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Brittons part
| Question | Answer |
|---|---|
| conditions for it to meet | Good neutron economy Need excellent corrosion resistance in High temp =water (~300°C) and High neutron flux (~1012 n/cm2s) High pressure (16MPa = 160 Atm) similar Δthermal expansion to fuel Long times (18 / 24 month refuelling cycles) |
| Why we need good neutron economy | Control/moderation can be entirely external – eg control rods + reactor design Enables lower enrichment of fuel (proliferation & reduced risk) Better long term performance Æ higher burn ups available |
| Whats good about zirconim | mechanically stable, – Low thermal neutron absorption cross section – Good mechanical properties, especially at high temperatures – Corrosion resistance – Radiation tolerance – Machinability (making long tubes) |
| Problems with zirconium | – Production of H (by taking oxygen out of water) / burning in HTemp steam – Long term corrosion resistance / H pick-up / hydride embrittlement – Anisotropic properties (HCP crystal structure) |
| extraction | zircon rich white sand (zironium oxide and silicates). v. reactive. kroll process. convert zirconium into zirc chlorine 4 and then exchange with magnesium and chemical separate -zirconium sponge. crush into pebbles. vacuum arc remelting remove impurity. |
| Ways to remove hafnium | – liquid-liquid extraction of thiocyanate-oxides (solubility in isobutyl ketone vs water) – Fractional crystallisation of potassium hexafluorozirconate (solubility in water) – Fractional distillation of tetrachlorides |
| tube formation | Extrusion – push through die (if soft). Then drill but ours is v.long Drawing– pulling through die. good length, not hollow. force on front so must be strong. Pilgering– cold (for surface finish) + mandrill. already have a hole and reduce thickness. |
| HCP anistropy | Harder along <c> axis than <a> axis – Stiffer along <c> axis than <a> axis – Creeps faster <a> than <c> – Thermal expansivity higher along <c> than <a> |
| Average orientation / texture affects: | – Mechanical properties – Irradiation creep – Thermal expansion – Thermal conductivity – Radiation transport – Elastic properties (sound, resonance) |
| Alloying materials changes: | – Strength (either through solution or second phase hardening) – Corrosion resistance (changing electrochemical performance) |
| What are these good for? Zircaloy4 Zirlo Zr2.5Nb ZrSn | Reduced hydrogen pick up optimisd corrosion resistance two phase CANDU Tin forms secondary phase particles |
| Role of Oxygen | Oxygen always present in HCP alloys • α-stabaliser (raise β to α transus) HCP • Potent interstitial solution strengthener • Opens up the β-phase field – Good for hot working (α pins β grain boundaries at HT) |
| Role of Tin | α-stabaliser • Dilute solution • Improves waterside corrosion resistance • Creep strengthener |
| Role of Niobium | Pure Nb is BCC • β-stabiliser • With moderate Nb (2.5wt%) often retained β formed • Enables microstructures similar to the Ti alloys to form |
| Role of Iron and Chromium | β-stabiliser • Usually forms small,SPPs, 0.1 - 1μm diameter • SPPs dispersed throughout the matrix • Lots of phases/compositions • Important for ion transport through the oxide layer during corrosion (ie improve corrosion resistance) |
| Role of Nickel | Forms SPPs • Zircaloy4 has 0.03-0.08 vs Zircaloy2 max 0.07 • Control Ni to improved hydrogen pickup and corrosion resistance – Change occupancy of the Zr 4+ sites in the oxide lattice changes ion transport |
| zircaloy-4 processing | ingot beta quench hot extrusion 14:1 first anneal 760 degrees for 4 hours first pilger 80% second anneal 650 degrees for 4 hours second pilger 70% 3rd anneal 575 degrees for 3 hours 3rd pilger 58% final anneal 500 degrees for 10 hours |
| what microsturcture do we want to control | alpha grain size and texture |
| delayed hydride cracking | Hydrides are orientated badly then you apply heat and the hydrides reorientate at the crack and then the crack progresses to elevate the stress. thermal fatigue. |
| How to reduce hdride cracking | we need no crack or cracks growing at a controlled rate thats fine. |
| Fuel rod fabrication | cladding tube inspection lower end plug decontamination plenmum weld chamber if solid end plug ... if hollow end do lower end plug weld and then upper.... pressurize with He and weld clean and etch |
| whats in the zironium safety case | waterside cracking, irradiation creep an delayed hydride cracking during loading, power transients and in transport |
| Difference between <a> interstitial loops and <c> vacancy loops | <a> interstitial loops, on prism planes • Early in burnup, size grows with irradiation temp up to 673K <c> vacancy loops, on basal planes • Late in burnup, large (>100nm) |
| creep is a function of | – Neutron flux (φ) – Applied stress (σ) – Time (t) – Activation energy (Q/R = activation temp) – Texture (f) (Kearns factor) – Grain size (G) – Dislocation density (ρ) – Alloy type (A) |
| what creep do we have at • Below 300°C • Above 300°C • Above 350°C • Above 400°C | • Below 300 – Pure irradiation creep is athermal • Above 300 – Thermal + irradiation creep • Above 350 – Thermal defects + irradiation defects compete – Annealing of vacancy clusters etc. • Above 400 – Thermal creep dominates (recombnations) |
| What microstructure will protect against creep | • Solute strengthening (eg Sn/Nb/O) – Inhibits dislocation motion – Sinks defects • Grain size and substructure – Dislocations can act as sinks – Cold worked creep faster than recrystal • Texture – Creep + growth anisotropy |
Created by:
emmaperry
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