Investigations on core basket bolts from a VVER 440 power plant

NDE investigations using ultrasonic inspections were performed on core basket bolts at a VVER 440 unit. Bolts with indications were replaced. Destructive investigations were performed on three bolts with indications. These investigations comprised of microstructural investigations using optical microscopy, hardness measurements and scanning electron microscopy of fracture surfaces. The bolts are M12 bolt manufactured from solution annealed Ti-stabilized stainless steel. Only one bolt had suffered from cracking. The washer of this bolt had been welded to the shielding plate, which had resulted in unintentionally high stresses. The observed cracking is regarded to be a result of intergranular stress corrosion cracking, enhanced by irradiation. The reason for the NDE indications in the other bolts were incomplete filling of the flat slot of the bolt head when the bolt was welded to the washer to prevent unscrewing of the bolt during operation