Strategies for Mitigating Swelling in Austenitic Stainless Steels in Fast Reactors NERS 521 – Final Presentation David Sirajuddin Nuclear Engineering & Radiological Sciences

Outline Definitions – voids, swelling Swelling dependencies Overview of Fast Reactor environment and material demands – Proposed material: Austenitic Stainless Steel Techniques for mitigating swelling: I.Cold-working (CW) II. Impurity atom introduction III. Compositional changes Summary & Conclusions

Swelling is a macroscopic effect of void formation and growth Voids = aggregation of vacancies Void formation and growth swelling Swelling can be quantified as percent volume change, D V/V [%], in a material Low swelling rate transient region Higher swelling rate steady state region ~ 1%/dpa

Void growth and formation dependents Approximate void growth equation[Brailsford and Bullough] Dose:Swelling increases with dose Dose Rates:Swelling decreases with dose rate Temperature: Maximum peak exhibited at intermediate temperature, minimum threshold for void growth ; [Was]

FR environment demands materials that can withstand harsher environments Approximate operating environments of Gen IV Fast Reactor (FR) systems [Allen] Austenitic stainless steels have been proposed for fuel cladding, baffles, etc. materials for FR components Reactor TypeCoolant Inlet Temperature ( o C) Coolant Outlet Temperature ( o C) Maximum Dose (dpa) PWR SCWR VHTR SFR LFR GFR MSR

What this means… Bad News: Swelling alters material properties and dimensions of austenitic stainless steels materials change from intended design parameters during operation! More Bad News: All operating temperatures of FRs encourage void formation and growth in austenitic stainless steels (SS) But, Good News: Swelling can be mitigated by material treatments, and material compositional changes

Objective Find treatments and changes that can be applied to austenitic stainless steels to make them more swelling resistant [Porolla, et al] [Encyclopedia Brittanica]

Swelling can be reduced by discouraging void growth General Strategy: Extend transient region of swelling vs. accumulated dose curve Specific Strategies: i.Cold-working (CW) ii.Addition of impurities iii.Fine-tune alloy composition iv.Use a different phase of steel!

Cold-working dampens void growth by extending the transient region Increased CW decreases material swelling by extending the transient region CW dampens the swelling peak in temperature dependence All same slope ! [Was, Dupuoy et al, Busboom et al]

Impurity introduction discourages void nucleation reduction of swelling Introduction of impurity atoms decrease swelling soluble atoms bind with point defects, reducing mobility and encouraging recombination Examples: Si, P, Hf (oversized) Trend shows increasing binding energy increased activation energy of voids void growth surpressed [Mansur et al, Was]

Impurity introduction contd Phosphorous and Silicon implantation decrease swelling [Garner, et al., Was]

Alloy composition can be fine-tuned to better accommodate swelling Increased Ni concentration extends the transient region This extension decreases swelling Decreasing trend continues until a minimum is reached at 50 at% [Was]. [Ukai, et al] [Garner et al, Was]

Conclusions & Summary Material composition changes and treatments dampen swelling! Impurity atoms inhibit void nucleation, Ni content increase extends incubation period CW prolongs transient region [Allen]

References 1. S. Ukai, et al. Swelling rate versus swelling correlation in 20% cold-worked 316 stainless steels. Journal of Nuclear Materials. 15 December E. R. Gilbert, et al. The influence of Cold-work level on the irradiation creep and swelling of AISI 316 stainless steel irradiated as pressurized tubes in the EBR-II fast reactor. Journal of Nuclear Materials. 3. N. Igata et al. Effect of light impurities on the early stage of swelling in austenitic stainless steel. Journal of Nuclear Materials (1998) Surh, Michael P. Vacancy cluster evolution and swelling in irradiated 316 stainless steel. Journal of Nuclear Materials 328 (2004) March G. S. Was. Fundamentals of Radiation Materials Science. Springer-Verlag Berlin Heidelberg. New York M. P. Surh, J. B. Sturgeon, W. G. Wolfer The Incubation Period for Void Swelling and its Dependence on Temperature, Dose Rate, and Dislocation Structure Evolution. 21st Symposium on Effects of Radiation on Materials, Tucson, AZ. 7. K.C. Russella. Void nucleation with embryo injection Departments of Materials Science and Engineering and Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA 8. E. P. Simonenb, S. M. Bruemmerb, L. Fournierc, B. H. Sencerc and G. S. Was The effect of oversized solute additions on the microstructure of 316SS irradiated with 5 MeV Ni++ ions or 3.2 MeV protons. Received 6 June 2002; accepted 11 November ; Available online 20 December 2003.

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