Thermal Gradient Effects on Stress-Rupture Behavior of Thin Walled Tubing
ROCKWELL INTERNATIONAL CANOGA PARK CA ATOMICS INTERNATIONAL DIV
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The results of stress-rupture tests of internally heated and pressurized thin-walled tubes of Type 304 stainless steel are presented. The tubes were immersed in high-purity sodium at 1200 deg F and at a flow rate of 20 ftsec. A heat flux of 10exp 6 Btusq ft-hr passed through the wall. The heat flux was applied in three modes steady 6-min on, 6-min off and 1-min on, 1-min off. The purpose of these tests is to examine the separate and combined effects on creep behavior of stainless steel fuel rod cladding from 1 Internal NaK pressurization 2 Thermal gradient through the cladding wall 3 Thermal cycling 4 Rate of thermal cycling and 5 Mass transfer. Control tests without internal heating showed that NaK versus helium pressurization and flowing sodium have no effect on the creep behavior. Steady internal heating also has little apparent influence on creep behavior even though evidence of mass transfer was found on the surface. Cyclic heating produced large changes in the microstructures, strain profiles, strain at failure, and the stress-rupture life. Both the reduced life and the attendant strain are explained semi-quantitatively in terms of a ratcheting mechanism using primary creep. These results support the concept that ratcheting of the cladding due to reactor load variations is a distinct possibility and that this must be considered in designing fuel elements for long life and high power density operation.
- Properties of Metals and Alloys