Neptunium behaviour in PUREX and GANEX processes C. Gregson, M. Carrott, D. Woodhead, R. Taylor SNEC 2014, Manchester Challenges in Np separations chemistry • Background • Neptunium extraction in an Advanced PUREX process • Neptunium extraction in a GANEX process Options for future fuel cycles R: Recycle plant FR: Transmutation reactor V: Vitrification FR R MA FP V U, Pu Heterogeneous Recycling Advanced PUREX + DIAMEX-SANEX or EXAm FR R FP V U, Pu, MA Homogeneous Recycling GANEX Requirements for advanced reprocessing Optimise recycling processes so the option to close the fuel cycle by ~2050 is competitive with other spent fuel management options • Reduced costs • Reduced wastes • Reduced environmental impact • Enhanced process safety • Enhanced proliferation resistance / safeguards • Enhanced public acceptability • Flexibility to process wider range of fuel types • Integrated with fuel fabrication and waste management PROCESS SIMPLIFICATION / INTENSIFICATION / INNOVATION Neptunium Np in the fuel cycle • Current closed fuel cycles Np in HLW glass • 237Np radiotoxicity in environment • Past use for 238Pu production • Future fuel cycles can burn Np in Pu/TRU fuels • Current reprocessing by PUREX process Np is split across several streams • Requires extra SX cycle to decontaminate • Advanced reprocessing direct Np to single stream • Reduced costs, wastes • Enhance proliferation ‘resistance’ Np redox chemistry in HNO3 NpO22+ NpO2 1 3 NO3 H ... 2 2 HNO3 / HNO2 Disproportionation ...NpO22 NpO2+ 1 1 HNO2 H 2O 2 2 2 NpO2 4 H HNO3 / HNO2 Np 4 NpO22 2 H 2O Np4+ 3 easily inter-convertible oxidation states in HNO3 Np(V) oxidation vs. Np(VI) reduction in HNO3 1 No HNO2 to catalyse the redox reaction Np(V) Rel. Abs. 0.8 HNO2 added catalyses Np(V) oxidation Addition of 1 mM NaNO2 0.6 0.4 Np(V)/Np(VI) equilibrium position defined by HNO3/HNO2 HNO2 added reduces Np(VI) 0.2 0 No HNO2 to catalyse the redox reaction 0 5 10 15 20 25 30 35 time (min.) Np(VI) with 1 mM NaNO2 Np(V) zero nitrite Np(V) with 1mM NaNO2 Np(VI) zero nitrite Reduction of Np(VI) and oxidation of Np(V) in 5 M HNO3 at 50oC with and without NaNO2 40 Np extraction in PUREX DNp [ Np ]organic [ Np ]aqueous D(NpVI) > D(NpIV) > D(NpV) Np(VI) = very extractable Np(IV) = extractable Np(V) = not extractable Need to stabilise the middle oxidation state Conventional PUREX process ~70 % Np UIV/N2H4 HNO3 UIV/N2H4 HS 50C 1BX 35C 1BXX 35C 1C 50C HNO3 UIV/N2H4 U Finishing Condition 20% TBP 20% TBP HAF HA 35C UP1 25C 1BS HAN 25C UP2 50C UP3 40C HNO3 HAN HAN Diluent wash Condition HAR 30% TBP PP1S 45C HNO3 PP2 30% TBP HA Feed PP1E 45C Diluent wash Condition Concentrate Finishing 30% TBP ~30 % Np How can we achieve 100 % extraction in future SX processes? -pulsed columns -centrifugal contactors Advanced PUREX process Dissolved SNF Simplified PUREX process •Single cycle SX flowsheet •Np control •In a single stream •With Pu-U product •With U then separated Extract/scrub Np follows U,Pu HLW (Am,Cm,FPs) U/Pu separation Option A. Np follows Pu A1: Pu,Np A2:Pu,U,Np Np barrier U backwash Option B. Np follows U B: Np U Minor actinide (Am,Cm) separations require additional SX cycles on HLW GANEX Process Dissolved SNF Co-separation of all TRU actinides U extraction An + Ln extraction An stripping Ln stripping U FPs Pu, Np, Am, Cm Ln GANEX 1st cycle GANEX 2nd cycle PUREX vs. GANEX processes • SX from nitric acid into an organic kerosene phase • Different extracting ligands PUREX O TBP O U P O O Pu Np EURO-GANEX TODGA O C8H17 N C8H17 O DMDOHEMA H3C N C8H17 N O C8H17 C8H17 O O U Pu Np Am Cm N C2H4 O C6H13 CH3 C8H17 Np extraction depends on • HNO3 • HNO2 • Extraction of HNO2 and Np(VI) into solvent • U saturation of the solvent • Chemical kinetics in aqueous and organic phases • Temperature • Any other redox active species • Residence time in contactor • Mixer-settlers > pulsed columns > centrifugal contactors Common problem to Advanced PUREX & GANEX processes Np extraction in Advanced PUREX • Experiments increasing in complexity • Single phase, 2-phase, single centrifugal contactor stage • Flowsheet trial • Modelling • …See talk by Hongyan Chen! • The Np(V) oxidation reaction is promoted by • Conditions in aqueous solution • High [HNO3], [HNO2]/[Np](aq) ~ 2, T = 50C • U loading reduces D(HNO2) • Within a flowsheet need to consider: • Sufficient [HNO2] in regions of low [U] during extraction • Increase [HNO3] across the extract section • Elevated temperature around the extract and scrub section NNL Np extraction flowsheet test SP1 S1 A2 FC1 HEATER 1 HS1 HEATER 4 5 HA1 8 9 HA2 HAF HA 13 HA3 14 AR1 A1 F1 12 F2 Miniature centrifugal contactors Simulant feed U+Np No hot test yet CEA Np extraction test HNO3 S HNO3 SP U,Np,Pu HAF •Spent fuel test •LWR UOx fuel •Pulsed columns E2 E1 HNO2 X 30% TBP 30% TBP HAR <0.11 % Np AR <0.25 % Np Dinh, B., et al. in Solvent extraction: fundamentals to industrial applications. Proceedings ISEC 2008 International Solvent Extraction conference pp.581-586 (2008). Key results – 2 successful tests NNL • >99 % Np extracted • U/Np test • HNO2 added • No radiation field • • • • • HM ~ 250 g/L Centrifugal contactors Lab scale Heated to 50 C (stages 1-8) 4.5 mol/L HNO3 feed CEA • >99.6 % Np extracted • Spent fuel test • No HNO2 feed required • Radiolytically generated HNO2 • HM ~250 g/L • Pulsed columns • Pilot scale • Ambient temperature • 4.5 mol/L HNO3 feed DNp DNp Np extraction in GANEX 1.E+04 1.E+03 Np(IV) 0.2M TODGA Np(VI) 0.2M TODGA Np(IV) 0.5M DMDOHEMA Np(VI) 0.5M DMDOHEMA 1.E+02 Np(IV) 1.E+03 Np(VI) Np(V) 1.E+01 1.E+02 1.E+00 1.E+01 1.E-01 0.1 1 10 [HNO3]aq,ini (M) 1.E+00 GANEX: Np(IV)>>Np(VI)>>Np(V) 1.E-01 1.E-02 0.1 1 [HNO3]aq,ini (M) 10 Np(IV): TODGA>DMDOHEMA Np(VI): DMDOHEMA>TODGA Np(V) more extractable than PUREX Optimising Np extraction Stability Np(IV)>Np(VI)>>Np(V) Promote extraction by promoting Np(V) disproportionation in flowsheet 0.3 Np(IV) Np(V)+(IV) 0.25 10 minutes following extraction Absorbance 0.2 17 hours following extraction at RT Np(IV)+(VI) 0.15 Np(VI) in DMODHEMA 0.1 0.05 0 600 Initial aq. = 5 M HNO3 700 800 900 1000 -0.05 Wavelength (nm) 1100 1200 1300 Np(V) disproportionation • Redox reaction residence time dependent • Faster in organic phase than aqueous phase • Faster in GANEX solvent than PUREX solvent • Rate increases with increased nitric acid concentration and temperature • Kinetic equation derived in 30 % TBP • Products Np(IV,VI) are both extractable • Promoting disproportionation should promote extraction “EURO-GANEX” process tests Solvent Feed Actinides Lanthanides Extract 1 FPs 12 13 Scrub Active feed 5.9 M HNO3 + 0.055 M CDTA SF solution (10 g/L Pu) Solvent Raffinate 0.5M HNO3 Loaded Solvent Feed 23 32 16 Ln Strip 29 0.01M HNO3 Lanthanides 28 0.5M HNO3 1M AHA 0.055M BTP 22 An Strip 17 Pu, Np, Am, Cm Solvent Feed Flowsheet results Glovebox test (NNL) Hot cell test (ITU) Contactors Centrifugals Centrifugals Feed Surrogate Fast reactor spent fuel Pu content in feed (g/L) 10 10 Pu recovery in E/S (%) > 99.9995 100.0 Am recovery in E/S (%) > 99.99 100.0 Np recovery in E/S (%) ~71 99.93 Np in HAR (%) ~ 29 Np in TRU product (%) ~69 99.9 Conclusions & outlook • Np control is one of main chemistry challenges for advanced reprocessing • Affects plant layout, economics, waste management, proliferation resistance, corrosion • Complex chemistry in nitric acid and organic phases • Extraction is residence time dependent • Now demonstrated that complete extraction is achievable by adjustment of acidity and temperature • In advanced PUREX flowsheets • In EURO-GANEX flowsheet • & with short residence time centrifugal contactors • Enhanced extraction under HA conditions (with SNF) • Predictive & mechanistic based models now required… Acknowledgments NNL Signature Research programme in Spent Fuel and Nuclear Materials EU Framework VII project “ACSEPT” Sellafield Ltd. UK Nuclear Decommissioning Authority Chris Maher, Chris Mason, Katie Bell, Jamie Brown, Mark Sarsfield, Zara Hodgson (NNL), Helen Steele (NNL/CEA) Danny Fox, Cécile Roube, Gill Crooks, Chris Jones, Eddie Birkett (ex-NNL) ACSEPT project – Andreas Geist (KIT), Rikard Malmbeck (ITU), Giuseppe Modolo, Andreas Wilden (FZJ), Xavier Hères, Stéphane Bourg, Manuel Miguirditchian (CEA) Colin Boxall, Scott Edwards (Lancaster University) Megan Jobson, Hongyan Chen, Andrew Masters (University of Manchester) Key references 1. 2. 3. M. J. Carrott, et al. Neptunium extraction and stability in the GANEX solvent (0.2 M TODGA / 0.5 M DMDOHEMA / kerosene), Solv. Extr. Ion Exch. 31, 463-482 (2013). R. J. Taylor et al. Progress towards the full recovery of neptunium in an Advanced PUREX process, Solv. Extr. Ion Exch. 31(4) 442-462 (2013). C. Gregson et al. Neptunium (V) oxidation by nitrous acid in nitric acid, Procedia Chemistry, 7, 2012, 398-403
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