XAPR中破口失水事故下堆芯自然循环冷却能力的功能可靠性研究

功能失效是导致自然循环系统运行失效的重要因素,需在其可靠性分析中予以考虑。针对多维不确定性参数及小功能失效概率问题,提出了一种将改进响应面法及重要抽样子集模拟法相结合的功能可靠性分析方法。以西安脉冲堆(XAPR)堆池水自然循环冷却为例,结合中破口失水事故,考虑输入参数及模型的不确定性,对其进行了功能可靠性评估和灵敏度分析。结果表明:XAPR堆芯自然循环功能失效概率为3.796×10~(-3),需充分考虑系统功能的可靠性。本文方法具有较高的计算效率,同时又能保证很高的计算精度,对XAPR堆芯自然循环非线性功能函数具有很强的适应性。     

基于最优化线抽样法的XAPR自然循环功能失效研究与分析

针对多维不确定性参数、小失效概率的功能可靠性分析,提出了一种优化线抽样的可靠性分析方法。该方法采用遗传算法求解约束条件的优化模型来寻求最优化重要方向,进而得到失效概率的高效估计。以西安脉冲堆（XAPR）自然循环冷却堆芯能力的可靠性评价为例,考虑模型与输入参数的不确定性,对中破口失水事故下的自然循环功能失效概率进行了量化分析。结果表明：与其他概率评估方法相比,本文方法具有很高的计算效率,同时又能保证很好的计算精度;对隐式非线性的功能可靠性分析是有效可行的,具有很强的适应性。     

功能可靠性分析方法在XAPR堆芯自然循环可靠性评价中的应用

功能失效是导致自然循环系统运行失效的重要因素,需要在其可靠性分析中予以考虑。基于功能可靠性评价流程,通过RELAP5程序模拟自然循环物理过程,对西安脉冲堆（XAPR）池水自然循环冷却堆芯能力的可靠性进行评价。结合中破口失水事故,根据包壳完整性的功能准则,确定影响自然循环的关键参数;采用拉丁超立方抽样确定输入参数组合,进行参数敏感性分析和功能可靠性评价,并将功能可靠性评价结果整合到概率安全评价（PSA）模型中。分析结果表明:在PSA模型中不仅需要考虑硬件可靠性,还应充分考虑功能可靠性,以更好地指导XAPR运行及提高其安全性。      

西安脉冲堆满功率运行工况内部始发事件一级概率安全评价

针对西安脉冲堆(XAPR)2 MW满功率运行工况,建立了内部始发事件一级概率安全评价(PSA)模型,对始发事件识别、事故序列分析及可靠性数据处理等进行了研究。应用小事件树-大故障树方法,在Risk Spectrum平台上完成XAPR堆芯损伤事故序列的定量分析。结果表明,XAPR内部事件导致的堆芯损伤频率(CDF)为4.14×10~(-6)/(堆·年),对CDF贡献最大的为堆水池堆芯高度处大破口失水事故,支配性事故序列是大破口失水事故后紧急排水系统失效。研究结果证明XAPR具有较高的安全性。     

西安脉冲堆燃料元件精细燃耗分布的蒙特卡罗方法研究

本文采用RMC模拟计算了西安脉冲堆（XAPR）稳态堆芯第1循环燃料元件的精细燃耗分布情况,根据XAPR运行温度制备了多温度点氢化锆的热化截面,计算了零燃耗下XAPR冷态和热态实验的k_eff。分别考虑燃料棒径向和轴向空间离散化下温度反馈的结果,确定了首循环脉冲堆三维燃耗最深的位置。结果表明,采用燃料棒径向燃耗分区的15 EFPD下D5和G14燃料棒燃耗计算结果较径向不分区的结果更接近实验值,RMC应用于XAPR精细燃耗计算具有较高可靠性,可用于脉冲堆物理计算与安全分析工作。     

CEFR在ULOF工况下的自然循环能力分析

自然循环能力是衡量钠冷快堆固有安全性的重要指标,堆芯布置、回路设计及工况参数等都会影响堆芯自然循环能力,因此不同堆型的自然循环能力有很大差异。为了保证堆芯事故得到有效缓解,中国实验快堆(CEFR)的设计中通过优化系统布置,重点考虑了堆芯自然循环。本文采用SAS4A程序对CEFR进行系统建模,分析了CEFR在无保护失流(ULOF)工况下的堆芯热工水力参数瞬态特性,验证了CEFR利用自身自然循环和负反馈设计进行事故缓解的能力,本文还对一回路流动阻力和二回路钠装量对堆芯自然循环的影响进行分析。计算结果表明,CEFR具有良好的自然循环特性,在ULOF工况下可以依靠其负反馈停堆,并能够建立起稳定的自然循环从而导出堆芯余热。     

基于数据挖掘的非能动系统功能可靠性评估方法研究

针对多维不确定性参数及小概率的功能失效问题,提出一种基于数据挖掘的功能可靠性分析方法。该方法将自举抽样响应面拟合模型及最优化线抽样技术相结合,进而高效获得非能动系统的功能可靠性。以西安脉冲堆为例,结合中破口失水事故,考虑输入参数及模型的不确定性,对其进行功能可靠性评价。结果表明,该自举抽样响应面模型具有较高的拟合度;最优化线性抽样技术具有很高的计算效率,同时又能保证很好的计算精度。因此,本研究的评价方法对非能动系统隐式非线性的功能失效分析具有很强的适应性。      

研究堆可靠性数据的收集与处理方法研究

面向开展研究堆概率安全分析(PSA)的需求,以研究堆通用可靠性数据库IAEA-TECDOC-930为通用数据源,基于贝叶斯方法建立了研究堆可靠性数据收集与处理方法,并以西安脉冲堆(XAPR)为对象设计开发了研究堆可靠性数据管理与分析系统,通过该系统的应用获取了不确定性较小且能反映本堆特征的部件可靠性参数。     

西安脉冲堆反应性引入事故瞬态安全特性分析

为研究西安脉冲堆（XAPR）在意外引入反应性且停堆系统失效事故下的瞬态安全特性,本文基于XAPR的结构和运行特点,建立了适用于XAPR的瞬态热工水力分析模型,并开发了用于XAPR安全特性分析的瞬态热工水力程序TSAC-XAPR。利用TSAC-XAPR程序对反应性引入事故进行模拟计算,结果表明：当XAPR在额定功率范围内运行时,发生反应性引入事故后,堆芯能依靠自身的固有反馈机制使脉冲堆重新达到稳定运行状态;当运行功率过高尤其是超过临界值时,反应性引入事故将导致脉冲堆关键热工水力参数发生振荡,无法再次达到稳态。此外,不同反应性引入方式将影响堆芯参数在反应性引入过程中的变化趋势,但并不影响其最终稳态值。     

非能动系统性能不确定性评价

本文提出了确定非能动安全功能不确定性的一种方法。实质是处理参照为先进轻水堆设计的迁移堆芯衰变热的非能动系统，非能动系统经由主管线连接到压力容器，依赖自然循环和浸在冷却水池中的热交换器将蒸汽和冷凝水的热量排人冷却水池热井。采用两种确定风险的定性方法，即使用失效模式与效应分析（FMEA）和风险与运行（HAZOP）的方法，对其结果相对地进行了比较，评价了物理失效的主要源项。确定了非能动系统性能的不确定性源项，并确定了启动失效机理的参数。最后，对概率项和系统热工水力性能响应最大作用的不确定性进行了评价。     

基于自适应重要抽样法非能动系统功能故障概率评估

针对非能动系统功能故障概率评估,提出一种新的自适应重要抽样方法。这种方法先对失效域进行预抽样,然后拟合出失效域中样本分布的密度函数,以之作为重要抽样密度函数。以1000 MW非能动先进压水堆(AP1000)非能动余热排出系统为研究对象,考虑模型和输入参数的不确定性,将响应面法和自适应重要抽样法相结合,对其进行功能故障概率评估。结果表明:与传统的概率评估方法相比,自适应重要抽样法具有较高的计算效率,同时又能保证很高的计算精度。     

Integration of functional reliability analysis with hardware reliability: An application to safety grade decay heat removal system of Indian 500 MWe PFBR

A passive system can fail either due to classical mechanical failure of components, referred to as hardware failure, or due to the failure of physical phenomena to fulfill the intended function, referred to as functional failure. In this paper a methodology is discussed for the integration of these two kinds of unreliability and applied to evaluate the integrated failure probability of the passive decay heat removal system of Indian 500 MWe prototype fast breeder reactor (PFBR). The probability of occurrence of various system hardware configurations is evaluated using the fault tree method and functional failure probabilities on the corresponding configurations are determined based on the overall approach reported in the reliability methods for passive system (RMPS) project. The variation of functional reliability with time, which is coupled to the probability of occurrence of various hardware system configurations is studied and incorporated in the integrated reliability analysis. It is observed that this consideration of the dependence of functional reliability on time will give significant advantages on system reliability. The integrated reliability analysis is also explained using an event tree. The impact of the provision for forced circulation in the primary circuit on functional reliability is also studied with this procedure and it is found that the forced circulation capability helps to bring down the total decay heat removal failure probability by lowering the peak temperatures after the reactor shut down.     

基于子集模拟法非能动系统功能故障概率评估

针对非能动系统多维不确定性参数和小功能故障概率问题,提出基于马尔可夫链蒙特卡罗子集模拟的可靠性分析方法。该方法通过引入适当的中间失效事件,将小功能故障概率表达为一系列较大的中间失效事件条件概率乘积的形式,进而利用马尔可夫链模拟的条件样本点来计算条件失效概率。以AP1000非能动余热排出系统为研究对象,考虑热工水力学模型和输入参数的不确定性,对其进行功能故障概率评估。结果表明：与其它概率评估方法相比,子集模拟法具有较高的计算效率,同时又能保证很高的计算精度;对非能动安全系统非线性功能函数有很强的适应性。     

非能动自然循环技术的发展与研究

非能动技术在核电工程领域越来越受重视，文章列举了非能动自然循环在核电系统中的应用，对其在运行过程中可能存在的问题进行了分析。介绍了非能动自然循环可靠性分析目前的数学研究方法，并简要总结了这些方法的优缺点。最后展望了非能动自然循环的发展方向。由于非能动自然循环存在失效的可能，在系统运行过程中应对非能动自然循环物理过程失效及其可靠性予以足够重视和积极研究；为了确保系统运行的安全性，系统运行过程中要能动与非能动相互结合，同时选择精确模型，完善非能动可靠性分析方法，准确实现理论计算与实验验证。     

Passive system reliability analysis using the APSRA methodology

In this paper, we present a methodology known as APSRA (Assessment of Passive System ReliAbility) for evaluation of reliability of passive systems. The methodology has been applied to the boiling natural circulation system in the Main Heat Transport System of the Indian AHWR concept. In the APSRA methodology, the passive system reliability is evaluated from the evaluation of the failure probability of the system to carryout the desired function. The methodology first determines the operational characteristics of the system and the failure conditions by assigning a predetermined failure criteria. The failure surface is predicted using a best estimate code considering deviations of the operating parameters from their nominal states, which affect the natural circulation performance. Since applicability of the best estimate codes to passive systems are neither proven nor understood enough, APSRA relies more on experimental data for various aspects of natural circulation such as steady-state natural circulation, flow instabilities, CHF under oscillatory condition, etc. APSRA proposes to compare the code predictions with the test data to generate the uncertainties on the failure parameter prediction, which is later considered in the code for accurate prediction of failure surface of the system. Once the failure surface of the system is predicted, the cause of failure is examined through root diagnosis, which occurs mainly due to failure of mechanical components. The failure probability of these components are evaluated through a classical PSA treatment using the generic data. Reliability of the natural circulation system is evaluated from the probability of availability of the components for the success of natural circulation in the system.     

Functional reliability analysis of Safety Grade Decay Heat Removal System of Indian 500 MWe PFBR

Passive systems are increasingly deployed in nuclear industry with an objective of increasing reliability and safety of operations with reduced cost. Methods for assessing the reliability of thermal-hydraulic passive systems, that is systems with moving working fluid, address the issues in natural buoyancy-driven flow that could result in a failure to meet the design safety limits under accident scenarios. This is referred as design functional reliability. This paper presents the results of functional reliability analysis carried out for the passive Safety Grade Decay Heat Removal System (SGDHRS) of Indian Prototype Fast Breeder Reactor (PFBR). The analysis is carried out based on the overall approach reported in the Reliability Methods for Passive System (RMPS, European Commission) project. Functional failure probability is calculated using Monte-Carlo method and also with method of moments.     

Passive system reliability analysis using Response Conditioning Method with an application to failure frequency estimation of Decay Heat Removal of PFBR

Innovative nuclear reactor designs include passive means to achieve high reliability in accomplishing safety functions. Functional reliability analyses of passive systems include Monte Carlo sampling of system uncertainties, followed by propagation through mechanistic system models. For complex passive safety systems of high reliability, Monte Carlo simulations using mechanistic codes are computationally expensive and often become prohibitive. Passive system reliability analysis using recently proposed Response Conditioning Method, which incorporates the insights obtained from approximate solutions like response surfaces in simulations to obtain computationally efficient and consistent probability estimates, is presented in this paper. The method is applied to evaluate the reliability of passive Decay Heat Removal (DHR) system of Indian Prototype Fast Breeder Reactor (PFBR). The accuracy as well as efficiency of the method is compared with direct Monte Carlo simulation. The variability of the reliability values is estimated using bootstrap technique. The system abilities, to prevent critical structural damage as well as to ensure operational safety, are quantitatively ascertained. The system functional failure probabilities are integrated with hardware failure probabilities and the inclusion of passive system unreliability in Probabilistic Safety Assessment is demonstrated.