H2O (at 550 °C)
• It is possible to produce it by reacting with
elemental F (F2 gas), but this is more expensive and
complicated.
• UF6 is a white compound. It is solid in room
temperature, but will sublime at 56.5 °C. It is
commonly formed by reacting dry UF4 with F2 gas at
elevated temperatures.
UF4,(s) + F2,(g)
UF6,(g)
42
Uranium Hexafluoride
• Because of its usefulness in the nuclear industry,
uranium hexafluoride has been extensively studied.
• It is used as the primary feed for uranium
enrichment plants separating 235UF6 from 238UF6.
• UF6 becomes a liquid at elevated pressure and
temperature (1137 torr, 64 °C), and the liquid is used
for pumping/pouring in process.
• UF6 is hygroscopic and reacts quickly with water,
and it is necessary to keep water or air out of any
process.
UF6,(g) + 3H2O
UO2F2 + H2O + 4HF
• UF6 is very corrosive and pipes may be lined with
teflon to avoid degradation.
43
Hydrides
• Uranium reacts with hydrogen to form hydrides:
U + 3/2 H2
UH3
• This is a reversible reaction and the rate is fastest
around 225 °C (for 1 atm of H2).
• Uranium hydride is very reactive and normally
ignites spontaneously in air. This may be prevented
by coordinating the hydride with various electrondonor compounds.
• Of the different hydrides, uranium-borohydride is of
special interest, U(BH4)4. This is one of the few
volatile uranium compounds (no comparison to
UF6) and it can be formed by reacting UF4 with
aluminum
UF4 + 2 Al(BH4)3
U(BH4)4 + 2Al(BH4)F2
44
Hydrides (cont.)
• Uranium-borohydride,
U(BH4)4.
• This compound can form
large networks, crystalline
structures of borohydride.
• It will only form in vacuum or
in inert atmosphere since air
(oxygen or moisture) will
quickly decompose it.
• In this example, the uranium
is coordinated by 12 hydrogen
atoms.
Chemistry of the Actinide Elements, 2nd ed.
45
Oxides
• Uranium has been reported in a large range of
different oxides. Examples are UO, UO2, UO3, as
well as non-stoichiometric complexes such as: U3O8,
U12O35, U16O37, etc.
• All types of oxides might not exist separately but
may be found as a mixture of different oxides.
• UO2 has been extensively studied as it is the ceramic
material used in most of the conventional nuclear
power reactors worldwide.
• UO3 and U3O8 also play a large role in uranium
production (yellowcake).
46
Mixed Oxides
• Uranium can also form mixed oxide complexes with
a number of metals.
– Examples include: MgO, CaO, SrO, BaO and rare earth
oxides.
– Other examples are systems with TiO2, ZrO2, CeO2.
• Especially important are ThO2 and PuO2 since these
can be used as a mixed-oxide fuel material in
nuclear reactors (thorium for potential breeding of
233U and Pu in conventional MOX-type fuel).
• Pu-U fuel (MOX) is used in a number of reactors in
the US today.
47
Reactions with U-Oxides
• Reactions between uranium oxides and some nonmetal complexes may form mixed oxides (UO2F2)
or other compounds (halides, U-ions, etc.).
Reagent
Temperature
Product with following oxide
(°C)
UO2
U3O8
UO3
CO
350
n.a.
n.a.
UO2
750
n.a.
UO2
UO2
HF(g)
550
UF4
UO2F2 + UF4
UO2F2
CCl4(g)
400
UCl4
UCl4 + UCl3
UCl4 + UCl3
S2Cl2
450
UCl4
UCl4
UCl4
H2SO4(aq)
25
n.a.
n.a.
UO22+
HNO3(aq)
25
UO22+
UO22+
UO22+
n.a. = no reaction or very slow.
Table reproduced from “Analytical Chemistry of Uranium”
48
Organometallics of U
• A large range of organometallic complexes have
been synthesized, stabilized, and studied in nonaqueous solvents. Too many to review here!
• Some of the classics are uranocene, a sandwich
structure of U between two cyclooctane rings.
• Other common structures are cyclopentadienylcontaining molecules with U.
Pictures from Wikipedia
49
Uranium in Solution
• When working with uranium in solution, it is
important to consider the oxidation state.
• For most practical purposes, only U(IV) and U(VI)
are important for solutions containing uranium
ions.
• In aqueous solutions, U(IV) is very prone to undergo
hydrolysis, much like Th4+ or Pu4+, and the free U4+
ion is only observed at fairly high acidity.
• The general formula for hydrolysis reactions with
U4+:
U4+ + xH2O
U(OH)x(4-x) + xH+
Where x = 1 to 5 has
been observed
Polynuclear species of the general formula U(UOOH)n(n+4) have
been suggested as well to form during U(IV) hydrolysis.
50
Complexes with U(IV)
UClO43+
U4+ + Cl-
UCl3+
U4+ + 2Cl-
UCl22+
U4+ + CNS-
UCNS3+
U4+ + 2CNS-
U(CNS)22+
U4+ + HSO4U4+ + 2HSO4U4+ + HPO42U4+ + HF
U4+ + 2HF
USO42+ + H+
U(SO4)2 + 2H+
UHPO42+
UF3+ + H+
UF22+ + 2H+
Stronger
U4+ + ClO4-
Weaker
• In aqueous solutions, U(IV) also forms complexes with halogens,
F-, Cl-, Br-. Charged complexes (ionic compounds) may also be
formed with sulfate, carbonate, thiocyanate, oxalate, etc.
51
Hydrolysis of U(VI)
• U(VI) will also hydrolyze, but it is less prone to do so than U(IV).
Extensive hydrolysis is not observed until neutral or higher pHvalues.
Chemical Thermodynamics of Uranium, OECD-NEA
52
Hydrolysis of U(VI) (cont.)
Structure and schematic of hydrolysed uranyl,
UO2(OH)42-, UO2(OH)53-.
Clark et al. Inorg. Chem. 1999, 38, 1456-1466
Structure of UO22+ coordinated to 5 water
molecules, UO2(H2O)52+
Vallet et al. J. Am. Chem. Soc. 2001, 123,
11999-12008
53
Hydrolysis of U(VI) (cont.)
• If other complexing reagents such as carbonate are included, the
behavior of uranyl in solutions may quickly become complicated. Mixed
(UO2)x(OH)y(CO3)z complexes may form.
54
Complexes with UO22+
UO22+ + ClO4-
UO2ClO4+
UO2Cl+
UO22+ + NO3-
UO2NO3+
UO22+ + HF
UO2F+ + H+
UO22+ + SO4-
UO2SO4
UO22+ + 2SO4-
UO2(SO4)22-
Stronger
UO22+ + Cl-
Weaker
• In general, aqueous complexes with UO22+ are much better studied
because of the higher stability of U(VI) in solution and the ease of
hydrolysis of U(IV).
55
Complexes with Uranyl
• Uranyl forms aqueous complexes with simple organic
compounds such as acetate, gluconate, carbonate, citric
acid, hydroxamic acid, etc.
• Several studies exist where stability constants for such
complexes have been found.
• Such complexes are important for environmental
reasons (carbonate especially), and for mining
purposes using carbonate leaching.
• Many studies to obtain stability constants were done by
solvent extraction techniques. This is a useful and
relatively simple method, but one must be careful if the
ionic strength and ionic medium change significantly as
this might introduce large errors.
56
Uranyl Complexes
• Di- and poly-carboxylic acids can form strong complexes
with uranium. Specifically, chelating complexes form
stable ring structures with the metal ion.
UO2-tris-oxalate ionic complex.
Most stable state (ground state).
Vallet et al., Inorg. Chem. 2003, 42, 1982-1993
57
Uranyl-chelates
• a through f: different isomers of UO2(oxalate)34- that can exist in solution.
Vallet et al., Inorg. Chem. 2003, 42, 1982-1993
58
Uranium in Non-aqueous Solvents
• Since Bucholtz’s discovery in 1805, numerous reports
have been written on different options for extracting
uranium, as uranyl, into different organic solvents.
• This often requires assistance of a lipophilic complexing
agent, extraction reagent, to coordinate with the uranyl,
resulting in a lipophilic complex.
• Examples of extracting reagents are tributyl-phosphate,
tetraalkylamines, phosphoric acids (DEHPA, HDBP).
• Some reagents are acidic and extract by an ion-exchange
mechanism, others are neutral and require anions to be
present in the final complex (to balance the +2 charge or
uranyl).
59
Extracting Reagents for UO22+
• Solvating complex with tributyl phospate, UO2(NO3)2TBP2
H3C
O
O
O
U
O
P
O
O
CH3
O
O
O
N
-
O
O
H3C
2+
O
P
O
O
H3C
N
O
O
CH3
-
CH3
• Chelating complex with dibutyl phosphoric acid, UO2DBP2HDBP2(*2H2O)
H3C
O
H3C
O
O H
O
P
O
• One (HDBP)(DBP) dimer is missing in this figure
-
2+
U
O
CH3
O
O
P
O
O
CH3
60
Analytical Tools for Uranium
• Mass spectrometery, ICP-MS
– ICP-MS gives no information of speciation, destructive
technique.
• Neutron Activation Analysis
– This gives no information of speciation. Sample might be
recovered, but will need to be decontaminated.
• UV-VIS spectrometry
– Can show difference in speciation and oxidation of
uranium.
• Fluorescence
– Can show difference in speciation. Can be very sensitive.
61
UV vis Spectra of U
U(VI)
Spectra of U(III), U3+,
U(IV), U4+ and
U(VI), UO22+
in hydrochloric acid
~420 nm
521 nm
U(III) 648 nm
U(IV)
The chemistry of the Actinide elements, 1st ed.
62
UV vis Spectra of U (cont.)
Spectra of uranyl nitrate (UNH) in:
I: Methyl ethyl ketone
II: Acetone
III: Diethyl Ether
IV: Water
Analytical Chemistry of Uranium
63
UV vis Spectra of U (cont.)
Spectra of uranyl nitrate (UNH) in:
I: Formamide
II: Tributyl Phosphate
III: Dioxane
Analytical Chemistry of Uranium
64
Uranium Fluorescence
This technique can be very
sensitive, but is also highly
dependant on the chemical
environment and what
uranium complex is
measured.
65
Uranium Chemistry on a Commercial Scale
www.world-nuclear.org
66
Uranium Mining
• Because natural uranium is always accompanied
by its daughter products, mining can be
potentially hazardous in terms of radiation
levels. High concentrations of radon may exist.
• This is also a problem after the first treatment
steps of the ore. Ponds of leaching solutions and
heaps of mill tailings can be quite radioactive
and need to be monitored and covered to reduce
the radiation to the environment.
67
Uranium Mining (cont.)
• Mining and recovery of uranium can be divided
into three principal steps:
– Concentration
– Purification
– Conversion
• In the first step, concentration, the ore particles
containing the valuable material (uranium) are
concentrated from a few % up to 85%-95%.
68
Uranium Mining (cont.)
• In the second step, purification, the last small
impurities are removed to obtain a pure
uranium compound. This is done at a uranium
refinery and is a liquid process. Any solid
uranium mineral still present must be dissolved.
• In the third step, conversion, the uranium in
solution is chemically converted to uranium
hexafluoride for enrichment.
69
Methods for Concentration
• Gravity concentration
– This is usually not used because it requires a very
special ore type with large uranium grains, only
present in a few mines.
• Flotation
– This method does not work well with uranium. It
does not float, or it does not float selectively.
• Leaching
– This is by far the most common method used for
concentration of uranium ore.
70
Recovery after Leaching
• Once the uranium is in the leach solution, it can
be recovered by precipitation, ion exchange, or
solvent extraction. For sodium carbonate, leach
solutions precipitation with NaOH can work.
• Acid leach has a tendency to include other
materials, so selective precipitation of U only is
difficult. Solvent extraction or ion exchange
works better.
71
Leaching
• In the leaching process, chemicals are used for
selectively leaching out uranium from the solid
ore. Common leaching agents are sulfuric acid or
alkaline carbonate solution. The choice of
leaching agent depends on the mineral. For
minerals that consume acid, such as limestone,
sodium or ammonium carbonate is good. For
example:
UO2 + ½ O2
UO3 (oxidation of uranium)
UO3 + Na2CO3 + 2 NaHCO3
Na4UO2(CO3)3 +H2O
72
Leaching (cont.)
• Although carbonate leaching is a more gentle
method, leaching with sulfuric acid is still very
common. Again, uranium is in the +4 oxidation
state in the ore so it needs to be oxidized to +6
UO22+. Solvent extraction is more common than
ion exchange for the recovery of UO22+ and can
be done by using lipophilic phosphoric acid
compounds or trialkyl amines.
UO22+ + SO42- + 2(R3NH)2SO4,org
UO22+ + 2(HDEHP)2,org
(R2NH)4UO2(SO4)3, org
UO2(HDEHPDEHP)2, org + 2H+
73
Leaching (cont.)
• There are, in principle, two methods for leaching
of uranium ore:
– “Heap” leaching: The ore is excavated and crushed
and the material is contacted with a leaching
solution for some time.
– In situ leaching: This method is becoming more
common since it does not require open mines.
Wells are drilled and the leach solution is pumped
underground and recovered downstream.
74
Uranium Mining
• In open-pit mines, the excavated ore is taken to a nearby
plant for concentration and purification.
Rössing mine, Namibia.
(www.world-nuclear-news.org)
• For in situ leaching, the leachate is recovered directly
and can be further concentrated and purified.
Beverly wellfield, Australia.
(www.world-nuclear.org)
75
Uranium Mining (cont.)
www.world-nuclear.org
76
Uranium Refining
• This is the overall process for
purifying the concentrate and
converting it to uranium
hexafluoride.
• In this process, yellowcake is
formed (UO3) or (U3O8).
• In this form, the uranium can
be shipped or handled. Since
this is a heavy metal powder,
care must be taken not to
ingest/inhale it. The radiation
levels are low since all
daughters have been removed.
Nuclear Chemical Engineering, 2nd ed.
77
Uranium Refining
• This is the overall process for
purifying the concentrate and
converting it to uranium
hexafluoride.
• In this process, yellowcake is
formed (UO3) or (U3O8).
• In this form, the uranium can
be shipped or handled. Since
this is a heavy metal powder,
care must be taken not to
ingest/inhale it. The radiation
levels are low since all
daughters have been removed.
Nuclear Chemical Engineering, 2nd ed.
78
Uranium Refining
• This is the overall process for
purifying the concentrate and
converting it to uranium
hexafluoride.
• In this process, yellowcake is
formed (UO3) or (U3O8).
• In this form, the uranium can
be shipped or handled. Since
this is a heavy metal powder,
care must be taken not to
ingest/inhale it. The radiation
levels are low since all
daughters have been removed.
Nuclear Chemical Engineering, 2nd ed.
79
Uranium Refining
• This is the overall process for
purifying the concentrate and
converting it to uranium
hexafluoride.
• In this process, yellowcake is
formed (UO3) or (U3O8).
• In this form, the uranium can
be shipped or handled. Since
this is a heavy metal powder,
care must be taken not to
ingest/inhale it. The radiation
levels are low since all
daughters have been removed.
Nuclear Chemical Engineering, 2nd ed.
80
Uranium Ore Concentration
• Ore is first crushed and then ground
with hot water until most of it passes a
fine mesh. The resulting fine slurry goes
into the leaching step. The first tank
dissolves most uranium. Particles that
are more resistant are dissolved in the
second step, where sodium chloride is
added to increase the reduction
potential. This causes iron to act as a
catalyst for uranium oxidation.
• The next set of steps separates the leach
liquor, now containing the uranium,
from the remaining suspended solids.
Cyclone separators, rakes, and
thickeners are used to remove all solids.
• The leach liquor is taken to solvent
extraction purification.
Nuclear Chemical Engineering, 2nd ed.
81
Uranium Ore Concentration
• Ore is first crushed and then ground
with hot water until most of it passes a
fine mesh. The resulting fine slurry goes
into the leaching step. The first tank
dissolves most uranium. Particles that
are more resistant are dissolved in the
second step, where sodium chloride is
added to increase the reduction
potential. This causes iron to act as a
catalyst for uranium oxidation.
• The next set of steps separates the leach
liquor, now containing the uranium,
from the remaining suspended solids.
Cyclone separators, rakes, and
thickeners are used to remove all solids.
• The leach liquor is taken to solvent
extraction purification.
Nuclear Chemical Engineering, 2nd ed.
82
Uranium Ore Concentration
• Ore is first crushed and then ground
with hot water until most of it passes a
fine mesh. The resulting fine slurry goes
into the leaching step. The first tank
dissolves most uranium. Particles that
are more resistant are dissolved in the
second step, where sodium chloride is
added to increase the reduction
potential. This causes iron to act as a
catalyst for uranium oxidation.
• The next set of steps separates the leach
liquor, now containing the uranium,
from the remaining suspended solids.
Cyclone separators, rakes, and
thickeners are used to remove all solids.
• The leach liquor is taken to solvent
extraction purification.
Nuclear Chemical Engineering, 2nd ed.
83
Uranium Ore Concentration
• Ore is first crushed and then ground
with hot water until most of it passes a
fine mesh. The resulting fine slurry goes
into the leaching step. The first tank
dissolves most uranium. Particles that
are more resistant are dissolved in the
second step, where sodium chloride is
added to increase the reduction
potential. This causes iron to act as a
catalyst for uranium oxidation.
• The next set of steps separates the leach
liquor, now containing the uranium,
from the remaining suspended solids.
Cyclone separators, rakes, and
thickeners are used to remove all solids.
• The leach liquor is taken to solvent
extraction purification.
Nuclear Chemical Engineering, 2nd ed.
84
Uranium Ore Concentration
• Ore is first crushed and then ground
with hot water until most of it passes a
fine mesh. The resulting fine slurry goes
into the leaching step. The first tank
dissolves most uranium. Particles that
are more resistant are dissolved in the
second step, where sodium chloride is
added to increase the reduction
potential. This causes iron to act as a
catalyst for uranium oxidation.
• The next set of steps separates the leach
liquor, now containing the uranium,
from the remaining suspended solids.
Cyclone separators, rakes, and
thickeners are used to remove all solids.
• The leach liquor is taken to solvent
extraction purification.
Nuclear Chemical Engineering, 2nd ed.
85
Uranium Concentrate Purification
• This is a liquid-liquid
extraction process for
purification of uranium using
tributyl phosphate.
• This particular process was
developed by Mallinckrodt.
Similar flow-sheets are used
by other refineries.
• The solvent extraction is
carried out using mixer-settler
batteries and the organic to
aqueous ratio is 13:1 in the
extraction step.
• Scrubbing is done to remove
all unwanted (non-U) metal
ions that were extracted.
Nuclear Chemical Engineering, 2nd ed.
86
Uranium Concentrate Purification
• The uranium concentrate from the
leach step enters the process and
the acidity is adjusted. In this
example the liquid-liquid
extraction reagent is tributylphosphate (TBP).
• The adjusted concentrate (the
aqueous feed ) is contacted with an
organic solvent containing TBP to
extract the uranium .
• Following the extraction are scrub
stages to remove unwanted
impurities and a uranium strip
stage to recover the uranium from
the organic solvent
• Finally, the organic solvent is
washed for reuse in the extraction
stage.
Nuclear Chemical Engineering, 2nd ed.
87
Uranium Concentrate Purification
• The uranium concentrate from the
leach step enters the process and
the acidity is adjusted. In this
example the liquid-liquid
extraction reagent is tributylphosphate (TBP).
• The adjusted concentrate (the
aqueous feed ) is contacted with an
organic solvent containing TBP to
extract the uranium .
• Following the extraction are scrub
stages to remove unwanted
impurities and a uranium strip
stage to recover the uranium from
the organic solvent
• Finally, the organic solvent is
washed for reuse in the extraction
stage.
Nuclear Chemical Engineering, 2nd ed.
88
Uranium Concentrate Purification
• The uranium concentrate from the
leach step enters the process and
the acidity is adjusted. In this
example the liquid-liquid
extraction reagent is tributylphosphate (TBP).
• The adjusted concentrate (the
aqueous feed ) is contacted with an
organic solvent containing TBP to
extract the uranium .
• Following the extraction are scrub
stages to remove unwanted
impurities and a uranium strip
stage to recover the uranium from
the organic solvent.
• Finally, the organic solvent is
washed for reuse in the extraction
stage.
Nuclear Chemical Engineering, 2nd ed.
89
Uranium Concentrate Purification
• The uranium concentrate from the
leach step enters the process and
the acidity is adjusted. In this
example the liquid-liquid
extraction reagent is tributylphosphate (TBP).
• The adjusted concentrate (the
aqueous feed ) is contacted with an
organic solvent containing TBP to
extract the uranium .
• Following the extraction are scrub
stages to remove unwanted
impurities and a uranium strip
stage to recover the uranium from
the organic solvent.
• Finally, the organic solvent is
washed for reuse in the extraction
stage.
Nuclear Chemical Engineering, 2nd ed.
90
Uranium Conversion
• For the conversion of uranium
concentrates to UF6, the UO22+
is first reduced to UO2.
HF is added to convert the
UO2 to UF4:
UO2 + 2HF UF4 + 2 H2O.
This is often done in fluidized
beds.
• The UF4 solid is fed into a
tower reactor together with F2
gas under high temperature,
and UF6 gas is formed.
• The gas mixture, UF6, F2, and
some diluent gases, are
separated to produce pure UF6
for the enrichment.
Nuclear Chemical Engineering, 2nd ed.
91
Uranium Conversion
• For the conversion of uranium
concentrates to UF6, the UO22+
is first reduced to UO2.
HF is added to convert the
UO2 to UF4:
UO2 + 2HF UF4 + 2 H2O.
This is often done in fluidized
beds.
• The UF4 solid is fed into a
tower reactor together with F2
gas under high temperature,
and UF6 gas is formed.
• The gas mixture, UF6, F2, and
some diluent gases, are
separated to produce pure UF6
for the enrichment.
Nuclear Chemical Engineering, 2nd ed.
92
Uranium Conversion
• For the conversion of uranium
concentrates to UF6, the UO22+
is first reduced to UO2.
HF is added to convert the
UO2 to UF4:
UO2 + 2HF UF4 + 2 H2O.
This is often done in fluidized
beds.
• The UF4 solid is fed into a
tower reactor together with F2
gas under high temperature,
and UF6 gas is formed.
• The gas mixture, UF6, F2, and
some diluent gases, are
separated to produce pure UF6
for the enrichment.
Nuclear Chemical Engineering, 2nd ed.
93
Uranium Conversion
• For the conversion of uranium
concentrates to UF6, the UO22+
is first reduced to UO2.
HF is added to convert the
UO2 to UF4:
UO2 + 2HF UF4 + 2 H2O.
This is often done in fluidized
beds.
• The UF4 solid is fed into a
tower reactor together with F2
gas under high temperature,
and UF6 gas is formed.
• The gas mixture, UF6, F2, and
some diluent gases, are
separated to produce pure UF6
for the enrichment.
Nuclear Chemical Engineering, 2nd ed.
94
Uranium Conversion
• For the conversion of uranium
concentrates to UF6, the UO22+
is first reduced to UO2.
HF is added to convert the
UO2 to UF4:
UO2 + 2HF UF4 + 2 H2O.
This is often done in fluidized
beds.
• The UF4 solid is fed into a
tower reactor together with F2
gas under high temperature,
and UF6 gas is formed.
• The gas mixture, UF6, F2, and
some diluent gases, are
separated to produce pure UF6
for the enrichment.
Nuclear Chemical Engineering, 2nd ed.
95
Final Steps of U-production
• The UF6 is taken to an enrichment plant where
enriched and depleted uranium is produced.
• Depending on the final use of the uranium,
different methods exist for reducing the uranium
hexafluoride and converting it back to a
condensed phase (solid metal, oxide ceramic).
96
Current and Future U-Chemistry
• Although uranium has been studied for
over 200 years, there are still
discoveries to be made.
• Recent work by J. Kiplinger from Los
Alamos National Laboratory explores
the possibility of preparing new
U-compounds.
• The 1,4-dioxane derivatives of uranium
iodides were prepared in glove boxes
and can be converted into a range of
other uranium compounds, including
alkoxides, halides, amides, nitrides and
carbides. Many of these may be
important for future advanced nuclear
fuel cycles.
http://www.rsc.org/chemistryworld/News/2011/March/28031101.asp
97
References
• Analytical Chemistry Uranium, 1st ed. Pavi. Ann ArbourHumphrey, 1970.
• The Chemistry of the Actinide Elements, 1st ed. Katz, Seaborg.
Methuen & co, 1957.
• The Chemistry of the Actinide Elements, 2nd ed. Katz,
Seaborg, Morss. Springer, 1986.
• The Chemistry of the Actinide Elements, 3rd ed. Katz, Morss,
Edelstein, Fuger. Springer, 2007.
• Nuclear Chemical Engineering, 2nd ed. Benedict, Pigford, Levi.
McGraw-Hill, 1981.
• Chemical Thermodynamics of Uranium, Grenthe, Konings.
OECD-NEA, 1992.
• www.world-nuclear.org (world nuclear association)
98
Future Webinars in Actinide
Chemistry Series
•
•
•
•
•
Plutonium Chemistry
Environmental Chemistry of Uranium and Plutonium
Analytical Chemistry of Uranium and Plutonium
Source Preparation for Alpha Spectroscopy
Sample Dissolution
• NAMP website www.inl.gov/namp