Information Notice No. 90-68: Supplement 1: Stress Corrosion Cracking of Reactor Coolant Pump Bolts
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 April 14, 1994 NRC INFORMATION NOTICE 90-68, SUPPLEMENT 1: STRESS CORROSION CRACKING OF REACTOR COOLANT PUMP BOLTS Addressees All holders of operating licenses or construction permits for pressurized- water reactors (PWRs). Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to the failure of Westinghouse reactor coolant pump turning vane cap screws at the Millstone Nuclear Generating Station, Unit 3. In this event, the failed cap screws were fabricated from material that has demonstrated susceptibility to intergranular stress corrosion cracking (IGSCC) as were the ones in the original information notice. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required. Description of Circumstances On September 2, 1993, the licensee for Millstone Unit 3 was inspecting the reactor lower core support plate before reloading fuel. The licensee discovered pieces of a locking cup for the Westinghouse model 93A-1 reactor coolant pump turning vane cap screws. The cap screws connect the flanged interfaces of the turning vane and thermal barrier. The joint is gasketed to prevent bypass leakage of reactor coolant beyond the thermal barrier. The licensee subsequently removed four turning vane cap screws for inspection. A visual and liquid penetrant inspection at the juncture of the head and body of the cap screws revealed cracks in two cap screws. One cap screw had no cracks. The head of the fourth cap screw was almost completely severed. The cap screws are made of alloy A286 stainless steel, designated by the American Society for Testing and Materials as A453 grade 660. The cap screw or cap screw head may deform, loosen, fracture, or fail the locking cup restraints. Cap screw failures could present a safety hazard because failed parts could enter the reactor coolant system and cause damage to vital components. The licensee has replaced all four Unit 3 reactor coolant pumps. The replacement pumps incorporate design improvements recommended by Westinghouse to reduce the susceptibility of the cap screws to IGSCC. These improvements include: 9404080152. IN 90-68, Supp. 1 April 14, 1994 Page 2 of 2 1. increased head-to-shank fillet radius between the cap and body of the cap screw to reduce stress concentration; 2. reduced thread engagement to decrease axial and bending stiffness; 3. reduced bolt shank diameter to lessen bending stresses; and 4. replacement of the A-286 alloy with cold-worked 316 stainless steel. Discussion Information Notice 90-68 described similar reactor coolant pump bolt failures at a foreign reactor. The failed bolts at the foreign reactor were also used to fasten the turning vanes of the reactor coolant pump. The bolts were of the same material as that of the Millstone turning vane cap screws. The information notice stated that considerable data have been available regarding the susceptibility of alloy A-286 to IGSCC. Cap screws fabricated of alloy A-286 are subject to IGSCC at peak stresses above 689 MPa [100 ksi]. The occurrence of IGSCC also appears to be a function of chromium content, fabrication practice, and environment. The Brookhaven National Laboratory conducted an extensive study of bolting. The results of the study are documented in NUREG/CR-3604, "Bolting Applications," May 1984. The report discussed the direct relationship between loading and IGSCC of bolts fabricated of alloy A-286. This report recommended that alloy A-286 not be used as a reactor structural material because of its susceptibility to IGSCC. This information notice requires no specific action or written response. If you have any questions about this matter, please contact the technical contact listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager. /S/'D BY CIGRIMES/FOR Brian K. Grimes, Director Division of Operating Reactor Support Office of Nuclear Reactor Regulation Technical contact: N. Fields, NRR (301) 504-1173
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021