STRESS CORROSION OF TYPE 304 STAINLESS STEEL IN SIMULATED SUPERHEAT REACTOR ENVIRONMENTS. I. Informal AEC Research and Develoment Report 568-TIO-2

A fuel jacket failure that occurred in May 1961 in the Type 304 stainless steel clad fuel element exposed in the Vallecitos Boiling Water Reactor superheated steam loop (SADE) was attributed to chloride stress corrosion cracking. In order to better understand the failure, a test program was carried out to try to reproduce the rapid stress corrosion attack in the simulated superheat reactor environment of the CL-1 superheat facility. The methods of corrosion testing under heat transfer conditions reported previously were modified: to apply a longitudinal stress on the test sheaths to produce a 0.1 per cent elongation in 1000 hours; to increase the chloride content of the moisture carryover with the steam by increasing the chloride in the recirculating water to 1.5 ppm; and to expose the solids deposits to various metal temperatures. After 1000 hours of exposure, no significant attack was noted on the test sheaths. The test procedures were further altered to simulate the significant amount of SADE fuel element exposure to saturated steam at varying temperatures with little to no superheat being generated. A 776-hour total exposure was carried out with the test conditions cycled several times. The entrance heater (calculated metal temperature during normal operation 800 to 900 deg F) was covered with numerous fine cracks that did not penetrate completely through the 0.030-inch sheath. The cracks were predominantly transgranular. Discontinuities were located by ultrasonic techniques in the middle (metal temperature ~ 1000 deg F) and exit (metal temperature ~ 1200 deg F) sheaths. The discontinuities were confirmed as intergranular penetrations of 0.006 to 0.010-inch depth respectively by metallography. Both types of attack were found on the SADE fuel element cladding failure. Chloride salts of chromium, copper, iron, and nickel were found by x- ray diffraction in the deposit taken from the test sheaths. Some laboratory tests with unsensitized and sensitized Type 304 stainless indicated that water solutions of copper or iron chloride salts chemically attacked the sensitized material intergranularly independent of stress. The test results indicated that chemical and chloride stress corrosion attack can act either singularly or in combination to produce the type failures experienced in the SADE and CL-1 tests. The presence of stress had little apparent effect on the uniform corrosion rate of the test sheaths, except when the stresses were high enough to cause creep. The creep resuited in scale spalling with some accelerated corrosion in the areas of scale cracking. (auth