Investigation of radionuclide behavior in integral experiments

In spite of the incompleteness of the measured data, caused by the above-mentioned failures of part of the instrumentation, it was shown that during the burst and refill phases (both phases could not be separately evaluated due to difficulties in instrumentation) the release fractions from the failed fuel rods amounted to 1.3% of the Xe, 0.8% of the and 0.2% of the Cs inventories of the failed rods. For the fission product noble gases, this fraction represented more than 90% of the total amount released over the course of the entire experiment, for iodine about 60% and for cesium about 10% the remainder of these radionuclides was transported to the coolant during the subsequent leaching phase, which in this experiment was conducted for about 12 hours. 

The Cs I atom ratios measured at the different sampling positions were in the range 2.0 to 3.5, roughly corresponding to the ratio of 3.0 which was calculated for the gap inventory of the failed fuel rods. These measured values strongly support the assumption of iodine transport in the primary circuit as Csl species. 

From the iodine fractions measured in the liquid and in the gaseous phases of the blowdown suppression tank, an iodine partition coefficient of 77 at a pH of the water phase of 4.8 was calculated. This figure agrees reasonably well with the experimental data obtained, for example, by Richter et al. (1985) at a pH of 5.3 and a water temperature of 100 to 200 °C however, because of the low pH of the water phase, its applicability in the modelling of iodine behavior in the containment seems questionable. Moreover, the particular geometry of the blowdown suppression tank, in which the volume ratio of the gas to the liquid phase was considerably smaller than in a real reactor containment, presumably influenced the iodine partition coefficient. 

During the recirculation phase, which lasted for 12 hours, 0.3% of the iodine inventory and 1.3% of the cesium inventory of the failed fuel rods were dissolved in the coolant. These figures are also within the expected range. 

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