Nuclear Forensics Summer School
Chemical behavior of isotopes and radioelements
• Radioisotopes and radioelements of concern

Fission products

Actinides

Actinide decay products
• Speciation in fuel
• Trends by periodic group

Cs

Sr

Lanthanides

Halides

Noble Gases

Polonium

Actinides
• Provide basis for understanding chemical behavior
6-1
Fission Products
• Fission yield curve varies with fissile isotope
• 2 peak areas for U and Pu thermal neutron induced fission
• Variation in light fragment peak
235U fission yield
• Influence of neutron energy observed
6-2
Burnup: LWR UO2
Element moles/kg % Element % Total
U
3.99E+00
91.85
91.85
Xe
4.69E-02
1.08
92.93
Zr
4.68E-02
1.08
94.01
Mo
4.05E-02
0.93
94.95
Nd
3.06E-02
0.71
95.65
Ru
2.73E-02
0.63
96.28
Cs
2.51E-02
0.58
96.86
Ce
2.34E-02
0.54
97.40
Sr
1.27E-02
0.29
97.69
Pd
1.26E-02
0.29
97.98
Ba
1.21E-02
0.28
98.26
La
1.04E-02
0.24
98.50
Pr
9.23E-03
0.21
98.71
Element moles/kg
% Element % Total
Tc
9.19E-03
0.21 98.93
Y
6.52E-03
0.15 99.08
Sm
5.39E-03
0.12 99.20
Kr
5.13E-03
0.12 99.32
Rh
4.86E-03
0.11 99.43
Rb
4.80E-03
0.11 99.54
Te
4.15E-03
0.10 99.64
Pu
4.14E-03
0.10 99.73
Np
2.66E-03
0.06 99.79
I
2.13E-03
0.05 99.84
Pm
1.31E-03
0.03 99.87
Eu
1.25E-03
0.03 99.90
39 MWd/kg (Siemens/KWU)
Ignore oxygen contribution
3
6-3
Burnup: Fast reactor
element distribution for different burnup
Element
U
Pu
Xe
Ru
Mo
Zr
Pd
Nd
Cs
Ce
Ba
La
Pr
Tc
Te
Sm
20%
53.84
12.40
4.05
3.72
3.58
3.05
2.78
2.74
2.60
1.80
1.73
0.94
0.83
0.58
0.55
0.54
Element
U
Pu
Xe
Ru
Mo
Zr
Pd
Nd
Cs
Ba
Ce
La
Pr
Cd
Te
Sr
Sm
40%
30.16
11.86
6.89
6.53
6.13
5.22
4.85
4.81
3.85
3.80
3.09
1.57
1.34
1.20
0.96
0.85
0.85
4
Element
U
Xe
Ru
Pu
Mo
Zr
Pd
Nd
Ba
Cs
Ce
La
Cd
Pr
Te
Gd
Sr
Sm
60%
6-4
15.50
9.03
8.75
8.54
7.98
6.86
6.43
6.39
5.89
4.30
4.10
2.01
1.98
1.65
1.28
1.08
1.05
0.98
Elements in fuel at burnup
• From oxide (39 MWd/kg)
 Actinides: U, Np, Pu
 Noble gases: Xe, Kr
 Group 1: Cs, Rb
 Group 2: Sr, Ba
 Group 4: Zr
 Lanthanides: (Y), Nd,
Ce, La, Pr, Sm, Pm, Eu
 Metal phase: Mo, Ru,
Pd, Tc, Rh
 Degree in metal
phase varies
 Non-metals: Te, I
• From fast reactor
• Actinides: U, Pu
• Noble Gases: Xe, Kr,
He
• Group 1: Cs, Rb
• Group 2: Ba, Sr
• Group 4: Zr
• Lanthanides: Nd, Ce,
La, Pr, Sm, (Y)
• Metal phase: Mo, Ru,
Pd, Rh, Tc
• Non-metals: Te, I
6-5
5
6-6
Speciation in Spent Fuel
•
•
•
Chemical form of actinides and fission products vary with fuel
Oxide fuel

Fuel for thermal reactors

Speciation dictated by reaction with oxygen
 Noble gases
 Oxides as solid solutions in UO2 (Ln, Group 1, Group 2, Zr,
Nb, Mo, Te)
 Separate oxide phase (Group 1, Group 2, Zr, Nb, Mo,Te)
 Metallic phases (Mo, Tc, Ru, Rh, Pd)

Specific behavior dependent upon concentration
 Related to burnup and fuel composition
Metallic fuel

Fuel for fast reactors, non-aqueous cooling of reactor

Elemental species most common

Solid solutions and intermetallic phases

Reactions with halides (I-)
6-7
7
Oxide Volatility
• For treatment of
Element Ref. 1 Ref. 2 Ref. 3
Ref. 4
Ag
80
0
0
0
oxide fuel
Cd
0
75
75
80
• UO2 oxidized to U3O8
Cs
99
90
100
99
In
0
75
0
75
 Heating to 400Ir
0
0
75
0
600 °C in O2
Mo
80
0
0
80
containing
Pd
80
0
0
0
atmosphere
Rh
80
0
0
0
Ru
80
90
100
80
 Around 30%
Se
80
0
0
99
volume increase
Tc
80
--- 0
0
• U3O8 reduction by
Te
99
75
75
99
1. AECL Technologies, Inc. “Plutonium Consumption Program-CANDU Reactor Projects,” Final
addition of H2
Report, July 1994.
SCIENTECH, Inc., Gamma Engineering Corp., “Conceptual Design and Cost Evaluation for the
• Kr, Xe, I removed 2.DUPIC
Fuel Fabrication Facility,” Final Report, SCIE-COM-219-96, May 1996.
3. Recycling of Nuclear Spent Fuel with AIROX Processing, D. Majumdar Editor, DOE/ID-10423,
 Some
December 1992.
4. Bollmann, C.A., Driscoll, M.J., and Kazimi, M.S.: Environmental and Economic Performance of
discrepancies
Direct Use of PWR Spent Fuel in CANDU Reactors. MIT-NFC-TR-014, 44-45, June 1998.
6-8
8
Element Volatility
• Melting points correlate
with vapor pressure
• Zone refining can
have applications
• Data for elements
• Need to consider solid
solutions and
intermetallics in fuel
Melting Points
Element
He
Kr
Xe
Cs
Rb
I
Te
Pu
Ba
Sr
Ce
Eu
°C
-272
-157
-111
29
39
114
450
640
725
764
795
822
Element
La
Pr
Nd
Pm
Sm
U
Y
Pd
Zr
Rh
Tc
Ru
Mo
6-9
9
°C
920
935
1010
1042
1072
1132
1523
1552
1852
1966
2200
2250
2617
Radionuclide Inventories
• Fission Products
 generally short lived (except 135Cs, 129I)
 ß,emitters
 geochemical behavior varies
• Activation Products
 Formed by neutron capture (60Co)
 ß,emitters
 Lighter than fission products
 can include some environmentally important
elements (C,N)
• Actinides
 alpha emitters, long lived
6-10
Fission products
• Kr, Xe
 Inert gases
 Xe has high neutron capture cross section
• Lanthanides
 Similar to Am and Cm chemistry
 High neutron capture cross sections
• Tc
 Redox state (Tc4+, TcO4-)
• I
 Anionic
 129I long lived isotope
6-11
Cesium and Strontium
• High yield from fission
• Both beta
 Some half-lives similar
• Similar chemistry
 Limited oxidation states
 Complexation
 Reactions
• Can be separated or treated together
6-12
6-13
Alkali Elements
• 1st group of elements
 Li, Na, K, Rb, Cs
 Single s electron outside noble gas core
 Chemistry dictated by +1 cation
no other cations known or expected
 Most bonding is ionic in nature
Charge, not sharing of electron
 For elemental series the following decrease
melting of metals
salt lattice energy
hydrated radii and hydration energy 6-14
ease of carbonate decomposition
Solubility
• Group 1 metal ions soluble in some nonaqueous phases
• Liquid ammonia
 Aqueous electron
very high mobility
• Amines
• Tetrahydrofuran
• Ethylene glycol dimethyl ether
• Diethyl ether with cyclic polyethers
6-15
Complexes
• Group 1 metal ions form oxides
 M2O, MOH
• Cs forms higher ordered chloride complexes
• Cs perchlorate insoluble in water
• Tetraphenylborate complexes of Cs are insoluble
 Degradation of ligand occurs
• Forms complexes with ß-diketones
• Crown ethers complex Cs
• Cobalthexamine can be used to extract Cs
• Zeolites complex group 1 metals
• In environment, clay minerals complex group 1 metal ions
6-16
6-17
Group 2 Elements
• 2nd group of elements
 Be, Mg, Ca, Sr, Ba, Ra
 Two s electron outside noble gas core
 Chemistry dictated by +2 cation
no other cations known or expected
 Most bonding is ionic in nature
Charge, not sharing of electron
 For elemental series the following decrease
melting of metals
* Mg is the lowest
ease of carbonate decomposition
Charge/ionic radius ratio
6-18
Complexes
• Group 2 metal ions form oxides
 MO, M(OH)2
• Less polarizable than group 1 elements
• Fluorides are hydroscopic
 Ionic complexes with all halides
• Carbonates somewhat insoluble in water
• CaSO4 is also insoluble (Gypsum)
• Nitrates can form from fuming nitric acid
• Mg and Ca can form complexes in solution
• Zeolites complex group 2 metals
• In environment, clay minerals complex group 2 metal
ions
6-19
6-20
Technetium
• Electronic configuration of neutral, gaseous Tc
atoms in the ground
• [Kr]4d55s2 [l] with the term symbol 6S5/2
• Range of oxidation states
 TcO4-, TcO2
• Tc chemical behavior is similar to Re
 Both elements differ from Mn
• Tc atomic radius of 1.358 Å
 0.015 Å smaller than Re
6-21
Technetium
• Tc and Re often form compounds of analogous composition and
only slightly differing properties

Compounds frequently isostructural

Tc compounds appear to be more easily reduced than
analogous Re species

Tc compounds frequently more reactive than Re analogues
• 7 valence electrons are available for bonding

formal oxidation states from +7 to -1 have been synthesized
• Potentials of the couples TcO4-/TcO2 and TcO4/Tc are intermediate
between those of Mn and Re

TcO4 – is a weak oxidizing agent
6-22
Polonium
• Chemistry of Po is similar to Te and Bi
• dissolves readily in dilute acids
• PoH2 volatile

melting point −36.1°C

Boiling point 35.3°C)
• Halides have structure PoX2, PoX4 and PoX6
• 2oxides PoO2 and PoO3 are the products of oxidation of
polonium.[14]
• Some microbes can methylate Po

Similar to Hg, Se and Te
• electron configuration of Po ground state atoms

5s25p65d106s26p4 (3P2)

analogous to the configurations of Se and Te

stable oxidation states of -2, +2, +4, and +6 would be
expected
6-23
Lanthanides
• Electronic structure of the lanthanides tend to be [Xe]6s24fn
• ions have the configuration [Xe]4fm
• Lanthanide chemistry differs from main group and transition elements
due to filling of 4f orbitals

4f electrons are localized
 Hard acid metals
* Actinides are softer, basis of separations

Lanthanide chemistry dictated by ionic radius
 Contraction across lanthanides
* 102 pm (La3+) to 86 pm (Lu3+),
 Ce3+ can oxidized Ce4+
 Eu3+ can reduce to Eu2+ with the f7 configuration which has
the extra stability of a half-filled shell
6-24
Lanthanides
• Difficult to separate lanthanides due to similarity
in ionic radius
 Multistep processes
 Crystallization
 Solvent extraction (TBP)
Counter current method
• larger ions are 9-coordinate in aqueous solution
• smaller ions are 8-coordinate
• Complexation weak with monodentate ligands
 Need to displace water
 Stronger complexes are formed with chelating
ligands
6-25
Actinides
• Occurrence
 Ac, Th, Pa, U natural
 Ac and Pa daughters of Th and U
 Traces of 244Pu in Ce ores
• Properties based on filling 5f orbitals
6-26
6-27
Actinide Electronic Structure
6-28
Electronic structure
• Electronic Configurations of Actinides are not always easy to confirm

atomic spectra of heavy elements are very difficult to interpret
in terms of configuration
• Competition between 5fn7s2 and 5fn-16d7s2 configurations

for early actinides promotion 5f  6d occurs to provide more
bonding electrons much easier than corresponding 4f  5d
promotion in lanthanides

second half of actinide series resemble lanthanides more closely
 Similarities for trivalent lanthanides and actinides
• 5f orbitals have greater extension with respect to 7s and 7p than do 4f
relative to 6s and 6p orbitals

The 5 f electrons can become involved in bonding
 ESR evidence for bonding contribution in UF3, but not in
NdF3
* Actinide f covalent bond contribution to ionic bond
* Lanthanide 4f occupy inner orbits that are not
accessible
• Basis for chemical differences between lanthanides and actinides 6-29
Electronic Structure
• 5f / 6d / 7s / 7p orbitals are of comparable energies over a range of
atomic numbers

especially U - Am
 Bonding can include any orbitals since energetically
similar
 Explains tendency towards variable valency
• greater tendency towards (covalent) complex formation than for
lanthanides

Lanthanide complexes tend to be primarily ionic
• Actinide complexes complexation with p-bonding ligands
• Hybrid bonds involving f electrons
• Since 5f / 6d / 7s / 7p orbital energies are similar orbital shifts may
be on the order of chemical binding energies

Electronic structure of element in given oxidation state may
vary with ligand

Difficult to state which orbitals are involved in bonding
6-30
Ionic Radii
• Trends based on ionic radii
6-31
Absorption Spectra and Magnetic
Properties
•
•
Electronic Spectra

5fn transitions
 narrow bands (compared to transition metal spectra)
 relatively uninfluenced by ligand field effects
 intensities are ca. 10x those of lanthanide bands
 complex to interpret
Magnetic Properties

hard to interpret

spin-orbit coupling is large
 Russell-Saunders (L.S) Coupling scheme doesn't work, lower
values than those calculated
* LS (http://hyperphysics.phyastr.gsu.edu/hbase/atomic/lcoup.html)
* Weak spin orbit coupling
 Sum spin and orbital angular momentum
 J=S+L

ligand field effects are expected where 5f orbitals are involved in
bonding
6-32
Pu absorbance spectrum
5
6+
Pu (835 nm)
4+
Absorbance
4
Pu (489 nm)
Normal
Heavy
Light
3
2
1
0
400
500
600
700
Wavelength (nm)
800
6-33
Oxidation states and stereochemistry
6-34
Hybrid orbitals
• Various orbital combinations similar to sp or d orbital
mixing
 Linear: sf
 Tetrahedral: sf3
 Square: sf2d
 Octahedral: d2sf3
 A number of orbital sets could be energetically
accessible
• General geometries
 Trivalent: octahedral
 Tetravalent: 8 coordination
6-35
Stereochemistry
C.N.
Geometry
O.N.
e.g.
4
distorted
+4
U(NPh2)4
5
distorted tbp
+4
U2(NEt2)8
6
octahedral
+3
An(H2O)63+, An(acac)3
+4
UCl62-
+5
UF6-, a-UF5
+6
AnF6
+7
Li5[AnO6] (An = Np, Pu)
+6
Li4UO5 , UO3
+5/+6
U5O8
+6
UO2(S2CNEt2)2(ONMe3)
distorted octahedral
6-36
Stereochemistry
8
cubic
+4
(Et4N)4[U(NCS)8], ThO2, UO2
+5
AnF83-
+4
ThI4, U(acac)4, Cs4[U(NCS)8],
+5
b-UF5
dodecahedral
+4
Th(ox)44-, Th(S2CNEt2)4
bicapped trigonal prismatic
+3
PuBr3
hexagonal bipyramidal
+6
UO2(h2-NO3)2(H2O)2
?
+6
UF82-
tricapped trigonal prismatic
+3
UCl3
capped square antiprismatic
+4
Th(trop)4(H2O)
square antiprismatic
9
6-37
Stereochemistry
10
bicapped square antiprismatic
+4
KTh(ox)4.4H2O
11?
fully capped trigonal prismatic?
+3
UF3
12
irregular icosahedral
+4
Th(NO3)62-
distorted cuboctahedral
+4
An(h3-BH4)4, (Np, Pu)
complex
+4
An(h3-BH4)4, (Th, Pa, U)
14?
6-38
Actinide metals
• Preparation of actinide metals

Reduction of AnF3 or AnF4 with vapors of Li, Mg, Ca or Ba
at 1100 – 1400 °C

Other redox methods are possible
 Thermal decomposition of iodine species
 Am from Am2O3 with La
* Am volatility provides method of separation
• Metals tend to be very dense

U 19.07 g/mL

Np 20.45 g/mL

Am lighter at 13.7 g/mL
• Some metals glow due to activity

Ac, Cm, Cf
6-39
Pu metal
Plutonium
a
b

d
d
e
Symmetry
monoclinic
monoclinic
orthorhombic
fcc
bc tetragonal
bcc
Stability
< 122°C
122-207°C
207-315°C
315-457°C
457-479°C
479-640°C
r / gcm-3
19.86
17.70
17.14
15.92
16.00
16.51
• Some controversy surrounding behavior of metal
http://www.fas.org/sgp/othergov/doe/lanl/pubs/0081803
0.pdf
6-40
6-41
Oxidation states
•
+2





•
+3





Unusual oxidation state
Common only for the heaviest elements
No2+ and Md2+ are more stable than Eu2+
 5f6d promotion
Divalent No stabilize by full 5f14
 Element Rn5f147s2
Divalent actinides similar properties to divalent lanthanides and
Ba2+
The most common oxidation state
The most stable oxidation state for all trans-Americium elements
except No
Of marginal stability for early actinides Pa, U (But: Group
oxidation state for Ac)
General properties resemble Ln3+ and are size-dependent
 Binary Halides, MX3 easily prepared, & easily hydrolyzed to
MOX
Binary Oxides, M2O3 known for Ac, Pu and trans-Am elements
6-42
Oxidation states
•
+4



•
•
•




+5



+6


+7

Principal oxidation state for Th
 similar to group 4
Very important, stable state for Pa, U, Pu
Am, Cm, Bk & Cf are increasingly easily reduced - only stable in certain complexes
e.g. Bk4+ is more oxidizing than Ce4+
MO2 known from Th to Cf (fluorite structure)
MF4 are isostructural with lanthanide tetrafluorides
MCl4 only known for Th, Pa, U & Np
Hydrolysis / Complexation / Disproportionation are all important in aqueous phase
Principal state for Pa (similar to group 5)
For U, Np, Pu and Am the AnO2+ ion is known
Comparatively few other AnV species are known
 fluorides fluoro-anions, oxochlorides, uranates,
AnO22+ ions are important for U, Np, Pu, Am UO22+ is the most stable
Few other compounds e.g. AnF6 (An = U, Np, Pu), UCl6, UOF4 etc..., U(OR)6
Only the marginally stable oxo-anions of Np and Pu, e.g. AnO536-43
Redox chemistry (Frost diagrams)
6-44
Redox chemistry
6-45
Redox chemistry
•
•
•
•
•
•
actinides are electropositive
Pa - Pu show significant redox chemistry

all 4 oxidation states of Pu can co-exist in appropriate conditions
stability of high oxidation states peaks at U (Np)
redox potentials show strong dependence on pH (data for Ac - Cm)

high oxidation states are more stable in basic conditions

even at low pH hydrolysis occurs

tendency to disproportionation is particularly dependent on pH

at high pH 3Pu4+ + 2H2O PuO22+ + 2Pu3+ + 4H+
early actinides have a tendency to form complexes

complex formation influences reduction potentials
 Am4+(aq) exists when complexed by fluoride (15 M NH4F(aq))
radiation-induced solvent decomposition produces H• and OH• radicals

lead to reduction of higher oxidation states e.g. PuV/VI, AmIV/VI
6-46
Actinide complexes
6-47
6-48
6-49
6-50
6-51
Organometallic
• Organometallic chemistry of actinides is
relatively recent
 Interest is expanding but still focused on U
• Similar to lanthanides in range of
cyclopentadienides / cyclooctatetraenides / alkyls
• Cyclopentadienides are p-bonded to actinides
6-52
•
•
•
•
•
•
Uranocene
Paramagnetic
Pyrophoric
Stable to hydrolysis
Planar 'sandwich'
Eclipsed D8h conformation
UV-PES studies show that bonding in uranocene has 5f & 6d
contributions
• e2u symmetry interaction shown can only occur via f-orbitals
6-53
Questions
1. What elements can be expected from nuclear
fission?
2. What actinides can naturally be found in the
environment?
3. What is a radioelement? Provide 10
examples.
4. What influences the speciation of actinides
and fission products in spent nuclear fuel?
5. What are the similarities and differences
between lanthanides and actinides?
6-54