Fuel Cycle Chemistry

Report
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
12-1
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    
12-2
Burnup
•
•
•
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
event.
 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
products

Neutron detection also used
12-3
 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
12-4
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
maximum
• Radionuclide fuel
distribution studied
 Fission gas release
 Axial distribution by
gamma scanning
 Radial distribution to
evaluate flux
12-5
Fission products
for MOX fuel
• Pu fuel has
higher
concentrations
of:
 Ru, Rh, Pd
• Fission product
behavior varies
 capture
12-6
Fuel variation during irradiation
12-7
Distribution in fuel
• Axial fission product
distribution corresponds very
closely to the time-averaged
neutron flux distribution
 PWR activity level in the
middle
 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
swelling
 BWR shows asymmetric
distribution
 Control rod positions
12-8
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
ratio)

Xe trapped in region with
high gas bubble density
12-9
Distribution in Fuel
• Increased Pu leads to
increased fission product
density
 Xe behavior
influenced by bubble
gas location
• Consumption of burnable
poison
 Gd isotopes 157 and
155 depleted in outer
zone
12-10
Distribution in fuel: Thermal behavior
• Mainly affects the gaseous and the volatile fission
products
 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
behavior
12-11
Iodine and Cs
•
•
•
•
CsI added to UO2

Both elements have
same maximum
location at 1000 °C
UO2+x

Iodine property
changes, mobility
to lower
temperature
regions
 Elemental I2
rather than IFormation in the range of
x to 0.02
No change in Cs
chemistry as it remains
monovalent
12-12
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
mechanism
attributed to fission product
atoms which already
migrated to grain
boundaries

UO2 lattice difficulty
in incorporating
large atomic radii
ions
12-13
Perovskite phase (A2+B4+O3)
• Most fission products
homogeneously distributed in UO2
matrix
• 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
Lanthanides
 Mono- and divalent elements
at A
• Mechanism of formation
 Sr and Zr form phases
 Lanthanides added at high
burnup
12-14
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
micron
• Concentration nearly
linear with fuel burnup
 5 g/kg at 10MWd/kg
U
 15 g/kg at 40
MWd/kg U
12-15
Epsilon Phase
• Formation of metallic phase
promoted by higher linear
heat
 high Pd concentrations
(20 wt %) indicate a
relatively low fuel
temperature
 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
compounds
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
fuel

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
12-17
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
12-18
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
12-19
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
12-20
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
12-21
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
12-22
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
12-23
12-24
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
12-25
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
12-26
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
12-27
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
12-28
Review
• 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
chemistry
• What are general trends in fission product
chemistry
12-29
Questions
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?
12-30
Pop Quiz
• Why do the metallic phases form in oxide fuel
12-31

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