The fission product transport in the primary circuit and in the containment was investigated in nuclear reactor severe accident conditions, where the reactor core is damaged and core materials are released from the reactor pressure vessel. The re-vaporization of fission products and core materials from the circuit surfaces was investigated with special emphasis on the effects of re-volatilization on iodine speciation and transport. Secondly, aerosol transport in the containment was studied in specific turbulent natural convective flow conditions. The reaction on primary circuit surfaces was studied in an experimental facility where the precursor was heated and the concentrations of aerosol and gaseous reaction products were analysed. In the experiments temperature, precursor and reaction crucible materials and the composition of the gas atmosphere were altered. The results indicated that with pure CsI in 650 °C temperature, significant fraction of released iodine was transported in gaseous form. The increase of H2 in gas atmosphere caused the release of gaseous I and CsI particles to decrease and increase, respectively. When different additives like Mo, B and Ag were mixed with CsI, the fraction of gaseous iodine in the release was collectively increased compared to the pure CsI case. The reduction of temperature from 650 °C reduced the aerosol release with all precursors, but especially with MoO3 + CsI precursor, significant release of iodine in gaseous form was detected. Overall, the results indicated the possibility of gaseous iodine release at the re-vaporization from the circuit in conditions present in the experiments. To investigate the aerosol particle transport in the containment, a cubical differentially heated cavity was used to experimentally investigate the deposition of spherical monodisperse SiO2 particles with diameters 1 and 2.5 micron from the enclosure atmosphere. The turbulent natural convective flow was induced by the temperature difference between two vertical isothermal walls. The measurement results of the particle depletion were compared to particle tracking simulation data, obtained using a validated CFD simulation of the experimental cavity. The results indicated that the smaller particles deposited faster from the cavity than what the theoretical "stirred settling" model predicted. Simulations using a cavity with ideal adiabatic boundary conditions suggested that the faster deposition was due to specific nature of the turbulent flow conditions in the cavity, not depicted by the stirred settling model. The results presented in this thesis can be utilized in the development of the severe accident computational tools. The composition and concentration of fission products transported and deposited in the containment are important for the accurate estimation of the source term in accident conditions.
|Translated title of the contribution||Fissiotuotteiden kulkeutuminen primääripiirissä ja suojarakennuksessa vakavissa ydinonnettomuuksissa|
|Publication status||Published - 2015|
|MoE publication type||G5 Doctoral dissertation (article)|
- severe accidents
- fission product
- primary circuit