Safety Assessment For Defective Welded Structure Of A Nickel-base Alloy Simulant Vessel Of Molten Salt Reactor

Molten salt reactor, in which the pressure vessel is made of Hastelloy C276, belongs to the generation IV nuclear energy systems. Lots of surface cracks perpendicular to the weld bead have been detected in the simulate vessel during the periodic inspections. These non-coplanar and parallel cracks take place in the welding structure of the barrel and heads. Owing to the high temperature and radioactivity of molten salt reactor, the vessel requires high tightness. The existence of cracks may cause leak of the structure, or even lead to environmental disaster and casualties. So there is a pressing need to research the safety of welding structure of molten salt reactor’s simulate vessel which contains cracks.Normally, the test reactor of molten salt reactor is operated at a pressure of 0.4 MPa and at a temperature of 700℃. Pressure fluctuation has been identified in the vessel. According to the research on the failure of the vessel and the causes of the cracks, it can be proved that reheating crack and fatigue failure both cause the destruction of the vessel.The applied stress distribution and multi pass welding process are calculated or simulated by finite element analysis method. Numerical simulation results indicate that circumferential stress is the highest primary stress in the welding structure. At this part, the highest primary stress reaches to 12.4MPa, while the highest welding residual stress is 253.8MPa, approaches the peak in the middle of the weld bead.Finite element analysis and the theory of fracture mechanics are combined to establish the interference principle of the non-coplanar and parallel cracks in the welding structure with J-integral. The interference principle shows that if the crack with maximum dimension is choosed to analyse, the result will be on the safe side.Different from the length of the cracks on the vessel, depth of the cracks can not be measured. Three depth values of the crack with maximum length are assumed for safety assessment. Fatigue assessment is conducted on these three cracks with same length and different depths. It is showed that these three cracks are free of fatigue assessment because the variations of the stress intensity factors are less than the threshold value. The final fracture assessment result demonstrates that cracks with 3mm depth and 6mm depth are acceptable, while the crack with a 9mm depth is unacceptable. At last, the maximum allowable sizes of the cracks in the welding structure are calculated by the failure assessment diagram method, and safety evaluation method for cracks of different sizes is provided.