Probabilistic risk analysis and risk-based priority scale for the tiles of the space shuttle

The thermal protection system of the space shuttle is one of its most critical subsystems because it protects the orbiter from heavy heat loads at reentry into the atmosphere. To optimize NASA's allocation of risk management resources, a probabilistic risk analysis model is developed for the black tiles, and a risk-criticality index is computed for each tile based on its contribution to the overall probability of loss of vehicle and crew (LOV/C). This assessment is based on the susceptibility of the tiles (i.e. their probabilities of debonding), and on the vulnerability of the orbiter to specific tile losses given the criticality of the subsystems under the aluminum skin in various locations. The two main initiating events are linked to the debonding of a tile, caused either by debris hits or by a weak bond because of poor tile installation. The PRA model relies on a partition of the orbiter's surface according to four parameters: the probability of debris hits, the probability of secondary tile loss once a first tile has debonded, the probability of burnthrough given a failure patch of specified size, and the probability of LOV given a hole in the orbiter's aluminum skin. The results show that the contribution of the tiles to the overall probability of LOV is about 10%. They also include a map of the orbiter's surface showing the relative risk-criticality of tiles at various locations. It was found that 85% of the risk can be attributed to 15% of the tiles, thus allowing the management to allocate more effort and resources to the maintenance of the most risk-critical tiles.     

Failure analysis of a thermal tile on the space shuttle Columbia

Several high-temperature reusable surface insulation (HRSI) tiles from the space shuttle Columbia were analyzed for their response to the catastrophic events during re-entry. The analysis of the HRSI tile focused on the outer surface, which was composed of silica tile with a reaction-cured glass (RCG) coating, and the inner surface strain isolation pad, which is bonded to the tile using a room-temperature vulcanizing silicone adhesive. Light optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were used to evaluate the tile surfaces. The tiles were found to have a heavily damaged outer surface, with much of the top RCG coating eroded by impact from shuttle debris and/or plasma flow during the shuttle’s breakup and re-entry. The RCG experienced slumping and the silica exhibited melting and glassification due to temperatures exceeding the glass transition temperature and the anticipated tile operating temperatures. Deep cavities present in the material were found to contain metal particles that are not normally present and suggest impact with shuttle debris. Silicone adhesive degradation was also found. The slumping of the silica, the degraded adhesive, and the erosion of the tile contributed to a complex process during which the individual tile delaminated from the shuttle airframe.     

大型LNG储罐罐底保冷力学强度校核设计浅析

罐底保冷设计及力学强度校核是大型液化天然气（LNG）储罐设计的核心技术之一。大型LNG储罐底部保冷层应能承受上部LNG产品及储罐本体重量，所以保温层应具有足够的抗压强度；同时罐底部保冷层可能受到横向作用力，因此设计需对罐底保冷层进行承载能力校核以确保投产后罐底泡沫玻璃砖不被压碎破坏。本文以国内某已建LNG项目储罐罐底保冷系统基本参数为例，对其在运行工况重量荷载、水压试验、地震等工况作用下进行了力学抗压强度校核设计，结果表明罐底保冷层具有较大安全系数，力学富裕量满足规范要求。     

A Cell Method Stress Analysis in Thin Floor Tiles Subjected to Temperature Variation

The Cell Method is applied in order to model the debonding mechanism in ceramic floor tiles subjected to positive thermal variation. The causes of thermal debonding, very usual in radiant heat floors, have not been fully clarified at the moment. There exist only a few simplified analytical approaches that assimilate this problem to an eccentric tile compression, but these approaches introduce axial forces that, in reality, do not exist. In our work we have abandoned the simplified closed form solution in favor of a numerical solution, which models the interaction between tiles and sub-base more realistically, when the positive thermal variation increases the volume of the sub-base. The thermal problem has been approached as a contact problem in a composite structure. In particular, the kinematic and equilibrium conditions have been imposed at the interface between lower part, which is the sub-base, and the upper part, which is composed by the adhesive, the tiles, and the grouting between the tiles. The failure condition has been studied in the Mohr-Coulomb plane by using the Leon criterion, a unifying criterion that combines the shear stress with traction and compression. Therefore, we employed a unique failure criterion both for the nodes at the interface between sub-base and adhesive (which undergo a shear/tensile failure or a shear failure) and the nodes at the interface between tiles and grouting (which undergo a tensile failure). This allowed us to model the tile debonding both in the horizontal and in the vertical interfaces, while previous FEM codes treated the tile debonding only on the horizontal interfaces. The numerical analyses were performed in parametric modality, by varying the geometric and mechanical characteristics of the model. Particular attention was devoted to the modeling of thin tiles, a new type of ceramic tiles, for which there are no yet consensus standards.     

Incorporating organizational factors into Probabilistic Risk Assessment (PRA) of complex socio-technical systems: A hybrid technique formalization

This paper is a result of a research with the primary purpose of extending Probabilistic Risk Assessment (PRA) modeling frameworks to include the effects of organizational factors as the deeper, more fundamental causes of accidents and incidents. There have been significant improvements in the sophistication of quantitative methods of safety and risk assessment, but the progress on techniques most suitable for organizational safety risk frameworks has been limited. The focus of this paper is on the choice of “representational schemes” and “techniques.” A methodology for selecting appropriate candidate techniques and their integration in the form of a “hybrid” approach is proposed. Then an example is given through an integration of System Dynamics (SD), Bayesian Belief Network (BBN), Event Sequence Diagram (ESD), and Fault Tree (FT) in order to demonstrate the feasibility and value of hybrid techniques. The proposed hybrid approach integrates deterministic and probabilistic modeling perspectives, and provides a flexible risk management tool for complex socio-technical systems. An application of the hybrid technique is provided in the aviation safety domain, focusing on airline maintenance systems. The example demonstrates how the hybrid method can be used to analyze the dynamic effects of organizational factors on system risk.     

Hypervelocity penetration of tungsten alloy rods into ceramic tiles: experiments and 2-D simulations

A series of terminal ballistics experiments and 2-D simulations, with small scale tungsten alloy penetrators, was performed in order to quantify the ballistic efficiency of confined ceramic tiles. The data includes both depth of penetration (DOP), into thick steel backing and X-ray shadowgraphs during the penetration process. Impact velocities ranged between 1.25 to 3.0 km/s. The size of the tiles varied in order to check their performance as a function of thickness and lateral dimensions. We found that the differential ballistic efficiency of alumina tiles is practically independent on impact velocity and tile thickness, within the ranges of velocity and thicknesses, investigated here. A detailed simulation study, using the Eulerian processor of the PISCES 2-D ELK code, was performed in order to better understand the interaction between long-rods and ceramic tiles, and particularly, to adjust a proper failure criterion to the tiles. We found that a simple version of the Johnson-Holmquist model, with a single parameter, is fairly adequate to account for most of the data. These include: lateral confinement, tile thickness and impact velocity influence on the penetration depth. We used the code to further investigate the influence of lateral dimensions on tile performance.     

Experiments and 2-D simulations of high velocity penetrations into ceramic tiles

This paper investigates the interaction of long-rod penetrators with thick ceramic tiles, sandwiched between steel plates, through several model experiments and 2-D simulations. Experimental data from low velocity penetrations have been used to calibrate the relevant properties of the ceramic specimens. The influence of increasing impact velocity on tile performance was then investigated through data and simulations of shaped charge jets penetrating the ceramic. We found that the ballistic efficiency of the ceramic tile is lower against high velocity (5 km/s) long-rods, in contrast with the common thesis that their improved performance against shaped charge jets is the result of their enhanced strength. On the other hand, our simulations clearly show that, for high strength ceramics, there is a radial motion of metal and ceramic debris towards the penetration axis. This effect is, probably, the main reason for the considerable improvement in the performance of ceramic tiles against shaped charge jets.     

Quasi-static impact damage in confined ceramic tiles

The effect of confinement on the failure mechanisms in dense alumina tiles during the penetration of a projectile was investigated and the role played by lateral mechanical confinement in inhibiting some failure modes was examined.Alumina tiles were placed in a confinement frame which simply and accurately allows high biaxial compressive pre-stresses. The confinement frame is a modular system which enables the application of various boundary conditions on the impacted tile to be generated. Tile samples were supported by semi-infinite support blocks made of steel or aluminum, and were confined by steel wedges.In the tests carried out under ballistic penetration conditions, the confined tiles were impacted by 0.3 cal. NATO AP (armor piercing) projectiles. These contained a hardened steel cone-shaped core with a sharp tip. In this investigation, the effect of reflected stress waves was small or even negligible, since the tile/support impedance mismatch was low, so that quasi-static failure mechanisms predominated. The effect of tile thickness on the damage mechanisms was also examined. Under the described test conditions, the major failure modes were radial cracks, cone crack, and fragmentation of the cone. Confined tiles exhibited reduced damage; damage in the form of radial cracks was reduced, and cone crack and fragmentation of the cone were inhibited. Based on the theory of a plate on an elastic foundation, the quasi-static loads that initiate the radial cracks were formulated. This model assists in understanding the initial damage mechanism.     

Probabilistic safety assessment support for the maintenance rule at Duke Power Company

The Nuclear Regulatory Commission (NRC) published the Maintenance Rule on July 10, 1991 with an implementation date of July 10, 1996 [1]. Maintenance rule implementation at the Duke Power Company has used probabilistic safety assessment (PSA) insights to help focus the monitoring of structures, systems and components (SSC) performance and to ensure that maintenance is effectively performed. This paper describes how the probabilistic risk assessment (PRA)¹ group at the Duke Power Company provides support for the maintenance rule by performing the following tasks: (1) providing a member of the expert panel; (2) determining the risk-significant SSCs; (3) establishing SSC performance criteria for availability and reliability; (4) evaluating past performance and its impact on core damage risk as part of the periodic assessment; (5) providing input to the PRA matrix; (6) providing risk analyses of combinations of SSCs out of service; (7) providing support for the SENTINEL program; and (8) providing support for PSA training. These tasks are not simply tied to the initial implementation of the rule. The maintenance rule must be kept consistent with the current design and operation of the plant. This will require that the PRA models and the many PSA calculations performed to support the maintenance rule are kept up-to-date. Therefore, support of the maintenance rule will be one of the primary roles of the PSA group for the remainder of the life of the plant.     

蓄能空调冷、热槽体保温方案优化及经济性分析

根据冷、热蓄能槽体的特点:工作温度、外形、槽体材料、内外保温和现有保温工艺,再结合保温材料的特性:工作温度、导热系数和抗压强度,比较分析了目前槽体保温工艺及材料选型,探讨了实用性与经济性相结合的保温方案,并进行技术和经济分析。     

The impact of assessing lower head integrity on boiling water reactor probabilistic risk assessments

Over the last several decades, much effort has been directed at estimating the likelihood of a large early release of radioactivity during a nuclear accident. This effort has culminated in the Individual Plant Examinations (IPEs) for the over 100 US nuclear power plants and the NUREG 1150 study. The large early release of radioactivity requires core damage with loss of primary containment integrity during the accident. Given a successful reactor scram, early containment failure coupled with a large release of radioactivity will only occur if the reactor core vessel is breached by core debris. Most IPE/PRA studies performed to date have not considered the possibility of quenching core debris in the lower plenum. Consequently, lower head failure is presumed to closely follow the onset of core damage. Therefore, these assessments did not address the role that in-vessel debris retention plays in preserving primary containment integrity, nor do they propose a criterion for evaluating the integrity of the vessel lower head given that core damage has occurred. Yet preserving the vessel lower head integrity is a necessary condition for satisfying the plant design and licensing basis. Therefore, a more complete treatment of the risk associated with nuclear plant operation includes an evaluation of the ability to retain the core debris in-vessel. This paper presents a performance requirement for vessel integrity to be used in probabilistic risk assessments; evaluates the impact the core damage progression and lower plenum quenching models have on the likelihood of terminating the damage progression in-vessel; documents the significant reduction in BWR containment failure probability that can occur when appropriate core damage and lower head quenching models are used; reviews the implications of core debris quenching in the lower head on BWR PRA modeling; argues why crediting the capability to maintain vessel integrity is necessary from a safety point of view. These results and conclusions are derived from consideration of a BWR 4 plant with a 251 inch vessel. However, the concepts are generally applicable and results specific to other BWR designs can be developed using the methodology presented in this paper.     

Human reliability analysis—why not turn to the human factors community?

In response to the Dougherty thesis that contemporary human reliability analysis (HRA) methods are inadequate, this paper proposes that credible assessments of these HRA methods adequacy can be obtained only by means of their full exploitation by human factors specialists as part of the probabilistic risk assessment (PRA) process. The paper traces the history of human factors in PRA. It concludes that regarding PRA, only peripheral attention has been given to human factors; further that there has been almost a total absence of human factors specialists involved in the PRA process. The paper introduces and discusses a recent US Nuclear Regulatory Commission research initiative on a task analysis-linked evaluation technique (TALENT) concept for integrating human factors expertise into the PRA process, and fully exploiting state-of-knowledge HRA methods and data. The paper concludes that by means of TALENT implementation: (1) more credible assessments of HRA methods adequacy can be made, and (2) more realistic estimates of the overall impact of human error on complex high reliability systems' reliability and risk can be achieved.     

全自动瓷砖包装机翻砖机理研究

研究瓷砖包装机翻砖装置的翻砖机理,探讨了瓷砖在翻转过程中速度、加速度的变化规律以及冲击力对瓷砖品质的影响,分析了瓷砖传送带在工作中拉力频繁变化对工作稳定性的影响,改进了瓷砖翻砖装置,使其更适合翻砖工况要求。     

Glossiness distribution over the surface of stoneware floor tiles due to the polishing process

The industrial process for polishing floor tiles requires several polishing stages in order to produce the desirable glossiness. Works on floor tile polishing with focus on the distribution of glossiness still lack in literature. The present work intends to measure and analyze the distribution of glossiness over the surface of porcelain stoneware tiles polished using forward speed of 7.5 cm s⁻¹ and lateral oscillation frequency and amplitude of 0.2 s⁻¹ and 12 cm. The glossiness pattern generated by the polishing process over the surface of six tiles were presented in grey-scale graphics, where each pixel was univocally associated with a portion of the tile surface. Correlations between the glossiness pattern found and the polishing kinematics were developed. Significant differences of glossiness were registered either between tiles polished under the same polishing condition, or within the surface of the same tile, between adjacent regions. The use of lateral oscillation motion caused the glossiness pattern over the tile surface to follow a waveform pattern, and two corroborative hypotheses were made in order to explain such fact, considering the light-surface interaction as well as the overlapping of trajectories of adjacent polishing heads.     

PRA importance measures for maintenance prioritization applications

Various importance measures are standardly calculated in a standard probabilistic risk assessment (PRA). Approaches are developed in this paper for using two of these measures—the minimal cutset contribution and the risk reduction importance—for prioritizing the risk importances of maintenances. One approach which is developed prioritizes maintenances based on the risk importance of the associated contributor. The second approach prioritizes maintenances based on the risk impact if the maintenance is ineffective. The core damage frequency is used as the risk measure for prioritization. The demonstration studies, which are carried out using the Surry Plant PRA, indicate that the two approaches give similar results if appropriate cutoff criteria are used. As an additional evaluation, risk unimportant maintenances are identified using the risk increase importance, or risk achievement worth, calculated in the PRA.     

Reducing facet nucleation during algorithmic self-assembly

Algorithmic self-assembly, a generalization of crystal growth, has been proposed as a mechanism for bottom-up fabrication of complex nanostructures and autonomous DNA computation. In principle, growth can be programmed by designing a set of molecular tiles with binding interactions that enforce assembly rules. In practice, however, errors during assembly cause undesired products, drastically reducing yields. Here we provide experimental evidence that assembly can be made more robust to errors by adding redundant tiles that "proofread" assembly. We construct DNA tile sets for two methods, uniform and snaked proofreading. While both tile sets are predicted to reduce errors during growth, the snaked proofreading tile set is also designed to reduce nucleation errors on crystal facets. Using atomic force microscopy to image growth of proofreading tiles on ribbon-like crystals presenting long facets, we show that under the physical conditions we studied the rate of facet nucleation is 4-fold smaller for snaked proofreading tile sets than for uniform proofreading tile sets.     

On the quantification of the effects of organizational and management factors in chemical installations

The quantitative effects of organizational and management factors in chemical installations are assessed through a linking of the results of a safety management audit with the basic events of a quantified risk assessment (QRA). A safety management audit establishes the relative position of the organizational and management aspects of a particular chemical installation with respect to an ideal management scheme, and for a number of failure causes and failure prevention combinations. A quantitative risk analysis including detailed system analysis offers a plant-specific decomposition of the plant-damage-state frequencies into events like hardware failures, maintenance-related failures, operation-related failures and so on. The basic events incorporated in the QRA are then categorized into classes similar to those explored by the management audit and are quantitatively linked to the audit results. Knowledge of these quantitative links would allow for the reflection of the deficiencies or strengths that might exist in the safety management system on the quantitative risk indices. A case study of an ammonia storage facility that has been audited demonstrates that the sensitivity of the risk indices to the value of the quantitative links is extremely high and that hence great care should be exercised in assessing these links.     

Beam tests of directly coupled scintillator tiles with MPPC readout

We report on the response to a proton beam of scintillator tiles directly coupled at the face of the tile to multi-pixel photon counters. Detailed measurements with protons show that flat tiles have high response near the photon counters while concave tiles have uniform response suggesting that tiles with this versatile configuration can be tailored to a desired uniformity. The beam response is in qualitative agreement with the response to a non-triggered radioactive source and reveals additional spatial features.     

Risk-based maintenance of ethylene oxide production facilities

This paper discusses a methodology for the design of an optimum inspection and maintenance program. The methodology, called risk-based maintenance (RBM) is based on integrating a reliability approach and a risk assessment strategy to obtain an optimum maintenance schedule. First, the likely equipment failure scenarios are formulated. Out of many likely failure scenarios, the ones, which are most probable, are subjected to a detailed study. Detailed consequence analysis is done for the selected scenarios. Subsequently, these failure scenarios are subjected to a fault tree analysis to determine their probabilities. Finally, risk is computed by combining the results of the consequence and the probability analyses. The calculated risk is compared against known acceptable criteria. The frequencies of the maintenance tasks are obtained by minimizing the estimated risk. A case study involving an ethylene oxide production facility is presented. Out of the five most hazardous units considered, the pipeline used for the transportation of the ethylene is found to have the highest risk. Using available failure data and a lognormal reliability distribution function human health risk factors are calculated. Both societal risk factors and individual risk factors exceeded the acceptable risk criteria. To determine an optimal maintenance interval, a reverse fault tree analysis was used. The maintenance interval was determined such that the original high risk is brought down to an acceptable level. A sensitivity analysis is also undertaken to study the impact of changing the distribution of the reliability model as well as the error in the distribution parameters on the maintenance interval.     

基于统计能量法的隔声瓦减振性能仿真研究

在水下结构表面敷设隔声去耦材料是应用最广泛也是非常有效的一种提高舰船隐身性能的方法。基于统计能量法开展了隔声瓦对复杂锥柱结构水下振动的影响研究,讨论了隔声瓦敷设方式对复杂锥柱结构水下振动的影响,分析了阻尼损失系数对隔声瓦减振效果的影响。研究表明,隔声瓦敷设方式、阻尼损失系数对隔声瓦减振效果有较大影响:当隔声瓦敷设在结构振动主导传递途径上时,其对传递途径下游结构的振动抑制效果较为明显,而对于振源及传递途径上游结构振动的影响较小;隔声瓦减振效果随敷设密度的增大而增加,随阻尼损失系数的增大而有所降低。