Chapter 9 - Neutron Damage Effects

Question 1

How are point defects created in a nuclear system?

Answer 1

By irradiation of neutrons.

Question 2

What is the rate of production of point defects per unit volume?

Answer 2

Q = nK/omega where n is the point defect survival efficiency, K is dpa/s, and omega is ?

Question 3

How is K (dpa/s) defined?

Answer 3

K = sigmavflux where sigma is the cross-section, v is the

Question 4

Where will low cycle fatigue (LCF) might come from in RPV?

Answer 4

fuel reloading

Question 5

Name all 3 point defects loss mechanisms in the balance equations?

Answer 5

1. Recombination
2. Absorption at voids for vacancies
3. Absorption at dislocations

Question 6

Are dislocations a sink for interstitials or vacancies?

Answer 6

interstitials

Question 7

Is the concentration of vacancies higher or lower relative to interstitials?

Answer 7

higher due to higher interstitial dislocation sink.

Question 8

Why do we observe microstructural changes in reactor materials?

Answer 8

Because of long range diffusion and clustering of displacement damage (defects)

Question 9

What type of defects cause microstructural change?

Answer 9

dislocation loops, stacking fault, tetrahedra, precipitates, bubbles, voids and solute segregation

Question 10

What are the benefits of martensitic steel?

Answer 10

fine boundaries and precipitates make the steel highly irradiation and heat resistance

Question 11

Is ferrite phase desired in irradiated materials? Why?

Answer 11

No, ferritic grains swell more than martensitic grains because of the bcc structure which have a smaller capture radius for vacancies.

Question 12

At what dose does swelling star to appear in martensite and in ferrite?

Answer 12

Martensite at 200 dpa and Ferrite at 50 dpa

Question 13

What morphologies can SIA clusters?

Answer 13

predominately only dislocation loop morphologies

Question 14

What morphologies can vacancy clusters?

Answer 14

3-D Void, 2-D loop or more complicated stacking fault tetrahedra

Question 15

What does SFT stand for?

Answer 15

stacking fault tetrahedra (SFT)

Question 16

Which is more mobile: vacancy clusetrs or SIA clusters

Answer 16

vacancy cluseters are much lower

Question 17

Which has higher binding energy? SIA clusetrs or vacancy clusters?

Answer 17

vacancy cluseters binding energy are much lower than for SIA clusters

Question 18

What is intergranular stress corrosion ?

Answer 18

is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion than their insides. (Cf. transgranular corrosion.)

Question 19

What is stress corrosion cracking?

Answer 19

is the cracking induced from the combined influence of tensile stress and a corrosive environment.

Question 20

What does IGSCC require?

Answer 20

1. Specimen loading (tensile stresses) and flaws
2. Susceptible alloy
3. Aggresive environment: corrosion potential (electrochemical potential) and coolant conductivity

Question 21

How can SS be converted into a susceptible alloy?

Answer 21

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

Question 22

What is sensizitation?

Answer 22

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.

Question 23

What is the effect of sensizitation?

Answer 23

Because chromium is lost near grain boudanries this region is more susceptible to corrosion.

Question 24

Where is sensizitation common?

Answer 24

near heat affected zones that have attained temperatures of 600 C during fabrication.

Question 25

What is irradiation assistes stress corosion cracking (IASCC)?

Answer 25

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.

Question 26

Is IASCC affect heat affected zones like IGSCC?

Answer 26

No, it happens on non heat affected zones of structural components

Question 27

Why does IASCC happen?

Answer 27

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

Question 28

What happend at davis besse?

Answer 28

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

Question 29

What was the thing that prevented a sever LOCA?

Answer 29

the stainless steel cladding which bowed upward and cracked indicating substantial plastic deformation

Question 30

What happened to davis besse 4 steps?

Answer 30

1. Leak through crack in weld or axial crack in tube 2. Leakage of borated water into annulus 3. Circumferential crack from OD back toward ID 4. Shrink fit may prevent leakage

Question 31

Why is vacancy concentration higher than dislocation?

Answer 31

Because interstitials are preferentially absorbed at dislocations, and vacancies are formed thermally also.

Question 32

What is martensite in the phase diagram?

Answer 32

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.

Question 33

In irradiation why is there less chromium in the grain boundaries ?

Answer 33

Because chromium is preferentially transported by vacancy exchange over Nickel and since grain boundaries are vacancy sinks, chromium is moved away from sink

Question 34

What material are the instrumentation and control drive mechanisms amde of?

Answer 34

nickel-based alloys

Question 35

What happen to the cladding/rpv in davis besse?

Answer 35

stress corrosion cracking

Question 36

What happen to the CRDM nozzle during removal for inspection? Why?

Answer 36

It tipped resting againsta adjacent CRDM. Because there was a big whole.

Question 37

Through where did the borated water leaked in davis besse?

Answer 37

Leaked onto the vessel head through cracks in the nozzle and the weld that attached nozzle 3 to the RPV head