Spent fuel dissolution results from completed project REDUPP and ongoing project Disco L.Z. EVINS, L. DURO, A. VALLS, C. CORKHILL, E. MYLLYKYLÄ, I. FARNAN, D. BOSBACH, V. METZ, P. MALDONADO
Introduction Spent Nuclear Fuel dissolution Grains= SNF matrix Fission gas bubbles Xe,Kr,I SKB Safety Asessment SR-SiteTR-11-01 Ca 1,8 Ga year old uraninite Spent Nuclear Fuel in cladding from Forsmark. Krall et al (2019) Scale bar 200 μ m 2
Introduction REDUPP • April 2011 – April 2014 • FP7 Collaborative Project • Reduce remaining uncertainty in the dissolution rate of spent uranium oxide fuel + train young scientists for future needs in our field Fluorite structure : CaF 2 , CeO 2 , ThO 2 , UO 2 Sample surface changes during dissolution, effects of ” high-energy sites” Effects of natural ground water on dissolution of alpha-doped UO 2 Experiments & Ab Initio modelling 3
Introduction DisCo • June 2017 – May 2021 • Horizon 2020 Collaborative Project • Improve understanding of spent fuel matrix dissolution in repository conditions • Test modern nuclear fuel types (doped & MOX) for comparison with conventional fuels: - Both real spent fuel and synthesized model materials • Disseminate the new knowledge : reach a wider community through training and mobility measures • Associated Group: CV Rez (CH), LEI (LT), MTA EK (HU), ICHTJ (PL), EIMV (SI), Subatech(FR) 4
Methods REDUPP Experimental Modelling • Synthesis of spent nuclear fuel • Density Functional Theory in first- analogues: fluorite structure, grain principles (Ab Initio) modelling size, porosity, defects … (L(S)DA & DFT+U) • Dissolution in various conditions & • Modelling a surface: 6-8 layers aqueous phases analyses • Stepped surfaces on fluorite materials: • Post-dissolution analyses of the solid terraces and steps phases • Ab initio molecular dynamics (AIMD) and atomistic thermodynamics simulations for different temperatures & water reactions on UO 2 surfaces Back-Scattered Electron (BSE) image of ThO 2 fragment, 4 weeks leaching 5
Methods DisCo Experimental Modelling • Real Spent Fuel dissolution experiments: • Improve existing models through used MOX, Cr-doped, Cr/Al-doped, and inclusion of Fe corrosion, hydrogen standard fuel. effect & metallic particles • Model materials: UO 2 with and without dopants (Cr, Al, Gd), with and without alpha- • Thermodynamics, chemical kinetics, emitter ( 233 U, 238 Pu) electrochemistry, reactive transport… • Dissolution experiments 1) oxidizing, SNF & air (as reference, SNF & Ar, Model materials & H 2 O 2 2) inert atmosphere & Fe (reducing) 3) under Hydrogen (reducing) • Post-dissolution analyses 6
Results REDUPP CeO 2 CeO 2-x CeO 2-x , 35 days Role of defects and grain boundaries dissolution in 0.01 M HNO 3 • Initial fast dissolution is focused on grain boundaries: misorientation angles & crystallographic control. • Intrinsic defects: oxygen vacancies replaced by oxygen during dissolution, Ce 3+ in CeO 2-x rapidly oxidized to Ce 4+ • Lattice strain and enhanced oxygen mobility, created by the removal of oxygen vacancies, resulted in the disintegration of particles, Corkhill et al 2014. Contribution of Energetically Reactive Surface Features to the Dissolution of CeO2 and ThO2 Analogues for Spent Nuclear Fuel Microstructures ACS preferentially along grain boundaries Appl. Mater. Interfaces, 6, 15, 12279-12289 Corkhill et al, 2016. Role of Microstructure and Surface Defects on the Dissolution Kinetics of CeO2, a UO2 Fuel Analogue. ACS Applied materials &Interfaces 8, 16, 10562- 10571 7
Results REDUPP Ab Initio 3 molecule water Water on UO 2 surfaces adsorption on the • Ab Initio Molecular Dynamics combined (221) surface of with atomistic thermodynamics UO 2 . • Dissociative adsorption: hydroxylated surface stable at environmental conditions • More reactive surfaces with steps and terraces: reaction accompanied by a modification of the step morphology. Maldonado et al , 2014. Ab initio atomistic thermodynamics of water reacting with uranium dioxide surfaces. The Journal of Physical Chemisty C 118, 8491−8500. 8
Results REDUPP ThO 2 Insight from isotope exchange Continuous change in isotopic ratio • Isotopic tracer ( 229 Th) to track surface processes: Continued isotopic exchange in spite of apparent chemical equilibrium • Continuous change of isotopic ratio 229 Th/ 232 Th : precipitation/dissolution reactions are still ongoing at the interface despite apparent chemical equilibrium • Alpha-spectrometry: surface layer, maximum 0,1 µm thick containing 229 Th and daughter nuclides of 229 Th and 232 Th decay series. Myllykylä et al 2017, Direct alpha spectrometry for analysing Myllykylä et al 2017 Dissolution of ThO 2 : study of dissolution leached ThO 2 pellets. Journal of Nuclear Materials, 493,2017, process with initial 229 Th spike. Journal of Radioanalytical and 69-76. doi.org/10.1016/j.jnucmat.2017.06.003 Nuclear Chemistry 311, 225-235. 9
Results REDUPP UO 2 in natural groundwater Dissolution rates vs Si-content Effects of natural groundwater 5,00E-07 Calc. fractional dissolution rate 4,50E-07 • 3 ground waters with different salinity 4,00E-07 3,50E-07 3,00E-07 • Experiments used isotopic exchange: 2,50E-07 2,00E-07 Rate Rates are calculated using change in 1,50E-07 1,00E-07 isotopic ratio 5,00E-08 0,00E+00 1,20E-06 • 0 0,05 0,1 0,15 0,2 0,25 Calculated dissolution rates highest in 1,10E-06 SiO 2 (mmol/L) 1,00E-06 fresh groundwater 5,00 9,00E-07 Calc. fractional dissolution rate 4,50 • This has highest silica and carbonate 8,00E-07 4,00 3,50 7,00E-07 content 3,00 0% 6,00E-07 2,50 5% 5,00E-07 • 2,00 Precipitates were found with U and Si 10% 1,50 4,00E-07 1,00 3,00E-07 5,00 2,00E-07 Ollila et al 2013. Dissolution rate of alpha-doped UO 2 in natural 0,00 1,00E-07 groundwater. Journal of NuclearMaterials 442 (2013) 320 – 325 0,00E+00 ONK-PVA1 OL-KR6 OL-KR5 Synthetic Evins L Z, Juhola P, Vähänen M, 2014. REDUPP. Final report. Fresh Brackish Saline Allard 0,01MNaCl 0,5MNaCl 1MNaCl Posiva Working Report 2014-12, Posiva Oy, Finland. 10
Results DisCo WP2: Sample preparation Spent nuclear fuel samples Model materials +( U,Pu)O 2 & • SNF samples prepared in Hot Cells at • UO 2 (as reference), UO 2 +Gd, 233 U-doped UO 2 already Studsvik , KIT INE, JRC & NNL UO 2 +Cr, UO 2 +Cr+Al, (U,Th)O 2 . available • Samples for dissolution prepared • Sample synthesis procedures have either as segments of a fuel rod, or as been optimized and samples fragments with the cladding removed characterized Cr-doped UO 2 prepared at FZ Juelich Spent nuclear fuel (MOX) prepared at KIT INE 11
Results DisCo WP3: Spent fuel dissolution 1 st leaching results available • Next year data will be available for inclusion in the chemical models (WP5) • Tests run in reducing conditions: H 2 or Mix of Ar/H 2 , plus reference test in air • Studsvik Example given here 12
Results DisCo WP4 : Model materials dissolution Ciemat example: Cr-doped UO 2 in H 2 autoclave. 1 st leaching results available [U] ~solubility limit • Next year data will be available for inclusion in the chemical models (WP5) • Some preliminary results given here from Juelich and Ciemat Juelich: H 2 O 2 simulate effects of alpha radiation 13
Results DisCo WP5: Chemical Modelling Example Amphos 21: 4 modelling approaches : Initial models Reactions involving O 2 , H 2 and H 2 O 2 available - further development ongoing occurring at the spent fuel surface • Thermodynamic calculations of ideal solid solution : Cr (III) in SNF *H₂+ (M) *O₂+ (M) *H₂O₂+ (M) [U] (M) a) • iCP: interface coupling COMSOL *H₂+ model *O₂+ model *H₂O₂+ model [U] model 1.E-03 Multiphysics and PhreeqC. Chemical kinetics and reactive transport 1.E-04 • Unirradiated MOX in Fe-containing 1.E-05 Callovo-Oxfordian water using CHESS- [ ] (M) HYTEC : kinetics of reactions at pellet 1.E-06 surface 1.E-07 • Electrochemical, mixed-potential 1.E-08 modelling developed to address oxidative 0 200 400 600 800 1000 dissolution in storage ponds Time (days) 14
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