PWR iodine speciation and behaviour under normal primary coolant conditions: An analysis of thermodynamic calculations, sensibility evaluations and NPP feedback

摘要

Iodine is one of the most important fission products due to its high fission yield, significant radiobio-logical hazard and potential volatility. Its environmental and biological risks have been extensively studied in case of a severe reactor accident. Nevertheless, little information is available about iodine behaviour under normal Pressurize Water Reactor (PWR) operating conditions. The work reported explores the behaviour of different iodine species (I~-, I_3,I_2, HOI and IO~- ) during full power periods, transient periods (power reductions and depressurizations) and shutdowns. Thermodynamic calculations were conducted, and their results are compared with previous predictions and with the experimental data provided by nuclear power plants (NPP). Based on thermodynamic calculations and NPP feedback, it was concluded that iodine speciation depends primarily on the redox potential and water radiolysis. 1.1 The experimental values confirm that the iodine ionic form 1" is the major species during normal operation (I_2 < 2%) and shutdowns (I_2 < 9%). 1.2 During shutdowns: 1.2.1 High [I2] (20—40%) can be observed in the presence of fuel failures following an iodine spike during power or pressure variations. The fuel oxidation by radiolysis products can lead to I_2 formation inside the gap and its subsequent release through cladding defects. 1.2.2 Once in the primary coolant, I2 is transformed into I~- or IO_3~- /IO_4~-, depending on the water oxidation conditions. 1.3 The lithium concentration and the primary coolant temperature seem to have a secondary influence on iodine speciation, while the existence of a redox potential threshold appears to be the main factor controlling the formation of volatile and non-volatile iodine forms. This paper summarizes the major results of the iodine thermodynamic studies and PWR feedback, permitting some possible recommendations for inclusion in the NPP guidelines in order to master iodine's behaviour. Future work is proposed. Redox potential measurements at high temperatures, coupled with thermodynamic estimations and radiolysis analysis, should be considered as useful tools to specify the optimal conditions for limiting iodine volatisation and I2 absorption.