Jenny’s D-9 Lit Summaries - 3/13/06

Raj, “Development of fuels and structural materials for fast breeder reactors”

Summary:

In order to reduce the fuel cycle cost of Fast Breeder Reactors, the burn-up of the fuel must be increased. This means that the FBR materials must be sufficient enough to handle the high temperature mechanical properties such as creep, low cycle fatigue, compatibility with liquid sodium coolant, stress corrosion cracking, void swelling, and weldability.

Experiments show that void swelling in 316 ss is too high, though titanium additions (thereby making D-9) help to lower the amount. Fine grains are preferred so as to maximize the mechanical properties. Tensile and creep properties of different heats were studied on three different Ti/C ratio samples. The ratio of 6 showed the best tensile properties, while 4 showed the best creep properties. The optimum level of cold working for D-9 that would not lead to softening of the material was found to be 20%. D9 is therefore determined to be the best choice for cladding and wrapper materials for FBR’s. Phosphorous, silicon, and boron also are added to improve resisance to void swelling, though they lower the weldability.

D9 has high temperature mechanical properties, compatibility with liquid sodium coolant, and good weldability, which makes it the overall choice for most parts of FBR’s, however, It’s thermal conductivity is relatively poor. Operational changes in the reacter such as shut downs, start ups, and power transients causes temperature gradients in the metal because mass transfer of the constituent elements occurs from high temp regions to low temp. This causes fatigue damage to the material. Thermal stresses need to be accounted for in design. Carbon helps impart high temperature strength.

Important note, these corrosion studies of D-9 and 316 were conducted in a liquid sodium environment, not LBE. This paper also states that precipitation of (Fe,Cr)23C6 at the grain boundaries and the consequent Cr depletion NEAR the grain boundary is the cause of inter granular corrosion.