Radiochemical studies of iodine behaviour under conditions relevant to nuclear reactor accidents

Changing the paradigm in modelling the Bray-Liebhafsky oscillatory chemical reaction

The Bray-Liebhafsky (BL) reaction is one of the simplest chemical oscillators consisting initially of only three components. Despite this, its mechanism is unknown for more than 100 years due to the absence of selective, sensitive, and fast experimental techniques for following all of the involved intermediates. The modeling of the BL mechanism assumes presumably mass action kinetics "adjustable" to oscillatory solutions by the application of mathematical stability analysis and treating the system as homogeneous. Such a basically mathematical approach need not suggest physically realistic kinetic parameters and is not unique since a number of models can be proposed. Based on recent experimental and computational results, a new model of the BL oscillatory reaction mechanism is constructed by including heterogeneous processes occurring in the system. The same set of equations is able to demonstrate not only the oscillatory evolution but also mixing effects on the oscillatory dynamics, and non-oscillatory stepwise-iodine oxidation and can rationalize other effects described in literature. Thus, the paradigm of treating the BL oscillatory system as a homogeneous one, described by formal kinetics only, is extended for a better understanding of the chemistry of this apparently simple system. The introduced ideas of energy redistribution may contribute to establishing an improved conceptual base for considering other complex oscillators in various fields of science.

Coupling between nucleation and chemical reactions and its importance in understanding the oxidation of iodine by hydrogen peroxide

We have investigated the reaction of iodine with hydrogen peroxide coupled to gas nucleation. A step-like increase in the nucleation rate with increasing amounts of dissolved oxygen can act as a trigger for the formation of highly reactive components and complete oxidation of iodine to iodate despite the large thermodynamic barrier for the whole process. Energetic coupling of nucleation with chemical reactions is based on local redistribution of energy by collapsing unstable nuclei. The developed model correctly describes the evolution of the measured reaction parameters. It offers a conceptual improvement of formal-kinetic models by drawing attention to important contributions of physical effects to the reaction mechanism.

Retention efficacy and release of radioiodine in fume hoods

Procedures to determine the release of hazardous gaseous substances including radioactive iodine are covered by different norms such as the European standard EN 14175 and the German national standard DIN 25466. The detection of sulphur hexafluoride (SF₆) is required to comply with the prescribed methodology. The detection limit of this test is 4.5·10^-7 mol/m³ in exhaust air. This detection limit would represent a very high activity in the region of 0.27 TBq/m³ leading to an unacceptable risk. We therefore developed a test using a filter system, consisting of a combination of filters capable of separating various chemical forms of airborne radioiodine. Air samples were collected directly in front of the fume hood and in the laboratory beside two different fume hoods of a similar construction with a final activated carbon filter for retention of radioiodine. Particular attention was therefore paid to air samples taken after passage over the filters. Significant differences in the degree of retention of iodine were found between the two fume hoods investigated. In one test a malfunction of the fume hood was demonstrated. In this case 0.148 × 10^-3% of the total released activity per m³ air was found 1 cm in front of the hood sash. A remarkably high fraction of the activity released in the fume hood (1.3 × 10^-3%/m³ air) was measured after the activated carbon filter. In the ambient air, values of up to 8.6 × 10^-6% pro m³ laboratory air sampled were measured, despite a 6-8-fold air exchange. The selected procedure is a factor of 10¹¹ (Schomäcker et al., 2001) more sensitive than the standard recommended methods (EN 14175). The standard test prescribed by the DIN/EN failed to reveal any inadequacy in the protective function of the radionuclide hood with respect to radioiodine retention.