Research On Structural Integrity Assessment Of Primary Loop Pipes In Pressurized Water Reactor Nuclear Power Plants

Complex loads such as thermal transient load,internal pressure,gravity,and earthquake need to be considered in the structure integrity assessment of pressurized water reactor(PWR)primary loop pipes.When defects are found in service,the time-consuming fracture mechanics assessment is required,which is difficult to meet the needs of the rapid analysis in the engineering.This will affect the overhaul period of the nuclear power plant,and then affect the economic benefits of power plant operation.At the same time,most of the existing safety assessment codes(such as French RCC-M Code and American ASME Code Section Ⅲ)are based on a single failure mode and lack of research on the unified assessment of multiple failure modes.When it is necessary to calculate the elastic-plastic fracture parameters under complex loads,complex finite element calculations or simplified plastic corrections of elastic fracture parameters are usually used,which is difficult to meet the needs of a rapid and accurate assessment in the engineering.On the other hand,the operation experience of the PWR shows that the actual transient parameters of the primary loop are often more complex than that in the design assumptions.The cracked pipes(or assumed to be cracked as needed)usually need to be replaced or simplified evaluated.There is still a lack of effective methods for a real-time crack propagation and safety assessment based on the monitoring transient parameters.In order to ensure the safe and economic operation of the nuclear power plant,the author studied comprehensive safety assessment technology of rapid assessment,analysis assessment and online assessment(based on the real-time monitoring of the transient parameters)according to the actual needs of the primary loop pipe in a large commercial PWR in this dissertation.The main conclusions of this dissertation are as follows:(1)For the primary loop pipes of austenitic stainless steel,the potential failure modes of fracture,plastic collapse and "Leak-Before-Break"(LBB)are studied.A technical process for rapid assessment of in-service defects based on the criterion of no-reduction of plastic limit load in the design presupposition is proposed.Based on the criterion of crack propagation on the critical condition,a programmed algorithm for calculating the critical crack is derived.The results show that both elastic-plastic fracture failure and plastic instability failure may occur in the hot-leg pipe after service,and the load range corresponding to the elastic-plastic fracture model is larger.The straight pipe of the hot leg has a large crack size in the critical failure state,which meets the requirements of monitoring leakage rate.So the pipe has LBB characteristics.The rationality of using the plastic limit load as the critical state of the structure is studied and demonstrated.On the premise of 11%hardening of the yield strength(approximately 1/10 of the difference between the tensile strength and the yield strength after service,or approximately 1/3 of the increase in the yield strength of the thermal aging plus cyclic hardening effect),the acceptable size information of the inner surface crack of the pipe based on the design plastic limit load is obtained(7.2%of wall thickness under the condition for the design reference crack shape).Based on the assumption of ideal plastic material and the maximum shear stress criterion,the design load safety margin of plastic limit load for the hot-leg straight pipe is assessed,and the minimum acceptable equivalent size of the design reference crack(on the premise that the crack shape parameters remain unchanged)is 7.3%of wall thickness.(2)Based on the failure assessment diagram(FAD)technology of the British R6 Code,a plastic correction analysis method for the elastic fracture parameter(stress intensity factor)is proposed.The study results show that the safety region of option 3 FAD of the hot-leg pipe under bending load is smaller than that under the internal pressure load.So it is more prone to fracture failure in the bending load condition.Under the same type of load,the crack size has little effect on the shape of FAD which means that the difference of plastic deformation at the crack front before the load reaches the limit load for the large-size and thick walled pipe is limited.In the application analysis,there are obvious differences in various forms of safety margins.In the study case,the elbow of the hot-leg pipe has a large safety margin.All forms of safety margins are greater than 1 under the conditions of two critical size cracks.The safety margin on load is the minimum one,so improving material tensile stress strength(or reducing structural load)is a more effective way to improve the comprehensive safety performance of the primary loop hot-leg pipe.Based on the theory of FAD,a set of calculation methods of elastic-plastic fracture parameters under complex loads(the case of only primary stress,and the case of both primary and secondary stresses)are proposed for the material stress-strain representation in uniaxial tensile meeting the R-O relationship.(3)According to different load characteristics of the pipe,a total stress calculation method suitable for online monitoring of operating parameters is proposed.Furthermore,an online safety assessment method based on monitoring data which can meet the requirements of in-service inspection code is given.The influence function of the pipe is proposed to be established at a fixed time interval which will make the computational complexity reduced significantly.With the help of computer parallel calculation and the scalable characteristics of the algorithm,the existing algorithm along the time axis is optimized to an algorithm that can be calculated to obtain the target parameters at each time point within the specified attenuation time range of the influence function.Then the calculation efficiency is significantly improved.The case study shows that the stress can be calculated quickly and accurately based on the influence function method,and the minimum deviation between the calculation results of the influence function and the finite element numerical simulation results is within 0.6%.At the same time,by compiling the criteria and calculation methods in the code,the calculation method of fracture parameters suitable for online monitoring is obtained,and the reliability is also verified by the analyzed case.Based on the technical results obtained from the research and the requirements in the in-service inspection code,the secondary development is carried out in the general commercial software platform.The software of "Online assessment software of cracked primary loop hot-leg pipe based on monitoring" is obtained.The deviation for the reference transient between the results of the software and the results in the original design report is less than 3%,and it is demonstrated that it has sufficient accuracy.The comprehensive safety assessment technology of cracked pipes(rapid assessment,analysis assessment and online assessment)which is described in this dissertation is general,and it can be used in the safety assessment of other similar vessels or other pipes.This study provides a useful reference for subsequent engineering practice.