Strategies for Mitigating Swelling in Austenitic Stainless Steels in Fast Reactors

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

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

3. 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, DV/V [%], in a material • Low swelling rate transient region • Higher swelling rate steady state region ~ 1%/dpa

4. 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]

5. FR environment demands materials that can withstand harsher environments • Approximate operating environments of Gen IV Fast Reactor (FR) systems • [Allen] • Austenitic stainlesssteels have been proposed for fuel cladding, baffles, etc. materials for FR components

6. 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

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

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

9. 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]

10. 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]

11. Impurity introduction cont’d • Phosphorous and Silicon implantation decrease swelling [Garner, et al., Was] [Garner, et al., Was]

12. 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]

13. 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]

14. References • 1. S. Ukai, et al. Swelling rate versus swelling correlation in 20% cold-worked 316 stainless • steels. Journal of Nuclear Materials. 15 December 2002. • 2. 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 258263 (1998) 1735-1739. • 4. Surh, Michael P. Vacancy cluster evolution and swelling in irradiated 316 stainless steel. • Journal of Nuclear Materials 328 (2004) 107114. March 2005. • 5. G. S. Was. Fundamentals of Radiation Materials Science. Springer-Verlag Berlin Heidelberg. New York. 2007. • 6. 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 2003. ; Available online • 20 December 2003.

15. Questions? • Does anyone have any questions?