1. 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

2. 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
I and 87-90Br as examples
Some neutron capture of fission products

3. 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
Need to minimize 244Cm

4. 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

5. 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

6. Fission products for MOX fuel
Pu fuel has higher concentrations of:
Ru, Rh, Pd
Fission product behavior varies
capture

7. Fuel variation during irradiation

8. 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

9. Distribution in Fuel Transuranics on fuel rim
Radial distribution of fission products mainly governed by thermal neutron flux profile .
Higher Pu concentration in outer zone of fuel
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
Transuranics on fuel rim

10. 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

11. 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. 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 I-
Formation in the range of x to 0.02
No change in Cs chemistry as it remains monovalent

13. 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

14. 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

15. 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

16. 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
Relative partial molar Gibbs free energy of oxygen of the fission product oxides and UO2

17. 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

18. Radionuclide Inventories
Fission Products
generally short lived (except 135Cs, 129I)
ß,emitters
geochemical behavior varies
Activation Products
Formed by neutron capture (60Co)
Lighter than fission products
can include some environmentally important elements (C,N)
Actinides
alpha emitters, long lived

19. 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

20. 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

21. 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

22. 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

23. 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

25. 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 Å
0.015 Å smaller than Re

26. 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

27. 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

28. 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

29. 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

30. Questions
What drives the speciation of actinides and fission products in spent nuclear fuel?
What would be the difference between oxide and metallic fuel?
Why do the metallic phases form in oxide fuel
How is the behavior of Tc in fuel related to the U:O stoichiometry?

31. Pop Quiz
Why do the metallic phases form in oxide fuel