Primary circuit Flashcards
What are the main components of the primary circuit?
The RPV and coolant pipes
Which elements are in stainless steel and why?
Fe - steel
C - strength
Cr - stainless - forms protective oxide layer
Ni - austenite former
Which elements stabilise ferrite/austenite?
Cr - BCC so stabilises alpha ferrite
C - interstitial element - stabilises gamma austenite
Ni - FCC - gamma austenite stabiliser
Mn - cubic - gamma austenite stabiliser
When are nickel alloys used for the primary circuit?
In steam generators since they are not susceptible to chloride cracking/pitting due to carbide network - inter granular preffered to intragranular.
What is the nature of a passive oxide film?
Very thin, adherent, non-porous
Oxide formed Cr2O3 at metal oxide interface - oxidation resistant
Mixed layer of Cr2O3 and Fe2O3 more porous
What is sensitisation?
Weld decay of stainless steels - most critical temperature 650-700 C - C diffuses to lattice defects eg gb or phase boundary where there is little coherency
Tends to form carbides of M23C6 where M is Fe or Cr
How can sensitisation be mitigated?
Redissolve carbides
Reduce the carbon level
Use steels with high carbon and cr affinity elements - Ti Nb V and Ta
How does irradiation sensitisation differ?
Cr depletion by segregation of elements - as neutron fluency increases depletion increases
How can we repair welds in service?
Using freezing seals - apply a jacket to the pipe either side of region, fill with CO2 or liquid nitrogen, this freezes water in region creating ice plug, region cut out and repaired
What are the requirements for SCC?
Material, tensile stress, environment
Where can spent fuel be stored?
Fuel dry store containers - usually near sea so Cl induced SCC is likely
What are the mechanics for SCC?
Intergranular or transgranular
What is pitting?
Local break down of the passive layer allows metal to come into contact with corrosive environment, metal dissolution and production of H+, attracting negative Cl-, bulk PH drop to 2-3
Cl- ions prevent the repassivation of the oxide film lead to surface pitting which lead to stress corrosion cracking
What are the stages of the slip dissolution model?
1 Slip plane intersects crack tip
2 Strain causes oxide film rupture and exposes bare metal
3 Anodic dissolution of crack tip extends the crack
4 Repassivation of the crack tip oxide occurs
5 Strain builds up at crack tip allowing more slip mechanism repeats
What are the conditions for the slip dissolution model?
Assumes that crack growth occurs by oxide film rupture at the crack tip
What is the effect of hydrogen?
Present due to the reduction corrosion process and attracted by Cl-, it enhances plasticity
Metal hydride formation under stress and temperature gradient
What are the steps in the corrosion enhanced plasticity model?
1 Stress intensity crack tip causes local plastic zone and activates slip
2 Vacancies and hydrogen produced diffuse along the plane and reduce crack blunting
3 Crack tip plastic zone contain hardened material with high dislocation density -> pile up
4 Hydrogen segregates to the dislocations decreasing the toughness then crack initiates by Stroh type mechanism
5 Hydrogen in some transition metals can reduce the decohesion stress along (111) causes cleavage cracking, dislocations emitted down a symmetrical plane shielding the new crack tip
6 cracking continues in zig-zag pattern
What is the effect of corrosion potential on SCC?
SCC occurs over a limited range of corrosion potential between active and passive, use pourbaix to determine
What is the effect of cold work on SCC?
Intergranular and transgranular observed at the sam time, cold work mechanically induce residual stress increasing the yield stress leading to higher elastic load, forming twins which are stress raisers and enhanced diffusion paths for species such as oxygen
What does irradiation result in?
Radiolysis of the water creating free radicals and enhancing the corrosion potential
Transmutation
Irradiation hardening by formation of defects
Cr Mo depletion, Ni Si enrichment
Irradiation creep and swelling
What is the effect of irradiation hardening on SCC?
BWR components intergranular cracking linked to irradiation hardening
How does silicon effect irradiation SCC?
Si is dragged to the GB by association with self-interstitial atoms, readily forming SiO2 which is soluble at high T
In which materials are the different SCC prominent?
Steels - sensitisation by chlorine gas
Ni - hydrogen embrittlement
Zr - hydrogen cracking and irradiation SCC
