Nuclear Semester 2 - Lectures 5-8

Define legacy waste

Waste generated during early reactor programmes in 1940’s and 1960’s

Brief history of Sellafield

• One and a half square mile complex

• Magnox Pile reactors and first fuel reprocessing plant during late 40s and 50s for weapons

• First civil electricity from Calder Hall opened in 1956

• Second gen fuel facility in 60s THORP in 90s

Describe Sellafield waste legacy

• 10 Magnox stations by 1966

• Increase pressure on storage facilities for spent nuclear fuel

• Cladding under water in silos

• Fission product stored as concentrated acidic waste

• Outdoor cooling ponds for spent fuel

• Facility open to the elements, dust dirt algae other biomaterial, partial/full corrosion of materials, sludge formation

Describe Hanford waste legacy

• Over 1600 different waste sites

• Main waste are precipitated secondary products from fuel reprocessing

• Over 2 million cubic meters of active sludge/effluent wastes generated between 40-80s

• More stored in 150 temporary single shell tanks

• Significant leaks has led to major concerns with contamination of water

Describe Le Hague and its waste

• Built in 1967 to reprocess plutonium for weapons

• Currently one of the worlds largest fuel reprocessing facilities

• Reprocesses fuel from Europe and Japan

• Plutonium turned into MOX fuel at Marcoule

• Reprocessing increased in 70s and 90s

• Waste generated from fuel reprocessing and related effluent treatment

• Uses co-precipitation method to remove radioactive ions from liquid combined with filter media and ion exchanges

Describe the types of waste

Liquids (effluents, extraction solvents, organic residues)

• Historical liquid effluent run-off into rives and oceans have been main environmental and health threats

• Modern effluent treatments can handle most dissolved radioisotopes

Sludges/slurries

• Precipitated salts, captured coagulated material, fuel cladding breakdown sludges

• Legacy sludge waste represents the major nuclear legacy problem for UK and USA

What are the origins of legacy sludge wastes?

Fuel reprocessing

• Main origin for USA legacy - varied precipitated salts and fission products from early fuel reprocessing

• Continued generation at current processing facilities world wide (THORP)

Effluent treatment

• At all stages of fuel cycle, effluents must be treated

• Flocculated/coagulated solids with bound radionuclides from early treatments

• Contaminated sand filter media/ion-exchange media from modern treatment systems

Fuel/Cladding storage

• In US/UK, early high volume elemental U fuel used

• Often stored long-term with fuel cladding in unsuitable silos and open ponds

• Major legacy problem for the UK

Describe liquid and aerial effluent regulation

• The regulation of non-radioactive discharges and disposals is the responsibility of the environmental agency and local authorities who regulate discharges under EPR 2010

• Control of radioactive wastes is subject to the provisions of the environmental permit for radioactive substances

• BSS 1996 relating to dose limits were incorporated into UK law

• BAT(NEEC) All discharges of radioactivity are subject to the requirement to use Best Available Technique to limit radioactive discharge

Annual dose of radiation to the public

• Natural background ~84%

• Medical ~15%

• Average dose is around 2700 uSv of which

  • 2230 natural

  • 410 medical

  • 6 occupational

  • 6 nuclear weapon fallout

  • 0.9 discharges and disposals

  • 0.1 cosumer product

What are the three main processing techniques of legacy liquid effluents

• Co-precipitation methods

• Ion-exchange/physical adsorption

• Membrane processes

What are the main mobile fission ions of concern for liquid effluents?

• Caesium-137

• Strontium-90

• Other trace fission products (Ce, Ru, Ba, Sb)

• Trace actinides

Describe the process of precipitation removal via co-precipitation reactions

• Various chemical precipitation reactions exist with differing affinity for particular elements

• Use co-precipitation ions to form main coagulants, as fission ions are not in highly enough concentrations to precipitate

• Radioions removed by two processes

  • Substitute for main precip ions and precip along with main reactions

  • Removed via physical adsorption onto formed precip

What is good for Caesium removal?

What is good for Strontium removal?

What is good for Strontium and actinide removal?

Describe the Ferric sulphate coagulants

• Iron salts

  • Need to pre-treat with quicklime and make pH neutral

  • Main processes at EARP for actinide removal

  • Forms highly voluminous sludge relatively easy to remove via traditional separation techniques

• Downsides - quite pH dependent, must neutralise first

Pros of co-precipitation methods

• Work well in high salt and variable environments

• Often do not have high selectivity

• All require further solid-liquid separation processes to produce clean water

• Low relative decontamination factors

Descirbe Ion-exchange as a method of processing liquid effluents

• Common in nuclear industry to use natural zeolite material

• Standard is clinoptilolite

• Zeolites have complex crystal structure with interstitial gap sizes that can trap ions at high capacity

• Zeolites also have large non-specific surface adsorption sites, hence removal is adsorption and exchange

Describe the IOX comparison for clinoptilolite

What is the langmuir isotherm equation

Cs+ - Qo = 214.1

Sr2+ - Qo = 98.13

What is the Freundlich isotherm equation

Cs+ - n = 2.529, Kf = 6.897

Sr2+ - n = 2.893, Kf = 7.566

Describe IOX column breakthrough kinetics

• Experiments in column tests can be also used to estimate ion-exchange capacity, but also breakthrough kinetics

• Normally, small scale columns are used and the ratio of the time outlet concentration (Ce) is measured against the initial concentration (Co) over time

Describe same ppm but one is Cs and one is Sr graph for IOX breakthrough

30 mins column residence time

Describe the same element but different ppm for 30 min residence time for IOX breakthrough

100 vs 200 ppm

Describe same element and same ppm but different residence time for IOX breakthrough

30 min vs 15 min

What is the Thomas model for IOX breakthrough

For the Thomas model, what is the plot, slope value and intercept value?

What is an enhanced actinide removal plant?

• Commissioned in 1994 to treat medium active concentrates and low active effluents with high levels of Fe

• pH adjustment - to 7-8 which precipitates Fe as Fe(OH)3 floc

• Cross flow filtration in 2 stages with ultra filters - final solids content is 50-100g/L

• Iron hydroxide co-precipitates with strontium and other fission products

• Powdered IOX can be added in batch to remove any caesium

• Complex aggregate formed

Describe a flow chart for Sellafield’s enhanced act rec plant (EARP)

Describe a cartridge filter design for EARP

• Cross flow ultrafiltration

• Can remove particles in the range of 0.001 micro-m to 0.1 micro-m

• Filter median is usually paper or other fibrous materials reinforced with a metal mesh

What is a SIXEP?

Site Ion Exchange Effluent Treatment Plant

• Commissioned in 1985 to treat Magnox fuel pond liquors

• Pressure filtration - sand bed filters to remove solids (Mg OH sludge)

• pH adjustment

• IOX - inorganic exchanged (clinoptilolite) giving DFs of up to 500 for Sr 90 and 2000 for Cs 137

Describe sand bed filter for liquid effluents

• Macroporous

• Can remove particles >10 micrometres

Ion exchange

• Media chosen for specific selectivity

• Finite IOX capacity

• Reduction in IOX efficiency over time

• Instantaneous DF could reduce from 2000 to <10

• bed replacement

Describe the Hanford tank side caesium removal system (TSCR) project

• Currently commissioning microfilter and IOX system

• Mobile station wil sit on the tank farms and remove elevated levels of Cs+ from low activity tank liquours

  • Minimum of 643k litres of waste from tanks will be treated and at least 3.7 × 10^15 Bq of 137Cs will be removed in this phase

  • Second phase will process up to 18.9 × 10³ m³ (5 Mgal) of waste feed

• IOX column uses crystalline silicotitanate (CST) IX median produced by UOP Honeywell

• Initial Steel membrane nanofilter, to ensure no solids past the IOX

Describe the focus on Fukushima for treatment

Treatment of highly radioactive effluent from reactor cooling waters (mainly sea water)

Multiple treatment systems set-up in stages

• Initial process was co-precipitation

  • Main process was to remove Cs

  • Metal ferrocyanide process

  • Low Sr removal and high level of secondary wastes meant new long term plans required

• Multistage co-precipitation and IOX plant installed (Toshiba, Kurion)

  • Initial stage uses Honeywell IOX materials for enhanced Cs removal

  • New high performance multinuclide removal usits uses iron hydroxide and carbonate co-precipitation and new proprietary IOX

What is SARRY?

Simplified active water recovery system - enhanced Cs removal (IOX)

What is ALPS?

Multinuclide removal facility - Secondary precipitation and IOX system for remaining radioactivity removal

How will legacy sludges be processed? (basic)

Sludge wastes must be transferred, processed and categorised before being sufficiently treated to allow cementation

• Sludge must be thickened to allow proper binding with cement

• As sludges are highly heterogeneous, very difficult to product a cement recipe that will work

• Both sludge composition and water content almost impossible to fully categorise

Another option is to mimic HLW processing, completely drying sludge to powder to allow easier cementation. Very energy intensive and requires off gas treatments

Why are sludges difficult to process?

Complex multiphase, multicomponent systems

• Creates very heterogeneous sludges with a large envelope of physical and chemical properties

A lack of well defined historical characterisation data

• Difficult to fully understand what is really present

• Site toxicology and radiology greatly limits sampling

Long term environmental changes

• Sludges constantly aging and changing

• Open air environments leads to potential high biological and organic contamination

Explain Inline cementation for sludge processing

Developed waste route, based on a wet-sludge mix being cemented

Wastes relatively ell characterised, as generally have been independently contained and separated

Different cementation routes must be designed for different sludge types

Briefly describe the 8 steps of the disposing of Hanford’s nuclear waste

1 - Liquid radioactive waste will be pumped from underground storage tanks into the low activity waste pretreatment facility

2 - Low activity waste pretreatment facility reduces the level of Cs and solids in the liquid waste, then piped underground to the vitrification facility

3 - The waste pipe runs into a manifold over a below-ground drain tank before reaching teh vitri facility. Drain tank allows transfer pipes to be gravity flushed after each waste transfer

4 - Waste is piped into concentrate receipt tank, each transfer is 9000 gal

5 - Waste is pumped into a mixing tank and combines with silica and other glass forming material

6 - The silica mixed waste is pumped into one of the two melters and heated to 21000 degrees farenheit overall several days forming molten glass

7 - Molten glass is poured into canisters, glass is cooled for several days until solid

8 - Canisters are sealed and exteriors are decontaminated, making them ready for permanent disposal

Sludge issues at Sellafield

• More than 90% of the nuclear hazard potential on the site

• 22% of all site programmes

• 35% of the total site costs during next 4 years

• 77% of the major project costs during the next 4 years

Describe Magnox Swarf Storage Silos (MSSS)

Original silos for de-canned Magnox fuel canister swarf

A number of very large waste deposits totaling 10000-15000

• Currently preparing for emptying, as part of risk reduction programme

• 20+ year programme to extract MSSS wastes using overhead grabs

• Waste to be filled in 3m³ boxes as interim storage pending final disposal

• Chronic corrosion of magnesium based wastes presenting 2 major issues:

  • Volume expansion

  • Pressurisation

• Procurement cost anticipated to be >£250m

What is an FGMFP?

• Main storage for fuel and cladding during civil power gen in 1960s

• Most fuel decladding in MSSS, but some 1500m cubed remains

• Cladding broke down into Mg (OH)2 based sludge

• Sludge retrieval for sludge packaging plant

• Issues

  • High and variable yield stress

  • Large particle size distribution

  • Open air pond - miscellaneous organic activity

What is a PFSP?

Pile fuel storage pond

• Made for temporary storage of windscale pile fuel for processing

• Waste consists of fuel, sludge and other debri due to open air pond

  • Sludge is low volume and yield stress

  • However very high organic and algal content with very complex behaviour

  • Complex heterogeneous aggregates

• Current plan is local sludge treatment plan

  • Sludge at 10% wt in corral transferred to buffer tank

  • 20% seen as max safe limit

  • Benefit - no need for further thickeners or dewatering

  • Cost - All efforts to consolidate sludge in corral only to tilute

What are HASTS?

Highly active storage tanks

• Impinging jet buffer tanks to hold highly active liquor by-product from THORP fuel reprocessing

• Mainly dissolved species - however precipitated fission products of Cs phosphomolybdate (CPM) and zirconium molybdate (ZM)

How do we categorise sludge systems?

Critical process zones : Zone 1

• Usually use form of liquid jets or eductors

• Critical to characterise jet particle - bed interaction

  • Consolidated bed eroded through high jet velocity pressures and shear

  • Eroded particulates must be mobilised sufficiently to allow transport

  • Erosion depends on YS, particle properties and jet properties

• Critical to understand jet dilution on sludge

  • Most sludge transported at up to 5 vol%

  • High dilution = High volume of effluent treatment

Give an example of zone 1 in critical process zones

HASTS (High active storage tanks)

• Erosion and mobilisation

  • Bed critical shear stress (Tc)

  • Particle cohesion, size, density, agrregation

• Particulate re-settling and transport

  • Size and density

  • Concentration

  • Suspension rheology

  • Turbulent interaction with jets and air-lifts

Normalise erosion

• Can consider erosion either

  • A momentum/pressure force acting on single particles

  • A shear force acing on a semi-solid homogeneous fluid of a given critical shear strength

Equation for erosion from particle basis

Equation for Final erosion length or radius

Critical relationship between sheer stress and erosion length

Equation for dimensionless critical bed shear stress

Describe zone 2 of the critical process zones

Sludge transport

• Critical balance between the fluid flow properties and particle properties

  • Require flow velocity to be high enough to maintain particles in suspension

  • If too great, the level of shear may break down the waste aggregates and structurally change the particle properties

• Normally consider balance of reynolds number (flow) and Froude number (particle) properties to understand critical deposition velocities

Describe zone 2 expanded theory

• Critical to characterise particle sedimentation re-suspension in pipes

  • Very complex turbulent interactions

  • High particle volume = more energy to suspend but increased hindered settling interactions

• Main variables to consider

  • Particle properties : size (d), fluid density (ro f), particle density (ro s), and concentration (phi)

  • Flow properties: critical flow velocity (Uc), kinematic viscosity (v), gravity (g)

• Therefore: number of variables = 7, number of dimensions = 3, number of dimensionless numbers to describe = 4

• Consider density ratio S = ps/pf as one parameter

Reynolds number equation

Describe zone 3 of the critical process zones

Sludge separation

• Sludges will separate and consolidate in storage

• Some form of separation will be required to thicken sludge and give consistent mixture to waste treatment plant (WTP)

• Sludges will change behaviour from that in ponds and silos due to pumping and transportation, hence prediction is difficult

• HASTS - settling of CPM, ZM, and ZMCA - critical to evaluate the rate of using jets for erosion

What is the Richard-zaki equation?

CPM - faster settling in low concs but hinders more as conc increases

ZM - sloer low conc settling, but does not hinder as much with conc increase

Richard-zaki in log form