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The use of both neutron and ion
irradiation to show the
microstructural origins of strong
flux-sensitivity of void swelling in
model Fe-Cr-Ni alloys
T. Okita, N. Sekimura and T. Iwai
University of Tokyo, Tokyo, Japan
F.A. Garner
Pacific Northwest National Laboratory, Richland, WA, USA
Outline of this presentation
• Neutron irradiation experiment on Fe-15Cr-16Ni
conducted in FFTF fast neutron reactor at ~ 426 ˚C at
seven dose rates between 8.9 x 10-9 to 1.7 x 10-6
dpa/sec.
• Ion irradiation experiment on the identical Fe-15Cr16Ni conducted with 4 MeV Ni3+ ions at 300, 400, 500,
and 600 ˚C at three different dose rates between 1.0
x10-3 to 1.0 x 10-4 dpa/sec.
Neutron irradiation ; FFTF/MOTA
dpa/sec in stainless steels
Materials Open Test Assembly
BELOW
CORE
BC
FFTF CORE
1
2
3
4
5
ABOVE
CORE
6
7
8
10-5
Irradiation at seven
positions in, below and
above the core.
10-6
10-7
10-8
10-9
-100 -75
Multiple specimens at
each condition.
-50
-25
0
25
50
75
100
120 150
Distance from midplane (cm)
2 cycles of irradiation in
FFTF cycles #11 and #12
to achieve two dose
levels at each dose rate.
Neutron irradiation conditions
426 ± 18 ˚C
Dose Rate, dpa/sec
Dose, dpa
Temperature, ˚C
#11
#12
#11
#11 & #12
#11
#12
1.7 x 10-6
1.4 x 10-6
43.8
67.8
427
408
7.8 x 10-7
9.5 x 10-7
20.0
32.4
390
387
5.4 x 10-7
8.4 x 10-7
14.0
28.8
430
424
3.1 x 10-7
3.0 x 10-7
8.05
11.1
411
410
9.1 x 10-8
2.1 x 10-7
2.36
6.36
430
431
2.7 x 10-8
6.6 x 10-8
0.71
1.87
434
437
8.9 x 10-9
2.2 x 10-8
0.23
0.61
436
444
Constant time experiment
2.59 x 10 7 sec (Cycle #11)
1.76 x 10 7 sec (Cycle #12)
Previous studies for the effect of dose rates
Authors
Reactors
Difference in
dose rate
Muroga et al.
RTNS-II
< 1 order
Muroga et al.
RTNS-II, JOYO
> 2 orders
Neustroev et al.
BOR-60
< 1 order
Garner et al.
EBR-II, FFTF
< 1 order
Lewthwaite et al.
DFR
> 1 order
Garner et al.
BOR-60
> 1 order
Garner et al.
BN-350
< 1 order
Porter et al.
EBR-II
< 1 order
Kruglov et al.
BR-10
< 1 order
Kozlov et al.
BN-600
< 1 order
Seran et al.
Rapsodie
< 1 order
Seran et al.
Pheonix
< 1 order
Grossbeck et al.
BR-2
< 1 order
Cole et al.
EBR-II
< 1 order
Walters et al.
EBR-II
< 1 order
Schneider et al.
Rapsodie
< 1 order
Allen et al.
EBR-II
> 1 order
Garner et al.
EBR-II
< 2 orders
Nanstad et al.
MTR, HFIR
> 1 order
Kitao
JMTR
< 1 order
Yanagida
KUR
< 1 order
Ferritic alloys
Garner et al.
EBR-II, FFTF
< 1 order
Nickel Alloys
Garner et al.
EBR-II
< 1 order
Alloys
Austenitic
alloys
A533B
Fe-Cu alloys
For austenitic alloys, there
are not enough studies, with
sufficiently detailed
databases for high dose rate
irradiation.
Typically, the effects of dose
rate have been investigated
covering less than one order
difference in dose rate.
Microstructural response
depending on such a small
difference in dose rate might
lie within the experimental
error bands, however.
Previous studies for the effect of dose rates
Authors
Reactors
Difference in
dose rate
Muroga et al.
RTNS-II
< 1 order
Muroga et al.
RTNS-II, JOYO
> 2 orders
Neustroev et al.
BOR-60
< 1 order
Garner et al.
EBR-II, FFTF
< 1 order
Lewthwaite et al.
DFR
> 1 order
Garner et al.
BOR-60
> 1 order
Garner et al.
BN-350
< 1 order
Porter et al.
EBR-II
< 1 order
Kruglov et al.
BR-10
< 1 order
Kozlov et al.
BN-600
< 1 order
Seran et al.
Rapsodie
< 1 order
Seran et al.
Pheonix
< 1 order
Grossbeck et al.
BR-2
< 1 order
Cole et al.
EBR-II
< 1 order
Walters et al.
EBR-II
< 1 order
Schneider et al.
Rapsodie
< 1 order
Allen et al.
EBR-II
> 1 order
Garner et al.
EBR-II
< 2 orders
Nanstad et al.
MTR, HFIR
> 1 order
Kitao
JMTR
< 1 order
Yanagida
KUR
< 1 order
Ferritic alloys
Garner et al.
EBR-II, FFTF
< 1 order
Nickel Alloys
Garner et al.
EBR-II
< 1 order
Alloys
Austenitic
alloys
A533B
Fe-Cu alloys
It has been possible to
achieve wider ranges of dose
rates from comparison of
results obtained in two or
three different reactors.
However, difficulties remain
in such comparative studies
because of simultaneous
changes in other important
irradiation parameters, such
as the neutron energy
spectrum or temperature
history.
Neutron irradiation conditions in this study
• Fast neutron irradiation in FFTF reactor
More than two orders difference in dose rates
achieved in one reactor.
Dose Rate, dpa/sec
Dose, dpa
Temperature, ˚C
#11
#12
#11
#11 & #12
#11
#12
1.7 x 10-6
1.4 x 10-6
43.8
67.8
427
408
7.8 x 10-7
9.5 x 10-7
20.0
32.4
390
387
5.4 x 10-7
8.4 x 10-7
14.0
28.8
430
424
3.1 x 10-7
3.0 x 10-7
8.05
11.1
411
410
9.1 x 10-8
2.1 x 10-7
2.36
6.36
430
431
2.7 x 10-8
6.6 x 10-8
0.71
1.87
434
437
8.9 x 10-9
2.2 x 10-8
0.23
0.61
436
444
Active temperature control system at ± 5 ˚C,
using a variation of He / Ar gas ratio in the gas gap.
Cavity microstructure in Fe-15Cr-16Ni
8.9 x 10-9 dpa/sec
3.1 x 10-7 dpa/sec
1.7 x 10-6 dpa/sec
0.23 dpa
8.05 dpa
43.8 dpa
0.61 dpa
11.1 dpa
67.8 dpa
1 cycle
2 cycles
100 nm
Enhanced swelling at lower dose rate
- Density change and microscopy data -
Swelling (%)
30
Fe-15Cr-16Ni, SA
408 - 444 ˚C
1%/dpa
5.4
20
387 ˚C
• Lower dose rate enhances
swelling by shortening the
incubation dose.
7.8
10
0.9
3.1
390 ˚C
0
0
10
20
30
40
• The steady state swelling
rate is not affected by the
difference in dose rate.
17 x 10-7
dpa/sec
50
60
Cumulative Dose (dpa)
70
Enhanced swelling at lower dose rate
Fe-15Cr-16Ni, SA
430 - 444 ˚C
Swelling (%)
5
• The steady state swelling
rate is also observed at
dose rates as low as < 10-7
dpa/sec and irradiated
less than 1 dpa.
1 %/dpa
4
3
9.1x 10-8
dpa/sec
2
0.89
1
2.7
0
0
2
4
6
8
Cumulative Dose (dpa)
10
• The incubation dose of
swelling can therefore
vary from < 1dpa to > 45
dpa when the dose rate
varies over more than two
orders of magnitude.
Enhanced swelling at lower dose rate
Fe-15Cr-16Ni, SA
430 - 444 ˚C
Swelling (%)
5
• The steady state swelling
rate is also observed at
dose rates as low as < 10-7
dpa/sec and irradiated
less than 1 dpa.
1 %/dpa
4
3
9.1x 10-8
dpa/sec
2
0.89
1
2.7
0
0
2
4
6
8
10
Incubation
Dose
Cumulative
Dose (dpa)
• The incubation dose of
swelling can therefore
vary from < 1dpa to > 45
dpa when the dose rate
varies over more than two
orders of magnitude.
Strong effect of dose rate on incubation dose
Incubation Dose (dpa)
100
Fe-15Cr-16Ni, SA
408 - 444 ˚C
390 / 387 ℃
10
1
0.1
10 -8
• The incubation dose
of swelling is almost
linearly proportional
to dose rate.
(dpa/sec)0.99
10 -7
10 -6
Dose Rate (dpa/sec)
10
-5
Total Dislocation Density (x1014 m-2)
Enhanced dislocation evolution
at lower dose rate
Fe-15Cr-16Ni, SA
408 - 444 ˚C
10
387 ˚C
7.8
8
390 ˚C
3.1
17 x 10-7
dpa/sec
6 0.9
• Lower dose rate
enhances dislocation
evolution.
5.4
4
2
0.27
0.089
0
0
10
20
30
40
50
60
Cumulative Dose (dpa)
70
• This effect arises
primarily from the
enhanced loop growth
at lower dose rate.
Dose rate dependence of loop density
Fe-15Cr-16Ni, SA
410 - 444 ˚C
At relatively low doses, the loop
density is proportional to
(dpa/sec)1/2.
1023
Loop Density (m-3)
(dpa/sec)1/2
1022
8.05
6.36
1.87
2.36
1021
11.1
14.0
0.61 0.71
0.23 dpa
1020 -9
10
10-8
10-7
Dose Rate (dpa/sec)
10-6
This is agreed with the previous
analysis that the saturated loop
density is proportional to
(dpa/sec)1/2.
Dislocation loop density is not
proportional to (dpa/sec)1/2 at
doses higher than ~ 10 dpa,
because loop unfaulting had
occurred.
Effects of dose rate on dislocation evolution
Dislocation Density
(x1014 m-2)
Network
Loop
10
8
5.4 x 10-7
dpa/sec
6
4
0.27
3.1
0.27
2
0.91
0.089
0
0
10
3.1
5.4 x 10-7
dpa/sec
0.91
0.089
20
0
10
20
30
Cumulative Dose (dpa)
• Loop line lengths seem to increase with dose below 10 dpa, and
decrease thereafter, because of loop unfaulting and network dislocation
formation above 10 dpa.
• There seem to be little effect of dose rate on these remaining loop line
lengths, because some loops at lower dose rate grow large enough to be
unfaulted and become network dislocation.
Effects of dose rate on dislocation evolution
Dislocation Density
(x1014 m-2)
Network
Loop
10
8
5.4 x 10-7
dpa/sec
6
4
0.27
3.1
0.27
2
0.91
0.089
0
0
10
3.1
5.4 x 10-7
dpa/sec
0.91
0.089
20
0
10
20
30
Cumulative Dose (dpa)
• The rate of network dislocation evolution is enhanced at lower dose
rates, caused by the enhanced nucleation and growth of dislocation
loops.
• Therefore, the total dislocation density, which includes both network
dislocations and loop line length is important to understand the dose
rate effects on microstructural evolution in the higher dose region.
Enhanced cavity nucleation at lower
dose rates
Fe-15Cr-16Ni, SA
408 - 444 ˚C
Cavity Density (x1022 m-3)
2.0
At a given dose rate, cavity
density increases with
dose.
1.5
3.1
1.0
0.9
17 x 10-7
dpa/sec
0.5
7.8
0.27
390 ˚C
0.089
0
0
10
Both the absolute value
and the rate of increase in
cavity density are higher at
lower dose rate.
387 ˚C
5.4
20
30
40
50
60
Cumulative Dose (dpa)
70
Low dose rates enhance
cavity nucleation.
Cavity Density (x1021 m-3)
Effect of dose rate on cavity size distribution
at 7.2 ± 0.8 dpa
5
4
Higher Dose Rate
Lower Dose Rate
8.05 dpa
3.1 x 10-7 dpa/sec
6.36 dpa
0.91 / 1.5 x 10-7 dpa/sec
3
2
1
0
0
10
20
30
0
10
20
30
40
Cavity Diameter (nm)
• Larger cavities can be observed only at the lower dose rate,
indicating that cavity growth is also enhanced at low dose rate.
• A higher density of small cavities can be observed at the lower
dose rate, indicating continuous operation of cavity nucleation.
Average Cavity Diameter (nm)
Average cavity diameter is not a good
measure of dose rate effects
Fe-15Cr-16Ni, SA
408 - 444 ˚C
50
7.8
40
387 ˚C
17 x 10-7
dpa/sec
390 ˚C
30
5.4
20 0.9
3.1
10
0.27
0.089
0
0
10
20
30
40
50
60
Cumulative Dose (dpa)
70
At similar cumulative dose
levels, cavities with larger
diameter caused by
enhanced cavity growth at
lower dose rate are offset
by the small cavities
caused by continuous
cavity nucleation, resulting
in little effect of dose rate
on average diameter.
Cavity Density (x1021m-3)
Interpretation of cavity size distribution
1 cycle irradiation
2.36 dpa, 0.91 x 10-7 dpa/sec
2 cycles irradiation
6.36 dpa, 0.91 / 2.1 x 10-7 dpa/sec
5
Recent Cavities
4
Earlier Cavities
3
2
1
0
0
10
20
30
0
10
20
30
40
Cavity Diameter (nm)
Recent cavities
Cavities nucleated during the 2nd cycle of irradiation
Earlier cavities
Cavities nucleated during the 1st cycle of irradiation
Diameters of “Earlier Cavities” (nm)
Enhanced cavity growth at lower dose rates
Fe-15Cr-16Ni, SA
408 - 444 ˚C
50
It is clearly observed that cavity
growth is strongly enhanced at
lower dose rates.
387 ˚C
40
1.7 x 10-7
dpa/sec
390 ˚C
7.8
30
0.9
At lower dose rate, accelerated
dislocation evolution provides
sufficient vacancies, resulting
in enhancements of both cavity
nucleation and growth.
3.1
5.4
20
10
0 0
0.27
0.089
10
20
30
40
50
60
Cumulative Dose (dpa)
70
Earlier cavities
Cavities nucleated
during the 1st cycle
Effects of dose rate on swelling in
ion-irradiated Fe-15Cr-16Ni
1.0 x 10-4 dpa/sec
4 MeV Ni3+ irradiation shows the dose
rate effect operates at all temperatures.
300˚C
400˚C
4.0 x 10-4 dpa/sec
1.0 x 10-3 dpa/sec
500˚C
600˚C
Swelling (%)
10
1
10
0
101
102
103
10-1
100
101
10-1
100
101
10-1
100
Irradiation Dose (dpa)
101
10-1
100
101
102
Summary
• Lower dose rate increases swelling by shortening the
incubation dose for swelling. The incubation dose is
proportional to dose rate.
• The steady state swelling rate is not affected by the
difference in dose rate.
• Lower dose rate enhances network dislocation
formation. This is caused by enhanced loop growth and
unfaulting.
• At lower dose rate, enhanced dislocation evolution
increases sink strength of interstitials. This is the major
reason to enhance cavity nucleation and growth at lower
dose rate.