HTR—10热气导管断裂事故下的扩散自然对流瞬态过程分析

热气导管断裂事故是１０ＭＷ高温气冷试验堆（ＨＴＲ－１０）的假想事故。为了分析该大破口事故初期的扩散自然对流的瞬态过程，本文提出了一个一维扩用自然对流模型，并用日本原子能研究院（ＪＡＥＲＩ）的倒Ｕ型管内的扩散自然对流实验验证了该模型。利用该模型分析了ＨＴＲ－１０热管导管断裂事故下的扩散自然对流过程。结果显示：经过４００００ｓ的时间延迟形成稳定的自然对流。     

10MW高温气冷实验堆反射层石墨毒物对平衡态堆芯特性影响分析

对１０ＭＷ高温气冷实验堆（ＨＴＲ－１０）反射层石墨毒物对平衡态堆芯特性的影响进行了敏感性分析计算，并且研究了反射层毒物浓度为５．２ｍｇ／Ｌ硼当量的情况下反应性的补偿手段。结果表明：毒物的存在，致使反应性下降，为了补偿这种效应，需要增大燃料中＾２３５Ｕ的富集度或者增大堆芯装料体积。本文工作可为ＨＴＲ－１０燃料中＾２３５Ｕ的富集度以及其它参数的选取提供参考依据。     

HTR－10高温气冷实验堆事故放射性释放及对环境的影响

本文介绍了ＨＴＲ－１０高温气冷堆可能发生的向环境释放较多放射性的三种事故的释放机制和释放量计算中的假设，并给出了释放量和对公众的辐射剂量的计算结果。这三种事故中，堆芯进水事故引起的公众辐射剂量最大，在离排放点２５０ｍ处公众个人受到的全身剂量为５．４４×１０￣（－１）ｍＳｖ，此剂量比核安全法规中规定的要采取隐蔽等场外应急措施的干预水平低１个量级。     

HTR-PM两根一回路连接管断裂的进气事故分析

进气事故是模块式高温气冷堆关注的超设计基准事故之一,石墨氧化腐蚀反应可能导致反射层结构强度减弱、燃料元件完整性和包容裂变产物能力被破坏,以及产生可燃气体等较严重后果。进气事故的分析研究对进一步掌握高温气冷堆的事故特性以及提高反应堆的安全设计具有重要意义。本文基于200MWe球床模块式高温气冷堆示范工程（HTR-PM）的初步设计,假设与一回路压力边界上、下相连的燃料元件进料管和卸料管同时发生断裂,从而形成烟囱效应并导致空气进入堆芯,利用高温气冷堆专用系统分析软件TINTE对自然循环建立及后续的进气腐蚀过程进行了研究,分析了自然循环流量、堆内石墨腐蚀速率、舱室氧气消耗量、燃料元件温度等关键参数的变化。结果表明,即使考虑腐蚀反应的不均匀性,事故后约60h时才会出现首个燃料包覆颗粒裸露现象,燃料元件最高温度峰值低于1620℃的设计限值,保持完好的燃料包覆颗粒仍具有包容放射性裂变产物的能力。同时,如果在相应的时间内采取措施切断进气源,使石墨腐蚀反应不能继续发展,将不会对反应堆的安全造成严重的影响。     

250 MW球床模块式高温气冷堆进水事故研究

基于250 MW球床模块式高温气冷堆(HTR-PM)的初步设计,以高温气冷堆专用系统分析软件TINTE程序为主要工具,对蒸汽发生器1根传热管双端断裂设计基准的进水事故进行了分析,研究了反应堆温度和压力的变化特性、球床石墨的腐蚀率以及安全阀开启所造成的可燃气体排放等.此外,还分析了风机挡板关闭失效情况下堆内温度分布差异所造成的自然循环对事故后果的影响.计算结果表明:在蒸汽发生器1根传热管双端断裂、最大进水量600 kg情况下,事故后燃料元件的最高温度远低于设计限值,化学反应所引起的石墨腐蚀不会造成反应堆结构强度的破坏和燃料元件的意外破损,释放到反应堆舱室的可燃气体含量也不存在爆炸危险.     

基于CFD的棱柱型模块式高温气冷堆典型事故研究

为验证和评估棱柱型模块式高温气冷堆设计的固有安全性,需针对代表性事故工况开展计算分析。目前针对棱柱型堆芯的模块式高温气冷堆尚缺少专用的事故分析程序。本研究基于通用CFD程序COMSOL针对堆芯活性区域和压力容器建立三维模型,包括燃料和冷却剂通道、石墨慢化剂、侧反射层以及压力容器;非能动余热排出系统采用对流边界条件简化模拟。采用C++编写点堆模块求解中子动力学,并通过动态链接库(DLL)与COMSOL实现耦合。首先计算了正常运行工况下的稳定状态;然后以该结果作为初始条件,选取3个典型事故瞬态工况开展了数值模拟,包括未失压丧失强迫流动冷却(PLOFC)事故、未失压丧失强迫流动冷却且未能停堆(PLOFC+ATWS)事故以及反应性引入且未能停堆(RIA+ATWS)事故;最后针对压力容器壁与非能动余热排出系统的辐射发射率开展了敏感性分析。计算结果表明：在本文分析的事故条件下,燃料最高温度均低于安全限值(1 620℃)且具有较大的裕量,因此均能保证堆芯燃料结构的完整性。对于PLOFC事故,提高非能动余热排出系统的换热能力能显著缓解事故后果,但对于ATWS类事故影响趋势则正好相反,需进一步开展综合分...     

HTR—10石墨球与燃料球均匀混合装料初装堆方案研究

分析了球床式高温气冷堆几种可能的初装堆方案的特点，选取石墨球与燃料球均匀混合作为１０ＭＷ高温气冷实验堆的初装堆方案。利用高温气冷堆物理设计程序ＶＳＯＰ进行计算，分析屯ＨＴＲ－１０从初始装料向平衡态过渡过程中的倒换料方式，最大单球功率及最大燃耗变化情况。     

板状燃料元件流道堵塞事故预防与探测技术研究

板状燃料元件用于研究堆中表现出良好的辐照性能。通过对国内外一些使用板状燃料元件研究堆堵流事故实例的调研,发现板状燃料元件板间的栅距通常很小,堆芯冷却剂流道狭窄,堵流事故的发生大都由异物进入流道或燃料肿胀引起。选取中国先进研究堆(China Advanced Research Reactor,CARR)作为特征研究对象,采用RELAP5/MOD3.2热工计算程序,对CARR堆芯、堆本体、单盒组件、堆外冷却回路等进行了热工水力模拟计算,结果表明:当反应堆功率提升时,堵塞的流道内燃料组件温度上升,冷却剂开始发生沸腾,功率会发生明显波动。通过中子注量率与功率的监控以及燃料温度的分析,有助于及早探知和预防堵流事故的进一步发展扩大。     

NHR—200堆芯旁通区三维流动传热数值分析

应用三维CFD软件PHOENICS－3．2,计算了200MW低温供热堆（NHR－200）堆芯旁通区及上腔室的流场和温场。分析了在堆芯与围板间的乏燃料存放区上端不同档板布置方案下的流场和温场,并考虑了旁通流量的影响。自然对流对流场和温场的影响不大,不会改变主流方向。在计算区域内,除主流外,还有由堆芯旁通区的下部流通面积突扩造成的一回流区及上腔室堆芯出口流通面积突扩和自然对流而形成的一大回流区。加挡板可阻挡上部大回流区对堆芯旁通区的影响,降低堆芯旁通区流体温度的变化。     

球床模块式高温气冷堆失冷事故特性研究

利用高温气冷堆专用系统分析软件THERMIX程序,对球床模块式高温气冷堆(HTR-PM)失冷失压和失冷不失压事故的动态特性进行了研究,分析了堆芯功率、燃料最高温度及堆舱水冷壁余热载出功率等关键参数的变化过程,并对影响余热排出功率和燃料最高温度的不确定性进行了评价.研究结果表明,在失冷事故下,堆芯余热可通过热传导、辐射和自然对流等非能动方式传至最终热阱大气,燃料元件和压力容器等重要部件的最高温度均在设计限值内.这为HTR-PM保持模块式高温气冷堆固有安全性不变的同时实现单堆250 MW的功率方案奠定了基础,也为后续高温气冷堆电站示范工程进一步的深入设计研究提供了依据.     

Consequences of delayed air ingress following a depressurization accident in a high temperature reactor

Air ingress is a specific event in a high temperature reactor (HTR). The potential threat posed by air ingress lies in the chemical reaction of oxygen with hot graphite at a temperature above 500 °C leading to reaction heat and graphite corrosion. In order to assess the consequence of air ingress into the reactor, it is postulated that breaks are present above and below the reactor core and that unobstructed ingress of air through them is possible. It is obvious that the air ingress incident has to be preceded by a depressurization accident. For this hypothetical scenario the maximum possible air flow rate through the core resulting solely from the pressure losses in the core is estimated as a function of the break cross-sections exposed above and below the core.In this paper, the thermal behavior of an HTR with prismatic fuel (operating inlet/outlet temperatures 450/850 °C) during air ingress accident conditions has been investigated. In particular, maximum temperatures and burn-off of the fuel and bottom graphite reflector for various air flow rates for the postulated hypothetical scenario have been analyzed. It also indicates the limiting time at which the graphite layer of fuel will be completely burnt-off and the fuel compacts exposed. In addition, the consequences of delayed air ingress after a core heat up following depressurization have been investigated.This paper, thus, throws light on the safety aspects of the new generation HTRs with prismatic fuels (e.g. NGNP/ANTARES) conceived for power generation and process heat application.     

Water-ingress analysis for the 200 MWe pebble-bed modular high temperature gas-cooled reactor

Water ingress into the primary circuit is generally recognized as one of the severe accidents with potential hazard to the modular high temperature gas-cooled reactor adopting steam-turbine cycle, which will cause a positive reactivity introduction, as well as the chemical corrosion of graphite fuel elements and reflector structure material. Besides, increase of the primary pressure may result in the opening of the safety valves, consequently leading the release of radioactive isotopes and flammable water gas. The analysis of such a kind of important and particular accident is significant to verify the inherent safety characteristics of the modular HTR plants.Based on the preliminary design of the 200 MWe high temperature gas-cooled reactor pebble-bed modular (HTR-PM), the design basis accident of a double-ended guillotine break of one heating tube and the beyond design basis accident of a large break of the main steam collection plate have been analyzed by using TINTE code, which is a special transient analysis program for high temperature gas-cooled reactors. Some safety relevant concerns, such as the fuel temperature, the primary loop pressure, the graphite corrosion, the water gas releasing amount, as well as the natural convection influence on the condition of failing to close the blower flaps, have been studied in detail. The calculation results indicate that even under some severe hypothetical postulates, the HTR-PM is able to keep the inherent safeties of the modular high temperature gas-cooled reactor and has a relatively good natural plant response, which will not result in environmental radiation hazard.     

高温气冷堆用石墨材料的腐蚀实验与软件模拟

在高温气冷堆进水进空气事故下,空气和水蒸气会与堆内的石墨材料发生化学腐蚀反应,从而可能影响反应堆的安全。为研究高温气冷堆内石墨材料的氧化腐蚀特性,本文利用气相色谱法实验测量了IG-110石墨在不同温度和不同气体组分配比情况下的腐蚀速率及腐蚀产物,并利用THERMIX/REACT软件对整个石墨腐蚀过程进行了模拟。研究结果表明：反应温度对石墨腐蚀的影响最为显著,腐蚀速率随着温度的升高而增大,同时随着温度升高,CO与CO₂的含量比也逐渐增大。通过与实验结果对比分析,验证了THERMIX/REACT软件用于高温气冷堆安全分析的可靠性。     

Study on the passive safe technology for the prevention of air ingress during the primary-pipe rupture accident of HTGR

A primary-pipe rupture accident is one of the design-basis accidents of a high-temperature gas-cooled reactor (HTGR). When the primary-pipe rupture accident occurs, air is expected to enter the reactor core from the breach and oxidize in-core graphite structures. This study is to investigate the air ingress phenomena and to develop the passive safe technology for the prevention of air ingress and of graphite corrosion. This paper describes the method for the prevention of air ingress into the reactor during the primary-pipe rupture accident. It is found that a safe cooling rate of the reactor core exists for the prevention of air ingress. The experimental results show that the natural circulation flow of air during the accident can be controlled by the method of helium gas injection into the reactor pressure vessel.     

Studies on diffusion and natural convection of two-component gases

A primary-pipe rupture accident is one of the design-based accidents of a high-temperature engineering test reactor (HTTR), which is being developed at JAERI. When the primary pipe ruptures, air is expected to enter into the reactor core from the breach by molecular diffusion and natural convection. In order to investigate the process of air ingress during the early stage of the primary-pipe rupture accident, experimental and analytical studies are performed on the conjugate phenomenon of the transient molecular diffusion and natural convection of a two-component gas mixture in two test sections, a reverse-U-shaped tube and a test model simulating simply the reactor. One-dimensional basic equations for continuity and momentum conservation are numerically solved to obtain a concentration change of gas species and an initiation time of a natural circulation of pure nitrogen in the reverse-U-shaped tube. Moreover, a modified numerical solution is proposed to reduce the computing time. A one-dimensional flow net work model is employed to calculate the transport process of air in the test model simulating the reactor. The calculated results agree well with the experimental ones on the concentration change of gas species and the initiation time of the natural circulation of pure nitrogen or pure air.     

Air ingress phenomena in a depressurization accident of the very-high-temperature reactor

The inherent properties of the very-high-temperature reactor (VHTR) facilitate the design of the VHTR with high degree of passive safe performances, compared to other type of reactors. However, it is still not clear if the VHTR can maintain a passively safe function during the primary-pipe rupture accident, or what would be a design criterion to guarantee the VHTR with the high degree of passively safe performances during the accident. The primary-pipe rupture accident is one of the most common of accidents related to the basic design regarding the VHTR, which has a potential to cause the destruction of the reactor core by oxidizing in-core graphite structures and to release fission products by oxidizing graphite fuel elements. It is a guillotine type rupture of the double coaxial pipe at the nozzle part connecting to the side or bottom of the reactor pressure vessel, which is a peculiar accident for the VHTR. If a primary pipe ruptures, air will be entered into the reactor if there is air in the reactor containment or confinement vessels. This study is to investigate the air ingress phenomena and to develop the passively safe technology for the prevention of air ingress and of graphite corrosion. The present paper describes the influences of a localized natural circulation in parallel channels onto the air ingress process during the primary-pipe rupture accident of the VHTR.     

Advanced pebble bed high temperature reactor with central graphite column for future applications

Design evaluations of the advanced pebble bed high temperature reactor, AHTR, with central graphite column are given. This reactor, as a nuclear heat source, is suitable for coal refinement as well as for electricity generation with closed gas turbine primary helium circuit. With this design of the central graphite column, it is possible to limit the core temperatures under the required value of about 1600°C in case of accident conditions, even with higher thermal power and higher core inlet and outlet temperatures. The designs of core internals are described. The after heat removal system is integrated in the prestressed concrete reactor pressure vessel, which is based on the principals of natural convection.Research work is being carried out, whereby the spherical fuel elements are coated with a layer of silicon carbide, to improve the corrosion resistance as well as the effectiveness of the fission products barrier.     

Air ingress benchmarking with computational fluid dynamics analysis

The air ingress accident is a complicated accident scenario that may limit the deployment of high-temperature gas reactors. The complexity of this accident scenario is compounded by multiple physical phenomena that are involved in the air ingress event. These include diffusion, natural circulation, and complex chemical reactions with graphite and oxygen. In an attempt to better understand the phenomenon, the FLUENT-6 computational fluid dynamics code was used to assess two air ingress experiments. The first was the Japanese series of tests performed in the early 1990s by Takeda and Hishida. These separate effects tests were conducted to understand and model a multi-component experiment in which all three processes were included with the introduction of air in a heated graphite column. MIT used the FLUENT code to benchmark these series of tests with quite good results. These tests are generically applicable to prismatic reactors and the lower reflector regions of pebble-bed reactors. The second series of tests were performed at the NACOK facility for pebble bed reactors as reported by Kuhlmann [Kuhlmann, M.B., 1999. Experiments to investigate flow transfer and graphite corrosion in case of air ingress accidents in a high-temperature reactor]. These tests were aimed at understanding natural circulation of pebble bed reactors by simulating hot and cold legs of these reactors. The FLUENT code was also successfully used to simulate these tests. The results of these benchmarks and the findings will be presented.     

The inherently-safe power reactor DYONISOS : Dynamic nuclear inherently-safe reactor operating with spheres

A philosophy of inherent safety is formulated and an inherently-safe thermal power reactor is presented. Solid fuel in the form of spheres a few centimetres in diameter is suspended under the hydrodynamic pressure of molten lead coolant in vertical channels within the graphite moderator. Loss of main pump pressure, or a loss-of-coolant accident (LOCA), results in immediate removal of the fuel to rigid sieves below the core, with consequent subcriticality. Residual and decay heat are carried away by thermal conduction through the coolant or, in the case of a LOCA, by a combination of radiation and natural convection of cover gas or incoming air from the fuel to the reactor vessel and convection of air between the vessel and steel containment wall. All decay heat removal systems are passive, though actively initiated external spray cooling of the containment can be used to reduce wall temperature. This, however, is only necessary in the case of a LOCA and after a period of 24 h.