| Passive systems are widely used in the design of the advanced reactors because they are effective in simplifying nuclear power plant designs and improving reactor safety.The natural circulation system,which is mainly used for reactor decay heat removal,is a passive safety system.The study of the reliability of the natural circulation systems can help identify crucial parameters and components in the systems,quantify the impact of these parameters and components on the system performance,and guide the design and analysis of the natural circulation systems.Quantitative reliability analysis improves the probability risk assessment of nuclear power plants and provides a holistic view of the risk assessment.However,the reliability analysis method of a natural circulation system is still under development.The characteristics of the natural circulation leave many problems yet to be discovered and studied.The author analyzed the epistemic uncertainty in the current methods that were found in the literature,and proposed an improved reliability analysis method for the design and analysis of new natural circulation systems.Based on this method,the author developed an uncertainty analysis package for molten salt systems.The improved method and the analysis package were applied to the functional reliability analysis of a nitrate salt natural circulation experimental loop,and the sensitivity analysis results were compared to the experimental results.Firstly,the author discussed the particularity of the reliability analysis of the natural circulation systems.The way in which the driving force is generated determines that one needs to pay special attention to the functional failures caused by various uncertain factors in the reliability analysis.The characteristics of the uncertainty determine that a combination of the best-estimate code and the uncertainty analysis focusing on the functional reliability becomes an effective method for reliability analysis of the natural circulation systems.Based on the analysis and comparison of the contemporary mainstream natural circulation system reliability analysis methods,the author summarized the problem of epistemic uncertainty in the selection of uncertain input parameters and models of these methods.The author then proposed an improved reliability analysis method for newly designed natural circulation systems.Through the complete analysis of the uncertainties of the input parameters,the standardized selection of the models,and the systematic sensitivity analysis,the improved method reduces the introduction of the epistemic uncertainties in the reliability analysis,and is applicable to the functional reliability analysis of the molten salt natural circulation systems.Based on the characteristics and the requirements of the nitrate salt natural circulation experimental loop,the author expanded the original uncertainty analysis package of the RELAP5-MS code and developed an uncertainty analysis package of the molten salt natural circulation system or NCUAPackage.The NCUAPackage realized the programed propagation of the input uncertainties,and served as an analysis tool for the functional reliability analysis of the loop.The author then completed the functional reliability analysis and sensitivity analysis of the loop using the NCUAPackage.The functional reliability analysis showed that the probability that the loop exceeds the maximum operating temperature is 0.0627,the probability that the loop exceeds the minimum operating temperature is 0.0753,and the total failure probability is 0.138 under the specified initial conditions and under 99% confidence level.Sensitivity analysis showed that the heat transfer coefficient of the airside of the salt-air heat exchanger is the most significant parameter causing system failure.Finally,the author designed and carried out two experiments in order to study the relations between the minimal salt temperature and the heat transfer coefficient of the airside of the salt-air heat exchanger,and the relations between the minimal salt temperature and the outside air temperature.Experiments showed that the two input parameters both affect the minimum salt temperature,and the impact of the heat transfer coefficient of the airside of the salt-air heat exchanger is significantly more than that of the outside air temperature.Consistency was found between the results of the experiments and the results of the sensitivity analysis. |
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