Uranic residue treatment

Transcription

1 Uranic residue treatment Howard Greenwood, Tahera Docrat, Sarah May, Sarah Allinson, Ruqqayah Sultan, Martin Watson, David Williamson, Paul Magee WMD

2 Front end uranic residues Residues arising from uranium purification, enrichment etc often poorly understood Differ from reprocessing residues in that: levels of radioactivity generally allow much simpler handling uranium contents are often much too high for economic encapsulation and disposal (and use up capacity at LLWR) Very large historic stocks in the UK (many tens of thousands of drums) and overseas-large liabilities

3 Overview of waste and residue processing Waste prevention Minimisation Reuse Sentence stored materials to established processing routes where practicable Develop and implement processing routes for more difficult to treat wastes and residues Preserve capacity at the Low Level Waste Repository (LLWR) Recycle Energy recovery Disposal Generally aim to send solid materials to landfill rather than LLWR; organics for incineration/recycle

4 Phased approach Phase 1: Historic (desktop) study Phase 1A: Inspection, sampling and characterisation Phase 2: Testing suitability of existing processes at laboratory and Pilot Plant scale Phase 3: (if necessary) development and testing of ad hoc process Full scale processing: via existing industrial plant via new ad hoc plant via flexible NNL Pilot Plant facilities

5 Typical work flow Material well understood and potential processing route extant Material suitable for processing via an extant route Phase 1 Historical Assessment Material well understood and processing route extant Material not well understood Direct processing via an existing route Phase 1A Characterisation Material well understood and processing route extant Material well understood and potential processing route extant Phase 2 Testing for processing via existing facility or process Material well understood but no processing route known Phase 3 Development of ad hoc Processes Material well understood but no processing route known Material not suitable for processing via an extant route Material suitable for processing via an extant route Phase 4 Testing and implementation of ad hoc process Direct processing via an existing but modified route

6 Examples of residues to which this methodology has been applied Process sludges/powders (filter cakes, alkaline precipitates, high U dissolver cakes) -very wide variety of materials Contaminated oils, solvents, oily residues and solvent extraction cruds Decommissioning residues (concrete, bricks, timber, metals, plastics, glass) Uranium metal and alloys (generally unusual forms) Uranic powders (UO 2, U 3 O 8, UO 3, UF 4 ) often unusual fingerprint Sintered pellets (UO 2, UC) generally atypical pellets HEPA and pre-filters Contaminated soft wastes (paper, plastic, clothing, gloves, mops) Contaminated graphite Incinerator ash and other vitrified uranic slags High U resinous floor coverings

7 Example: RAPTOR Phase 1 desktop assessment of a historic residue c. 2 te material c. 1 te enriched uranium at useful power reactor level enrichments poorly understood history; suspicion of unusual contaminants Phase 1A characterisation study identified components likely to be problematic in terms of disposal of residual undissolved solids identified presence of Np, Pu and Tc at levels well in excess of fuel specifications ( Bq Np + Pu/g U, or up to c. 600x specification for refeed as fuel) Some batches high Tc Phase 2 process testing leachability issues in nitric acid (required leach conditions outside of regime of normal operating plant) Front end flowsheet TBP/OK solvent extraction had little effect on Np, Pu or Tc

8 RAPTOR Phase 3 development Phase 3 ad hoc method development Laboratory work to devise useful nitric acid leach regime (yielding low U residual solids and impure UNL for onward treatment) Main aim to devise a method of sufficiently separating Np, Pu and Tc from U to permit recycle of U to fuel Without excessive cost (i.e. minor mods to existing plant) Initial separative work funded under NNL Signature Research Programme (potential wider applicability) Later funding by customer (NDA liability) Considered: Ion exchange (new equipment required) Solvent extraction (equipment available)

9 RAPTOR development-1 RAPTOR Residual Actinide, Process TO Remove Literature survey of possible chemistry to be applied Solvent extraction magic bullet approach to allow simple modification of existing front end SX plant Add a chemical or chemicals to alter chemistry of Np and Pu to prevent coextraction with uranium

10 RAPTOR development-2 Required characteristics for a magic bullet Convert Np and/or Pu to nonextractable oxidation state (IV) and (VI) states extractable to TBP/OK, (III) and (V) states nonextractable Also Convert Np(VI) to Np(V), Pu(IV) to Pu(III) AND/OR Form non-extractable complex Useful reaction rate Bulk availability/cost Chemical stability Environmental acceptability

11 RAPTOR development-3 Standard front end solvent extraction equipment (mostly mixer-settlers) Usually configured with: Extract Section Contact of impure uranyl nitrate/nitric acid solution with TBP/OK Strip section Contact of U loaded solvent with e.g. 1 M nitric acid or pure UNL Backwash section Contact of stripped solvent with very weak (c M nitric acid) Initial considerations suggested most advantageous point to add magic bullet would be at strip acid injection point

12 RAPTOR (Reduction times) Example reduction times for: hydrazine and alkyl substituted hydrazines hydroxylamine and substituted hydroxylamines oximes Reduction times used to determine if reduction could occur in typical strip sections of extant plant Table 1. Np(VI/V) and Pu(IV/III) reduction reaction times for various reductants 0.1 M reductant in 1 M HNO 3 Time for 99 % reduction of N 2 H 4 (hydrazine) (CH 3 ) 2 N 2 H 2 (1,1-dimethylhydrazine) (CH 3 ) 3 CN 2 H 3 (tert-butylhydrazine) NH 2 OH (hydroxylamine) (C 2 H 5 ) 2 NOH (N,N-diethylhydroxylamine) HOC 2 H 4 (C 2 H 5 )NOH (N,N-ethyl(hydroxyethyl)hydroxylamine) CH 3 CH=NOH (acetaldoxime) C 3 H 7 CH=NOH (butyraldoxime) Time for 99 % reduction of Pu(IV) Np(VI) to Np(V) to Pu(III) 198 s at 25 C - (only 0.13 % reduced in 198 s) 60 s at 25 C - (only 0.24 % reduced in 60 s) 522 s at 25 C - 17 s at 20 C 3.3 s at 35 C 146 s at 20 C 38 s at 35 C 27 s at 20 C 12 s at 35 C 18 s at 20 C 5.1 s at 35 C 19 s at 20 C 4.9 s at 35 C (only 0.10 % reduced in 522 s) 175 hrs at 20 C 4 hrs at 35 C 12 s at 20 C 1.2 s at 35 C 4 s at 20 C 0.3 s at 35 C 23 mins at 20 C 3.1 mins at 35 C 4 hrs at 20 C 12 mins at 35 C State Scientific Center of Russian Federation A. A. Bochvar All-Russia Research Institute of Inorganic Materials (VNIINM) Research Report, Research of Selective Reductants for Np(VI) and Pu(IV) in Purex- Process, V. S. Koltunov, 1997.

13 RAPTOR (Rate constants) Comparison of rate constants for various magic bullets Acetohydroxamic acid and other reductants for Np(VI) to Np(V) reduction Table 2. Np(VI/V) reduction reaction rate constants for selected reductants Reductant Temp. ( C) Rate constant (s -1 ) Acetohydroxamic acid Formohydroxamic acid U(IV) N,N-ethyl(hydroxyethyl)hydroxylamine Hydroxylamine Acetaldoxime ,1-dimethylhydrazine N,N-diethylhydroxylamine Hydrazine iso-butyraldehyde Butyraldehyde Oregon State University presentation, Redox Chemistry of Neptunium in Solutions of Nitric Acid, A. Paulenova, M. Precek, K. Hartig and N. Knapp.

14 RAPTOR (Electrode potentials) Known actinide electrode potentials were compared with the onset oxidation potentials for various reducing agents Example: acetohydroxamic acid onset potential V vs SHE Thus acetohydroxamic acid can reduce actinides with reduction potentials greater than V i.e. reduce: Np(VI) to Np(V) Pu(IV) to Pu(III) Table 3. Formal potentials for selected actinide couples Oxidation Numbers SHE: Standard Hydrogen Electrode Reaction Formal potential vs. SHE in 1 M HClO 4 (V) VI-V UO e - = UO VI-IV UO H + + 2e - = U H 2 O VI-III UO H + + 3e - = U H 2 O V-IV UO H + + e - = U H 2 O V-III UO H + + 2e - = U H 2 O IV-III U 4+ + e - = U VI-V NpO e - = NpO VI-IV NpO H + + 2e - = Np H 2 O VI-III NpO H + + 3e - = Np H 2 O V-IV NpO H + + e - = Np H 2 O V-III NpO H + + 2e - = Np H 2 O IV-III Np 4+ + e - = Np VI-V PuO e - = PuO VI-IV PuO H + + 2e - = Pu H 2 O VI-III PuO H + + 3e - = Pu H 2 O V-IV PuO H + + e - = Pu H 2 O V-III PuO H + + 2e - = Pu H 2 O IV-III Pu 4+ + e - = Pu The Chemistry of the Actinides, S. Ahrland, J. O. Liljenzin and J. Rydberg, Pergamon Press, New York, 1975, pages

15 RAPTOR (Complex formation) Example data for complex formation Acetohydroxamic acid, is a complexing reductant Table 4. Stability constants for actinide acetohydroxamate complexes µ = ionic strength of medium Actinide log β 1 log β 2 log β 3 Medium Pu C, µ = 2.0 M ClO 4 - Forms strong complexes with Np(IV) and Pu(IV) potentially rendering them inextractable by TBP/OK UO C, µ = 0.1 M NO 3 - Np C, µ = constant ClO 4 - Pu C, µ = 2.0 M NO 3 - The Chemistry of Acetohydroxamic Acid Related to Nuclear Fuel Reprocessing, B. S. Matteson, PhD Thesis, Oregon State University, May 2010.

16 RAPTOR (Stability in nitric acid) Ideal magic bullets : have sufficient life in nitric acid to be practical in existing systems do not yield hazardous or problematic degradation products Acetohydroxamic acid Table 5. Half life of acetohydroxamic acid in nitric acid [HNO 3 ] (M) t ½ (hrs) 0.5 ~ ~3 Westinghouse Savannah River Company, Aiken, Report WSRC-TR report, Radiation chemistry of acetohydroxamic acid in the UREX process, D. G. Karraker, Breaks down to acetic acid and hydroxylamine

17 RAPTOR Testing On the basis of the above criteria, two reductants were selected for laboratory testing Acetohydroxamic acid Acetaldoxime Plus a back-up Ascorbic acid/hydrogen peroxide

18 RAPTOR-laboratory testing Small scale laboratory experiments Proof of principle Used real residue samples Results were not discouraging, without being gee-whiz Thought to be due to magic bullet breakdown during cumbersome manual tests Continuous tests in actual equipment deemed essential Easier to carry out Pilot Plant than continuous laboratory SX experiments Customer funded

19 RAPTOR-EAGL testing EAGL is a small scale dissolution/filtration/sx plant in the NNL Preston Laboratory Pilot Plant Separation of U and Gd Solvent extraction unit has 6 forward extract stages and 10 strip stages in one unit 16 Back extraction stages in a second unit Twice the size of our laboratory unit (less flexible, but less labour intensive, especially regarding dissolution) 10 kg uranic feed/day

20 RAPTOR via EAGL testing-1 U 3 O 8 feed dissolved in 3 M HNO 3 to 200 g U/dm 3 (70 Bq Np + Pu/g U vs specification of 0.2 Bq/g U) UNL fed to solvent extraction at 2 dm 3 /hr, moderately high solvent loading flowsheet Loaded solvent stripped with normal 1 M nitric acid strip with aqueous solution magic bullet fed to same feed point as strip acid to minimise contact time with nitric acid (very simple modification) Run No Magic bullet 1 Acetohydroxamic acid 2 Acetohydroxamic acid Np removal (%) Pu removal (%) 94.0 > > Acetaldoxime 96.4 >99.8 Both reductants were very successful and reduced the Np and Pu on U basis to the point where the material can be fed back to the fuel cycle for re-use Otherwise plant ran as normal 25 hrs/run

21 RAPTOR-future FY 2015/16 Additional testing of RAPTOR using high Tc materials and materials containing species likely to generate cruds on SX Potential for programme lasting c. 1 year to operate RAPTOR via EAGL to clear a particular liability which contains very considerable uranium value which is current a liability rather than an asset Potential to deploy RAPTOR for a range of similar materials

22 Phased approachgeneral track record-1 25 year programme in terms of UK residues-also similar work for organisations in USA, Europe, Japan For the UK alone, over 50,000 drums categorised via Phase 1 Assessment Over 30,000 drums cleared for processing in customers own plants (generally nitric acid leaching) Various processes implemented in new full scale plant owned by customers (e.g. NAWF, CUTLASS) Over 2,500 drums of highly intractable wastes and residues processed via the NNL flexible Pilot Plant 100 s m 3 oil and solvent cleaned to free release levels Millions of of uranium value recovered to the fuel cycle as well as discharging even larger taxpayer liabilities

23 Phased approachgeneral track record-2 Work carried out in partnership with Springfields Fuels Ltd, URENCO, DSRL etc Success due to: Availability of a unique and vital facility offering the only available treatment/ disposal route for many UK residue liabilities Unique fusion of basic science with state of the art laboratories and flexible pilot plant plus knowledge of the capabilities of available full scale plant through plant support work Enlightened customers committed to a long term strategy and programme Dedicated team of c. 12 chemists, chemical engineers and plant operators, most with many years experience in the field