Ruthenium release from fuel in accident conditions

During a hypothetical nuclear power plant accident, fission products may be released from the fuel matrix and then reach the containment building and the environment. Ruthenium is a very hazardous fission product that can be highly and rapidly released in some accident scenarios. The impact of the atmosphere redox properties, temperature, and fuel burn-up on the ruthenium release is discussed. In order to improve the evaluation of the radiological impact by accident codes, a model of the ruthenium release from fuel is proposed using thermodynamic equilibrium calculations. In addition, a model of fuel oxidation under air is described. Finally, these models have been integrated in the ASTEC accident code and validation calculations have been performed on several experimental tests. © Oldenbourg Wissenschaftsverlag, München

Fission product release from nuclear fuel II. Validation of ASTEC/ELSA on analytical and large scale experiments

This article is the second of two articles dedicated to the mechanisms of fission product release from a degraded core. The models of fission product release from nuclear fuel in the ASTEC code have been described in detail in the first part of this work (Brillant et al., this issue). In this contribution, the validation of ELSA, the module of ASTEC that deals with fission product and structural material release from a degraded core, is presented. A large range of experimental tests, with various temperature and conditions for the fuel surrounding atmosphere (oxidising and reducing), is thus simulated with the ASTEC code. The validation database includes several analytical experiments with both bare fuel (e.g. MCE1 experiments) and cladded fuel (e.g. HCE3, VERCORS). Furthermore, the PHEBUS large-scale experiments are used for the validation of ASTEC. The rather satisfactory comparison between ELSA calculations and experimental measurements demonstrates the efficiency of the analytical models to describe fission product release in severe accident conditions. © 2013 Elsevier B.V. All rights reserved

Fission product release from nuclear fuel I. Physical modelling in the ASTEC code

This article is the first of a series of two articles dedicated to the mechanisms of fission product release from a degraded core as they are modelled in the ASTEC code. The ASTEC code aims at simulating severe accidents in nuclear reactors from the initiating event up to the radiological consequences on the environment. This code is used for several applications such as nuclear plant safety evaluation including probabilistic studies and emergency preparedness. To cope with the requirements of robustness and low calculation time, the code is based on a semi-empirical approach and only the main limiting phenomena that govern the release from intact rods and from debris beds are considered. For solid fuel, fission products are classified into three groups, depending on their degree of volatility. The kinetics of volatile fission products release depend on the rate-limiting process of solid-state diffusion through fuel grains. For semi-volatile fission products, the release from the open fuel porosities is assumed to be governed by vaporisation and mass transfer processes. The key phenomenon for the release of low volatile fission products is supposed to be fuel volatilisation. A similar approach is used for the release of fission products from a rubble bed. An in-depth validation of the code including both analytical and integral experiments is the subject of the second article. © 2013 Elsevier B.V. All rights reserved

An expert system based on a Bayesian network for fire safety analysis in nuclear area

International audienceFor 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. © 201

Energy balance in a confined fire compartment to assess the heat release rate of an electrical cabinet fire

International audienceElectrical cabinet fires are a major concern in nuclear power plants due to the significant impact on plant safety. This paper investigates electrical cabinet fires in highly confined and mechanically ventilated compartments. In particular, it addresses the issue of experimentally determining the heat release rate of an electrical cabinet fire through realistic large-scale testing. The fire source is highly complex in nature due to the heterogeneity, stochastic geometry and location of the combustible materials within the electrical cabinet. After providing a detailed description of the experiments and apparatus, this paper focuses on the thermal modeling necessary to assess the heat release rate based on the energy balance in the fire compartment, the ventilation system and the electrical cabinet itself. The experimental method is then applied to a propane gas burner and an analytical cabinet containing PMMA tiles with homogeneous spatial distribution in order to validate the thermal model and heat measurements. Finally, the method is used to assess the heat release rate and combustion efficiency of an electrical cabinet. The various terms of the energy balance are described in detail to show the major role of the thermal inertia of both the electrical cabinet and the compartment walls. © 2012 Elsevier Ltd. All rights reserved

Numerical method for determining water droplets size distributions of spray nozzles using a two-zone model

International audienceThe 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. © 2017 Elsevier B.V

Characterization of closed-doors electrical cabinet fires in compartments

An important cause of fire departure in industrial facilities is due to electrical origin and particularly to electrical cabinets. The investigation of such fires has been scarce up to now and has been investigated exclusively in the nuclear industry. The Institut de Radioprotection et de Sûreté Nucléaire (IRSN) conducted a large number of experiments involving electrical cabinets burning either under a calorimetric hood or inside a mechanically ventilated compartment to investigate this topic. Calorimetric hood experiments demonstrated that the most important parameter is the size of the vents of the cabinet and that the time to flashover depends on many factors and seems somewhat random with regard to the observable parameters. The influence of the compartment on the fire behavior depends on the temperature of the surrounding atmosphere of the cabinet and on the oxygen content in the compartment at the level of the inlet vent of the cabinet. The compartment strongly impacts the pyrolysis of the combustible, affecting the fire duration, but has a weak effect on the Heat Release Rate (HRR). Experiments were usually remarkably reproducible, opening the way to a phenomenological description of this type of fire. A semi-empirical model based on the coupled solution of ventilation limit and excess pyrolysate could then be developed. This model was introduced in a zone code, and an ad-hoc modeling of the fire extinction based on a critical surfacic mass loss rate is proposed. The major features of the compartment fire experiments such as characteristic HRR and fire duration could then be reproduced with acceptable error. The development of such a semi-empirical model is a common practice in fire safety engineering concerned with complex combustibles. © 2011 Elsevier Ltd. All rights reserved

Characterisation of open-door electrical cabinet fires in compartments

International audienceIntroduction The study of electrical fires is a major concern for fire safety in the industry and more particularly for fire safety in nuclear facilities. To investigate this topic, IRSN conducted a large number of real-scale experiments involving open-door electrical cabinets burning firstly under a calorimetric hood and then inside a mechanically-ventilated compartment. The main challenges are to determine accurately the heat release rate of such a complex fire source in a vitiated atmosphere and to provide an experimental database for validating properly the combustible modelling, taking into account the oxygen depletion in an enclosure. After providing a detailed description of the fire scenarios and of the experimental apparatus, this paper focuses on the characteristic stages of the cabinet fire development, essentially based on the heat release rate time evolution of the fire. The effects of the confinement, of the outlet branch location, of the ventilation management and of the fire barrier on the fire source were then investigated. The reproducibility of electrical cabinet fires is also studied. A new model for complex fire source (applied in this study for open-door electrical cabinet fires) was then developed. This model was introduced in the zone code SYLVIA and the major features of the compartment fire experiments, such as characteristic heat release rate with effect of oxygen depletion and over-pressure peak were then calculated with a rather good agreement for this complex fire source (i.e. electrical cabinet). © 2015 Elsevier B.V. All rights reserved

Experimental and numerical approaches of aerosol removal in spray conditions for containment application

TOSQAN is an experimental program undertaken by the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) in order to perform thermal hydraulic containment studies. The TOSQAN facility is a large enclosure devoted to simulate typical accidental thermal hydraulic flow conditions in nuclear pressurized water reactor (PWR) containment. The TOSQAN facility, which is highly instrumented with non-intrusive optical diagnostics, is particularly adapted to nuclear safety code validation. The present work is devoted to study a water spray injection used as a mitigation means in order to washout aerosol fission products. © 2008 Elsevier B.V. All rights reserved