Overview of the facility and experiments performed in Phébus FP

The international Phébus FP programme was initiated in 1988 by the French "Institut de Radioprotection et de Sûreté Nucléaire" (IRSN2), in cooperation with the European Commission (EC), to investigate key phenomena involved in light water reactor (LWR) severe accidents. In addition to IRSN and EC, other partner countries contributed to this programme. The Phébus facility was operated by the French "Commissariat à l'Energie Atomique" (CEA) at Cadarache, providing prototypic reactor conditions which allowed to study fuel degradation, the release of fission products (FPs) and their transport and behaviour through the reactor coolant system (RCS) and in the containment building. A specific attention was paid to iodine radiochemistry due to its major impact on the consequences of any radioactivity release to the environment. A preliminary re-irradiation period allowed to build up a short lived fission product inventory typical of a reactor fuel. The experimental phase comprised two main parts: during the fuel degradation phase, the experimental bundle was heated-up, then fission products were emitted and transmitted through the circuit towards the containment. In the second part, the containment was isolated from the circuit and the long term phase consisted of investigating first, the aerosol physics in the containment atmosphere, and next, the iodine chemistry. A large amount of instrumentation was implemented to measure on the one hand thermal-hydraulic parameters as temperature, pressure, humidity, hydrogen, oxygen and carbonaceous gases concentrations, pH, and on the other hand, fission products release, either through on-line γ-spectrometers, or with sampling devices coupled with off-line post test analysis. Attention was focused on the instrumentation devoted to iodine measurements to allow the distinction between aerosol particles, gaseous molecular iodine and other gaseous iodine species like organic iodides. After the test, a non-destructive examination campaign of the test section allowed to describe the final degradation state. Further destructive examinations were performed on selected parts of the fuel bundle to give information on the interaction between materials, and also on selected circuit sampling devices to obtain data on the long lived fission products and to characterise aerosols. © 2013 Published by Elsevier Ltd

Iodine behaviour in the containment in Phébus FP tests

Amongst the fission products that can be released in the environment during a severe accident, iodine causes great concern because of its volatility and of its potential radiological impact on population The Phébus FP program provides an extensive database on iodine behaviour in the containment of prime importance for a more realistic evaluation of the iodine source term that can be released in the environment in case of a severe accident The four main results regarding the iodine behaviour in the containment are: an early presence of gaseous iodine in the containment, the importance of painted surfaces exposed to the containment atmosphere in adsorption/desorption of volatile iodine, the key role of silver in preventing the iodine volatilisation from the sump when in excess over iodine (i.e in FPT0/1 tests) and the low steady state levels of gaseous iodine concentrations reached on the long term, independently of the amount of gaseous iodine present early in the containment During the fuel degradation, an early presence of gaseous iodine in the containment was observed in all tests The interpretation of this result suggests that this gaseous iodine was likely formed in the primary circuit (RCS), probably linked to non equilibrium chemical effects In FPT3, performed with a B4C control rod instead of Ag-In-Cd control rod for the other tests, the gaseous iodine fraction was much higher pointing to the essential role of control rod material on iodine chemistry in the RCS In all tests, more than half of the iodine fuel inventory reached the containment under aerosol and gaseous forms The Phébus FP tests give evidence that the sump did not contribute as much as expected to the production of gaseous iodine in the containment FPT0 and FPT1 results highlight the key role played by silver produced mainly by the control rod degradation Silver rapidly reacts with iodine to form non soluble AgI Thus, despite acidic pH, in so far as silver is in excess compared to iodine, it greatly reduces any gaseous iodine re-volatilisation from the sump either by radiolytic oxidation of iodide or by organic iodide formation from the immersed painted surface For FPT2, the very low volatilisation of gaseous iodine is attributed to the alkaline pH For FPT3, where the silver amount was negligible (Ag only present as FP) and the sump acidic, no significant iodine volatilisation from the sump was observed, due to low amounts of iodide aerosols transferred in it Phébus FP results underline the importance of painted surfaces in the containment atmosphere through their interaction with gaseous iodine Adsorption of gaseous iodine leads to the large decrease of the gaseous iodine concentration and to the formation of volatile organic iodides In some conditions, organic iodides gain in importance and become the predominant volatile iodine species notably in the long term for FPT0 and FPT1 This is of particular significance for radiological consequences when operating filtered containment venting as organic iodide are difficult to remove by filtration The Phébus FP experiments show that whatever the early gaseous iodine fraction was in the containment, low long-term steady concentrations are established This indicates that an equilibrium between iodine formation/destruction mechanisms and/or reversibility of iodine adsorption/desorption processes yield a similar steady-state concentration in the long term in all the tests The paper details the main results obtained and their up-to-date understanding © 2013 Elsevier B.V All rights reserved

Containment behaviour in Phébus FP

The international Phébus FP (fission product) programme investigated core degradation and radioactive product release originating from a core melt-down accident in Light Water Reactors. Five integral in-pile tests were performed at Cadarache. During these tests, the main processes which affect the severe degradation of fuel and control rods, the release of fission products and control materials, their transport through the reactor coolant circuit as well as their deposition as aerosols in the containment vessel were assessed. Various physico-chemical processes such as aerosol agglomeration and various deposition modes, such as gravitational settling, diffusiophoresis and deposition on the stainless steel walls were analysed during each test considering containment components dimensioned to guarantee conditions representing as far as possible the reactor case. The radiochemistry of fission products in the sump and the gaseous atmosphere of the containment were scrutinized in order to determine volatile forms of fission products, which could be released to the environment due to a containment leakage or a deliberate venting during an accident. The experimental data base obtained contains a wealth of information to validate the computer codes used for safety and accident management assessment. Observations made in Phébus-FP containment vessel led to a thorough understanding of the in-containment source term and its coupling with thermal hydraulics under unsaturated but condensing atmospheres. This paper summarizes the main results of interest concerning the containment vessel experimental data obtained during the whole PHEBUS FP programme. © 2013 Published by Elsevier Ltd

Modelling of bituminized radioactive waste leaching. Part I: Constitutive equations

This paper presents theoretical work aiming at studying the behaviour of leached bituminized waste materials and the development of a saline solution-filled pore structure (called permeable layer) in the material. In Part I of this paper, the constitutive model COLONBO which describes mathematically the evolution of the pore structure, the water uptake, the salt and RN release of leached bituminized waste is presented. By the introduction of adequate simplifications, an analytical solution of the problem is obtained which gives the main relations between measurable leaching indicators (water uptake, soluble salt release, porosity profiles and apparent thickness of the permeable layer) and material (soluble salt content, bituminized waste density) or environmental parameters (chemical activity of water in the leachant). In the companion paper (Part II), the validity of these relations is investigated experimentally. © 2005 Elsevier B.V. All rights reserved

Containment behaviour in Phebus FP

The international Phébus FP (Fission Product) programme was investigating core degradation and radioactive product release originating from a core melt-down accident on a Light Water Reactor Plant. Five integral in-pile tests have been performed at Cadarache. During these tests, the main processes which affect the severe degradation of fuel and control rods, the release of fission products and control materials, their transport through the reactor coolant circuit as much as their deposition as aerosols in the containment vessel were assessed. Various physico-chemical processes such as aerosol agglomeration and various deposition modes, such as gravitational settling, diffusiophoresis and deposition on the stainless steel walls were analyzed during each test considering containment components dimensioned to guarantee conditions representing as far as possible the reactor case. The radiochemistry of fission products in the sump and the gaseous atmosphere of the containment were scrutinized in order to determine volatile forms of fission products, which could be released to the environment due to a containment leakage or a deliberate venting during an accident. The experimental data base obtained contains a wealth of information to validate the computer codes used for safety and accident management assessment. Phébus-FP containment vessel led to a thorough understanding of the in-containment source term and its coupling with thermal hydraulics under unsaturated but condensing atmospheres. This paper summarizes the main results of interest concerning the containment vessel experimental data set during the whole PHEBUS FP programme.JRC.F.5-Nuclear Reactor Safety Assessmen

Reprint of "containment behaviour in Phébus FP"

The international Phébus FP (fission product) programme investigated core degradation and radioactive product release originating from a core melt-down accident in Light Water Reactors. Five integral in-pile tests were performed at Cadarache. During these tests, the main processes which affect the severe degradation of fuel and control rods, the release of fission products and control materials, their transport through the reactor coolant circuit as well as their deposition as aerosols in the containment vessel were assessed. Various physico-chemical processes such as aerosol agglomeration and various deposition modes, such as gravitational settling, diffusiophoresis and deposition on the stainless steel walls were analysed during each test considering containment components dimensioned to guarantee conditions representing as far as possible the reactor case. The radiochemistry of fission products in the sump and the gaseous atmosphere of the containment were scrutinized in order to determine volatile forms of fission products, which could be released to the environment due to a containment leakage or a deliberate venting during an accident. The experimental data base obtained contains a wealth of information to validate the computer codes used for safety and accident management assessment. Observations made in Phébus-FP containment vessel led to a thorough understanding of the in-containment source term and its coupling with thermal hydraulics under unsaturated but condensing atmospheres. This paper summarizes the main results of interest concerning the containment vessel experimental data obtained during the whole PHEBUS FP programme. © 2013 Elsevier B.V. All rights reserved

Thermodynamic and Kinetic Studies of lodine and Caesium Transport in Nuclear Severe Accident by High Temperature Mass Spectrometer

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Modelling of bituminized radioactive waste leaching. Part II: Experimental validation

This paper presents experimental work aiming at studying the behaviour of leached bituminized waste materials and the development of a saline solution-filled pore structure (called permeable layer) in the materials. Standard leach tests are performed on two synthetic BW materials containing different soluble salt contents and leached by solutions of varying chemical activities. The water uptake and the sodium nitrate leach rates are measured continuously during the tests. Porosity profiles are obtained from ESEM image analyses performed on the same BW samples leached during periods of varying durations. These experimental results provide a precise description of the internal structure of BW materials during leaching. The impact of the leaching duration, of the soluble salt content, and of the chemical activity of water in the leachant on the overall behaviour of leached BW materials is by this way assessed and compared to the theoretical trends presented in Part I of this paper. © 2005 Elsevier B.V. All rights reserved