Insights into iodine behaviour and speciation in the Phébus primary circuit

Abstract

The Phébus FP integral test series studies a large spectrum of the phenomenology of severe accidents in water-cooled nuclear reactors. These tests represent a unique source of representative integral source term data, covering fuel rod degradation and behaviour of fission-products released via the coolant system into the containment. The present analysis concerns the behaviour of iodine in the test circuit representing the Reactor Coolant System (RCS) which reaches gas temperatures of nearly 1600°C at the circuit entrance and descending to 150°C before entry into the containment. The stake in the data analysis is a better understanding of iodine phenomenology in RCS. This is indeed all the more serious as iodine is one of the most radiological important fission products released from the fuel and may exist under highly volatile forms even within cold leg thermal–hydraulics conditions.Complex and coupled phenomena arise in the primary circuit during the tests as the temperature decreases (drops) from the inlet of the circuit to the outlet. These are respectively for the iodine vapours and aerosols: chemical transformation, condensation on walls/aerosols, homogeneous nucleation into aerosols and agglomeration, deposition by thermophoresis. Depending on the location in the primary circuit, a combination of these phenomena occurred simultaneously.The phenomenological behaviour of iodine in RCS2CFD: Computational Fluid Dynamics; c.i.: Containment inventory; i.i.: Initial inventory (in the bundle); FP: Fission Product; PTA: Post-Test Analysis; PWR: Pressurized Water Reactor; RCS: Reactor Coolant System; TGT: Thermal Gradient Tube; TL: Transition lines.2 will be appraised through the analyses of the iodine transport, retention, vapour speciation and gaseous occurrence in the Phébus FP primary circuit during the four Phébus-bundle tests. In these tests, the impact of different oxido-reducing and thermal–hydraulics conditions prevailing in the primary circuit as well as the impact of boron and control rod materials on iodine behaviour has been investigated. In this test series, iodine behaviour in the FPT3 primary circuit clearly departed from the others because a much higher iodine retention was observed upstream the steam generator (due to partial boron-rich blockage after 14,500s) but above all a much higher gaseous iodine fraction in hot and cold legs was formed as compared to other tests.In the three other Phébus FP tests, iodine was generally poorly retained in the primary circuit (70% of released iodine reaching the containment vessel). Two main zones of significant deposition were identified coinciding with sections in which temperatures dropped rapidly. These were the fuel bundle exit (upper plenum and vertical line) where the gas cooled from very high fuel temperatures down to 700°C and the steam generator riser (upstream part and hot leg entrance) where temperatures cooled from 700°C to 150°C.As expected, iodine was mainly transported under vapour forms in the circuit hot leg. However, Phébus FP tests provided new insights into iodine transport, as several volatile iodine vapour species not associated to caesium were evidenced.In all tests, a significant amount of iodine under a gaseous form was found in the containment early during the bundle transient phase, implying that this gas was originated from the circuit. Except for FPT3, measurements of gaseous iodine in the circuit, from discrete samplings, were however more contradictory as only negligible amounts of gaseous iodine were generally measured in the cold leg. Due to the limitations of such measurements (trapping efficiencies, limited period of samplings) the gaseous iodine occurrence in the primary circuit during FPT0/1/2 could neither be stated nor refuted.Finally, Phébus FP test analyses that were performed using equilibrium gas-phase chemistry models evidenced that it becomes necessary to reconsider iodine species behaviour along their transport in the RCS not only as a function of oxido-reducing conditions, material release kinetics, but also in the light of potential kinetics limitations in vapour chemical transformations. Indeed, even if a strong connection between B, Cs, Mo, Cd and I chemistry was evidenced; in general, calculations were only partly satisfactory in reproducing the main aspects of the observed iodine/caesium behaviour and speciation. A better prediction of the volatile iodine speciation, the level of association of I to Cs and the gaseous iodine occurrence are the main objective of the experimental international and cooperative programme CHIP launched by IRSN in support of the Phébus FP programme interpretation. This programme was especially dedicated to investigate the kinetic limitations of iodine chemical reactions in a model primary circuit.