Investigations on core basket bolts from a VVER 440 power plant

Non-destructive examinations using ultrasonic inspections were performed on core basket bolts at a VVER 440 unit. The four M12 bolts, manufactured from solution annealed Ti-stabilized stainless steel, had been replaced due to NDE indications. Destructive investigations were performed on three of these bolts. One bolt had suffered from intergranular stress corrosion cracking, enhanced by irradiation. Irradiation induced grain boundary segregation was observed in the material. The washer of the bolt with cracking had been unintentionally spot welded to the shielding plate, restricting the free movement of the components to adapt to the differences in thermal expansion during e.g. start-up. The stress situation is modeled using finite element computation, and the location and magnitude of high stresses due to spot welding is clearly shown. 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.