New Insights on the potential void swelling of AISI 304 and 316 … · 2016-07-29 · Advance...

New Insights on the potential void swelling of AISI 304 and 316 stainless steels in the

PWR baffle-former assembly

F. A. GarnerRadiation Effects Consulting

Texas A&M University

Advance Summary

• The void swelling equation used by the light water industry for annealed 304 stainless anticipates that eventually a steady-state swelling rate of ~1%% per dpa will be reached at all temperatures and dpa rates.

• The equation is based solely on EBR-II data >370°C, extrapolating to lower temperatures, but most of the PWR internals exist below 370°C.

• Recent Russian data suggest that such a high swelling rate is not reached even after ~150 dpa at temperatures <370°C.

• Is it possible that 1% per dpa does not apply to all combinations of dpa rate and temperature relevant to PWR internals?

• Analysis of large data field suggests that there are two swelling rate regimes of different magnitude and PWRs may lie primarily in the lower rate regime.

4

PWR core internals

• Constructed primarily from annealed 304

stainless steel.

•~2 cm maximum thickness

• The dpa rates in PWRS are one to two orders

lower than dpa rates of fast reactors.

• Void swelling is very sensitive to dpa rate.

• Temperature is also an important variable for

swelling.

75-100 dpa in

40 years

5

Plates are on the order of 1-2 cm thick.

PWR core internals are constructed from SA304 and CW316 stainless steels (baffle-former-barrel)

Stainless steels

6



280-

290°C

345°C

Coolant temperatures

7



280-

290°C

345°C

Steel can become hotter than

345°C due to gamma heating

and thermal resistivity.

Large volumes

of water inside

the assembly

Construction of PWR baffle-former-barrel assembly

8

Strongest heating by

gamma rays from fuel

Localized hot spots can

also arise from coolant

eddies and peaks in

“local” gamma heating.

280 to ~480°C

Local gamma heating arises from neutron thermalization in water.

9

Voids observed at ~340°C and 12 dpa in PWR Tihangebaffle-former bolt (cold-worked 316 stainless steel)

D. J. Edwards, F. A. Garner, B. A. Oliver and S. M. Bruemmer, 2001

This steel was previously thought to be very resistant to swelling under LWR-relevant conditions.

The majority of the internal structure of LWRs is composed of annealed 304 stainless which is much more prone to swelling.

~0.2% swelling

10

Voids observed at ~340°C and 12 dpa in PWR Tihangebaffle-former bolt (cold-worked 316 stainless steel)

D. J. Edwards, F. A. Garner, B. A. Oliver and S. M. Bruemmer, 2001

This steel was previously thought to be very resistant to swelling under LWR-relevant conditions.

The majority of the internal structure of LWRs is composed of annealed 304 stainless which is much more prone to swelling.

~0.2% swelling

Low dpa rate, low dose, low temperature compared to conditions in EBR-II.

11

Microstructure of 304L BWR Core Shroud

after 25 years, reaching 1.9 dpa at 290ºC

Thomas, Edwards, Asano, Ooki and Bruemmer, 2007.

Fresnel contrast image <111> Relrod image (Frank loops)

12

Voids in austenitic pressure vessel of BR-10 at 1.9 x 10-9 dpa/secPorollo, Konobeev and Garner, 2005

12Х18Н9Т steel at 0.64 dpa and 350ºC left-hand - large voids on sub-grain boundaries

right-hand - spatial distribution of smaller voids.

50 nm 100 nm

Russian analog of AISI 321

13

Swelling of EI-847 stainless steel (variant of 316 SS)

in BN-350 at relatively low temperaturesPorollo, Konobeev, Garner, 2000

• Is it realistic to expect a swelling rate of 1%/dpa to develop at PWR-relevant temperatures?

• Some Russian evidence suggests it is possible.

At temperatures as low as 330-340°C, it appears that swelling eventually attains the ~1%/dpa steady-state swelling rate known to be characteristic of austenitic alloys.

C. Cawthorne and E. J. Fulton, “Voids in

Irradiated Stainless Steel,” Nature, Vol. 216,

November 11, 1967

After swelling was discovered at ~1.5 % in the U.K., annealed 304 stainless steel ducts surrounding safety and control rods were pulled from EBR-II with swelling as high as 11% swelling at ~60 dpa.

Fish et al.,ASTM STP 529 1973

Many examples of significant swelling in 304 stainless steel were found thereafter.

Long pressurized creep tubesWalters et al. J. Nucl. Mater. 43 1972

Bottom half of long creep tubesPorter and GarnerASTM STP 8701985

~5% swelling

Decreasing dpa rate strongly accelerates the onset of high rate swelling.

17

Voids and carbide precipitates observed in annealed AISI 304 irradiated in EBR-II fast reactor at 380°C to 21.7 dpa

0.84 x 10-7

dpa/sec

1.0%

swelling

Bond, Sencer, Garner, Hamilton, Allen and Porter, 2001

The early U. S. LMR program needed a predictive swelling equation for annealed 304 stainless steel.

• Three or four equations were in circulation, but the most popular was the Foster-Flinn equation.

• Data base involved a number of heats from different experiments, introducing material and environmental scatter.

• Swelling equation contained only neutron fluence and irradiation temperature but not dpa rate.

The early U. S. LMR program needed a predictive swelling equation for annealed 304 stainless steel.

• Three or four equations were in circulation, but the most popular was the Foster-Flinn equation.

• Data base involved a number of heats from different experiments, introducing material and environmental scatter.

• Swelling equation contained only neutron fluence and irradiation temperature but not dpa rate.

• Due to its high swelling behavior 304 was dropped from the LMR program and there was no further incentive to develop improved equations.

• When swelling later became an issue for the APWR the Foster-Flinn equation was chosen.

Need for a new equation for APWR

• Foster-Flinn equation was derived from data above 370°C, the inlet temperature of EBR-II.

• Most of PWR internals are below this temperature.

• Neutron fluence had been replaced by dpa.

• Dpa rate was now known to be as important as temperature.

Need for a new equation for APWR

• Foster-Flinn equation was derived from data above 370°C, the inlet temperature of EBR-II.

• Most of PWR internals are below this temperature.

• Neutron fluence had been replaced by dpa.

• Dpa rate was now known to be as important as temperature.

• It was proposed to develop a new equation using five EBR-II hex-ducts that contained reflector blocks in rows 8-14.

• Involved a single heat of steel with more data available in the region 370-400°C, hopefully allowing better extrapolation below 370°C.

• For design continuity the funding sponsor mandated that the temperature dependence of the Foster-Flinn equation be retained for comparison.

22

Flux-dependent swelling equation for SA 304 was developed for APWR predictions

Garner et al., 2005

Two Row 10 assemblies on opposite sides of the reactor

5 hexagonal ducts from identical SA 304 heat

Rows 8-10 in reflector and row 14 of blanket of EBR-II

2 cm disks punched for density change and microscopy

Total of 280 disks

372-440°C and 2-32 dpa

Range of PWR-relevant dpa rates.

23

Flux-dependent swelling equation for SA 304 was developed for APWR predictions

Garner et al., 2005

Two Row 10 assemblies on opposite sides of the reactor

5 hexagonal ducts from identical SA 304 heat

Rows 8-10 in reflector and row 14 of blanket of EBR-II

2 cm disks punched for density change and microscopy

Total of 280 disks

372-440°C and 2-32 dpa

Range of PWR-relevant dpa rates.

ΔV/V =A(T) (dpa rate)-0.731(dpa)2

With 1%/dpa as maximum rate

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 5 10 15 20 25 30 35

DPA

Sw

ellin

g (

%)

U9009

U9007

U9009

0.38-0.96 x 10-7

dpa/sec

U9007

0.44-1.12 x 10-7

dpa/sec

~

Comparison of swelling in two Row 10 assemblies on opposite sides of the reactor

Data shown only from lower half of the assemblies

~16% difference in dpa rate.

Lower dpa rate leads to earlier swelling.

Data shown from bottom half of ducts only to keep temperature as close to PWR-relevant as possible.

Comparison of swelling observed in Rows 8-14

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 5 10 15 20 25 30 35

Sw

ell

ing

(%

)

DPA

U1603

U9009

U8972

U9807

U1603

Row 140.062 - 0.156 x 10-7

U9009

Row 100.38 - 0.96 x 10-7

U8972

Row 91.00-2.05 x 10-7

U9807

Row 81.25-3.60 x 10-7

dpa/sec

Decrease in the dpa rate leads to a progressive shortening in the duration of the transient regime.

Highest swelling rate in this data set is ~0.3%/dpa.

Data shown from bottom half of ducts only to keep temperature as close to PWR-relevant as possible.

S = (dpa)2 (dpa rate)–0.731 F(T)

where F(T) = EXP (22.106 – (18558/(T + 273.15)))

with swelling S given in %, temperature T in °C and dpa rate is in units of 10–7 dpa/sec.

Transient curvature dictated by (dpa)2 term and swelling rate will eventually exceed 1%/dpa.

Swelling rate is constrained not to exceed 1%/dpa, but this rate is expected to occur at all temperatures of PWR interest.

Modified Foster-Flinn equation for swelling of annealed 304 stainless steel

Two questions need to be addressed.

• Is it realistic to expect that ~1%/dpa is a universal steady-state for this steel under all “high” temperature conditions?

• Does this requirement also apply at “low” PWR-relevant temperatures.

• We now have more data on “high” temperature behavior.

• No PWR or EBR-II data can answer the second question!

• There are some data that suggest that the steady-state swelling rate might be lower below 370°C.

Two questions need to be addressed.

• Is it realistic to expect that ~1%/dpa is a universal steady-state for this steel under all “high” temperature conditions?

• Does this requirement also apply at “low” PWR-relevant temperatures.

• We now have more data on “high” temperature behavior.

• No PWR or EBR-II data can directly answer the second question!

• There are some data that suggest that the steady-state swelling rate might be lower below 370°C.

Reirradiation experiments on annealed 304 stainless steel with stress changes

Garner, Flinn and Hall, 2009

Stressed to unstressed Unstressed to stressed

Long unstressed components spanning a peak in dpa rate but increasing in temperature to produce “swelling loops” in several irradiation experiments on 304 stainless steel (Garner, 2000—2008)

Long capsules EBR-II safety rod duct

Long 304 SS fuel pins spanning a peak in dpa rate but increasing in temperature to produce “swelling loops” (Garner, 2000-2008)

Low power pins High power pins

As temperature rises the loops become tighter, indicating a relative independence of temperature at higher temperatures.

Similar behavior observed in 316 SS, reinforcing the conclusion thar 300 series stainless steels approach 1%/dpa at temperatures >380°C.

Long 304 SS fuel pins spanning a peak in dpa rate but increasing in temperature to produce “swelling loops” (Garner, 2000-2008)

Low power pins High power pins

As temperature rises the loops become tighter, indicating a relative independence of temperature at higher temperatures.

Similar behavior observed in 316 SS, reinforcing the conclusion that 300 series stainless steels approach 1%/dpa at temperatures >380°C.

Might there be a lower intrinsic swelling rate of 300 series stainless steels at PWR-relevant fluxes and

temperatures?

34

After 41 years in BOR-60 Row 10 the Э-65 duct reflector assembly (Russian analog of AISI 321) swelling did not exceed 2% at 330-360°C and 108-150 dpa

№1

№3

№5

№8

№11

Э-65

Level of examination dpaAverage irradiation

temperature, оС

1 56 380

3 91 370

5 131 360

8 150 345

11 108 33015

75

№1

№3

№5№8№11

-1

1

3

5

7

9

0 200 400 600

Sw

ell

ing

, %

Distance from bottom of duct, mm345°C, 150 dpa

Dpa rates are just above PWR internal conditions.

35


№1

№3

№5

№8

№11

Э-65


temperature, оС

1 56 380

3 91 370

5 131 360

8 150 345

11 108 33015

75

№1

№3

№5№8№11

-1

1

3

5

7

9

0 200 400 600

Sw

ell

ing

, %

Distance from bottom of duct, mm

Above 360°C a higher swelling rate regime starts to develop, such that at 380°C swelling of 6% is exceeded at only 56 dpa.

Dpa rates are just above PWR internal conditions.

36


№1

№3

№5

№8

№11

Э-65


temperature, оС

1 56 380

3 91 370

5 131 360

8 150 345

11 108 33015

75

№1

№3

№5№8№11

-1

1

3

5

7

9

0 200 400 600

Sw

ell

ing

, %

Distance from bottom of duct, mm

Above 360°C a higher swelling rate regime starts to develop, such that at 380°C swelling of 6% is exceeded at only 56 dpa.

Might there be two temperature regimes of swelling?

Swelling of annealed 304 at 390C over a wide range of dpa rates

Johnson and Hofmann, 1977

Dependence of swelling in annealed 304 stainless steel as observed in Rows 10, 12 and 14 of EBR-II

Allen and coworkers, 2006


Eventual “breakaway” to higher swelling rate regime depending on environmental variables.

Two ducts in row 9 of EBR-II but with ~16% difference in dpa rate

As dpa rate decreases there is a tendency to form loops, indicating a transition from low swelling rate to higher swelling rate.

Swelling loops seen on Row 10 duct used to establish the current swelling equation for 304 SS


Note that high flux side of duct stays on one path but low flux side breaks away to a higher swelling rate.


Very low flux duct

Swelling observed in four faces of a hexagonal flux thimble tube in row 5 of EBR-II (dpa rates intermediate to PWR values)

Temperatures are very similar for different duct faces but there are significant differences in neutron flux.

Swelling observed in four faces of a hexagonal flux thimble tube in row 5 of EBR-II (dpa rates intermediate to PWR values)

Dependence of transient duration on dpa rate is seen in high temperature part of faces.

Very high swelling observed in six faces of a Fe–18Cr–10Ni–Tihexagonal fuel duct irradiated in the BOR-60 fast reactor

Neustroev and Garner, 2008

Low inlet temperatureOf 320°C

Very high swelling observed in six faces of a Fe–18Cr–10Ni–Tihexagonal fuel duct irradiated in the BOR-60 fast reactor

Neustroev and Garner, 2008

Low inlet temperatureOf 320°C

Breakaway in 360-390°C interval

Swelling of cold-worked M316 fuel cladding in DFRC. Cawthorne, U.S./U.K. Exchange, 1979

Increasing temperatureVery low inlet

temperature of ~270°C

Swelling of cold-worked M316 fuel cladding in DFRC. Cawthorne, 1979

Increasing temperature

breakaway

Breakaway temperature is ~370°C

Conclusions

• The modified Foster-Flinn equation is an improvement over the earlier Foster-Flinn equation in that it explicitly accounts for the effect of dpa rate.

• Lack of material variability in current data bases allows better description of influence of reactor variables.

• Both versions are based on data only above 370°C, introducing

considerable uncertainty into predictions at lower temperatures.

• 1%/dpa still appears to be a universal swelling rate at “high” (>360-380°C) temperatures.

• For “lower” PWR-relevant temperatures a bounding estimate might be ~0.07%/dpa, serving as a universal rate for the transient regime.

• The “breakaway” to higher rate appears to be in the range 360-380°C.

• Much of the PWR internals may never reach a swelling rate of ~1%/dpa.

New Insights on the potential void swelling of AISI 304 and 316 … · 2016-07-29 · Advance...

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