The design of high nickel austenitic steels for the core materlals of the fast reactors was performed following the d-electrons concept devised on the basis of molecular orbital calculations of transition-metal based alloys. In this design two calculated parameters are mainly utilized. The one is the d-orbital energy level (Md) of alloying transitlon elemehts, and the other is the bond order (Bo) that is a measure of the covalent bond strength between atoms. Using the Md-Bo phase stability diagram accurate prediction become possible for the phase stability of the austenite phase and 5% swelling at 140dpa for nickel ions. Here. Md and Bo are the compositional average of Md and Bo parameters, respectively. On the basis of the phase stability diagram and preliminary experiments, guidelines for the alloy design of carbo-nitraides precipitated high nickel austenitic steels were constructed. Following the guldelines several new austenitlc steels were designed for the fast reactors core material.
