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 to 1.7 x dpa/sec. Ion irradiation experiment on the identical Fe-15Cr- 16Ni conducted with 4 MeV Ni 3+ ions at 300, 400, 500, and 600 ˚C at three different dose rates between 1.0 x10 -3 to 1.0 x dpa/sec.

Neutron irradiation ; FFTF/MOTA FFTF COREABOVE CORE BELOW CORE Distance from midplane (cm) dpa/sec in stainless steels BC Irradiation at seven positions in, below and above the core. 2 cycles of irradiation in FFTF cycles #11 and #12 to achieve two dose levels at each dose rate. Materials Open Test Assembly Multiple specimens at each condition.

Dose Rate, dpa/secDose, dpaTemperature, ˚C #11#12#11#11 & #12#11# x x x x x x x x x x x x x x Neutron irradiation conditions Constant time experiment 2.59 x 10 7 sec (Cycle #11) 1.76 x 10 7 sec (Cycle #12) 426 ± 18 ˚C

Previous studies for the effect of dose rates 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. AlloysAuthorsReactors Difference in dose rate Austenitic alloys 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 A533B Fe-Cu alloys Nanstad et al.MTR, HFIR> 1 order KitaoJMTR< 1 order YanagidaKUR< 1 order Ferritic alloys Garner et al.EBR-II, FFTF< 1 order Nickel Alloys Garner et al.EBR-II< 1 order

Previous studies for the effect of dose rates AlloysAuthorsReactors Difference in dose rate Austenitic alloys 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 A533B Fe-Cu alloys Nanstad et al.MTR, HFIR> 1 order KitaoJMTR< 1 order YanagidaKUR< 1 order Ferritic alloys Garner et al.EBR-II, FFTF< 1 order Nickel Alloys Garner et al.EBR-II< 1 order 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.

Dose Rate, dpa/secDose, dpaTemperature, ˚C #11#12#11#11 & #12#11# x x x x x x x x x x x x x x Neutron irradiation conditions in this study Fast neutron irradiation in FFTF reactor More than two orders difference in dose rates achieved in one reactor. 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 100 nm 1 cycle 2 cycles 0.23 dpa8.05 dpa43.8 dpa 0.61 dpa11.1 dpa67.8 dpa 8.9 x dpa/sec3.1 x dpa/sec1.7 x dpa/sec

Cumulative Dose (dpa) Fe-15Cr-16Ni, SA ˚C Lower dose rate enhances swelling by shortening the incubation dose. The steady state swelling rate is not affected by the difference in dose rate. Enhanced swelling at lower dose rate Swelling (%) 17 x dpa/sec ˚C 390 ˚C Density change and microscopy data - 1%/dpa

Cumulative Dose (dpa) Swelling (%) Enhanced swelling at lower dose rate The steady state swelling rate is also observed at dose rates as low as < dpa/sec and irradiated less than 1 dpa. The incubation dose of swelling can therefore vary from 45 dpa when the dose rate varies over more than two orders of magnitude. Fe-15Cr-16Ni, SA ˚C

Cumulative Dose (dpa) Swelling (%) Enhanced swelling at lower dose rate Incubation Dose The steady state swelling rate is also observed at dose rates as low as < dpa/sec and irradiated less than 1 dpa. The incubation dose of swelling can therefore vary from 45 dpa when the dose rate varies over more than two orders of magnitude. Fe-15Cr-16Ni, SA ˚C

Dose Rate (dpa/sec) Incubation Dose (dpa) Strong effect of dose rate on incubation dose The incubation dose of swelling is almost linearly proportional to dose rate (dpa/sec) / 387 ℃ Fe-15Cr-16Ni, SA ˚C

Enhanced dislocation evolution at lower dose rate Lower dose rate enhances dislocation evolution. This effect arises primarily from the enhanced loop growth at lower dose rate. Cumulative Dose (dpa) Total Dislocation Density (x10 14 m -2 ) x dpa/sec ˚C 390 ˚C Fe-15Cr-16Ni, SA ˚C

Dose Rate (dpa/sec) Loop Density (m -3 ) Dose rate dependence of loop density At relatively low doses, the loop density is proportional to (dpa/sec) 1/2. 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 (dpa/sec) 1/ dpa Fe-15Cr-16Ni, SA ˚C

Effects of dose rate on dislocation evolution 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. 5.4 x dpa/sec Loop Network Cumulative Dose (dpa) Dislocation Density (x10 14 m -2 ) 5.4 x dpa/sec

Effects of dose rate on dislocation evolution 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. 5.4 x dpa/sec Loop Network Cumulative Dose (dpa) Dislocation Density (x10 14 m -2 ) 5.4 x dpa/sec

Cumulative Dose (dpa) Cavity Density (x10 22 m -3 ) Enhanced cavity nucleation at lower dose rates At a given dose rate, cavity density increases with dose. Low dose rates enhance cavity nucleation. Both the absolute value and the rate of increase in cavity density are higher at lower dose rate. 17 x dpa/sec ˚C 390 ˚C Fe-15Cr-16Ni, SA ˚C

Cavity Diameter (nm) Cavity Density (x10 21 m -3 ) Effect of dose rate on cavity size distribution at 7.2 ± 0.8 dpa 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.

Cumulative Dose (dpa) Average Cavity Diameter (nm) Average cavity diameter is not a good measure of dose rate effects 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 x dpa/sec ˚C 390 ˚C Fe-15Cr-16Ni, SA ˚C

Interpretation of cavity size distribution 2 cycles irradiation 6.36 dpa, 0.91 / 2.1 x dpa/sec 1 cycle irradiation 2.36 dpa, 0.91 x dpa/sec Cavity Diameter (nm) Cavity Density (x10 21 m -3 ) Recent Cavities Earlier Cavities Recent cavities Cavities nucleated during the 2nd cycle of irradiation Earlier cavities Cavities nucleated during the 1st cycle of irradiation

Cumulative Dose (dpa) Diameters of “Earlier Cavities” (nm) Enhanced cavity growth at lower dose rates It is clearly observed that cavity growth is strongly enhanced at lower dose rates. At lower dose rate, accelerated dislocation evolution provides sufficient vacancies, resulting in enhancements of both cavity nucleation and growth. Earlier cavities Cavities nucleated during the 1st cycle x dpa/sec ˚C 390 ˚C Fe-15Cr-16Ni, SA ˚C

Irradiation Dose (dpa) Swelling (%) Effects of dose rate on swelling in ion-irradiated Fe-15Cr-16Ni 300˚C ˚C500˚C600˚C 4 MeV Ni 3+ irradiation shows the dose rate effect operates at all temperatures. 1.0 x dpa/sec 4.0 x dpa/sec 1.0 x dpa/sec

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.