Statistical Characterization of Heat Release Rates from Electrical Enclosure Fires for Nuclear Power Plant Applications

Since the publication of NUREG/CR-6850/EPRI 1011989 in 2005, the US nuclear industry has sought to re-evaluate the default peak heat release rates (HRRs) for electrical enclosure fires typically used as fire modeling inputs to support fire probabilistic risk assessments (PRAs), considering them too conservative. HRRs are an integral part of the fire phenomenological modeling phase of a fire PRA, which consists of identifying fire scenarios which can damage equipment or hinder human actions necessary to prevent core damage. Fire ignition frequency, fire growth and propagation, fire detection and suppression, and mitigating equipment and actions to prevent core damage in the event fire damage still occurred are all parts of a fire PRA. The fire growth and propagation phase incorporates fire phenomenological modeling where HRRs have a key effect. A major effort by the Electric Power Research Institute and Science Applications International Corporation in 2012 was not endorsed by the US Nuclear Regulatory Commission (NRC) for use in risk-informed, regulatory applications. Subsequently the NRC, in conjunction with the National Institute of Standards and Technology, conducted a series of tests for representative nuclear power plant electrical enclosure fires designed to definitively establish more realistic peak HRRs for these often important contributors to fire risk. The results from these tests are statistically analyzed to develop two probabilistic distributions for peak HRR per unit mass of fuel that refine the values from NUREG/CR-6850, thereby providing a fairly simple means by which to estimate peak HRRs from electrical enclosure fires for fire modeling in support of fire PRA. Unlike NUREG/CR-6850, where five different distributions are provided, or NUREG-2178, which now provides 31, the peak HRRs for electrical enclosure fires can be characterized by only two distributions. These distributions depend only on the type of cable, namely qualified versus unqualified, for which the mean peak HRR per unit mass is 11.3 and 23.2 kW/kg, respectively, essentially a factor of two difference. Two-sided, 90th percentile confidence bounds are 0.0915 to 41.2 kW/kg for qualified cables, and 0.0272 to 95.9 kW/kg for unqualified cables. From the mean (~70th percentile) upward, the peak HRR/kg for unqualified cables is roughly twice that for qualified, increasing slightly with higher percentile, an expected phenomenological trend. Simulations using variable fuel loadings are performed to demonstrate how the results from this analysis may be used for nuclear power plant applications.     

Statistical Characterization of Cable Electrical Failure Temperatures Due to Fire for Nuclear Power Plant Risk Applications

Single-value failure temperatures for fire loss of electrical cable functionality have been the norm for Fire Probabilistic Risk Assessments since the publication in 2005 of NUREG/CR-6850. If the calculated exposure temperature matches or exceeds the cable failure temperature, electrical failure is always assumed; if not, no failure is assumed. While this can be relaxed somewhat if a distribution for the exposure temperature is estimated, use of a distribution on the cable failure temperature itself more readily enables such relaxation and, therefore, a more realistic assessment. This paper develops probability distributions for different generic cable types (based on insulation) using data from the US Nuclear Regulatory Commission tests. Results indicate mean failure temperatures considerably higher than those used deterministically, 252°C, 421°C and 383°C, respectively for thermoplastic, thermoset and Kerite-FR^®. This suggests considerable relaxation from the conservatism inherent using the deterministic failure temperatures could be achieved. The paper then postulates two hypothetical distributions on the exposure temperature from applying a fire phenomenological model in a statistical way to estimate the possible relaxation using the distributed cable failure temperatures to enhance the realism of the assessment. Examples show that use of probabilistically-distributed cable failure temperatures (in conjunction with similar for exposure temperatures) can reduce the probability of electrical failure for a normally-distributed exposure temperature with a mean of 350°C and standard deviation of 58.3°C by factors of approximately three and eight for Kerite-FR^® and thermoset cables, respectively. The reduction would be less pronounced for thermoplastic cables, although larger reductions would be possible here as well for lower exposure temperatures (e.g., a factor of two).     

PVC电缆及其护套原料燃烧性能的对比

利用锥形量热仪对PVC电缆护套原料和对应电缆试样进行了实验研究，对比分析两组实验的点燃时间（TTI）、热释放速率（HRR）等主要燃烧参数。探讨了两组实验结果之间的相关性。实验结果表明：在相同实验条件下，PVC护套原料与对应电缆试样的HRR在燃烧的初始阶段（约240s内）有很好的相关性。因此，以护套原料的实验数据预测电缆样品在燃烧初期的HRR具有可行性，这对电缆护套原料的选用、降低新产品的开发成本以及制定电缆护套原料阻燃标准都具有实际意义。     

Design Fire Characteristics for Probabilistic Assessments of Dwellings in England

In England, there are no fixed requirements on the parameters adopted when considering residential design fires, and analyses undertaken are often deterministic with limited consideration given to probabilistic assessments and the sensitivity of parameters. The Home Office dwelling fires dataset has been analysed, considering the fire damage area and the time from ignition to fire and rescue service arrival. From this, lognormal distributions for the maximum heat release rate (HRR) and fire growth rate of residential fires have been approximated. The mean maximum HRR ranges from 900 kW to 1900 kW, with a standard deviation ranging from 2000 kW to 3700 kW, depending on property type and room of fire origin. The mean growth rate, assuming a t2 relationship, ranges from 0.0022 kW/s2 to 0.0034 kW/s2, with a standard deviation ranging from 0.0071 kW/s2 to 0.0132 kW/s2. When considering incidents which result in immediate fire and rescue service call out following ignition, the mean growth rate increases to a range of 0.0058 kW/s2 to 0.0088 kW/s2. As a result of the analyses, design fire distributions are provided which can be adopted for probabilistic assessments. For deterministic analyses, it is proposed that an approximate 95th percentile fire may be adopted, aligning with a medium growth rate of 0.0117 kW/s2 and a maximum fuel-limited HRR in the region of 3800 kW to 4400 kW, depending on whether the dwelling is a house or an apartment. A 95th percentile design fire broadly aligns with values already specified in guidance, helping to substantiate the existing recommendations.     

隧道内火灾大小和火灾蔓延模拟研究

隧道内火灾的热释放速率对火灾蔓延和烟气生成起着关键作用。影响热释放速率的关键参数包括：燃烧物特性、隧道形状、通风条件以及车辆流量。综述了几年来热释放速率对这些参数的依赖性所做的研究成果。设计了贝页斯概率模型来模拟火灾热释放速率受隧道形状以及纵向通风的影响，设计了定性模型来模拟火灾在类似海底隧道内从一个物体蔓延到另一个物体的情况，通风条件同样是纵向通风，并给出了此次研究的初步成果。     

Design fire curves for tunnels

Various ways exist to represent a design fire curve for tunnels. These can include different fire growth rates or combinations of fire growth rates with constant levels of heat release rate (HRR) coupled to a decay period. This means that the curve has to be represented with different mathematical expressions for different time periods. A more convenient way is to describe the design fire curve with a single mathematical expression. Such a curve has been presented by the author (H. Ingason, Fire development in large tunnel fires, 8th International Symposium on Fire Safety Science, Beijing, China, 18–23 September 2005, pp. 1497–1508), but it does not include a constant HRR period. This paper presents a new, single exponential, design fire curve with a constant maximum HRR. A presentation of available design curves is given as well.     

110 kV高压电缆着火过程数值模拟研究

为了研究综合管廊局部空间内电缆接头内热源作用下的着火过程,通过运用数值模拟软件FDS三维传热和热解模型对110 kV高压电缆接头着火过程建模分析.研究了内外热源对电缆着火过程的不同影响,分析了不同运行状态下的温度分布情况,对比了不同缆芯材料的热传导作用对电缆着火过程的影响.结果表明,正常运行和短时过载状态下,电缆的火灾...     

Investigation of the effect of tunnel ventilation on crib fires through small-scale experiments

This paper adopts a series of 1:20 scale tunnel experiments based on a series of large-scale tunnel experiments to study the influence of forced ventilation on fires. The small-scale tunnel has dimensions of 0.365 m (W)×0.26 m (H)×11.9 m (L). Cribs using a wood-based material provide the fuel source and forced ventilation velocities from 0.23 to 1.90 m/s are used. From the study of the measured heat release rate (HRR) and mass loss rate data it is found that the forced air velocity affects the fire spread rate and burning efficiency and further affects peak HRR values at different air velocities. A simple model to describe these influences is proposed. This model is used to reproduce the enhancement of peak HRR for cribs with different porosity factors noted by Ingason [1] and to assess the effects of using different length of cribs on peak HRR. The results from these analyses suggest that different porosity fuels result different involvement of burning surface area and result different changes in peak HRR. However, no significant difference to the enhancement on fire size is found when the burning surface area is similar. It is also found that the trend in the enhancement on fire size by using sufficiently long crib and available ventilation conditions matches the predictions of Carvel and Beard [2] for two-lane tunnel heavy goods vehicle fires.     

区间隧道列车火灾通风临界时间研究

计算地铁区间列车火灾人员所需安全疏散时间,与模拟所得可用安全疏散时间对比,确定区间人员疏散策略及通风临界时间。研究表明：地铁列车外部中间位置着火停靠在区间,火源功率分别为5、7.5、10 MW,需启动纵向通风排烟系统,组织人员向上风向疏散。火源功率为5 MW,纵向通风风速为2.0 m/s时,150~180 s 开始通风可保证人员安全疏散;火源功率为7.5、10 MW,纵向通风风速分别为2.4、2.6 m/s 时,120~180 s 开始通风可保证人员安全疏散。风机由静止转换为事故工况的通风临界时间为120 s,由运转转换为事故工况的通风临界时间为90 s。     

Experimental Study on the Burning Behavior of Pool Fires in Rooms with Different Wall Linings

An experimental test series, comprising 10 experiments with varying pool sizes, lining materials and amounts of liquid burning, was conducted under free burn and room burn conditions. The thermal feedback from the enclosure (ISO 9705 Room Corner Test facility) enhanced the burning rate of the pools and resulted in a thermal runaway in some of the runs. The onset of the thermal runaway, which can be associated with flashover, varied with all the input parameters. The lining with the lowest thermal inertia lead to the fastest increase in the heat release rate (HRR) in the enclosure and caused flashover in the shortest time. Given the profound difference between the enclosure tests and the free burn tests and also between enclosure tests with different linings, it is recommended to show great caution if free burn tests are to be used in design fire scenarios.     

Using Sensor Signals to Analyze Fires

Building fire sensors are capable of supplying substantially more information to the fire service than just the simple detection of a possible fire. Nelson, in 1984, recognized the importance of tying all the building sensors to a smart fire panel [1]. In order to accomplish a smart fire panel configuration such as envisioned by Nelson, algorithms must be developed that convert the analog/digital signals received from sensors to the heat release rate (HRR) of the fire. Once the HRR of the fire is known, a multiroom zone fire model can be used to determine smoke layers and temperatures in the other rooms of the building. This information can then be sent to the fire service providing it with an approximate overview of the fire scenario in the building.This paper will describe a ceiling jet algorithm that is being developed to predict the heat release rate (HRR) of a fire using signals from smoke and gas sensors. The prediction of this algorithm will be compared with experiments. In addition, an example of the predictions from a sensor-driven fire model, SDFM, using signals from heat sensors, will be compared with measurements from a full-scale, two-story, flashover townhouse fire.     

不同影剧院座椅连排燃烧特性试验研究

利用10 MW大尺度量热计对不同影剧院座椅进行连排燃烧试验研究.建立座椅燃烧试验台,将15把影剧院座椅分3排台阶布置,通过对多人影剧院软质座椅排列在一起进行燃烧试验,提供燃烧产生的热释放速率、总热释放量、产烟速率、总产烟量相关特性信息.试验中,分别选取普通影剧院座椅与阻燃影剧院座椅分别进行连排燃烧试验,并将燃烧测试得到...     

Assessing the Sprinkler Activation Predictive Capability of the BRANZFIRE Fire Model

This paper describes an investigation into the sprinkler response time predictive capability of the BRANZFIRE fire model. A set of 22 fire/sprinkler experiments are simulated where the sprinkler activation time and the heat release rate (HRR) for each individual experiment had been determined. The experiments provided data for use in validating the sprinkler activation prediction algorithms in the BRANZFIRE zone model. A set of base case values were chosen and input files constructed for the simulations. The experiments were then simulated by the fire model using both the NIST/JET ceiling jet and Alpert’s ceiling jet options (which are the two ceiling jet correlations available in the BRANZFIRE zone model). The fire model included a heat transfer calculation for the temperature of the heat sensitive sprinkler element. Different sprinkler operational parameters such as the conduction factor, response time index (RTI) and the sprinkler depth below ceiling were also varied to assess the sensitivity of their effect on the activation time. Results showed that using the NIST/JET ceiling jet algorithm gave a closer prediction of the sprinkler response time in a small room than Alpert’s correlation. This was expected, since the former includes the effect of a hot upper layer while the latter applies to unconfined ceilings. The experiments available for comparison had been conducted inside an enclosure with a developing hot upper layer. The findings also signified that changing the sprinkler operational parameters can change the predicted sprinkler activation time significantly.     

考虑人员逃生公路隧道火灾控制风速研究

公路隧道火灾人员逃生与控制风速关系密切。本研究基于PHOENICS软件,建立了矩形、圆形及马蹄形断面下二、三及四车道9种计算模型,选取了大客车(20 MW)及无载重货车(30 MW)2种火源释放率, 选取了2.0 m/s、2.5 m/s、3.0 m/s、3.5 m/s及4.0 m/s的入口风速共计40种主要常见火灾工况,考虑了纵向通风对人体极限温度承受值的影响,采用了杨涛修正的动态火源释放率曲线及周勇狄修正的克拉尼公式,选用了适当的人员逃生条件,给出了每种工况8个特征时刻的10个特征点的温度值及曲线图,给出了燃烧5 min、12 min、30 min后火源处的纵横断面温度云图及中轴面烟气云图,给出了对应于火源燃烧位置上下游8个特征位置下人员逃生的忍受时间与逃离时间。研究得出:在基于人员逃生条件下矩形断面隧道在火源释放率为20 MW时二车道控制风速为3.0 m/s,三、四车道均为2.5 m/s;30 MW时二、三、四车道控制风速均为3.5 m/s,圆形与马蹄形断面隧道在火源释放率为20 MW时二、三、四车道控制风速均为3.5 m/s,30 MW时二车道控制风速均为4.0 m/s,三、四车道均为3.5 m/s。在火灾发生1 min后,人员以1 m/s从火源上下游进行疏散均可安全逃生。     

核电厂地震概率风险评估研究综述

日本福岛核事故之后,核电厂抗震安全评估受到广泛关注。地震概率风险评估方法是核电厂抗震安全评估方法之一。该文针对地震概率风险评估方法进展做了全面综述：首先介绍核工程领域地震概率风险评估方法的发展历史;并总结核电厂地震概率风险评估方法的基本原理和研究进展;然后论述三种改进的地震概率风险评估方法：基于SMA的地震概率风险评估方法、基于性能的概率地震风险评估方法和先进的地震概率风险评估方法;最后对全文做了总结和展望。     

Numerical studies on evaluation of smoke control system of underground metro rail transport system in India having jet injection system: A case study

The rail based urban transport system is being developed for national capital of India, New Delhi. The smoke control using ventilation in case of fire inside the tunnel, similar to Delhi Metro corridor has been investigated using computational fluid dynamics technique. A section of tunnel having dimensions 400 m long, 5.5 m wide and 6 m high is considered for simulation. The analysis has been carried out by assuming a variable fire source with a peak heat release rate (HRR) of 16 MW, located at the center of the tunnel. Ventilation ducts are located in the ceiling near the tunnel portals and are inclined at 10 degrees to the plane of the ceiling through which fans discharge air. The influence of the fire HRR curve slope on the smoke flow dynamics in a realistic tunnel model fitted with jet injection type longitudinal ventilation system has been investigated. In case of fire two cases are studied: (1) fans activated immediately after detection, (2) fans activated at delayed times to take into account the response time for the fans to achieve its maximum speed. The velocity of supply and exhaust fans necessary to remove smoke in 30 s from the upstream direction is determined. The velocities of fan required to produce desired critical velocity in the longitudinal direction for different HRR of fire is predicted.     

CALFIRE: An interactive model for fire calculations

CALFIRE, the acronym for CALculate Fire In Room and Enclosure, is a knowledge-based mathematical formulation of analytical and numerical procedures to predict the consequences of a fire in a room or enclosure. CALFIRE is a well-knit and integrated computer model that offers menu items such as heat release rate (HRR), flame height, vent size, and room temperatures of closed rooms, and rooms with natural and forced ventilation. Warnings and checks have been provided to prevent the misuse of the model. Care has been taken to require minimal keyboard responses in order to make CALFIRE a truly user-friendly, interactive fire model.     

基于CONE的超高温耐火电缆火灾危险性分析

利用锥形量热仪对超高温耐火电缆在不同辐射功率下的点燃时间（TTI）、热释放速率（HRR）、质量损失速率（MLR）和燃烧残余物进行了研究。研究表明,随着辐射功率增加,耐火电缆的TTI逐渐缩短,HRR和MLR逐渐增大,火灾危险性逐渐增加。超高温耐火电缆在35 kW/m2和50 kW/m2辐射功率下火灾性能指数相比于25 kW/m2分别增加了44.4%和176.5%,火灾增长指数分别增加了30.4%和83.0%。结合理论分析可以得出,耐火电缆的临界辐射功率为3.61 kW/m2、零辐射平均热释放速率为36.5 kW/m2,表现出较低的火灾危险性。     

Experimental investigation on transverse ceiling flame length and temperature distribution of sidewall confined tunnel fire

This paper presents an experimental investigation on the transverse ceiling flame length and the temperature distribution of a sidewall confined tunnel fire. The experiments were conducted in a 1/6th scale model tunnel with the fire source placed against the sidewall, 0 m, 0.17 m and 0.35 m above the floor, respectively. Experiments of fire against a wall without a ceiling, 0.35 m above the floor in a large space, were also conducted as a control group. Results shows that for small heat release rate (HRR), the flame is lower than the ceiling and extends along the sidewall. With the increase of HRR and elevation of burner height, the flame gradually impinges on the ceiling and spreads out radially along it. The flame impingement condition and the flame shapes of the wall fire with and without ceiling are presented. From the viewpoint of the physical meaning of flame impinging on the ceiling, the horizontal flame length should be a function of the unburned part of the fuel at the impinging point. Based on the proportional relation between the flame volume and HRR, the effective HRR (Q_ef) at the ceiling is determined and the effective dimensionless HRR, Q^*_ef is defined to correlate the horizontal ceiling flame length. Additionally, predictive correlations of transverse ceiling temperature distribution are proposed for the continuous flame region, the intermittent flame region and the buoyant plume region under the ceiling, respectively.     

综合管廊电缆舱灭火系统效果研究

针对某综合管廊的电缆舱室,采取全尺寸实验和数值模拟结合的方法,探究综合管廊内电缆舱的火灾特性,对比分析干粉灭火系统和高压细水雾灭火两种灭火系统的灭火效果.将火源功率为0.7 MW的乙醇火设置于电缆架底部,观察火灾发展和烟气蔓延情况,分析电缆舱火灾特性;对比两种灭火系统开启后管廊内的温度和烟气变化规律,分析灭火效果.在实...