Technical Challenges in the Immobilisation of UKAEA ILW

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Technical Challenges in the Immobilisation of
UKAEA ILW

• Michelle Wise, Ann Goldsmith

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Outline

• UKAEA Policy on Waste Management
• Overview of UKAEA Wastes
• Example Technical Challenge

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UKAEA Policy

• Compliance with regulator requirements and
  expectations
• Aim for passive safety
• Prioritise on basis of hazard, aim for
  progressive reduction
• Demonstrate BPEO/BPM as required
• Conditioned wastes consistent with future long
  term management

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UKAEA Sites
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Overview of Wastes- Solids

• Stored, Untreated Wastes
• Often incomplete records
• Inappropriate storage/ disposal arrangements
• silo Dounreay shaft
• mild steel containers
• designed without sampling in mind
• Sea disposal packages
• Wastes from decommissioning old facilities
• Incomplete records
• Time drivers
• need to avoid creating new problems
• Fuels

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Harwell RHILW

• Historical waste in Tube Stores
• concerns over condition (mild steel containers)
• uncertainties over contents
• Accessibility
• retrieval machine, further requirements
• Waste Requiring Additional Treatment (WRATs)
• records may be misleading
• Assay

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B462 Head End Cells
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Dounreay Silo and Shaft

• Retrieval even more difficult
• Throughput orders of magnitude higher
• Little or no link to records
• if contained, no numbers!
• Parallel sludge and solids treatment
• little sludge characterisation
• Criticality and analysis issues
• Have to think in completely different way

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Overview of Wastes- Liquids

• Minor process residues
• organic solvents, may be in solid waste stream
• Floc from Effluent Treatment
• Harwell Sludges borderline LLW
• Specific Waste Streams
• Harwell Liquors
• Winfrith Sludges
• Dounreay Raffinates
• New waste streams
• e.g. from NaK/ sodium treatment

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Harwell Sludges

• From floc treatment of low-medium liquors
• Accumulated over decades- settled
• Main problem is mobilisation of solid mass
• are samples representative
• New plant designed and built
• Incorporates active cementation lab

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SGHWR Sludge Issues

• Variety of Sources
• ion exchange material (Powdex)
• filter aid
• decontamination operations
• Radionuclide Inventory
• analysis to date not comprehensive
• difficult to predict
• questionable how representative
• Chemical Inventory
• not comprehensive, e.g. wrt complexants

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Resolution

• Trials to develop envelope using real samples
• takes into account ageing
• formulation based upon solids range
• Conservative scoping inventory estimates
• Scoping encapsulation trials to examine effect of
  complexants
• Comprehensive sampling and fingerprinting
  protocol for retrieved wastes
• Test blocks from each batch

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Dounreay Process Liquors

• High Risk- Drive to Process
• Removal and Treatment of Liquors
• PFR Raffinate (203m3)
• DFR Raffinate (219m3)
• MTR Raffinate (807m3)
• currently being encapsulated in DCP
• ADU Floc (116m3)
• Heel in Tanks
• getting it out
• content
• chemical and radiological

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HALS Tank
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Example Challenge- PFR Raffinate

• PFR fuel reprocessed at Dounreay
• 1980-1996
• Fuel in basket dissolved in conc. nitric
• basket introduces Zn and Cu
• Solvent extraction used to separate U/Pu
• PFR raffinate is the aqueous effluent from the
  first stage washing
• numerous batches, with different sources
• batches/ contents concentrated by evaporation

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Composition of Raffinate

• Uncertainty
• only old samples- further additions/ removals
• analysis not comprehensive- e.g. no H-3, acidity
  or Tank 15- potentially inconsistent
• re-analysed/ re-assessed- found chloride
• re-sampling planned
• Previously considered as HLW on basis that future
  campaigns would process higher burn-up fuel and
  be concentrated further- this didn't happen

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Options for Immobilisation

• Vitrification
• standard for HLW
• until recently reference strategy
• difficult to transport, or require gt40M plant!
• Cementation
• feasible because heat output is relatively low
• tanks incorporate cooling, but not required
• product would be within Nirex limits, i.e. can be
  considered as ILW, not HLW
• DCP unsuitable (inflexible), DCP2 required

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Wasteform Development Lab at Harwell
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Small-scale Scoping Trials

• Preparation of simulant cement mixes at 150
  ml scale
• use Kenwood Chef and polypots
• Determination of time to initial set
• Measurement of mix viscosity
• Measurement of bleed water
• Observation of change in physical appearance

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Variables Examined

• Options for neutralising acid
• NaOH solution
• Ca(OH)2 powder
• mix NaOH and Ca(OH)2
• Cementation trials with
• (over) neutralised nitric acid solution
• neutralised worst-case simulant liquor
• effects of key cations independently
• 11 PFAOPC and 11 BFSOPC

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Results

• PFA performed better than BFS
• 75 Ca(OH)2 (where precipitation started),25
  NaOH sequence best- no excess
• Effect of cations varied with cement and with
  neutralising agent
• in one case, Zn delayed set gt14 days
• Defined envelope for confirmatory tests

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Formulation Working Envelope
1.221
C "fluid mix
recommended formulation
PFA/OPC Ratio
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A
B "thick mix"
0.821
0.40
0.42
0.38
W/S Ratio
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Product Evaluation Tests

• Larger scale mixes of about 1 litre
• Profile of heat release
• Mix viscosity/ rheology
• Matrix density
• Compressive strength
• Dimensional stability
• measure expansion/ contraction
• Confirm previous measurements
• Bleed water, Initial set time

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Calorimeter
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Viscometer
Paddle and Pot
Automatically generates rheogram
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Compression Testing Machine
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In-situ Microstrain Measurement
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Rate of heat release
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Cumulative heat release
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Microstrain measurements
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Conclusions

• UKAEA has a range of historic wastes, and wastes
  from new operations
• design process to minimise WM problems
• Unique problems presented by relatively
  small-volume (sub)streams
• Frequently difficult to characterise
• Develop robust formulation envelopes
• Need for on-hand cementation and analytical
  facilities