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Comportement des produits de fission dans le réservoir pendant l'essai FPT2


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Titre du congrès :CSARP 2007 Cooperative Severe Accident Research Program Ville du congrès :Albuquerque Date du congrès :18/09/2007

Résumé

The international Phébus FP programme was initiated in 1988 by the French "Institut de Radioprotection et de Sûreté Nucléaire" (IRSN), in cooperation with the European Commission (EC), to investigate key phenomena involved in light water reactor (LWR) severe accidents. This experimental programme consists in 5 in-pile integral experiments performed by IRSN in the Phébus facility operated by the CEA on the site of Cadarache. Its aim is to study the fuel degradation, the release of fission products and their transport through the reactor coolant system to the containment building. For safety concern, attention is focused on the iodine radiochemistry as it can directly impact on the consequences of any radioactivity release to the environment. Ultimately, the Phébus FP programme results will be used to validate the different computer simulation codes used in water reactor safety analysis.
The purpose of the presentation is to provide an overview of the fission product behaviour in the containment during the FPT2 experiment, performed on 12th October 2000, emphasizing iodine data collected in the containment vessel. Unlike the previous FPT0 and FPT1 tests, FPT2 was performed in steam poor conditions (0,5 g/s steam flow rate instead of 2 g/s) with an alkaline sump (pH 9) under evaporating conditions instead of a non evaporating acidic
sump (pH 5), so varying two important parameters impacting physico-chemical phenomena.
The FPT2 test scenario was in agreement with the pre-defined test protocol including a 8-day irradiation phase, a fuel bundle degradation phase which lasted about 5 hours and a 4-day long term phase that consisted of an aerosol stage dedicated to the analysis of aerosol
deposition mechanism inside the containment vessel and a chemistry phase devoted to the analysis of the iodine chemistry. Regarding containment inventories, the agreement between the cold leg mass flow rates and the containment on-line measurements is very satisfactory.
The overall aerosol mass conveyed to the containment vessel is estimated to 55g (taking into account the contribution of oxygen) which is much lower than for FPT1 (160g). The major elements contributing to the aerosol mass are fission products (Cs, Mo), control rod materials (Cd, Ag, In) and structure material (Sn). The bundle inventory fractions injected in the containment are of the same order than in FPT1 for almost all the most volatile fission products (I, Cs, Mo and Rb) except Te for which high deposits were observed in the upper plenum and the hot leg. Nevertheless, control rod materials (Cd and Ag) and fuel materials (U, Pu) release fractions are notably lower than in FPT1. Once the aerosols were injected in the containment vessel, they were submitted to three deposition processes, which were assumed to be of the first order regarding the aerosol concentration:
· Deposition by gravitational settling accounted for the major part (74 %) of the overall deposits in the containment, with a deposition kinetics slower than in FPT1;
· Deposition by diffusiophoresis represented about 12% of the overall deposits, with a deposition kinetics which is consistent with FPT1 regarding the various steam condensation rates;
· Deposition on the vessel vertical walls and top lid were estimated to 11% of the containment inventory, which is significantly higher than for FPT1.
Throughout the FPT2 experiment, size and dispersion of the aerosol particles tended to grow as a function of time, probably because of agglomeration processes, but remained smaller than in FPT1.
Despite the very low efficiency of the washing phase, mass balance closures performed respectively at the end of the aerosol phase, at the end of the chemistry phase and after decontamination operations are quite satisfactory as they stand between 87% and 111% for all elements.
The whole set of measurements dedicated to the iodine characterization appears selfconsistent and provides new data for the iodine behaviour in the containment vessel. During the transient, 57% of the iodine bundle initial inventory (i.i.) was injected in the containment.
Iodine, which was mainly in aerosol form, was submitted to the same deposition processes than the other elements and did not exhibit any peculiar behaviour regarding the various deposition kinetics. Nevertheless, interactions with the paint of the condenser walls were noticed as well as some desorption from the vertical walls of the containment vessel. It was also observed that iodine water solubility changed throughout the experiment, passing from a soluble state during the transient and the aerosol phase to a partly non soluble state during the chemistry phase, the soluble fraction highly depending on the sump temperature. The gaseous iodine fraction reached its highest value (0.3% i.i.) during the second oxidation phase, then it
decreased down to 0.01% i.i. at the end of the experiment. Unlike the previous tests, molecular iodine accounted for the major part of the gaseous iodine during the entire experiment.
After further interpretation and transposition to the reactor case, these data should be relevant for a more accurate estimation of the iodine impact on potential source term to the environment in case of a hypothetical severe accident.