Nuclear Semester 2

Define open fuel cycle

Fuel is used once in reactor, wet interim stored and ultimately dry stored for disposal.

Define partially closed fuel cycle

Fuel is reprocessed to separate U, Pu, U-Pu made into MOx. Main U not recycled.

Define fully closed fuel cycle

Reprocessed U-Pu made into MOx fuel and  U fuel fully recycled.

Chemical formula for yellow cake

U3O8

Explain the process of Uranium mining

• Ore is crushed and ground

• Acid leaching is predominantly used (H2SO4)

• Uranium oxidised to get it into hexavalent form

• Leaching solution is fltered and concentrated using solvent extraction

• Precipitated as yellow cake 70-80% using ammonia/magnesia/caustic soda

Explain how to get from yellow cake to fuel ready Uranium

• Yellow cake (U3O8) to UO3 conversion and purification

• Conversion of UO3 to UF4

• UF4 to UF6 (high temp)

• Enrichment of U235 - leads to significant depleted uranium by-product

• Back conversion of UF6 to UO2 granule solid for fuel

Give the advantages and disadvantages of a closed fuel cycle

ADV

• Smaller final waste volumes

• Lower long term inventories of Pu

• Reduced requirement for new U mining

DIS

• Cost

• U price not increased sufficiently to warrant recycle

• Nuclear proliferation risk

Explain why open fuel cycles help nuclear proliferation

• Pu stockpiling due to non recycling

• If Pu is not separated, fuel cannot be used for nuclear weapons

How long do we have to store spent fuel for?

• At reactor cooling for 12 months

• Central pond facility - cooling for reprocessing and handling

• Long term (decades) usually dry (prior to disposal)

Describe decay heat

• Created from beta and gamma emissions from fission products

• Amount and type of fission depend on number of factors

  • Initial enrichment

  • Irradiation time

  • Irradiation history

  • Cooling time

Compare wet and dry storage of fuel

• Wet store - active system, requiring engineering systems to maintain environment

• Dry store - passive system requiring no intervention

  • Getting fuel completely dry is difficult

  • New storage racks holding 63 fuel cans in THORP will allow greater storage capacity for wet

  • Number of routes to longer term dry storage being assessed

Generally, how do we dispose of spent fuel?

• GDF (Geological Disposal Facility)

• Multiple barriers

• Secure against unauthorised removal of nuclear material

• Has to be safe for 1000s of years

Advantages and Disadvantages for solvent extraction

ADV

• Safety

• Mature technology

• Low cost

• Scale up easy

• Selectively good

• High purity achievable

• Low temp

• Re-use solvent

• High HM loading capacity

DIS

• Degradation of solvent

• Entrainment

• Not always CHON only

• Waste management

What are the general principles of solvent extraction?

• Use affinity of solute for specific solvent to concentrate solute

• Reverse operation to recover solute

MIX - SETTLE - SEPARATE

What does PUREX stand for?

Plutonium Uranium Reduction EXtraction

PUREX Flow sheet

What are the equations for the U and Pu extraction? (reversible)

Equation for equilibrium constant for Uranium extraction

Why is the equilibrium constant of U extraction favoured by high NO3-?

Le Chatelier’s principle, higher nitrate core will push equilibrium to the right, run normally at high acid concentrations 3-5 M, runs with high TBP (20-30%)

Describe the partitioning of U and Pu (PUREX)

• Based on reducing Pu(IV) to Pu(III) which is an inextractable in solvent

• U remains in the solvent phase, the Pu goes into the aqueous phase

• The reductant used is uranous nitrate (U(IV)), stabilised by hydrazine

Describe the back extraction of U (PUREX)

• No need to change valence state

• High back extraction greatly favoured by diltue NO3-

• Run back extraction using very dilute acid, will shift the reaction towards the release of U complex, run 0.01-0.1M acid in back extraction

What are the downsides to TBP?

• Very susceptible to radiolitic breakdown, owning to phospohorus atom

• Leads to breakdown products that are less efficient, and limits TBP recycle

• TBP is not very volatile, good for safety perspective but difficult to recycle via thermal separation

Describe pulsed columns

• Approx 1m of column needed for one theoretical stage

• Requires large amount of head space but little floor space

• Moderate solvent inventory

• Long residence times

Describe mixer settlers

• Physical and theoretical stages nearly equivalent in efficiency

• Simple to design and operate

• Large settler volume

  • long residence time

  • high process inventory

• Poor geometry for critically (high volume hold up)

• Tolerant of solids

Describe centrifugal contractors

• A process intensified PI technology

• Each unit near one theoretical stage

• Requires little headroom or floor space, requires remote maintenance capability

• Small solvent inventory

• Short residence times

Overview of the PUREX process

Describe step 1 of the PUREX system (Fuel Dissolution)

• Shear assembly to expose fuel

• Dissolve fuel in nitric acid

• Centrifuge dissolver liquor to remove solids

• Store clarified liquor for monitoring and feed to solvent extraction

Describe step 2 of the PUREX system (Solvent Extraction)

• Primary separation

  • Separation of fission products from U/Pu

  • Separation of Pu from U

  • Decontamination of solvents

  • Treatment of fission product stream

Describe primary separation (PUREX)

• Extract U and Pu into solvent phase

• Split U and Pu, U in solvent, Pu in aqueous

• Clean Pu steam of any carried over solvent

Describe Uranium purification (PUREX)

• Decontamination of U stream from Np

• Decontamination of solvents

• Uranium purification

  • Initial U and Np back extraction to aqueous

  • Adjust temperature/conc to ensure U and Np in correct oxidation states

  • Solvent extraction to remove Np from U

  • Back strip with dilute acid to recover U

Describe Plutonium purification (PUREX)

• Decontamination of Pu stream from Tc

• Pu nitrate evaporation

• Decontamination of solvents

• Plutonium purification

  • Use NOx to oxidise Pu(III) to Pu(IV)

  • Solvent extraction to separate Pu(IV) from Tc

  • Backwash Pu(III) into acid

  • Clean out any entrained solvent from acid

  • Evaporate to reduce volume for storage

Describe Uranium finishing (PUREX)

• Steam strip acid to remove entrained solvent

• Evaporate to remove water and acid

• Use hot air to breakdown U nitrate to U oxide

• Cool, blend and store U oxide powder in drums

Describe Plutonium finishing (PUREX)

• Final conditioning to ensure Pu(IV)

• Add oxalic acid to precipitate Pu

• Separate liquid and solids

• Dry and calcine to convert solids to Pu dioxide

• Blend, sample and store in cans

• Wet Pu oxalate is fed continuously to a 2-stage drying calcining furnace

• Drying/calcination operates at up to 600 degrees celcius in electrically heated furnaces with counter current air flow

Define heterogenous recycling

U, Pu actinides as separate products

Define homogeneous recycling

U, Pu actinides in a single product

Describe homogeneous recycling

GANEX 1st Cycle

• Solvent extraction process

• Use of ligand that only extracts Uranium

• DEHiBA in diluent

• Still have nitric acid dissolution = head end is the same

• Uranium is still extracted as a UO2 uranyl ion

GANEX 2nd Cycle

• EURO-GANEX is a solvent-extraction process aiming to co-separate all transuranic elements

• For this, a solvent consisting of TODGA and DMDOHEMA in kerosene is used

Define adsorption

Mass transfer from liquid/gas to solid phase

Define leaching

Mass transfer from solid to liquid phase

Define solvent extraction

Mass transfer from one liquid phase to another

Pros and Cons of solvent extraction (SX)

Pros

• Using the correct ligand is generally straight forwards as long as it dissolves in the solvent

• High ligand concentration usually okay if solubility/miscibility in solvent/diluent is high

• Chemicals are usually recoverable

• Superior when the target species conc in water is >2g/L

Cons

• Generally unavoidable entrainment of organic and aqueous phases

• More prone to degradation

Pros and cons of adsorption

Pros

• Adsorbent regeneration is generally straight forward

• Less pron to degradation

• Generally superior for anion separation

• Superior when the target species conc in water is <2g/L

Cons

• Attatching the correct ligand to the adsorbent not always easy

• Cannot have a very high ligand concentraion or physical properties are affected

• Can last long time but no generally recycled

Define ionic bonding

Complete transfer of valence electrons between bonding partners to create a lattice of cations and anions

Define covalent bonding

Chemical bonds are formed from both bonding partners sharing valence electrons

Define coordinate bonding

Chemical bonds are formed from one bonding partner donating valence electrons

Difference between trans and cis ligands

Trans are on opposite diagonals, cis are on the same side of a complex

Explain HSAB theory

• Dissolved metal ions in solution are considered lewis acids (capable of accepting a lone pair to form a chemical bond)

• Likewise, potential ligands are lewis bases (capable of donating a lone electron pair to form a chemical bond

• Uranyl *UO2 (2+) is slightly unusual, being a polyatomic cation

Describe hard acids and bases

• High charge density

• Not very polarizable

• Bonding interactions more ionic in nature

• Smaller ionic radius (relative to mass)

Describe soft acids and bases

• Low charge density

• Highly polarizable

• Bonding interactions more covalent in nature

• Larger ionic radius

Given some examples of soft hard and intermediate acids/bases

Define and describe chelation

A chelation or cherlating interaction occurs when a single ligand bonds to a metal ion via multiply heteroatoms

Bonding can be very strong, and its sometimes not a helpful for processes such as solvent extraction

Common chelating agent is EDTA

Active ingredient in chemical first-aid sprays found in labs

Describe selective separations

• Uses a carefully chosen ligand to target specific chemical species

• Applications usually based on removing just the valuable component of a wastestream

• Reagents can sometimes be expensive and/or toxic

• Example : Leaching of gold mind tailings

Describe Non-selective separations

• Large variety of chemical species transferred from one phase to another

• Applications usually based on removing hazardous components of a wastestream, prior to discharge to environemnt

• Example : ion-exchange column to produce potable water

Equation for the distribution coefficient (AKA partition coefficient) for separation processes

Equation for the separation factor

What are the requirements of a good organic phase for nuclear separations?

• Able to extract some components to a larger extent than the rest of the components in the aq phase

• Reaction should be stable but reversible

• The compound formation should be easily reversed so that the solvent phase can be reused

• Density of solvent phase at all times should be different from the aq phase to effect the separation

• Inexpensive and cost effective with more than one supplier

• Low toxicity

• Low viscosity to aid mixing

• High boiling point

Why is TBP a good ligand for PUREX?

• P=O bond is strongly polarised meaning electron rich

• Hydrophobic alkyl groups allow for good solvation in the organic phase

• Hydrophobic parts of the complex interact with the organic phase and keep the hydrophilic parts contained

Describe acid hydrolysis and radiolysis of TBP

Describe the degradation of TBP by hydrolysis and radiolysis

Advantages of CHON ligands

• Still prone to radiolysis but

• Degradation products have low affinity compared to other fission products

• Have relatively low boiling points so no issues residue when spent solvent is incinerated

• More selective for U/Pu than TBP, overall lower capacity due to solubility limits of the complexes in the organic phase

What is the importance of relative densities of phases?

• In SX, must always be a density difference between the aqueous and organic phase

• System must also account for the changes in phase density that occur during mixing, due to mass-transfer

• Phases being close in density less problematic for mixer/settlers and pulse columns

• Can be very problematic for centrifugal contract set up

Describe the issues with Iodine-129

• I129 and I131 are major contaminants in spend nuclear fuel

• Complex chemical speciation and volatility means radioiodine is present in most stages of fuel recycke

• Gas and aqueous streams require treatment

• I129 is most significant dose contributor to plant emissions

What is the solution to I129 issues in separation?

• Apply HSAB theory to the radioiodine management problem

• Designing a selective solvent extraction process for non-metals is very difficult

• Easier to design a solid-phase adsorbent and contact this material with a gaseous or aqueous stream

• Want the binding to be close to irreversible

• Radioiodine loaded material can then be processed as solid waste and sent for geological disposal