Reliable models for the behaviour of iodine in a reactor containment following a severe nuclear reactor accident are essential to 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. These aspects have been addressed in the ICHEMM programme.
ICHEMM is a 3-year shared-cost action programme which was completed in February 2003. The main achievements of the work are as follows:
- Provision of new experimental data on the rate of radiolytic decomposition of gaseous CH3I under conditions relevant to reactor containments;
- Development of mechanistic and empirical models of gaseous iodine (I2 and CH3I) behaviour under irradiation;
- Provision of new experimental data on the destruction of ozone at containment surfaces, and assessment of the impact of this reaction on the gaseous iodine behaviour;
- Identification of chemical additives with potential to reduce the volatility of organoiodide in the containment atmosphere under severe accident conditions;
- Provision of new data and models on the interaction of gaseous and aqueous iodine with reactive metals found in BWR containments;
- Integral tests to extend the database on iodine volatility and organic iodide formation to higher temperature and dose rate, as well as evaporating/condensing steam conditions.
- A State-of-the-Art report on iodine chemistry and mitigation mechanisms;
- Development of new models based on the experimental data generated the other work packages, and incorporation into containment chemistry modelling codes;
- Assessment of the impact of the new data, models and procedures on the calculated iodine source term for representative PWR and VVER accident sequences.
The new data and models produced in this programme will contribute to reducing uncertainties associated with the consequences of severe reactor accidents. Improvements in the reliability of predictions of the volatile iodine speciation and concentration in reactor containment could contribute to the optimisation of accident management devices and strategies.
(1) AEA Technology plc, Winfrith, Dorchester, Dorset GB-DT2 8DH, GB
(2) Framatome ANP, Freyeslebenstrasse 1, PO Box 3220, DE-91050 Erlangen, Germany
(3) Paul Scherrer Institut, 5232 PSI-Villigen, Switzerland
(4) Chalmers University of Technology, 41296 Gotheburg, Sweden
(5) IRSN, CE Cadarache, FR-13108 Saint Paul lez Durance, France
(6) JRC/IE, Nuclear Safety Unit, Postbus 2, 1755 ZG Petten, The Netherlands
(7) NRIR Rez, 25068 Rez, Czech Republic