Multiple reactor vessel internals bolt failures were discovered during the 1981 and 1982 in-service inspections performed at three pressurized water reactor nuclear power plants. All the failures were limited to bolts that fastened the lower portion of the reactor vessel internals thermal shield to the lower grid assembly. Subsequent examinations during 1982, 1983, and 1984 revealed bolt failures at four additional plants. These failures included bolts that fastened the core barrel to the core support shield and lower grid assembly. All the affected fasteners were fabricated from Alloy A-286 [American Society of Testing Materials (ASTM) A-453, Grade 660] material. The results of an extensive evaluation program revealed the failure mechanism to be environmentally assisted intergranular stress cracking. It has also been concluded that Alloy A-286 bolting material will operate successfully at stress levels below the material’s yield strength. At stress levels greater than the material’s yield strength, the probability for this form of cracking is greatly increased.