There is widespread interest in the effect of neutron bombardment on the mechanical properties of the austenitic stainless steels because of their frequent application in reactor structures and fuel claddings. For this reason, a columbium-stabilized austenitic stainless steel (20Cr-25Ni-bal Fe) developed in the United Kingdom for gas-cooled reactor service has been tested inreactor for its stress-rupture characteristics.

The tests were performed by internally pressurizing tubes at temperatures of 550, 650, 750, and 850 C in a neutron flux of approximately 5.0 x 10¹² nv fast (>2.9 Mev) and 6.0 ¹³ 1013 nv thermal. Duplicate tests were performed ex-reactor. Brief details of test procedures and a discussion of the experimental results are presented.

Only a small effect of irradiation on material strength was seen, but a strong effect of irradiation and temperature on the fracture ductility was observed.