[1] Univ London Imperial Coll Sci Technol & Med, Dept Mat, London SW7 2AZ, England
[2] Commiss European, Joint Res Ctr, Inst Transuranium Elements, D-76125 Karlsruhe, Germany
[3] Australian Nucl Sci & Technol Org, Inst Mat Engn, Menai, NSW 2234, Australia