Zirconium Flashcards
Why is Zirconium used in nuclear fuel?
Cladding must be of good neutron economy, high neutron flux, generate heat, minimal creep
Zirconium - low thermal neutron absorption cross section, good mechanical properties, corrosion resistance, radiation tolerance, machinability
What are the problems associated with Zirconium?
Production of H, burning int high temperature steam
Long term corrosion resistance, H pick up, hydride embrittlement
Anisotropic properties HCP structure at low T
How is Zirconium extracted?
Kroll - zirconium silicate/oxide
2Mg (l) + ZrCl4 -> Zr (s) + 2MgCl2 (s)
Vacuum arc melting to alloy
Why is HF removed from Zr?
Hf has much higher neutron cross section - can be used in control rods
How are Zr tubes formed?
Extrusion, drawing or pilgering
Pilgering most commonly used - cold + mandrill
What are the crystallographic structures of Zr?
HCP - ABABA, 6 fold symmetry
BCC - no close packed
What are the crystallographic planes in HCP?
Basal - (0002)
Prismatic - {1 -1 00}
Pyramidal (1st) - {1 -1 0 1}
What is the burgers orientation relationship?
During phase transformation of high T beta BCC to low T alpha HCP - close packed planes and directions overlap
Shared planes {0001}A//{110}B
What are the two deformation modes of HCP metals?
Dislocations - line defects on discrete directions and planes
Twins -
What are the different types of dislocations?
Screw
Edge
Mixed
How can yield occur by dislocations?
Existing dislocations become mobile - Schmid
New dislocations emitted - Frank Read sources
What is a double arm Frank-Read source?
Applied shear stress moves dislocation
Sessile at either end, mobile middle portion
Dislocation glides and opens up (two screw sources)
Continues till bows so far to pinch itself
Terminates on existing dislocation loop
New loop forms
Which crystal orientations are associated with soft and hard grains?
Hard - load parallel/perpendicular to basal/prismatic -> pyramidal only active slip system of much higher CRSS
Soft - basal or prismatic active
What is crystal twinning?
At low temp and high stress rates, especially in hard grains, significant lattice shear due to rotation of the lattice occurs
What are the two twinning modes in Zr and their consequences?
Contraction twin - C1 - under tension -> rotation 85 of c axis
Extension twin - T1 - under compression -> rotation 65 of c axis
How can crystallographic texture be analysed?
EDX - pole figures
Stereographic projection
EBSD
What are the effects of texture?
Mechanical properties Irradiation creep Thermal expansion, conductivity Radiation transport Elastic properties
How can texture be described?
Kearn’s factor
volume fraction of axis along each of the three major poles
How is texture formed?
Due to crystal rotation as crystal deforms by dislocations, twinning
Soft grains deform first and reorientate
Texture changes with heat treatment - recovery, recrystallisation
What are the strain paths for pilgering?
Radial strain - reduce radius
Hoop strain - reduce thickness
Biaxial - both simultaneously
Why alloy Zr?
To change mechanical and corrosion properties
What is the effect of oxygen?
Alpha stabiliser - raises beta -> alpha transus
potent interstitial solution strengthener
What is the effect of tin?
Alpha stabiliser
Dilute solution, improves waterside corrosion resistance
Creep strengthener
What is the effect of niobium?
BCC - beta stabiliser
enables microstructures similar to Ti to form
What is the effect of Fe/Cr?
Beta stabiliser
Forms small secondary phase particles
Fe - improves corrosion resistance due to ion transport through oxide layer
What is the effect of nickel?
Forms secondary phase particles
Improves hydrogen pick up - corrosion resistance by changing the occupancy of Zr4+ hence changes ion transport
From the melt how is Zr4 processed to obtain desired texture?
Hot ingot
Quench - little pinning of beta grains
Extrusion - high temperature so soft to reduce high ratio - 5% stored dislocation - hardening so must recover
Annealing - recovery - anihilates dislocations of opposite signs
80% reduction pilgering - cold
Aneal - cant push too far since might crack
70% pilger - colder - engineering decission to go hot to cold
Anneal
56% pilger - cooler (reducing % and T each time)
Final anneal - globular structure, fine grains texture orientation towards (0002) sample normal
What affects the performance of Zr alloys?
Alpha grain size
Texture
Heat treatments/Welding
Alloy chemistry
What is the evolution of Zr texture?
Big beta grains -> alpha grains (BOR)
Beta grain refinement through processing - annealing
Texture switches to (0002) towards sample normal
Control re-crystallisaton/growth new grains
What is the evolution of Zr microstructure?
Prior beta grains with alpha laths
What are secondary phase particles?
Small spherical particles of different crystallographic structure decorating Zr matrix, distributed within alpha matrix eg Fe/Ni/Cr
Important for corrosion resistance due to hydrogen pick up
How does H affect Zr?
When H is introduced to the system - by low quality alloys or wet fuel
H beta stabiliser
Forms solid hydrides - brittle - delta FCC hydrides - ZrH2 with CaF2, Zr on corners, H on (1/4 1/4 1/4)
gamma - FCT - c/a ratio >1, metastable
What is the effect of the morphology of hydrides?
Hydrides grow as a needle-like precipitate (lenticular) - do not fit in Zr structure <a> strain 5%, strain 7% - incoherent interface grow outwards - local hardening, loss in ductility
eruption of dislocation loops to balance out misfit strain</a>
How do hydrides form crystallographically?
Needles grow along <a> directions - can determine orientation of Zr by hydriding structure
Form on gbs (intergranular) or within (intragranular)
Low H conc 200 ppm - intragranular only, ageing -> intergranular
500 ppm hydride plates feathering
5000 ppm H both type hydrides FCC and FCT</a>
How do hydrides effect the mechanical properties?
Increasing H conc step drop in modulus
Yield stress similar but more complex - reduces by 1/3
Increasing H reduces K1C
How does the location of hydrides effect cracking?
Hollow rod shape hence to prevent cracking from hoop stress must align hydrides perpendicular to stress to prevent ‘unzipping’
How can the location of hydrides be controlled?
Texture
How can hydrides be removed?
Niobium picks up hydrogen before hydrides can form - lowers reduction of water
Reduces K1c decrease massively
with increasing T H remains locked in beta phase so K1c doesn’t change much when Nb is present
What is delayed hydride cracking?
Hydrides orientated badly for crack propagation
Heat -> hydrides at crack tip reorientate
Crack progress, stress alleviates
Repeat
How does temperature cycling effect hydrides?
Reduction in toughness due to dissolution of hydrides at high T then precipitate at low T
H diffuses to regions of cold, forms preferentially at areas of high tensile stress
What does the concentration of Hydrides at cracks depend on?
Concentration of hydrogen
volume of hydrides
hydrostatic stress
Temperature
What are the different stages of crack growth?
Stage 1 - no growth - manageable
Stage 2 - stable growth - manageable, not good
Stage 3 - unstable crack growth - fast unmanageable
What is the process zone model?
Crack processes based upon ‘strip yield’
delayed hydride cracking occurs when critical displacement vc achieved
hydride is thin region of constant cohesive strength
process zone -> external stress + cohesive stress = 0
stress reduces as PZ moves through hydride
What are the main steps in fuel rod fabrication?
Cladding tube, inspection Insert lower end plug - optional weld Pellet stack make up and length check load pellets and weigh Measure length Decon tube open end Insert plentum spring Insert tube with open end into weld chamber Solid/hollow end plug
When is welding required?
During the solid end plug or hollow end plug stage
Grid/guide tube, spacer water rods
Fuel channel
What are the different welding techniques?
Tungsten inert gas Resistance Laser beam Electron beam Spot welding
What are the principles of tungsten inert gas welding?
TIG - tungsten electrode, arc of electrons, contained with inert shielding gas (argon/helium)
Cheap but requires very skilled welder
What are the principles of upset welding?
Friction, two parts of similar size pushed together, heat+displacement = fusion
What are the principles of laser beam welding?
High power laser, local melting (small spot size, reduces HAZ, dissipated heat small -> reduced HAZ)
Clean, automatic
What are the principles of electron beam fusion?
Sample in vacuum, part welded with high flux of electrons, locally heats+fuses
Clean but expensive
What are the principles of spot welding?
Geometry important
Cheap, repeatable
How does welding effect the metallurgy and mechanical properties?
Large thermal sink -> large columnar grain in fusion zone
HAZ - globular grains
High residual stress due to thermal expansion
How can residual stress be measured?
Hole drilling
Contour drilling
Diffraction method
How do the conditions in the reactor affect the fuel cladding?
High T and P
Steam
High neutron flux
-> Waterside corrosion and irradiation creep
What are the mechanisms of corrosion in Zr?
High T, steam
H forms at cathode in half cell - proton released from water, recombines with electrode forming hydrogen
H diffuses towards tensile stress, lower chemical potential of hydrogen
oxide layer grows during transport
What is irradiation growth?
Neutron bombardment generates lots of point defects - vacancies and interstitials
Accumulation of defects at traps/sinks - dislocations -> climb -> irradiation growth
Occurs in absence of applied stress
Crystallographically anisotropic - <a> grows contracts</a>
How does irradiation damage affect the microstructure?
Creates vacancy loops on basal plane, <a> interstitial loops on prism planes
Forest of loops - dislocation interaction harden
Sweep of dislocations creates soft region next to hard material
</a><a> basal flows through channels common </a>
What is irradiation creep?
Elevated temperature deformation - enhanced by defects
Occurs due to pellet clad interactions - causes tubes to collapse and rupture
What are the stages of creep?
Primary - transient
Secondary - steady state
Tertiary - unstable
What does creep depend on?
Neutron flux applied stress time activation energy texture grain size dislocation density alloy type
How can creep be mitigated?
Solute strengthening - Sn/Nb/O - inhibits dislocation movement, sinks for vacancies and self interstitials
Grain size and substructure - cold worked creep faster than recrystallised
Texture - creep and growth anisotropy
