Jouni Hokkinen

Studies on steam condensation and particle diffusiophoresis in a heat exhanger tube

Abstract In this paper, studies on steam condensation and aerosol behaviour in a heat exchanger are presented. The heat exchanger is a model of one single tube of a Passive Containment Condenser (PCC) that is used in the European Simplified Boiling Water Reactor (ESBWR). A hot carrier gas containing nitrogen and steam with Ag and CsOH particles flows through the heat exchanger. The walls of the heat exchanger are cooled with a water jacket, thus causing steam condensation and diffusiophoretic particle deposition. The amount of condensed steam is measured, as well as the temperatures, particle mass concentrations and size distributions before and after the heat exchanger. The experiments are done with different proportions of steam and nitrogen in the carrier gas. Heat and mass transfer in the system are modelled with well known engineering correlations, producing results that agree nicely with the experimental results. The diffusiophoretic particle deposition velocity is shown to be proportional to the steam condensation rate, as expected.

A Scoping Study of the Chemical Behavior of Cesium Iodide in the Presence of Boron in the Condensed Phase (650°C and 400°C) Under Primary Circuit Conditions

Abstract This work is a contribution toward understanding the chemical reactions on the primary circuit surfaces involving gaseous iodine release during a severe nuclear reactor accident. Cesium iodide was used as a nonradioactive precursor material in order to highlight the effects of carrier gas composition (Ar/H2O, Ar/H2O/H2, and Ar/Air), temperature, the initial cesium/iodine (Cs/I) molar ratio by adding cesium hydroxide, and the presence of boron on the molar composition of the deposited iodine compound and on the release of gaseous iodine from the deposit. The results from the experiments involving only cesium iodide as a precursor revealed a slight decomposition of cesium iodide and a release of gaseous iodine. Furthermore, the measured gaseous iodine mass decreased with the addition of hydrogen to the carrier gas at 650°C. At 400°C, the amount of released material (aerosol and gas) was decreased. However, whereas at 650°C the sampled iodine existed mainly as aerosols, the mass concentration recorded from the experiment at 400°C indicated a predominance of gaseous iodine. When the initial Cs/I molar ratio was significantly greater than unity (1.5 < Cs/I < 4.5), the mass of produced gaseous iodine was barely detectable, suggesting a reaction between cesium hydroxide and the gaseous iodine released from cesium iodide decomposition. In the presence of boron, the transport of gaseous iodine was increased as a result of the formation of glassy cesium borate in the evaporation crucible. The presence of steam and its quantity were shown to have an enhancing influence on the cesium borate formation and on the release of gaseous iodine.

A Complementary Study to the Chemical Behaviour of Caesium Iodide in Presence of Boron in Condensed Phase (650 °C and 400 °C) under Primary Circuit Conditions: Differential Thermal Analysis and Thermogravimetric Studies

Abstract This work is a contribution to the work performed in a paper on the understanding of the chemical reactions between cesium iodide and boron oxide in condensed phase, under conditions close to the ones prevailing in the primary circuit of a nuclear power plant in case of a severe accident. The thermal degradation of samples made from cesium iodide or cesium iodide and boron oxide mixtures has been investigated using the techniques of thermogravimetric analysis and differential thermal analysis at temperatures from 20°C to 1000°C. The boron-to-cesium molar ratio in the investigated mixture was fixed at about the value of 5 (B/Cs = 5). Apart from the dehydration of boric acid, evidence is presented for the formation of a vitreous compound at 360°C to 420°C, depending on the atmosphere composition. Carrier gas composition also seemed to influence the behavior of the precursor mixture. While under air and argon, the recorded thermograms are similar. In the presence of argon/water vapor, a specific behavior and difference on reactivity is noticed, due to the adsorption of water from the carrier gas at the beginning of the process. It was also pointed out that the addition of water or oxygen delayed the glass formation process.