Analysis of Role and Pros and Cons of Hydraulic Test in Pressure Equipment Safety Assessment
Some nuclear power plants are instructed according to French RCC-M standard materials and design methods in China,and in-service inspections are carried out according to the"Rules for In-service Inspection of Mechanical Equipment in the Nuclear Island of Nuclear Power Plants(RSE-M)",which requires in-service hydraulic test for the reactor coolant system every 10 years after the plant is put into operation.The test pressure is 1.2 times the design pressure of the reactor pressure vessel.ASME Ⅺ,"Rules for In-service Inspection of Nuclear Power Plant Components"eliminates the requirement for in-service hydraulic tests based on fracture mechanics analysis to determine the defect detection capability of hydraulic tests,concluding that hydraulic tests at no more than 1.1 times the operating pressure can detect defects that non-destructive inspection is unlikely to miss.Compared with the in-service hydraulic test required by ASME Ⅺ,test pressure of periodic hydraulic required RSE-M is higher and the test temperature is lower.Also there are differences in the calculation methods of stress intensity factor in fracture mechanics between the two standards.In this paper,the stress intensity factor was calculated using the methods of the two standards separately,and the detection ability of the in-service hydraulic test required by the RSE-M standard for defects was analyzed using fracture mechanics.During the defect evaluation,a more realistic calculation method was employed,which took into account a safety factor of 1.Additionally,the analysis also considered the impact of neutron irradiation on the toughness of the pressure vessel's reactor-type cylinder material,as well as the inhomogeneous distribution of neutron fluence within the vessel wall thickness.The analysis primarily focused on calculating Type Ⅰ failure,assuming that the defects were planar and perpendicular to the direction of maximum stress.Both the methods outlined in ASME Ⅺ and the RSE-M standard were utilized to calculate the KI value.Furthermore,the limiting defects under conditions of 1.2 times the design pressure and operating pressure were calculated separately.The analysis results show that it is impossible to maintain both high detection capabilities of defects and equipment loads at a designed safety coefficient in regular hydraulic tests at 1.2 times of design pressure.Based on long-term operational experience,with the implementation of appropriate non-destructive inspection,the role of the regular hydraulic test at 1.2 times of design pressure in the RSE-M standard is limited in the regular evaluation of the pressure-bearing capacity of Class Ⅰ nuclear safety equipment,while it reduces the operational safety coefficient of pressure vessels and increases the risk of equipment fatigue damage and crack propagation.Taking into account the pros and cons of implementing the regular hydraulic test,it is recommended to cancel the ten-year cycle of the 1.2 times design pressure regular hydraulic test and replace it with a shorter test cycle for a seal test.
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维普
