Feasibility for Slag Processing in UTSF

Scheme for Slag Processing in UTSFM. A. ShettyK. Kumarguru P. S. DhamiY. C. RaoSourceGenerated during metallurgical process of plutonium

Mixture of calcium fluoride and magnesium crucible containing plutonium and associated americium.

Plutonium + Americium + Pure Calcium + Calcium fluoride + Magnesium crucible.

1/3rd of the material is calcium fluoride2/3rd of the material is Magnesia crucible

Present stock of slag is more than 5 Te. Approximate amount of Plutonium in slag = 3.5 4.5 gm/kgApproximate amount of Americium in slag = 15-120 mg/kg as per the aging.

Aim of slag processingTo recover plutonium from slag and to convert it to oxide form To recover associated americium and to convert it to oxide formRest of the solution containing calcium and magnesium to be transferred to WMD for further treatment and disposal.

Proposed flowchart Dissolution of slag and crucible material in nitric acid.Filtration of dissolution product.Extraction and purification of Plutonium and Americium.Liquid waste management.

Options available for DissolverPDF DissolverPresent Thermosyphon dissolver in cell-4Installation of new dissolver in cell-4New dissolver installation on terrace

Evaluation of options for dissolverPDF DissolverFeasibility of equipment:Dissolver and related off gas system can be used for slag dissolution.Problems and solutions:Handling of Plutonium:Alpha tight containment for whole systemFiltration of dissolved product: Installation of filtration system inside containmentProviding an alpha tight containment for the whole area may be a time consuming step.Installation of new dissolver on terraceFeasibility of equipment:New dissolver equipment with associated equipments and systems can be installed on terrace.Problems and solutions:Handling of Plutonium:Alpha tight containment for whole system as plutonium handling.

Additional set up with system piping and alpha tight containment will be a time consuming step. New Dissolver in cell-4Feasibility of equipment:New dissolver vessel with filtration system can be designed fabricated and installed in cell.Problems and solutions:Dismantling existing thermosyphon dissolverFabrication and installation of new dissolver vesselExisting thermosyphon dissolver has to be discarded as it is, without attempting for any use.Thermosyphon DissolverFeasibility of equipment:Dissolver and related off gas system can be used for slag dissolution.Problems and solutions:Filtration of dissolution product: Installation of filtration system and a product receiver tank in cell. Being inside cell, system has complete containment Same dissolver with slight modification can be used. This dissolver may not be used for any other operation hereafter, hence before discarding, this can be utilized for slag dissolution.Thermosyphon dissolverView 1

Thermosyphon dissolver view 2

Dissolver details.Dia of charging limb : 350 mmDia of storage tank : 750 mmVolume of dissolver : 1600 LDissolution batch size can be kept as 1200 L

Dissolution of slagBasis: 100 kg of slag per batch of dissolution ( ~ 1 drum)Plutonium content ~ 350 450 g per 100 kg of slag solidAmericium content ~ 1.5 12 g per 100 kg of slag solid

Parameters:Dissolution in 5 M HNO3 and 1.3 M Al(NO3)3 mixtureSolid to liquid ratio 1:12 (1kg : 12L)Temperature to be maintained ~ 100 oC Time for dissolution ~ 8 hrs.

Dissolution product composition:Acidity = 2.5- 3.0 M[Pu] = 300- 400 mg/l[Am]= 1- 10 mg/l[Ca]= 15-20 gm/l[Mg]= 25-30 mg/l[Al]=35-40 gm/lUn-dissolved solid content ~ 5%

Dissolution product handling5% undissolved solids in dissolution product can choke the transfer lines and hinder mixer settler operationDissolution product needs to be filtered before processingFiltration using polypropylene cloth with two cartridges of different pore sizes.The filter cloth needs to be replaced after every batch processing as the non-dissolving solids form gummy material which can choke the filter cloth and hinder filtration.

Filtration of dissolved product : Mechanism 1

Filtration of dissolved product : Mechanism 2

Processing for Pu extractionPUREX processMixer settler unitsOrganic solvent : 20% TBP in n-Paraffin.Scrubbing agent: 2.5-3.0 M HNO3.Stripping agent: 0.3 M HAN in 0.6 M HNO3.Steps involved:Extraction of Plutonium in 20% TBP while Am and other metal ions go in raffinateScrubbing of plutonium loaded organic stream with nitric acid scrub solution to remove impuritiesStripping of Plutonium from organic using 0.3 M HAN in 0.6 M HNO3 as a stripping agent.Plutonium product will then be purified with anion exchange resin to meet required purity specification before sending to reconversion step

Plutonium Extraction Cycle

Extraction of PlutoniumExtraction of Plutonium in 20 % TBP while Am and other metal ions go in raffinateA:O ratio = 3 : 14 no. of stages gave 99.98 % extraction in four stages.Available no. of stages in UTSF for extraction = 12

Am containing Raffinate volume generated will be approximately 1.11 times that of aqueous feed processed. This will be stored in WT1 and WT2 tanks inside cell before processing for Am recovery and purification.It is proposed to transfer the Thorium product solution from WT1 & WT2 to UST by providing S.S. lined dyke.Scrubbing of Plutonium loaded organic streamImpurities in Plutonium loaded organic streamScrubbing: To increase purity of product streamFor Scrubbing A : O ratio = 1 : 3Scrubbing operation was not carried out during laboratory experimentsNo. of stages available in UTSF for scrubbing = 18

Stripping of Plutonium

Stripping of PlutoniumStripping of plutonium from loaded organic using 0.3 M HAN in 0.6 M HNO3.For stripping: A:O ratio = 1:34 no. of stages in laboratory experiment gave 99.99% of product stripping.12 no. of stages are available in UTSF for stripping. Plutonium concentration in product will be around 2.5-3.0 gm/lImpurities in the product need to be analyzed.Further purification by anion exchange process.

Final purification of PlutoniumFinal purification by anion exchange process.Conditioning of feed solution Acidity = 6.8 MNaNO2 conditioning to get > 99% conversion to Pu(IV)Feed volume = 150 litDOWEX 1 x 4 resin

Ion exchange process for Pu purificationConditioning: Column conditioned with 6.8 M nitric acid.Loading:Loading of plutonium solution in resin column Washing:Washing of resin column with 6.8 M nitric acid to remove all loaded impuritiesElution:Elution of column with 0.5 M nitric acid to get concentrated plutonium product. ( 15-20 gm/l)

Effluent acid from IX cycleEffluent acid generated in IX cycle is around 450 lit of 6.5 M per batch of processingThis acid will be recycled back to dissolver to use for dissolution of next batch of slag dissolution after adjusting its strength to 5 MThis will take care of acidic ILW from IX cycle.

Modifications required in Ion exchange processExisting resin column: Cation exchange resinRequired resin column: Anion exchange resinInstallation of tank ( tank space) for supply of wash acid to column Effluent acid storage tank installation in cellPiping for recycling effluent acid to dissolver

Sr. No.Problems encounteredPossible solution1.Charging port size of existing J rod Thermosyphon dissolver is of 4 diameter and slag lumps are of size varying up to 4-5. Modification of the port hole plug to increase its size to bigger one.2.Choking of ejector routes due to undissolved solid finesProvision for transfer of product solution from nozzles at elevated position

Jetting out of dissolver after every batch with intense air sparging to ensure complete fluidization of particles.Sr. No.Problems encounteredPossible solution3.Clogging of filter cloth during filtration of un-dissolved productAs this cant be avoided and can hinder the filtration of next batch dissolution product the filter cloth can be changed after every batch filtration.5.Only single mixer-settler cycle is available and the process chemicals for Pu purification and Am purification are differentInitially only plutonium can be recovered and purified and Am containing raffinate will be stored inside cell in WT1 & WT2 which will be processed at later stage.Sr. No.Problems encounteredPossible solution6.Solvent and diluent are different for Pu purification and Am purificationOnly Pu purification can be taken up first and Am can be stored in cell and after completing 5 batches of slag dissolution and processing change of process can be done after proper decontamination of system7.WT1 and WT2 contain TLR at present and are not available for storing Am raffinateTLR can be shifted to UST and this tank can be provided with a proper barrier and containment.8.Only one IX column & anion exchange resin required for Pu & another resin column is required for Am. As on today only one purification can be done.Sr. No.Problems encounteredPossible solution9.Lean plutonium solution generated in reconversion laboratory to be recycledTank is being installed which can be used for this.10. Degradation of organic and recycling of used organicExtent of degradation due to alpha has to be studied in laboratory. Based on the quality of organic it can be either recycled back to purification cycle or can be discarded.11.Quality of Pu and Am to be sent to laboratoryStudy of the impurities in the plutonium and Am solutions after final purification cycle has to be done. And limits are to be fixed.Americium ProcessingRaffinate from extraction cycle for plutonium contains Americium.

Concentration of Am depends upon the storage time for the particular lot of slag

Steps for Purification of Americium:Extraction StrippingPurification using ion exchange resinUranium Thorium Separation FacilityRetrofitted in old Evaporator PlantProcessing of Th metal/Thoria rods jacketed with aluminiumTHOREX processDecladding & dissolutionSeparation of U233 from Th232 and other fission productsIon exchange cycle for purification of U233.

Process steps & available equipments Decladding:Using caustic solution, sodium nitrateDissolution:14 M Nitric acid in presence of NaF catalyst.Dissolver:Thermosyphon type Charging limb: 600 lit (dia: 350 mm)Storage limb : 1000 lit ( dia 750 mm)

Dissolver related equipmentsDissolverDown draft condenserAmmonia scrubberCaustic scrubberDeclad Waste TankCaustic tankConditionerCD-condenserCD-condensateOff gas system with chiller, condenser, cyclone separator, heater, fiber glass filter beds & blowers

Proposed processProblems for dissolution sideSize of the slag ranges from few mm to around 4 for which dissolver charging port size has to be modifiedOn reheating the undissolved solids form gummy material which chokes the filter cloth making filtration for next batch difficult.

Dissolution Product Filtration

Scrubbing for plutonium purification cycle for PHWR spent fuel reprocessingDesign decontamination factor for scrub column in solvent extraction cycle for plutonium purification cycle (i.e. 2AS column) is around 2.