Development of IFAC-1 SS: An Advanced Austenitic Stainless Steel for Cladding and Wrapper Tube Applications in Sodium-Cooled Fast Reactors

Fuel cycle cost of sodium cooled fast reactors (SFRs) is strongly dependent on the in-reactor performance of core structural materials, i.e., cladding and wrapper tube materials of the fuel subassembly, which are subjected to intense neutron irradiation during service, leading to unique materials problems like void swelling, irradiation creep and helium embrittlement. In order to increase the burnup of the fuel and thereby reduce the fuel cycle cost, it is necessary to employ materials which have high resistance to void swelling as well as better high temperature mechanical properties. The Indian fast reactor program began with the commissioning of the 40 MWtFast Breeder Test Reactor (FBTR). The core structural material of FBTR is 20% cold worked 316 austenitic stainless steel (SS). For the 5000 MetPrototype Fast Breeder Reactor (PFBR) which is in an advanced stage of construction at Kalpakkam, 20% cold-worked alloy D9 (14Cr-15Ni-Ti SS) has been selected as the cladding and wrapper tube material for the initial core. The target burnup of the fuel is 100 GWd/t. Advanced austenitic stainless steel and oxide dispersion strengthened steels are being developed for achieving fuel burnup higher than 100 GWd/t. An advanced alloy D9 containing controlled amounts of titanium, silicon and phosphorous has been developed. This alloy named as IFAC-1 (Indian Fast Reactor advanced Clad-1) SS is aimed at thermal creep properties comparable to that of alloy D9, and superior void swelling resistance upto a target burn-up of about 150 GWd/t. The nominal chemical composition of IFAC-1 SS is 14Cr-15Ni-.25Ti-.75Si-.04P. The chemical composition has been optimized after extensive evaluation of the tensile, creep and microstructural stability of fifteen laboratory heats with different amounts of titanium, silicon and phosphorous. Void swelling behavior was studied using ion irradiation. IFAC-1 SS contains higher levels of low melting eutectic phase forming elements such as phosphorous, and so is susceptible to solidification cracking. Extensive pulsed TIG welding trials have been carried out on IFAC-1 SS/316LN SS weld joints with varied weld parameters to find out the feasibility of obtaining solidification crack-free welds and the optimum welding parameters have been established. This paper gives an overview of the development of this advanced core structural material for SFRs.

Recrystalization Behavior of Coldworked 14Cr-15Ni-2Mo Austenitic Stainless Steel Under Tensile Deformation

Recrystallization behavior of 20% coldworked alloy D9 (14Cr-15Ni-2Mo austenitic stainless steel) during high temperature tensile deformation has been assessed. The alloy was produced by vacuum induction melting followed either by vacuum arc refining (VAR) or electroslag refining (ESR). Tensile studies were carried out at various temperatures between ambient and 1073 K at an interval of 50 K and at strain rate of 1.2 × 10−3 s−1. The peak in the stress-strain curve was observed for the tensile tests conducted at 973–1073 K. This is attributed to dynamic recrystallization during tensile deformation. The values of the apparent activation energy were estimated as 384 kJ/mol for ESR grade alloy D9 and 372 kJ/mol for VAR grade alloy D9 by hyperbolic-sine Arrhenius equation, which agree well with that required for dynamic recrystallization in stainless steels. TEM studies confirmed extensive recovery and dynamic recrystalization in both the alloys.