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Development of an Effluent Treatment Process for the
Silos Direct Encapsulation Plant
Thomas Jones
SDP Delivery Team / Nuvia Ltd
SDP Project
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Aims
• Explain the requirement for the Silos Direct encapsulation Plant (SDP)
• Describe the SDP process and how effluent is generated
• Outline how an effluent treatment technology was selected
• Explain the SDP Effluent Treatment Plant (ETP) process
• Outline some of the R&D trials underpinning plant development
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• MSSS store legacy ILW (magnox swarf and
misc β/γ waste including organic materials)
• Magnox swarf disposal ~ 1964 to 1992
• Building in care and maintenance pending
decommissioning
• Waste including organic materials has
degraded over time
• Magnox swarf decomposes in water to
produce Mg(OH)2 sludge and H2 gas
• Silos must be emptied and decommissioned to
remove a major hazard from Sellafield
• Suitable long-term storage solution for the
waste that meets modern regulatory standards
is required
Magnox Swarf Storage Silos (MSSS)
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The Silos Direct Encapsulation Plant (SDP)
• Sludge and small waste items
encapsulated by tumble mixing
within Undersize Mixing Vessel
(UMV)
• Large items are mechanically
removed and encapsulated by
flood grouting into product boxes
SDP will immobilise waste
retrieved from the MSSS in a
form suitable for safe long
term storage
3 m3 product box
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SDP Process
Undersize
Route
Receipt of
flask to
SDP
Waste
tipped into
UMV
Skip
washings
UMV
decant
after
settling
Waste
placed in
box linerOversize
Route
Liner
decant
Mixing in
UMV
Flood
Grouting
Mixture
tipped
into liner
Grout
cures
Wet grout
(GGBS & CEM)
addition
ETP
Concentrate
addition
Grout
cures
Capping
Grout
added
Bleed water decant
Bleed water
decant
Decant
Waste liner is
boxed, lidded,
decontaminat
ed and
monitored
before
transfer to a
product store.
Capping
Grout
added
GGBS and
CEM powder
addition
Capping
grout
cures
Capping
grout
cures
Decant
Wet grout
(GGBS &
CEM)
addition
Wet grout
(GGBS & CEM)
addition
GGBS: Ground Granulated Blast Furnace Slag
CEM: Cement powder
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Undersize Mixing Vessel
Prototype UMV UMV Mixing Station
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Requirement for Effluent Treatment
• SDP will generate ~ 20 m3 per
day of aqueous effluent
• Encapsulation of effluent
without volume reduction is
unsustainable
• Treatment of SDP effluents
will:
• Minimise the secondary waste
generated by SDP (fewer boxes
of waste generated)
• Minimise activity in liquid effluent
discharges (to meet sea
discharge limits)
Volume of effluent generated by
SDP exceeds volume of water
consumed via waste
encapsulation
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Selection of an Effluent Treatment Technology
• All available effluent treatment
technologies collated
• Immature technologies discounted and
all permutations of the remaining options
evaluated
• Progression of precipitation, solid liquid
separation, evaporation and ion
exchange technologies.
Chosen technology:
Solids Separation by Settling, Followed by Acid
Evaporation.
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Acid Evaporation
Carbonic Acid (H2CO3)
Bicarbonate ion (HCO3-)
Carbonate ion (CO3-2)
• Carbonates/hydroxides in feed
limit the achievable volume
reduction
• Addition of HNO3 acidifies effluent
and destroys carbonate (released
as CO2)
• Solubility of concentrate solution
and therefore the achievable
volume reduction (without solids
precipitation) is increased
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The SDP Evaporator
• ‘Simplified HA Evaporator’ design
(coils removed)
• Minimal entrainment and transfer of
contamination to the distillate
• Reduced pressure operation
(prevents corrosion & maintains
containment)
• Design / operating conditions to
minimise:• Fouling
• Foaming
• Solids formation
• Corrosion
Packing
Effluent
Feed in
Steam Condensate out
Low
Pressure
Steam
Evaporator
Concentrate
Out
Overheads to
condensers and
vacuum ejectors
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Outline of the Effluent Treatment Plant (ETP) Process
Evaporator
Condensers
Distillate
Collection
Vessel
Nitric Acid
Sentencing
Vessels
Sodium
Hydroxide
Condenser Overheads
to SDP for Abatement
and Discharge
Grout
Free
Effluent
SDP Encapsulation Process
Collection
Vessels
Buffer
Vessels
Transfer
VesselSettling
Vessel
Settled Solids for
Encapsulation
Neutralised
Distillate
discharged via
Existing Site
Effluent
Infrastructure
Grout
Bearing
Effluent
Sample Point
Sample Point
Sample Point
pH
Adjustment
Vessel
Concentrate
Collection
Vessel
Sodium
Hydroxide
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Small Scale Evaporation Trials
• Initial hot plate trials to determine
the maximum achievable volume
reduction
• 5 litre Heidolph Laborta 20
Evaporator trials to:• Underpin acid evaporation process
• Validate various models
• Optimise mode of acidification
• Determine effect of evaporator process
conditions on: • foaming
• solids formation
• acid carryover to the distillate
• Evaluate alternative process conditions as
potential mitigation against corrosion
• Determine the effect of silo organics and
process additives (e.g. foaming & fouling)
Heidolph
Evaporator
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Corrosion Testing of Evaporator Materials
• Evaporator Concentrate is the most corrosive environment on plant
• Corrosion trials undertaken by NNL specialists
• Significant concentration of Cl- and SO42- ions are present in nitric acid at
elevated temperature
• Preferred evaporator materials of construction (NAG 304L and Uranus 65
stainless steels) were both shown to be unsuitable for process conditions
• Alternative materials and process conditions evaluated
• Inconel 625 selected as material of construction for the evaporator and
concentrate vessels
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Fouling Rig Trials
• Evaporator throughput potentially
reduced by fouling of heat transfer
surfaces
• NNL trials underway using a
specially designed fouling rig
• Steam heated Inconel 625 plate
heat in contact with ‘test material’
process liquors
• Results to date show minimal
fouling which is removable using
HNO3
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Acknowledgements
I would like acknowledge all those who provided advice and assistance in
preparing my paper and presentation, in particular:
• Sellafield Ltd and the SDP Delivery Team for permission to present
• The National Nuclear Laboratory (NNL) & NSG Environmental Ltd (NSG)
for allowing inclusion of their SDP trials work
