Abstract
(English)
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Reliable models for the behaviour of iodine in a reactor containment following a severe nuclear reactor accident are essential for the prediction of the potential release to the environment, and thus to the development and qualification of appropriate mitigation strategies and devices. Whilst most aspects of iodine chemistry are now adequately understood, particularly for PWR conditions, some outstanding issues remain. Firstly, some of the processes leading to the destruction of volatile forms of iodine are not well quantified. An improved knowledge of these destruction rates will allow their importance to be assessed, in terms of natural mitigation processes and accident management interventions. Secondly, the effects on the iodine behaviour of certain materials and conditions, which are specific to BWR systems, are unknown. An understanding of these specific effects will allow data and models developed mainly for PWR systems to be applied with confidence to BWR source term predictions.
The ICHEMM project work programme comprises the following main elements.
i. Provision of new kinetic data on volatile iodine destruction or transmutation reactions, which are not routinely included in severe accident iodine chemistry modelling codes. This will involve experimental measurements of the rate of molecular iodine destruction by ozone in the gas phase, and of the rate of methyl iodide destruction under irradiation in the gaseous and aqueous phases
ii. Investigation of other possible mitigation mechanisms or accident management measures to favour the conversion of volatile iodine species to non?volatile forms under severe accident conditions. This will involve experimental studies of the effects of candidate additive materials on iodine volatility from irradiated iodine solutions.
iii. Provision of experimental data on iodine behaviour under conditions specific to BWR containments under accident conditions, including the effect of reactive materials, and
iv. Quantification of the effects of the identified mitigation mechanisms on the predicted iodine source term for representative accident sequences. This will include the development of kinetic models based on the results of the experimental programmes, incorporation into severe accident modelling codes and evaluation of the impact on the calculated source term for some prototypical accident sequences.
PSI contribution to the project is conduction of the second main element. Since a new, effective and practical composition of additives will be established based on the outcome of in-situ b-irradiation of organic iodides, it is a novel work with a significant contribution to the reactor safety since the ultimate goal is to develop and verify the additive composition for a complete destruction of organic iodide produced in reactor sump and effective fixation of iodine liberated from the destruction.
Status of PSI contribution per 1.2.2002
Following activities, as planned in the EU contract, have already been conducted:
· . Modification of the in-situ b-irradiation facility
· Development of procedures for experiment conduct
· Preparation of a safety report to obtain clearance to conduct experiments
· Development of dedicated measurement techniques to separate and detect iodine species
· Development and testing of a technique to produce liquid labelled organic iodide
· Development of measurement technique to measure inactive organic iodide using a mass spectrometer
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