For fire safety studies in nuclear installations, IRSN uses the SYLVIA software. The SYLVIA two-zone model was developed by IRSN to simulate a full ventilation network, fire scenarios in a highly ...confined and mechanically ventilated facility, and airborne contamination transfers inside nuclear installations. In order to take into account the different sources of uncertainty coming from initial and boundary conditions as well as from model parameters, the SYLVIA software is associated with the SUNSET statistical software. However, such a use of SYLVIA software has a major drawback: it requires a large number of runs and a significant statistical analysis what is not always compatible with the requirements of safety assessments in terms of deadlines. To overcome this difficulty, IRSN is currently developing an expert system based on a SYLVIA database. This approach allows deriving the most likely diagnosis or prognosis in a very short time, but also deriving a more complex form of reasoning intertwining prognostic and diagnostic inferences. The proposed expert system is based on the Bayesian Belief Network (BBN) methodology and consists in two steps: First, a large database obtained from SYLVIA runs allows the estimation of Conditional Probability Tables. Then, a message passing algorithm is used to exploit dynamically this data base. The illustrating example is based on the study of pressure effects due to fire scenarios in nuclear facilities and the database is made up of 1,600,000 runs of the SYLVIA software. The goal of this paper is to detail the methodology and process to carry out an expert system for fire safety studies, and is supported by one example showing how it can be used as a decision support tool for fire safety analysis in nuclear area. To our opinion, the development of expert systems represents a new generation of computational tools in the field of probabilistic fire simulation.
•Expert system, a tool that aims to simulate the cognitive mechanisms of an expert.•Expert system based on the Bayesian Belief Network.•Expert system, a new generation of computational tools in the field of probabilistic fire simulation.•Expert system, a decision support tool for fire safety analysis in nuclear area.
Field simulation results of a fire, centered in a small scale enclosure (1.04m3) with an open doorway, performed with the CFD computer code ISIS, are presented. Three heat release rates of 10.6, 15.5 ...and 21.7kW, provided with a propane gas burner, are numerically studied and compared to experimental results. Turbulence and soot modeling are first validated by simulating the gas burner fire in an open atmosphere for the three fire powers. Due to the complex role of buoyancy in production of turbulence inside a pool fire plume, anisotropic modeling, through the generalized gradient diffusion hypothesis, is considered in the standard k–ε model. The comparisons between experimental measurements and numerical simulation results, for the enclosure fire, concern temperature and velocity profiles at the doorway and temperature profiles inside the enclosure. Velocity measurements at the open doorway are performed using a stereoscopic particle image velocimetry (SPIV) technique, allowing a full comparison with computational fluid dynamics (CFD) results. For the three heat release rates, the simulation results agree well with experimental measurements. General flow patterns, provided by CFD simulations, are reported for the highest fire power and supply useful information for understanding enclosure fires.
The water spray systems are widely used for fire safety area and is a well-established technique for providing safety and protection of nuclear installations and also industrial facilities. One major ...challenge is to be able to properly determine the technical features of the water spray system that are required for predictive simulations. For that, a Phase Doppler Interferometer (PDI) device, that is a complex and challenging laser technique, is often used to measure the water droplets size distributions and the water droplets velocities. However, some usual water spray models can require as input parameters only an overall water droplets size distribution and water droplets initial velocity and some statistical methods are needed to determine them from local accurate measurements. In this paper, it is addressed a new calibration approach for assessing the input parameters of this modeling by using large-scale and well-controlled fire tests. Then, by introducing some correlations to take into account different operating conditions of the pressure at the spray nozzle head, this technique is validated on other large-scale fire tests. After discussing thoroughly the results, this new method shows that it can be a valuable and efficient tool for determining the overall features of water spray systems linked with the modeling of water spray system used in this study.
The thermal oxidation of polyethylene films in air at 80
°C and 90
°C has been studied by tensile testing, IR spectrophotometry and molar mass determination from rheometric measurements. In the ...conditions under study, the polymer predominantly undergoes chain scission and embrittles suddenly when the weight average molar mass reaches a critical value (90
kg
mol
−1), far before significant damage of the entanglement network (
M
e
=
1.9
kg
mol
−1) in the amorphous phase.
The following embrittlement mechanism is proposed: chain scission in the amorphous phase induces chemicrystallization. The thickness of the interlamellar amorphous layer (
l
a) decreases until a critical value of the order of 6–7
nm, below which plasticity cannot be activated and the polymer behaves in a brittle manner, as previously shown for virgin polyethylene. Using (
l
a,
M
W) maps, it is possible to explain the differences observed in the embrittlement behaviour of semi-crystalline polymers predominantly undergoing chain scission.
The study deals with the data regression methods used for validating zone code modeling in comparison with experimental fire tests. These methods aim at determining both the temperatures of upper and ...lower zones and the interface location calculated from experimental vertical temperature profiles. A new data regression method is proposed to improve the determination of upper and lower temperatures and position of the interface. This new method merges both the thermal stratifications showing constant temperature in the upper layer as often encountered for well ventilated fire (approach of Quintiere et al. 4) and those showing constant gradient as often observed for fire in closed and forced ventilated compartment (approach of Audouin et al. 8,9). These two latter existing methods and the new former one are then applied to large scale fire tests experiments performed in IRSN facilities. The major outcomes indicate that the most appropriate regression method depends on the basic shapes of the experimental vertical temperature profiles. The interface height is the most sensitive variable to the choice of the regression methods. Indeed, discrepancy higher than 100% can be found concerning this last variable. In contrast, the temperature of lower zone appears to be the less sensible variable. The discrepancy on the upper temperature is found also significant in some cases. From these investigations, guidelines are proposed to improve the data regression process needed for validating zone codes and are supported by a thorough analysis of these three data regression methods using experimental data from large-scale fire tests.
•Data regression methods for analyzing temperature stratification.•Thermal stratifications with constant temperature gradient.•Application for fire in closed and forced ventilated compartment.•Methodology for identifying the most suitable data regression method.
The inverse kinematics technique, applied to radioactive beams and combined to the Coulomb excitation method, is a powerful tool to study low-energy fission. A novel experimental setup was developed ...within the R3B/SOFIA (Reactions with Relativistic Radioactive Beams/Studies On FIssion with Aladin) collaboration to identify in mass and atomic numbers both fission fragments in coincidence. These new data provide elemental, isobaric, and isotonic yields for the fission along the thorium isotopic chain. Results are also compared to previous measurements using either the same reaction mechanism or thermal-neutron induced fission. This latter comparison permits to probe the influence of the excitation energy in the fission process.
Reliable neutron-induced-reaction cross sections of unstable nuclei are essential for nuclear astrophysics and applications but their direct measurement is often impossible. The surrogate-reaction ...method is one of the most promising alternatives to access these cross sections. In this work, we successfully applied the surrogate-reaction method to infer for the first time both the neutron-induced fission and radiative capture cross sections of 239Pu in a consistent manner from a single measurement. This was achieved by combining simultaneously measured fission and γ-emission probabilities for the 240Pu (4He, 4He′) surrogate reaction with a calculation of the angular-momentum and parity distributions populated in this reaction. While other experiments measure the probabilities for some selected γ-ray transitions, we measure the γ-emission probability. This enlarges the applicability of the surrogate-reaction method.
Taking benefit of the R3B/SOFIA setup to measure the mass and the nuclear charge of both fission fragments in coincidence with the total prompt-neutron multiplicity, the scission configurations are ...inferred along the thorium chain, from the asymmetric fission in the heavier isotopes to the symmetric fission in the neutron-deficient thorium. Against all expectations, the symmetric scission in the light thorium isotopes shows a compact configuration, which is in total contrast to what is known in the fission of the heavier thorium isotopes and heavier actinides. This new main symmetric scission mode is characterized by a significant drop in deformation energy of the fission fragments of about 19 MeV, compared to the well-known symmetric scission in the uranium-plutonium region.
This paper investigates the pressure variations induced by a pool fire in a well-confined and force-ventilated enclosure. This study finds practical applications to fire safety in the nuclear ...industry in which some compartments are often highly confined and ventilated by means of a ventilation network. In this paper, the question is to study and understand whether the pressure effect could be high enough to cause fire barriers to fail (fire door, dampers, etc.) and/or to release radioactive material inside the nuclear facility in spite of the pressure drop cascade strategy usually considered. Relying on two sets of large-scale fire tests performed by IRSN, this work quantifies and discusses the impact of pressure effects caused by hydrocarbon pool fires on the fire compartment and on the ventilation network. Pressure histories are presented for experiments involving 0.3–3.1-MW liquid pool fires and a fire room connected to an industrial ventilation system that includes both inlet and exhaust branches. The analysis of experimental data is supported with a theoretical approach in order to describe in detail the physical mechanisms that contribute to pressure variations. Then, a parametric analysis allows us to determine the effects of the fire heat release rate and the air flow resistance inside the ventilation network on the pressure peaks. Finally, the last part of this paper focuses on a correlative approach to estimate the overpressure peak at fire ignition.
► Investigation of pressure variations from experimental and theoretical approaches. ► Physical mechanisms that induced pressure peaks at ignition and extinction. ► Parameters are the fire, the ventilation network set-up and the enclosure properties. ► Experimental correlation for the prediction of overpressure peak at fire ignition.
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