Extensive experimental investigations found3 that the ferritic steels, whose high temperature mechanical properties are far inferior to austenitic stainless steels, displayed excellent radiation resistance. The ferritic — martensitic steels (9-12% Cr) have, therefore, been chosen for clad and wrapper applications, in order to achieve the high burn-up of the fuel. This is based26-29 (Table 5) on their inherent low swelling behavior. The 9Cr-1Mo steel, modified 9Cr-1Mo (Grade 91), 9Cr-2Mo, and 12Cr-1MoVW (HT9) have low swelling rates at doses as high as 200 dpa. For example, HT9 shows 1% swelling at 693 K for 200 dpa. The threshold dose for swelling in ferritic steels is as high as nearly 200 dpa in contrast to 80 dpa for the present generation D9 austenitic stainless steel. It is established that the void swelling depends crucially on the structure of the matrix lattice, in which irradiation produces the excess defects.

Extensive basic studies have identified19,30-33 the following reasons as the origin of superior swelling resistance in ferritic steels:

1. The relaxation volume for interstitials, that is, the volume of the matrix in which distortion is introduced by creatingan interstitial, in bcc ferrite is larger19 than fcc austenite. For every interstitial introduced, the lattice distortion is high and hence the strain energy of the lattice. Hence, the bias toward attracting or accommodating interstitials in the bcc lattice is less. This leaves higher density of‘free’ interstitials in the bcc lattice than fcc lattice. As a result, recombina­tion probability with vacancies increases significantly and supersaturation of vacancy reduces. Conse­quently, the void nucleation and swelling is less.

2. The migration energy of vacancies in bcc iron is only 0.55 eV, against a high value in fcc austenite, 1.4 eV. Vacancies are more mobile in bcc than fcc, increasing the recombination probabilities in bcc ferrite. Another factor is the high binding energy between carbon and vacancy in bcc iron (0.85 eV), while it is only 0.36-0.41 eV in austenite. This leads19 to enhanced point defect recombina­tion in bcc than fcc, due to more trapping of vacancies by carbon or nitrogen.

3. In bcc iron, it is known30 that there is a strong inter­action between dislocations and interstitials solutes, forming atmospheres of solutes around dislocations.

The formation of ‘atmospheres’ around dislocations makes them more effective sinks for vacancies than interstitials, resulting in suppression of void growth,

V Ka і

(d)

Figure 5 Initial structure24 of normalized and tempered modified 9Cr-1Mo steel: (a) Monocarbides (MC) and M23C6 along lath boundaries in a carbon extraction replica of the sample and (b) Microdiffraction of fine particle marked B, confirming the crystal structure of MC. Energy dispersive analysis of X-rays (EDAX) identifying the MC particles (B) to be rich in

(c) V and (d) Nb.

provided the following conditions are satisfied: ‘atmo­spheres’ comprise of either oversized substitutional atoms or interstitials, dislocations have high binding energy with solutes, and concentration of solute atoms at the core of the dislocation exceeds a critical value. On the other hand, if‘atmosphere’ is made up of undersized atoms like Si or P, the voids can grow. The ‘atmosphere’ of interstitials reduces the dislocation bias for additional capture and inhibits dislocation climb, thus converting them to saturable sinks. Such a scenario would increase the recombination prob­abilities, suppressing the void growth.

These fundamental differences in the behavior of solutes and point defects in bcc lattice make ferritic steel far superior to austenitic steels, with respect to radiation damage.

The challenging task for materials scientists to use ferritic steels directly in fast reactor fuel assembly was with respect to enhancing the high temperature mechanical properties of the ferritic steels, especially high temperature creep life and irradiation creep resistance.