Primary circuit

Question 1

What are the main components of the primary circuit?

Answer 1

The RPV and coolant pipes

Question 2

Which elements are in stainless steel and why?

Answer 2

Fe - steel
C - strength
Cr - stainless - forms protective oxide layer
Ni - austenite former

Question 3

Which elements stabilise ferrite/austenite?

Answer 3

Cr - BCC so stabilises alpha ferrite
C - interstitial element - stabilises gamma austenite
Ni - FCC - gamma austenite stabiliser
Mn - cubic - gamma austenite stabiliser

Question 4

When are nickel alloys used for the primary circuit?

Answer 4

In steam generators since they are not susceptible to chloride cracking/pitting due to carbide network - inter granular preffered to intragranular.

Question 5

What is the nature of a passive oxide film?

Answer 5

Very thin, adherent, non-porous
Oxide formed Cr2O3 at metal oxide interface - oxidation resistant
Mixed layer of Cr2O3 and Fe2O3 more porous

Question 6

What is sensitisation?

Answer 6

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

Question 7

How can sensitisation be mitigated?

Answer 7

Redissolve carbides
Reduce the carbon level
Use steels with high carbon and cr affinity elements - Ti Nb V and Ta

Question 8

How does irradiation sensitisation differ?

Answer 8

Cr depletion by segregation of elements - as neutron fluency increases depletion increases

Question 9

How can we repair welds in service?

Answer 9

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

Question 10

What are the requirements for SCC?

Answer 10

Material, tensile stress, environment

Question 11

Where can spent fuel be stored?

Answer 11

Fuel dry store containers - usually near sea so Cl induced SCC is likely

Question 12

What are the mechanics for SCC?

Answer 12

Intergranular or transgranular

Question 13

What is pitting?

Answer 13

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

Question 14

What are the stages of the slip dissolution model?

Answer 14

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

Question 15

What are the conditions for the slip dissolution model?

Answer 15

Assumes that crack growth occurs by oxide film rupture at the crack tip

Question 16

What is the effect of hydrogen?

Answer 16

Present due to the reduction corrosion process and attracted by Cl-, it enhances plasticity
Metal hydride formation under stress and temperature gradient

Question 17

What are the steps in the corrosion enhanced plasticity model?

Answer 17

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

Question 18

What is the effect of corrosion potential on SCC?

Answer 18

SCC occurs over a limited range of corrosion potential between active and passive, use pourbaix to determine

Question 19

What is the effect of cold work on SCC?

Answer 19

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

Question 20

What does irradiation result in?

Answer 20

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

Question 21

What is the effect of irradiation hardening on SCC?

Answer 21

BWR components intergranular cracking linked to irradiation hardening

Question 22

How does silicon effect irradiation SCC?

Answer 22

Si is dragged to the GB by association with self-interstitial atoms, readily forming SiO2 which is soluble at high T

Question 23

In which materials are the different SCC prominent?

Answer 23

Steels - sensitisation by chlorine gas
Ni - hydrogen embrittlement
Zr - hydrogen cracking and irradiation SCC