Fuel Cycle Chemistry

Radiochemistry in reactor
• Readings: Radiochemistry in Light Water Reactors,
Chapter 3
• Speciation in irradiated fuel
• Utilization of resulting isotopics
• Fission Product Chemistry
• Fuel confined in reactor to fuel region
 Potential for interaction with cladding material
 Initiate stress corrosion cracking
 Chemical knowledge useful in events where fuel is
outside of cladding
• Some radionuclides generated in structural material
Fission process
Recoil length about 10 microns, diameter of 6 nm
About size of UO2 crystal
95 % of energy into stopping power
 Remainder into lattice defects
* Radiation induced creep
High local temperature from fission
 3300 K in 10 nm diameter
Delayed neutron fission products
0.75 % of total neutrons
 137-139I and 87-90Br as examples
Some neutron capture of fission products
eff    
Measure of extracted energy
Fraction of fuel atoms that underwent fission
 %FIMA (fissions per initial metal atom)
Actual energy released per mass of initial fuel
 Gigawatt-days/metric ton heavy metal (GWd/MTHM)
 Megawatt-days/kg heavy metal (MWd/kgHM)
Burnup relationship
Plant thermal power times days divided by the mass of the initial fuel loading
Converting between percent and energy/mass by using energy released per fission
 typical value is 200 MeV/fission
 100 % burnup around 1000 GWd/MTHM
Determine burnup
Find residual concentrations of fissile nuclides after irradiation
 Burnup from difference between final and initial values
 Need to account for neutron capture on fissile nuclides
Find fission product concentration in fuel
 Need suitable half-life
 Need knowledge of nuclear data
* cumulative fission yield, neutron capture cross section
 Simple analytical procedure
 137Cs(some migration issues) 142Nd(stable isotope), 152Eu are suitable fission
Neutron detection also used
 Need to minimize 244Cm
Radionuclides in fresh fuel
• Actual Pu isotopics in MOX fuel may vary
Activity dominated by other Pu isotopes
Ingrowth of 241Am
MOX fuel fabrication in glove boxes
Fuel variation
during irradiation
Chemical composition
Radionuclide inventory
Pellet structure
Higher concentrations of
Ru, Rh, and Pd in Pu fuel
• Total activity of fuel
effected by saturation
 Tends to reach
• Radionuclide fuel
distribution studied
 Fission gas release
 Axial distribution by
gamma scanning
 Radial distribution to
evaluate flux
Fission products
for MOX fuel
• Pu fuel has
 Ru, Rh, Pd
• Fission product
behavior varies
 capture
Fuel variation during irradiation
Distribution in fuel
• Axial fission product
distribution corresponds very
closely to the time-averaged
neutron flux distribution
 PWR activity level in the
 Activity minima from
neutron shielding effect
of spacer grids
 local decrease in
fission rates
 Fuel density effects
 Dishing at end of fuel
 Disappear due to fuel
 BWR shows asymmetric
 Control rod positions
Distribution in Fuel
• Radial distribution of
fission products mainly
governed by thermal
neutron flux profile .
• Higher Pu concentration in
outer zone of fuel
Transuranics on fuel rim
Epithermal neutron
capture on 238U
Small influence of
thermal migration on Cs
 Gaseous and volatile
fission products
 Influence by fuel initial
composition (O to M
Xe trapped in region with
high gas bubble density
Distribution in Fuel
• Increased Pu leads to
increased fission product
 Xe behavior
influenced by bubble
gas location
• Consumption of burnable
 Gd isotopes 157 and
155 depleted in outer
Distribution in fuel: Thermal behavior
• Mainly affects the gaseous and the volatile fission
 linear heat rating
 pellet temperatures during reactor operation
 stoichiometry of the fuel
• Halogens and alkali elements
 Cs and I volatility
High fission yields
Enhanced mobility
 Can be treated similarly, different chemical
Iodine and Cs
CsI added to UO2
Both elements have
same maximum
location at 1000 °C
Iodine property
changes, mobility
to lower
 Elemental I2
rather than IFormation in the range of
x to 0.02
No change in Cs
chemistry as it remains
Iodine and Cs
release of cesium and iodine
from fuel at 1100 to 1300 K
release rates increase with
increasing temperature
2100 K largest
fraction released
after 60 seconds
Both elements released at
significantly faster rate
from higher-burnup fuel
Different release
attributed to fission product
atoms which already
migrated to grain
UO2 lattice difficulty
in incorporating
large atomic radii
Perovskite phase (A2+B4+O3)
• Most fission products
homogeneously distributed in UO2
• With increasing fission product
concentration formation of
secondary phases possible
 Exceed solubility limits in UO2
• Perovskite identified oxide phase
 U, Pu, Ba, Sr, Cs, Zr, Mo, and
 Mono- and divalent elements
at A
• Mechanism of formation
 Sr and Zr form phases
 Lanthanides added at high
Epsilon phase
• Metallic phase of fission
products in fuel
 Mo (24-43 wt %)
 Tc (8-16 wt %)
 Ru (27-52 wt %)
 Rh (4-10 wt %)
 Pd (4-10 wt %)
• Grain sizes around 1
• Concentration nearly
linear with fuel burnup
 5 g/kg at 10MWd/kg
 15 g/kg at 40
MWd/kg U
Epsilon Phase
• Formation of metallic phase
promoted by higher linear
 high Pd concentrations
(20 wt %) indicate a
relatively low fuel
 Mo behavior controlled
by oxygen potential
 High metallic Mo
indicates O:M of 2
 O:M above 2, more
Mo in UO2 lattice
12-16of the
Relative partial molar Gibbs free energy of oxygen
fission product oxides and UO2
Grouping of behavior
Experiments performed between 1450 and
1825 °C
trace-irradiated UO2 fuel material
Limit formation of fission products
4 categories
Elements with highest electronegativities have
highest mobilities
Te, I
Low valent cations and low fuel solubility
Cs, Ba
Neutral species with low solubility
Xe, Ru, Tc
Similar behavior to low valent cations
(xenon, ruthenium,
polyvalent elements were not released from
Nd, La, Zr, Np
Ions with high charges and dimension of fuel
remain in UO2
Neutral atoms or monovalent fission products
are mobile in fuel fission products
Evident at higher temperatures
 higher fuel rod heat ratings
 accident conditions
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
Fission products: General chemistry
• 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
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
• 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
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
• 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
• 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
• 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
• 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
• 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
• How is uranium chemistry linked with
chemistry in fuel
• What are the main oxidation states of the
fission products and actinides in fuel
• What drives the speciation of actinides and
fission products in fuel
• How is volatility linked with fission product
• What are general trends in fission product
1. What drives the speciation of actinides and
fission products in spent nuclear fuel?
2. What would be the difference between oxide
and metallic fuel?
3. Why do the metallic phases form in oxide fuel
4. How is the behavior of Tc in fuel related to the
U:O stoichiometry?
Pop Quiz
• Why do the metallic phases form in oxide fuel

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