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The test facility THAI (thermal-hydraulics, hydrogen, aerosol, and iodine) aims at addressing open questions concerning gas distribution, behaviour of hydrogen, iodine and aerosols in the containment of light water reactors during severe accidents. Main component of the facility is a 60 m 3 stainless steel vessel, 9.2 m high and 3.2 m in diameter, with exchangeable internals for multi-compartment investigations. The maximal design pressure of the vessel is 14 bar which allows H 2 combustion experiments at a severe accident relevant H 2 concentration level. The facility is approved for the use of low-level radiotracer I-123 which enables the measurement of time resolved iodine behaviour. The THAI test facility allows investigating various accident scenarios, ranging from turbulent free convection to stagnant stratified containment atmospheres and can be combined with simultaneous use of hydrogen, iodine and aerosol issues. THAI experimental research also covers investigations related to mitigation systems employed in light water reactor containments by performing experiments on, e.g. pressure suppression pool hydrodynamics, performance behaviour of passive autocatalytic recombiners, and spray interaction with hydrogen–steam–air flames in phenomenon orientated and coupled-effects experiments. The THAI experimental data have been widely used for the validation and further development of Lumped Parameter and Computational Fluid Dynamics codes with 3D capabilities, e.g. International Standard Problems ISP-47 (thermal hydraulics, gas distribution) and ISP-49 (hydrogen combustion), EU-SARNET/SARNET2 code-benchmark exercises involving THAI data on iodine/surface interactions, iodine mass transfer, passive autocatalytic recombiner performance, iodine oxide behaviour and iodine transport in multi-compartment behaviour. The present paper provides an overview of the THAI experiments related to hydrogen and fission products issues performed in the frame of national and international projects. From the comprehensive THAI experimental database, a selection of typical results is presented to illustrate the multi-functionality of the THAI facility and the broad variety of the experimental investigations.

The conversion of gaseous molecular iodine into iodine oxide aerosols has significant relevance in the understanding of the fission product iodine volatility in a LWR containment during severe accidents. In containment, the high radiation field caused by fission products released from the reactor core induces radiolytic oxidation into iodine oxides. To study the characteristics and the behaviour of iodine oxides in large scale, two THAI tests Iod-13 and Iod-14 were performed, simulating radiolytic oxidation of molecular iodine by reaction of iodine with ozone, with ozone injected from an ozone generator. The observed iodine oxides form submicron particles with mean volume-related diameters of about 0.35 μm and show low deposition rates in the THAI tests performed in the absence of other nuclear aerosols. Formation of iodine aerosols from gaseous precursors iodine and ozone is fast as compared to their chemical interaction. The current approach in empirical iodine containment behaviour models in severe accidents, including the radiolytic production of I 2 -oxidizing agents followed by the I 2 oxidation itself, is confirmed by these THAI tests.

In the case of a severe accident in a nuclear Light Water Reactor (LWR), the high radiation fields reached in the reactor containment building due to the release of fission products from the reactor core would induce air radiolysis. The air radiolysis products (ARP) could, in turn, oxidise gaseous molecular iodine (I2) into aerosol-borne iodine–oxygen–nitrogen compounds, abbreviated as iodine oxides (IOx). These reactions involve the conversion of a gaseous iodine compound resulting in a change of the iodine depletion rate from the containment atmosphere. Kinetic data were produced within the first part of PARIS project on the air radiolysis products formation and destruction. The second part of the PARIS project as presented in this paper deals with the impact of the ARP on the conversion of I2 into IOx. The objective was to provide a database to develop new or to validate existing kinetic models of formation and destruction of iodine oxides. The iodine tests of the PARIS project, performed at very low, realistic iodine concentrations, constitute an important database to further develop or validate empirical and mechanistic models on radiolytic I2 oxidation. In the presence of painted surface areas or silver aerosol surface areas, radiolytic I2 oxidation is negligible compared to I2 adsorption on these surfaces for the conditions examined. However, radiolytic I2 oxidation remains very efficient if surface areas are small or if they are made of the relatively non-reactive stainless steel.

In case of a hypothetical severe accident in a nuclear LWR (light water reactor), the high radiation fields reached in the reactor containment building due to the release of fission products from the reactor core could induce air radiolysis. The air radiolysis products could, in turn, oxidise gaseous molecular iodine into aerosol–borne iodine–oxygen–nitrogen compounds. Thereby, this reaction involves a change of iodine speciation and a decrease of iodine volatility in the reactor containment atmosphere. Kinetic data were produced within the PARIS project on the air radiolysis products formation and destruction, and on their reaction with molecular iodine, with the objective of developing and validating existing kinetic models. The current paper includes the non-iodine tests of the PARIS project whose objective was to determine the rates of formation and destruction of air radiolysis products in the presence of both structural containment surfaces (decontamination coating (“paint”) and stainless steel), aerosol particles such as silver rich particles (issued from the control rods) in boundary conditions representative for LWR or PHEBUS facility containments. It is found that the air radiolysis products concentration increases with dose and tend to approach saturation levels at doses higher than about 1 kGy. This behaviour is more evident in oxygen/steam atmospheres, producing ozone, than in air/30% (v/v) steam atmospheres, the latter favouring the model-predicted on-going production of nitrogen dioxide even at very high doses. No significant effect of temperature, dose rate and hydrogen addition (4%, v/v) was observed. Furthermore, the inserted surfaces do not exhibit significant effects on the air radiolysis concentrations. However, these “non-noticeable influence” could be due to a masking of small effects by the appreciable scattering of the experimental air radiolysis product concentrations. The PARIS results are then analysed using two different kinetic models, an empirical and a mechanistic one. The kinetic constants within an empirical model including formation and destruction of air radiolysis products, derived from PARIS results, are in reasonable agreement with those determined previously for lower steam fractions. From the mechanistic model IODAIR-IRSN, it is concluded that ozone is the predominant air radiolysis product at low doses in air/steam atmospheres. At doses higher than 1 kGy, nitrogen dioxide becomes increasingly important, both due to an increase in its concentration and due to a simultaneous decrease in ozone concentration.

A project on thermochemical data and modelling of ex-vessel corium behaviour has been conducted as part of the European Commission Fourth Framework Programme on Nuclear Safety. The objective of the study was to provide critically assessed thermochemical data and develop calculational methods for use in assessment studies of the consequences of melt–basemat interactions. The work programme comprised the critical assessment and experimental determination of thermochemical data for species of key importance to melt interactions. A database has also been developed to model the multi-component system and calculations performed to predict the melt composition. Experimental determinations for silica and zirconia-based systems, for example measurements of the chemical activities of Ce and La in melts, have been performed to assist in the validation of the models. The sensitivity in the predictions of the effects of ex-vessel interactions to factors such as accident conditions (e.g., temperature, concrete type) and, uncertainties in the thermochemical and kinetic data has also been assessed.

International audience; Plant assessments have shown that iodine contributes significantly to the source term for a range of accident scenarios. Iodine has a complex chemistry that determines its chemical form and, consequently, its volatility in the containment. If volatile iodine species are formed by reactions in the containment, they will be subject to radiolytic reactions in the atmosphere, resulting in the conversion of the gaseous species into involatile iodine oxides, which may deposit on surfaces or re-dissolve in water pools. The concentration of airborne iodine in the containment will, therefore, be determined by the balance between the reactions contributing to the formation and destruction of volatile species, as well as by the physico-chemical properties of the iodine oxide aerosols which will influence their longevity in the atmosphere. This paper summarises the work that has been done in the framework of the EC SARNET (Severe Accident Research Network) to develop a greater understanding of the reactions of gaseous iodine species in irradiated air/steam atmospheres, and the nature and behaviour of the reaction products. This work has mainly been focussed on investigating the nature and behaviour of iodine oxide aerosols, but earlier work by members of the SARNET group on gaseous reaction rates is also discussed to place the more recent work into context. © 2014 Elsevier Ltd. All rights reserved.

Owing to large surface areas, the reaction of volatile molecular iodine (I 2 ) with steel surfaces in the containment may play an important role in predicting the source term to the environment. Both wall retention of iodine and conversion of volatile into non-volatile iodine compounds at steel surfaces have to be considered. Two types of laboratory experiment were carried out at Siemens (KWU) in order to investigate the reaction of I 2 at steel surfaces representative for German power plants. 1. (1) For steel coupons submerged in an I 2 solution at T = 50, 90 or 140 °C the reaction rate of the I 2 −I − conversion was determined. No iodine loading was observed on the steel in the aqueous phase tests. I 2 reacts with the steel components (Fe, Cr or Ni) to form metal iodides on the surface which are all immediately dissolved in water under dissociation into the metal and the iodide ions. From these experiments, the I 2 −I − conversion rate constants over the temperature range 50–140 °C as well as the activation energy were determined. The measured data are suitable to be included in severe accident iodine codes such as IMPAIR. 2. (2) Steel tubes were exposed to a steam-I 2 flow under dry air at T = 120 °C and steam-condensing conditions at T = 120 and 160 °C. In dry air, I 2 , was retained on the steel surface and a deposition rate constant was measured. Under steam-condensing conditions there is an effective conversion of volatile I 2 to non-volatile I − which is subsequently washed off from the steel surface. The I2−T − conversion rate constants suitable for modelling this process were determined. No temperature dependence was found in the range 120–160 °C.

In the SARNET2 WP8.3 THAI Benchmark the capability of current accident codes to simulate the iodine transport and behavior in sub-divided containments has been assessed. In THAI test Iod-11 and Iod-12, made available for the benchmark, the distribution of molecular iodine (I2) in the five compartments of the 60 m3 vessel under stratified and well mixed conditions was measured. The main processes addressed are the I2 transport with the atmospheric flows and the interaction of I2 with the steel surface. During test Iod-11 the surfaces in contact with the containment atmosphere were dry. In Iod-12, steam was released, which condensed on the walls.Nine post-test calculations were conducted for Iod-11 and eight for Iod-12 by seven organizations using four different codes: ASTEC-IODE (CIEMAT, GRS and TUS), COCOSYS-AIM (AREVA, FZ-Jülich and GRS), ECART (Pisa University) and MELCOR (Pisa University and VTT). Different nodalizations of the THAI vessel with 20–65 zones were applied.Generally, for both tests the analytical thermal-hydraulic results are in a fairly good agreement with the measurements. Only the calculated local relative humidity deviates significantly from the measured values in all calculations. The results in Iod-11 for the local I2 concentration in the gaseous phase are quite diverse. Three calculations show only minor deviations from the measurement, whereas the others are substantially different from the measured I2 concentrations. For Iod-12, no calculation delivers a satisfactory evolution of the I2 concentration in all five compartments of the vessel. There are three mediocre results standing out in the Iod-11 exercise which are from the same user–code combinations. The discrepancies derive from various reasons which are discussed in the paper.In the benchmark a significant user effect was detected, i.e. results achieved with the same code differed considerably.This work highlights the need of a detailed iodine adsorption/desorption model and precise thermal-hydraulic modeling for an accurate simulation of I2 transport in a sub-divided containment, as well as experienced users or straight forward user guidelines.

International audience; The Phebus Fission Product (FP) Program studies key phenomena of severe accidents in water-cooled nuclear reactors. In the framework of the Phebus program, five in-pile experiments have been performed that cover fuel rod degradation and behaviour of fission products released via the coolant circuit into the containment vessel. The focus of this paper is on iodine behaviour during the Phebus FPT2 test. FPT2 used a 33 GWd/t uranium dioxide fuel enriched to 4.5%, re-irradiated in situ for 7 days to a burn-up of 130 MWd/t. This test was performed to study the impact of steam-poor conditions and boric acid on the fission product chemistry. For the containment vessel, more specifically, the objective was to study iodine chemistry in an alkaline sump under evaporating conditions. The iodine results of the Phebus FPT2 test confirmed many of the essential features of iodine behaviour in the containment vessel provided by the first two Phebus tests, FPT0 and FPT1. These are the existence of an early gaseous iodine fraction, the persistence of low gaseous iodine concentrations and the importance of the sump in suppressing the iodine partitioning from sump to atmosphere. The main new insights provided by the Phebus FPT2 test were the iodine desorption from stainless steel walls deposits and the role of the evaporating sump in further iodine depletion in the containment atmosphere. The current paper presents an interpretation of the iodine behaviour in the FPT2 containment vessel based on dedicated small-scale analytical experiments and computer codes calculations. Other investigations dealing with primary circuit and sump chemistry are also reported. These could help to scale the results of Phebus-FP tests to reactor accidents. Modelling studies were generally successful when a gaseous iodine injection from the primary circuit was assumed. Indeed, though each of the iodine codes has specific iodine chemistry features that should be further developed and each approach to the modelling is distinct, the overall iodine behaviour in the FPT2 containment is generally well reproduced by the models that predicta low final gaseous iodine concentration in the containment atmosphere,a predominant iodine concentration in the sump and to a lesser extent a significant iodine deposition on containment surfaces. The main code-to-code differences, in the results obtained in gaseous iodine speciation, come from the various treatments of gaseous radiolytic reactions. Calculations that include the radiolytic conversion of volatile iodine into iodine oxide particulate show there is a persistence of both gaseous iodine and iodine oxide particles in the atmosphere. There are also some variations between the predicted organic iodine concentrations that depend mainly on the initial assumptions. A key aspect of the Phebus FPT2 test interpretation is that the long term iodine behaviour in the containment can be explained by exchanges between organic iodide released from painted surfaces and inorganic iodine released from deposited aerosol on the containment walls. Further studies of regulatory significance on sump chemistry showed that the gaseous iodine control that was evident in the Phebus tests through silver release and/or alkaline buffered sump solutions may not be assured. As most of the past iodine aqueous chemistry studies were done with rather pure systems and because of the uncompleted understanding of the gaseous iodine speciation, the results may not be extrapolated easily to conditions of reactor accidents thus necessitating deeper investigations. © 2011 Elsevier B.V. All rights reserved.

The large-scale iodine test Iod-9 of the German Thermal hydraulics, Hydrogen, Aerosols, Iodine (THAI) program was jointly interpreted by means of post-test analyses within the THAI Circle of the Severe Accident Research NETwork (SARNET)/Work Package 16. In this test, molecular iodine (I 2 ) was injected into the vessel dome of the 60 m 3 THAI vessel to observe the evolution of its distribution between water, gas, and surfaces. The main processes addressed in Iod-9 are (a) the mass transfer of I 2 between the gas and the two sumps, (b) the iodine transport in the main sump when it is stratified and then mixed, and (c) the I 2 adsorption onto, and desorption from, the vessel walls in the presence and absence of wall condensation. The codes applied by the THAI Circle partners were the Accident Source Term Evaluation Code (ASTEC)-IODE (IRSN, Saint Paul Lez Durance, France), Containment Code System (COCOSYS)-Advanced Iodine Model (AIM) (GRS, Garching, Germany), and Library of Iodine Reactions in Containment (LIRIC; AECL, Chalk River, ON, Canada). ASTEC-IODE and the Advanced Iodine Model (AIM) are semi-empirical iodine models integrated in the lumped-parameter codes ASTEC and COCOSYS, respectively. With both codes multicompartment iodine calculations can be performed. LIRIC is a mechanistic iodine model for single stand-alone calculations. The simulation results are compared with each other and with the experimental measurements. Special issues that were encountered during this work were studied in more details: I 2 diffusion in the sump water, I 2 reaction with the steel of the vessel wall in gaseous and aqueous phases, and I 2 mass transfer from the gas to the sump. Iodine transport and behavior in THAI test Iod-9 are fairly well simulated by ASTEC-IODE, COCOSYS-AIM, and LIRIC in post-test calculations. The measured iodine behavior is well understood and all measured data are found to be consistent. The very slow iodine transport within the stratified main sump was simulated with COCOSYS only, in a qualitative way. Consequently, this work highlighted the need to improve modeling of (a) the wet iodine adsorption and the washdown from the walls, (b) the I 2 mass transfer between gas and sump, and (c) the I 2 /steel reaction in the gaseous and aqueous phases. In any case, the analysis of the large-scale iodine test Iod-9 has been an important validation step for the codes applied.

The 5th FWP EURSAFE Project highlighted iodine chemistry in the containment as one of the issues requiring further research in order to reduce source term uncertainties. Consequently, a series of studies was launched in the 6th FWP SARNET Project aimed at improving the predictability of iodine behaviour during severe accidents via a better understanding of the complex chemical phenomena in the containment. In particular, SARNET has striven to foster common interpretation of integral and separate effect test data, production of new or improved models where necessary, and compilation of the existing knowledge of the subject. The work has been based on a substantial amount of experimental information made available from bench-scale projects (PARIS and EPICUR), via intermediate-scale tests (CAIMAN) to large-scale facilities (SISYPHE, THAI and PHEBUS-FP). In the experimental field, particular attention has been paid to two specific issues: the effects of radiation on both aqueous and gaseous iodine chemistry, and the mass transfer of iodine between aqueous and gaseous phases. Comparisons between calculations and results of the EPICUR and CAIMAN experiments suggest that the aqueous phase chemistry is reasonably well understood, although there are still some areas of uncertainty. Interpretation of integral experiments, like PHEBUS-FPT2, indicated that radiation-induced conversion of molecular iodine into particulate species (IxOy) could be responsible for the gaseous iodine depletion observed in the long-term. However, the results of much simpler, small-scale experiments have shown that further improvements in understanding and modelling are still needed. Mass transfer modelling has been extended to cover evaporating sump conditions based on SISYPHE data; however, application of this model to the larger scale THAI experiments seems not to be straightforward. In addition to these two major issues, some specific studies have been carried out concerning the potential effect of passive autocatalytic hydrogen recombiners on iodine volatility. The RECI analytical experiments have shown that metal iodides (namely CsI and CdI2) are not stable and yield gaseous iodine when heated, in a humid atmosphere, at temperatures representative of recombiner operation. Another important undertaking successfully carried out has been the compilation of an Iodine Data Book, which provides a critical review of the experimental data and modelling approaches that have been used in the development of iodine source term methodologies. This should assist in a proper use of such models, and inform their future development.

The International Phebus Fission Product programme, initiated in 1988 and performed by the French "Institut de Radioprotection et de Sûreté Nucléaire" (IRSN), investigates through a series of in-pile integral experiments, key phenomena involved in light water reactor (LWR) severe accidents. The tests cover fuel rod degradation and the behaviour of fission products released via the primary coolant circuit into the containment building. The results of the first two tests, called FPT0 and FPT1, carried out under low pressure, in a steam rich atmosphere and using fresh fuel for FPT0 and fuel burned in a reactor at 23 GWdt-1 for FPT1, were immensely challenging, especially with regard to the iodine radiochemistry. Some of the most important observed phenomena with regard to the chemistry of iodine were indeed neither predicted nor pre-calculated, which clearly shows the interest and the need for carrying out integral experiments to study the complex phenomena governing fission product behaviour in a PWR in accident conditions. The three most unexpected results in the iodine behaviour related to early detection during fuel degradation of a weak but significant fraction of volatile iodine in the containment, the key role played by silver rapidly binding iodine to form insoluble AgI in the containment sump and the importance of painted surfaces in the containment atmosphere for the formation of a large quantity of volatile organic iodides. To support the Phebus test interpretation small-scale analytical experiments and computer code analyses were carried out. The former, helping towards a better understanding of overall iodine behaviour, were used to develop or improve models while the latter mainly aimed at identifying relevant key phenomena and at modelling weaknesses. Specific efforts were devoted to exploring the potential origins of the early-detected volatile iodine in the containment building. If a clear explanation has not yet been found, the non-equilibrium chemical processes favoured in the primary coolant circuit and the early radiolytic oxidation of iodides in the condensed water films are at present the most likely explanations. Models that were modified or developed and embodied in the computer codes for organic iodide formation/destruction in the gas phase and Ag-I reactions in the sump lead, in agreement with the Phebus findings respectively to greatly enhanced organic iodide formation kinetics and long term concentration in the containment atmosphere on one hand and, in the conditions of Phebus experiments, to significantly limited molecular iodine volatilisation from the sump in so far as silver was in excess compared to iodine, on the other hand. Organic iodides then quickly gain in importance and become the predominant volatile iodine species at long term. © 2006 Elsevier B.V. All rights reserved.

A shared-cost action on Organic Iodine Chemistry has been completed as part of the CEC 4th Framework programme on Nuclear Fission Safety. Organisations from four EC countries are involved in an integrated programme of experiments and analysis to help clarify the phenomenology, and to increase confidence in the modelling of iodine behaviour in containment. The project is focused on identifying the main routes for organic iodine formation, and providing new experimental kinetic data which will be used to improve existing models and to stimulate code development.