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Wenmans part

QuestionAnswer
Describe the different materials used in a PWR primary circuit – including composition, microstructure, mechanical properties and corrosion resistance. Ni Alloy 600 and 690; primary circuit materials austenetic used for weld repairs Fuel dry store containers; 304L/316L stabilised austenitic stainless steels (not heat treated so high residual tensile stress),sea air, initially high temp.
Define the term “sensitisation” with respect to both thermal and irradiation conditions. Heating leads to C diffusion to gran boundaries where they form carbides and strip the metal from the lattice. the lattice is Cr denuded and therefore less corrosion resistant. Cr depletion by segregation of elements - affects a smaller region
With respect to thermal sensitisation describe mitigation strategies (leads to stabilised steels) Solution treatment to redissolve the carbides 1100°C and quench. avoid high temp Reduce the carbon to below where carbide precipitate still sensitisation if extended periods at 650°C. add elements with a greater affinity for C; Ti, Nb, V and Ta.
Describe the use of freeze seals explaining how they work and what their strengths/weaknesses are. For fully ductile austenitic steel or nickel alloy (no ductile-brittle transition and high toughness at all temp.) apply a jacket to pipe either side and fill with cooling medium (CO2 / N) to locally freeze the water in the pipe.
Irradiation dose of possible onset of swelling 50 dpa
Describe the three components necessary for SCC and the characteristics of stress corrosion cracks. specific material plus specific environment plus tensile stress (residual or applied).
Discuss corrosion pit formation a local break down of the passive film... corrosive environment.. metal dissolution and evolution of H+ ions ... ph change. Cl makes the pit grow faster and pit nucleation rather than repassivaion. Plastic deformation or cold work increase pitting
Explain and discuss the various theories of SCC including slip-dissolution Slip plane touches the crack tip The plane slips which causes the oxide film to break Anodic dissolution extends the crack The oxide is repassivide strain builds up until the plane slips again. strain builds by fatigue or creep. max at high strain.
Describe the mechanisms of irradiation assisted SCC. – Radiolysis of the water -- free radicals -- enhanced corrosion potential. – Transmutation e.g. B to Li + He, Ni to Fe + He. – more defects. – Cr + Mo depletion + Ni and Si enrichment of the grain boundaries. – Irradiation creep and swelling.
Describe the effects of corrosion potential on SCC of power plant materials. SCC occurs over a limited range of corrosion potential between active and passive. Pourbaix diagrams can be used to determine the active and passive regions at different potentials and pH.
Describe SCC occurrences in real nuclear plant • SCC of stainless steel primary circuit components has been observed leading to costly outages. • PWR Baffle/former bolts have shown cracking
Discuss corrosion pit transition to stress corrosion cracks. affected by pH, pit geometry and competition between intergranular corrosion forming a sharp crack compared to dissolution of the grain leading to further pit growth.
Explain and discuss the various theories of SCC including hydrogen embrittlement. attract anions (Cl-)reduce oxide passivation rates. Hydrogen enhanced plasticity at the crack tip. Metal hydride formation under stress and temperature gradients.
How does Hydrogen enhance plasticity adsorption of hydrogen reduces bond strength and so inc dislocation movement. Reduces crack blunting. dislocations travel where mobility is easier but then hardened region. A portion of the crack tip stress is added to the pile-up.
How does Hydrogen enhance plasticity second crack tip decreases the local value of toughness (KIC) and a crack embryo can form by a Stroh type mechanism reduce the decohesion stress along (111) plane and the normal stress to this plane causes cleavage cracking at an angle Result in (111) plane zigzag
compare corrosion in a PWR to a BWR (see pourbaix diagram) In general due to the boiling environment and therefore high oxygen levels BWR are at high corrosion potential. PWR has injected H2 to remove O2 and is therefore at low corrosion potential.
Describe the effects of cold work on SCC of power plant materials. Cold work - strain hardening - Increased yield stress- higher elastic loading - transgranular crack. leads to dislocation structures/bands that contain twins. These act as both stress raisers and enhanced diffusion paths for species such as oxygen.
Explain the choice of low alloy steel (quenched and tempered bainitic) for RPVs and the manufacturing route. large ignot melt, chop off top and bottom. upset forging and put a hole in the middle. enlarge, square and draw on a mandrel. enlargement. rough machine and HT (quench and temper). submerged arc weld. HT to remove H. stainless steel inner layer
Describe the composition of RPV steel. Carbon level of a mild steel Mn and Mo extra strength. Ni hardenability for thick sections and decrease the ductile-to-brittle transition temp. Sulphur and phosphorous levels controlled as they can lead to intergranular embrittlement
Describe the fabrication process of an RPV and how modern RPVs differ to older vessels explaining the pros and cons of these choices. Forging reduces the number of welds around andremoves all longitudinal and high flux areas so less flaws But not that many places can make them difficultQuenching thick sections due to temp gradients, martensite and austenite have differnt vol - crack
2 steels used in RPV A533B (plate) and A508 (forging).
AP1000 RPV 4 forgings no weld in high flux 1500 tonnes 12x3 m
EPR RPV 7 forgings Weld in high flux, use metal to half the flux 2000 tonnes 10x5m
Describe the phenomenon of a ductile-to-brittle transition in ferritic steel. The RPV is a ferritic steel which suffers from a loss of ductility with temperature and hence brittle failure is possible
Describe why microstructure is important in the fracture process of steels. Scatter originates from the microstructure and hence we should never apply statistical fits without thinking about microstructure and linking to mechanistic failure.
Discuss the use of Weibull statistics for small data sets and fit data to a two parameter Weibull distribution. can learn the method if i realy need to? The Weibull distribution is often used for failure analysis and is derived from extreme value statistics. If one assumes that fracture occurs from one particle (weakest link argument) then Weibull should describe the toughness distribution
Describe how understanding of microstructure is crucial to the interpretation of statistical fracture data. could end up with negative toughness otherwise.
Calculate a simple plastic collapse load. applied force to cause net section yielding = Area left to crack x yield stress Lr = stress per unit of material /Yeild stress
Calculate a simple stress intensity factor and determine whether brittle failure will occur. K1 = shape factor x applied stress x sqrt(pi x crack length) and compare to K1c
Draw a FAD for a simple case calculating and plotting (Lr,Kr) on the FAD and determine the factor of safety see picture with the old square and the new line
Define and explain the term primary stress Normal or shear stresses developed by an imposed load to satisfy the laws of equilibrium of internal/external forces/moments. not limiting or self-equilibrating. primary stresses which exceed yield can produce gross distortions and plastic collapse.
Describe Submerged arc welding and discuss what they are appropriate for. pour flux over the weld area, filler wire goes under. Good; automated, fast and large components but; insulated = slow cool = large grains nd ductile. Filler goes into the turbulent weld pool. Horizontal only. multipass=reheat=complex microstructure.
Describe a multipass submerged arc weld microstructure and its associated mechanical properties. in ferrites As-deposited; columnar structures of allotriomorphic ferrite, Widmanstätten (depending on cooling)and acicular is toughest Reheated; recrystallisation=equiaxed ferrite w/ small particles of FeC. more boundaries = tougher. lower hardness
Explain why welding leads to residual stresses in components. As liquid cools it contracts, If constrained this can lead to residual stresses as well as other defects such cracks and entrapped non-metallic particles (inclusions).
Describe issues related to welding such as reheat cracking resulting from creep. next to the weld in the HAZ, intergranular cracking, at high temp. check by borland test
Describe the problems associated with dissimilar metal welds eg between RPV (low alloy steel) and primary circuit (stainless steels/Ni alloys) and inside RPV problems; thermal mismatch. carbon tend to diffuse from ferrite (0.2% C) to austenite (0.03%)-sentisation. so use of a Ni alloy weld middle trap C
HAZ in austentic stainless steels and feritic steels In austenitic stainless steels this region sees Cr depletion In ferritic steels the HAZ is associated with intergranular cracking.
Describe MIG welding and discuss what they are appropriate for. Metal Inert Gas, consumable electrode is the filler metal and shield using an inert gas. can be automated, vertical and clean. Expensive though
Widmanstätten ferrite and acicular ferrite fast cooling rate/low temp, Iron can't diffuse fast. carbon is rejected from bcc (less soluble), needles help reject more. Shear process = strain between grains; balance by grains in diff directions. Widmanstätten = grain boundaries. acicular = intra
Describe TIG welding and discuss what they are appropriate for. Tungsten Inert Gas. non-consumable tungsten electrode. inert gas shielding. Disadvantages; low deposition rates, workpiece must be clean Helium gives hotter arc and argon better shielding as it is heavier.
Define and explain the term secondary stress Normal/shear stress by constraint of adj material or self-constraint of structure (geometrical features). Residual stresses are a secondary stress. self-limiting; after yield point, load /stress relationship is broken. will not plastic collapse alone
Residual stresses are typically categorised into three types dependent on the length scale over which they equilibrate: I - macro stresses. equilibrate over the whole component. caused by machining processes. II - intergranular. equilibrate over a few grains. III - intergranular. a few inter atomic distances. Caused by point defects and dislocations.
Weld residual stress measurement techniques Some techniques are destructive (contour method) whilst others are non- or semidestructive (hole drilling). Some techniques are portable. Ultrasonic, eddy current, hole drilling. Some are expensive e.g. synchrotron X-ray diffraction.
There are three stages to creep: 1 Transient creep ( rapid) 2 Steady state creep (decrease due to recovery of dislocaions is matched by rate of hardening due to increasing strain) 3 Tertiary creep and fracture. rate accelerates due to cavities at the GB and failure when link up
underclad cracking 2nd weld reheats HAZ of 1st weld and relieves the stress by straining material in HAZ; it isnt ductle enough and cracks. carbide formers pin the material so that the strain concentrates near the grain boundary which cracks it annealing reduces it.
Explain the mechanism of pellet-clad interactions in AGR fuel. pellet expands more than cladding. the gap is gone and the radial cracks in the pellet apply stress to the cladding creep fatigue mchanism pushes the crack open more.
explain the mechanism of pellet-clad interactions in water cooled reactor fuel. Initial gap is greater in PWR (~80-100 mm) cf (~30-50 mm) in AGR. closes within a fifth of the fuel life
Describe the embrittlement of AGR cladding by helium. Ni in stainless steel absorbs neutrons and then because Ni 60whcih goes o iron and helium. Helium bubbles go to grain boundaries and embrittle.
Explain the texture effect and initiation of I-SCC on Zr. Powr increase increase iodine, volatile and goes to cladding through fuel cracks. zirconium is sensitive. texture helps the hydrogen and iondine problem to have Cdirection parellel to slow diffusion down. hard grain(slip) next to a soft grain = crack.
Describe mitigation strategies for PCI. chromia dope and alumina dope and MOX to change the grain sze in the fuel. harder fuel makes more cracks to release thermal which makes the load distribution more even. BWR coated fuel tubes with zirconia to blunt cracks.
Key differences between a PWR fuel pin and an AGR fuel pin AGR are annula, steel cladding, spirals to gas cool, and no spring, need a spacer blocker instead so that we dont affect with welding
PWR fuel failures grid fretting; vibrations cause fuel to wobble in grid and you wear a hole debris; corrosion products get into reactor PCI so small
BWR fuell failures PCI crud deposition
factors in fuel performance code, fuel Fission gas release and its effect on swelling stresses. Evolution of properties under irradiation: Young’s mod, thermal conductivity, creep behaviour, porosity, oxygen stoichiometry and microstructure changes.
factors in fuel performance code, cladding Mechanical properties evolution: Elastic, plastic and creep (irradiation and thermal effects) Microstructure Oxidation (AGR, BWR, PWR) and hydration behaviour (PWR and BWR). Sensitivity to crack growth through SCC (BWR, PWR) or creep-fatigue (AGR)
componets of the heat transfer pressure, conduction, radiation, oxide
AGR fuel annular UO2 pellets (64 per pin) stacked in a Nb stabilised austenitic stainless steel clad. Nb forms carbides to provide pinning particles to reduce creep Stainless steel is necessary due to the higher temperatures in an AGR of ~750°C.
Problems with predicting dose damage relationships Problems with predictions: • Too many variables (some unknown) • E.g. LBP, effect of microstructure. • A lot of noise in the data/missing data • Overfitting (works for the data you’re fitting to but will not extrapolate)
contour method to measure residual stress Wire cut sample and the surface will deform to relive the residual stress. point measurement of the surface and map it to see the height. FEM and force all nodes back.
Created by: emmaperry