[en] For the commonly used subset simulation (SS) method based on modified Metropolis-Hastings (MMH) algorithm, the acceptance rate of candidate samples decreases and the error of failure probability estimate increases at lower failure probability level, and the algorithm's robustness is also poor. For the accurate, efficient and robust evaluation of thermal-hydraulic reliability of passive system using SS, a new Markov chain Monte Carlo (MCMC) method based on adaptive conditional sampling (aCS) was introduced and a reliability evaluation method based on adaptive MCMC and SS was proposed. Taking the experimental facility of a secondary side passive residual heat removal system as an example, the performance comparison of SS based on MMH (MMH-SS) and SS based on aCS (aCS-SS) was given at different failure probability levels. Calculation results show that, under lower failure rate level, aCS-SS can make the acceptance rate of candidate samples stably near the target value, and accuracy and robustness of the failure probability estimate are higher than those of MMH-SS. (authors)

[en] Sensitivity analysis is applied to uncertainty analysis and reliability evaluation of the thermal-hydraulic process for reactor passive system, which can quantitatively identify uncertainty input parameters that have important influence on system's thermal-hydraulic behavior. Based on hybrid of random balance designs and Fourier amplitudes sensitivity test (HFR), the passive residual heat removal experimental system of a nuclear power plant is used as an example to carry out the global sensitivity analysis. The applicability and accuracy of HFR method are demonstrated by simulation results. The change rule of parameter importance with time and the parameter importance ranking when the system is running stably are given by sensitivity analysis, which help to guide passive system's design optimization and operation management. (authors)

[en] Under the framework of RMPS (Reliability Methods for Passive Safety Functions), thermal-hydraulic reliability (TH-R) of an experimental facility is evaluated, and sensitivity analysis is applied for uncertainty parameters which has effects on the TH-R of the passive systems. Related conclusions contribute to understand the uncertainty of the thermal hydraulic processes of the passive residual system from the view of reliability. Results of reliability and sensitivity analysis can be used to guide the system optimal design and operation management. (authors)

[en] Latin hypercube sampling (LHS) method with better space-filling property is often used for computer simulation, to solve the problem of huge computation cost for complex systems, simulation and establish more accurate substitution models. LHS method's advantages in passive system reliability analysis are introduced in this paper. Improved Latin hypercube sampling, optimized Latin hypercube sampling and extension method of samples are summarized. LHS method's applications and deficiencies in nuclear field are proposed. Lastly, LHS method's future application and development direction in passive system reliability analysis are suggested. (authors)

[en] On the basis of reliability analysis mathematical model, combined with the operating data from an experimental facility and improved thermal-hydraulic codes, the uncertainty of input parameters is identified. Compared with the accuracy and the goodness of different Neural Network Response Surface methods, the one optimized with PSO is analyzed by classification accuracy. The results show that PSO response surface has relatively better fitting performance and can evaluate the reliability of the passive system accurately. (authors)

[en] In view of the difficulty in constructing the importance sampling density in the reliability evaluation of passive system and the defects of the standard multi-level cross entropy method in practical engineering application, an improved multi-level cross entropy method is proposed by improving the algorithm structure and introducing the Halton sequence sampling method with better uniformity. Taking an experimental facility for passive residual heat removal system of a certain marine nuclear power plant as an example, the performance verification analysis and example analysis of the IMCE method are implemented. The calculation results show that, the relative error distribution of the improved method is more convergent and the distribution of the coefficient of variation is similar, and the improved method is with better estimation accuracy and robustness with less computation. Moreover, the improved method is with stronger applicability in practical engineering application, because there is no need to set additional smoothing parameter, and the evaluation process can be completed earlier according to the system characteristics and sampling conditions. (authors)

[en] The thermal stratification of marine reactor surge line is divided into four types of typical transient based on the operating data:power increase, power decrease, off-design conditions, and small spray flow. The dimensionless Richardson number (Ri) analysis and the numerical simulation calculation of transient conditions are performed separately for these typical transients, generating the thermal stratification section length, duration and maximum temperature difference along the horizontal line section of the surge line under these typical transients. As indicated by the results, the thermal stratification under power increase and decrease transient conditions extends through the surge line just once, and the surge line safety may be affected by the circulating thermal fluctuation at the joint under the power increase transient condition, the thermal stratification of long section, long time and large temperature difference in the horizontal section under the small spray flow transient condition, and the thermal stress fluctuation caused by the off-design conditions. The method of study on thermal stratification of surge line based on operating data, proposed in this paper, lays a foundation for the subsequent thermal stress and thermal fatigue analyses, and also provides reference for the thermal stratification studies of other volumetric equipment. (authors)

[en] Taking the flashing phenomenon in the low pressure natural circulation system as the research object, the change law of void fraction in the stage of stable two-phase natural circulation flow driven by flashing is analyzed. Through the analysis, the following law is found that the inlet fluid temperature of the rising section and the liquid level height of the water tank will cause the flow to affect the flashing vaporization process, which makes the radial and axial distribution of the void fraction different. Through the analysis, it is concluded that the main factor affecting the vaporization process is the fluid superheat degree. It is known that reducing the fluid degree of subcooling at the inlet of the rising section, the flashing starting point will move down, and the flashing two-phase section will become longer. Along with the continuous flashing vaporization, the fluid degree of subcooling decreases gradually, the axial void fraction distribution increases rapidly first and then gradually slows down, and the radial void fraction changes from 'wall peak' type to 'nuclear peak' type. Then, based on the change of local fluid degree of subcooling, fitting gives the calculation formula of axial void fraction under different cases. By comparing with the experimental data, it is found that the fitting is good, and the relative error is within ± 15%. (authors)