Chapter 9 - Neutron Damage Effects Flashcards
How are point defects created in a nuclear system?
By irradiation of neutrons.
What is the rate of production of point defects per unit volume?
Q = nK/omega where n is the point defect survival efficiency, K is dpa/s, and omega is ?
How is K (dpa/s) defined?
K = sigmavflux where sigma is the cross-section, v is the
Where will low cycle fatigue (LCF) might come from in RPV?
fuel reloading
Name all 3 point defects loss mechanisms in the balance equations?
- Recombination
- Absorption at voids for vacancies
- Absorption at dislocations
Are dislocations a sink for interstitials or vacancies?
interstitials
Is the concentration of vacancies higher or lower relative to interstitials?
higher due to higher interstitial dislocation sink.
Why do we observe microstructural changes in reactor materials?
Because of long range diffusion and clustering of displacement damage (defects)
What type of defects cause microstructural change?
dislocation loops, stacking fault, tetrahedra, precipitates, bubbles, voids and solute segregation
What are the benefits of martensitic steel?
fine boundaries and precipitates make the steel highly irradiation and heat resistance
Is ferrite phase desired in irradiated materials? Why?
No, ferritic grains swell more than martensitic grains because of the bcc structure which have a smaller capture radius for vacancies.
At what dose does swelling star to appear in martensite and in ferrite?
Martensite at 200 dpa and Ferrite at 50 dpa
What morphologies can SIA clusters?
predominately only dislocation loop morphologies
What morphologies can vacancy clusters?
3-D Void, 2-D loop or more complicated stacking fault tetrahedra
What does SFT stand for?
stacking fault tetrahedra (SFT)
Which is more mobile: vacancy clusetrs or SIA clusters
vacancy cluseters are much lower
Which has higher binding energy? SIA clusetrs or vacancy clusters?
vacancy cluseters binding energy are much lower than for SIA clusters
What is intergranular stress corrosion ?
is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion than their insides. (Cf. transgranular corrosion.)
What is stress corrosion cracking?
is the cracking induced from the combined influence of tensile stress and a corrosive environment.
What does IGSCC require?
- Specimen loading (tensile stresses) and flaws
- Susceptible alloy
- Aggresive environment: corrosion potential (electrochemical potential) and coolant conductivity
How can SS be converted into a susceptible alloy?
If heated above 600 C because carbon and chromium in solution form carbides that remove Cr from the matrix and this occurs near grain boundaries and is called sensizitation
What is sensizitation?
refers to the precipitation of carbides at grain boundaries in a stainless steel or alloy, causing the steel or alloy to be susceptible to intergranular corrosion or intergranular stress corrosion cracking.
What is the effect of sensizitation?
Because chromium is lost near grain boudanries this region is more susceptible to corrosion.
Where is sensizitation common?
near heat affected zones that have attained temperatures of 600 C during fabrication.
What is irradiation assistes stress corosion cracking (IASCC)?
Irradiation-assisted stress corrosion cracking (IASCC) is an age-related degradation mechanism where materials exposed to neutron radiation become more susceptible to SCC with increasing fluence.66 IASCC, like PWSCC, is a distinctive subset of SCC.
Is IASCC affect heat affected zones like IGSCC?
No, it happens on non heat affected zones of structural components
Why does IASCC happen?
because of irradiation induce point defect production which causes subsequent long range diffusion and microstructural evolution. As point deffects diffuse so do metal impurities and solute atoms. Cr is preferentially transproted by vacancy exchange so vacancy absroption at sinks leads to preferential movement of Cr away from sync (grain boundaries) which is simarl to thermal sensisitazion
What happend at davis besse?
leaking of primary coolant and boric acid outisde control rod drive mechanisms (reactor pressure vessel head) due to cracking in CRDM nozzles due to corrosion of RPV head
What was the thing that prevented a sever LOCA?
the stainless steel cladding which bowed upward and cracked indicating substantial plastic deformation
What happened to davis besse 4 steps?
- Leak through crack in weld or axial crack in tube
- Leakage of borated water into annulus
- Circumferential crack from OD back toward ID
- Shrink fit may prevent leakage
Why is vacancy concentration higher than dislocation?
Because interstitials are preferentially absorbed at dislocations, and vacancies are formed thermally also.
What is martensite in the phase diagram?
Martensite is not shown in the equilibrium phase diagram of the iron-carbon system because it is not an equilibrium phase. Equilibrium phases form by slow cooling rates that allow sufficient time for diffusion, whereas martensite is usually formed by very high cooling rates.
In irradiation why is there less chromium in the grain boundaries ?
Because chromium is preferentially transported by vacancy exchange over Nickel and since grain boundaries are vacancy sinks, chromium is moved away from sink
What material are the instrumentation and control drive mechanisms amde of?
nickel-based alloys
What happen to the cladding/rpv in davis besse?
stress corrosion cracking
What happen to the CRDM nozzle during removal for inspection? Why?
It tipped resting againsta adjacent CRDM. Because there was a big whole.
Through where did the borated water leaked in davis besse?
Leaked onto the vessel head through cracks in the nozzle and the weld that attached nozzle 3 to the RPV head
