Radiochemistry in reactor

Transcription

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 12-1

2 Fission process Recoil length about 10 microns, diameter of 6 nm About size of UO 2 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 Br as examples Some neutron capture of fission products eff 12-2

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 137 Cs(some migration issues) 142 Nd(stable isotope), 152 Eu are suitable fission products Neutron detection also used Need to minimize 244 Cm 12-3

4 Radionuclides in fresh fuel Actual Pu isotopics in MOX fuel may vary Activity dominated by other Pu isotopes Ingrowth of 241 Am MOX fuel fabrication in glove boxes 12-4

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 12-5

6 Fission products for MOX fuel Pu fuel has higher concentrations of: Ru, Rh, Pd Fission product behavior varies capture 12-6

7 Fuel variation during irradiation 12-7

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 12-8

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 238 U 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

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 12-10

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 12-11

12 Iodine and Cs CsI added to UO 2 Both elements have same maximum location at 1000 C UO 2+x Iodine property changes, mobility to lower temperature regions Elemental I 2 rather than I - Formation in the range of x to 0.02 No change in Cs chemistry as it remains monovalent 12-12

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 UO 2 lattice difficulty in incorporating large atomic radii ions 12-13

14 Perovskite phase (A 2+ B 4+ O 3 ) Most fission products homogeneously distributed in UO 2 matrix With increasing fission product concentration formation of secondary phases possible Exceed solubility limits in UO 2 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

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 12-15

16 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 UO 2 lattice Epsilon Phase Relative partial molar Gibbs free energy of oxygen of the fission product oxides and UO 2

17 Experiments performed between 1450 and 1825 C trace-irradiated UO 2 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 UO 2 Neutral atoms or monovalent fission products are mobile in fuel fission products Evident at higher temperatures higher fuel rod heat ratings accident conditions Grouping of behavior 12-17

18 Radionuclide Inventories Fission Products generally short lived (except 135 Cs, 129 I) ß, emitters geochemical behavior varies Activation Products Formed by neutron capture ( 60 Co) ß, emitters Lighter than fission products can include some environmentally important elements (C,N) Actinides alpha emitters, long lived 12-18

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 (Tc 4+, TcO 4- ) I Anionic 129 I long lived isotope 12-19

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 12-20

21 Complexes Group 1 metal ions form oxides M 2 O, 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

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 12-22

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 CaSO 4 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

24 12-24

25 Technetium Electronic configuration of neutral, gaseous Tc atoms in the ground [Kr]4d 5 5s 2 [l] with the term symbol 6 S 5/2 Range of oxidation states TcO 4-, TcO 2 Tc chemical behavior is similar to Re Both elements differ from Mn Tc atomic radius of Å Å smaller than Re 12-25

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 TcO 4- /TcO 2 and TcO 4 /Tc are intermediate between those of Mn and Re TcO 4 is a weak oxidizing agent 12-26

27 Lanthanides Electronic structure of the lanthanides tend to be [Xe]6s 2 4f n ions have the configuration [Xe]4f m 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 (La 3+ ) to 86 pm (Lu 3+ ), Ce 3+ can oxidized Ce 4+ Eu 3+ can reduce to Eu 2+ with the f 7 configuration which has the extra stability of a half-filled shell 12-27

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 12-28

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 12-29

30 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

31 Pop Quiz Why do the metallic phases form in oxide fuel 12-31