VITRIFICATION DEVELOPMENT AND PRODUCT QUALITY

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VITRIFICATION DEVELOPMENT AND PRODUCT QUALITY

• N R Gribble
• Nexia Solutions
• RWIN 2005

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Regulatory Requirements

• Convert the waste into a monolith which is both
  safe and convenient for engineered storage
• Produce an assurance that that the product can be
  safely and conveniently transported from a store
  to a repository
• Produce an assurance that the product will
  satisfy anticipated disposal requirements
• Demonstrate waste complies with the National
  Strategy for Radioactive Waste Management

3
Product Quality Assurance

• In order to satisfy the Regulatory demands we
  need to demonstrate
• that when making products
• Process operates in a stable manner
• Waste incorporation within the matrix can be
  controlled to a high degree
• Process is tolerant to variations in plant and
  flowsheet conditions
• Satisfactory product over a range of conditions
  which lie within the plant control capabilities

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Assurance of Product Composition

• Two methods exist
• Sampling
• Prior demonstration of concise knowledge of feed
  composition and plant control during production

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Some Problems Associated With Sampling

• Complex operation to sample from some processes
• After the event
• How representative is the sample ?
• Difficulties defining the number of samples
  required
• Potential interruption of the process
• Analysis expensive and time consuming

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Approach to Product Quality Adopted

• Define the limits of product acceptability
• Define the process envelope at full scale
• Demonstrate process envelope on plant during
  commissioning
• Operate the plant within the defined envelope

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Development Work - Defining the Limits of Product
Quality

boundary for acceptable waste
limit of process operation
process envelope
process flowsheet
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QA Philosophy

• Product properties need to conform to those
  expected from inactive development and hence
  comply with the required specification
• Based upon controlling/recording data for
  variables impacting on product quality
• Adopt philosophy for development work to give a
  fully auditable experimental database
• Extends beyond plant operation

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Auditable Experimental Database

• All prime data, eg
• calibrations
• measurements
• instrument readings
• analytical results
• Work proposals / amendments
• Meeting / event records
• Run Schedules
• Reports

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The Objectives of the Development Programme

• Determine the characteristics of the waste
• Develop representative simulants
• Select an encapsulation matrix for the waste
• Produce a database of information from initial
  waste processing to disposal
• Establish the process envelopes
• Develop the case for product quality

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Waste Characterisation

• Characterisation of the waste material is the
  vital pre-requisite in any Waste Retrieval and
  Treatment Programme. An in-depth knowledge of
  the properties of the waste is required so that a
  meaningful process can be devised
• Allows development of realistic inactive
  simulants cost and time savings
• Any programme of work started without agreed
  base-line characterisation data is heading for
  trouble.

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Characterisation Strategy

• Comprehensive and detailed
• Determined chemistry and general properties of
  wastes
• Three Main Areas
• Physical Characteristics.
• Chemical Analysis.
• Radiochemical Analysis.

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Simulant Formulation -- 1

• Waste Simulation is a critical stage in the
  development of any process.
• Aid to process testing and development
• Inactive simulation enables large scale
  evaluation and testing of chosen processes.
• Most cost effective option.
• Minimise risk to projects.
• Simulants formulated to mirror range of
  properties relevant to process being evaluated -
  fit for purpose

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Simulant Formulation -- 2

• Simulant formulations based upon best available
  data from FISPIN, historical records, plant
  flowsheets and results of characterisation work
• Simulant formulations are not static. Reviewed
  on a case by case basis to ensure they are still
  relevant to work being done
• Simulant formulations used in process development
  work are peer reviewed by independent body prior
  to use.
• Peer Review Group made up of people with
  relevant background and experience
• Agreed with customer

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Factors Influencing the Choice of a Suitable
Matrix

• Nature of the waste
• Flexibility of the matrix to variations in waste
  composition
• Waste content in final product
• Nature of any additives in waste
• Leach resistance
• Radiation stability
• Thermal stability
• Compatibility
• Mechanical strength
• Final storage/Disposal system
• Product integrity with time
• Previous experience

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Reasons for choosing glass

• Majority of Alternatives cannot withstand
  radiation and heat
• front runners
• glasses
• ceramics (although less tolerant to waste
  composition change)
• synthetic minerals (early stage of development -
  unproven)
• Glass option preferred
• ease of processing
• quality/stability of product
• available technology

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Glass Network Structure
(A)
A) Crystalline SiO2
B) Glassy SiO2
(B)
C) Mixed glass with M ions.
(C)
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Why borosilicate glass

• Borosilicate glasses preferred world-wide
• formation temperature / favourable reaction
  kinetics
• tolerance to wide range of waste compositions
• low expansion
• less corrosive (to melters)
• durability

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Structure of the Development Programmes
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Small Scale Inactive Work

• Defined the range of limiting parameters of
  product composition and properties
• Extended from simplified to detailed inactive
  simulation
• Exceeded the boundaries of the process to fully
  understand the behaviour of the product
• wide range of process conditions
• feedstock variations
• key elements
• incorporation level
• Identified simulants for full scale work
• Exceeded the boundaries of proposed full scale
  operation
• Used active sources to assess product radiation
  stability

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What properties are of interest?

• Viscosity
• Density
• Homogeneity
• Insoluble solids content
• Glass transition temperature
• Thermal conductivity
• Leach resistance
• Thermal stability

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Crystallisation

• As glass is metastable, above Tg, it will form
  crystalline phases. Quantities are dependent on
  time and temperature and composition.
• For historical reasons, we choose a temperature
  of 650C to characterise crystal growth.
• The amount of crystallisation is measured by
  microscopy/image analysis means, after grinding
  and polishing the surface of a thin-section.

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Typical Microstructures
As-cast
Heat-treated
1 total solids
5 total solids
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Soxhlet Leach Test
BLR Sample Wt. Loss S.A. x Time
ELR El. Wt. Loss x Wt. Sample S.A. x Time
Wt. El. in Sample
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Soxhlet Leach Test
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Small Scale Active Work

• Confirmed the accuracy of small scale inactive
  simulants
• hard to simulate some active components
• Ranged from trace active to fully active material
• Provided important link between inactive work at
  all scales and the fully active vitrification
  plant product properties

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Creation of a Process Envelope

• Cannot be determined by Laboratory studies alone
• Requires a pilot plant facility
• Laboratory studies define initial process
  envelope and highlight limitations
• Process envelope established on pilot plant
• Process envelope mapped onto Vitrification Plant
  during commissioning

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Description of Vitrification Process
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Large Scale Inactive Work --1

• Established and confirmed flowsheet
• Determined accuracy and precision of delivery
  systems
• Developed understanding of the plant
• Defined the process envelope within which PQ is
  guaranteed
• Demonstrated process stability and
  reproducibility over extended periods of
  operation
• Demonstrated ability of plant to respond to
  operating conditions and process disturbances (eg
  feed delivery problems, temperature and sparge
  variation, compositional variation)

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Large Scale Inactive Work --2

• Established off-gas system DFs and secondary
  wastes
• Determined differences between large and small
  scale processes and/or product properties
• Determined homogeneity by sampling throughout the
  product
• Determined incorporation by analysis
• Measured chemical and thermal stability of
  selected samples
• Confirmed links with small scale inactive
  laboratory studies and plant commissioning data
• Long term assessment of full scale inactive
  products
• Provided process knowledge and understanding

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Calcination Conditions

• Optimisation of calcination temperatures
• Tube temperature profile
• Rotational speed
• Waste type variation
• Throughput
• Sugar concentration (ruthenium retention)
• Mal-operations

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Product Analysis - Calcine

• Bulk density
• Particle size
• Residual nitrate content
• Weight loss at 1000 oC
• Chemical analysis
• Solubility experiments (chemical characteristics)
• Reactivity (with base glass)

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Melter Operations

• Waste variation
• Key element variation
• Incorporation rate
• Throughput
• Base glass delivery
• Temperature variations
• Sparging (mixing trials)
• Mal-operations

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Vitrified Product Container
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Non-Active Product Sampling
x
x
x
x
x
x
3 positions / pour 10 pieces / position 30
pieces / pour
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Non-Active Product Analysis

• Techniques used
• density determination
• glass analysis (elemental)
• visual assessment
• SEM studies
• stereo microscopy
• optical microscopy
• image analysis (volume fraction of solids)
• variety of physical property measurement
  techniques

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Vitrified Waste (WVP Commissioning)
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Commissioning - RD Aims

• Provided input into commissioning test programme
• Supplied all simulant (gt100 m3)
• Monitored performance in critical areas
• melter
• calciner
• glass and HAL feed systems
• incorporation level control
• off-gas performance
• product quality
• Provided independent review of plant and product
  data
• Proved similarity of performance between test
  rigs and WVP
• thus demonstrate applicability of development
  work

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Purpose of the VTR

• To increase the throughput of WVP lines 1,2 and 3
• To provide underpinning Product Quality data to
  process changes
• To improve understanding of the waste
  vitrification process and the limitations of the
  operating envelope
• To identify and validate process improvements
• Use infrastructure to develop alternative
  technologies

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VTR Calciner
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Summary

• Determined the characteristics of the waste
• Selected the right matrix
• Investigated waste-matrix interactions
• Understood the effects of scale up
• Determined Process Envelope based on many
  integrated activities
• Defined Product Quality envelope
• Identified plant operational limitations