Investigation on Revaporization from CsI Deposited Particles in the Primary Circuit in Nuclear Severe Accident Conditions

Since the Fukushima Dai-Ichi (1F) Nuclear Power Station accident, a renewed attention has been posed on the evaluation of delayed source terms, especially from Reactor Coolant System (RCS) surface deposits and on the boron carbide control rods behaviour. After fission products have been released from the overheated and molten fuel, they are transported through the reactor coolant system and fission products will reach areas at lower temperature. Therefore, vapour condensation and particle nucleation processes take place in the gas flow. If vapour condensation takes place close to the surfaces of the primary circuit, a layer of condensate can be formed on it. Particles in the gas flow may also deposit on the circuit surfaces together with control rod and structural materials. The objectives of the work was to investigate the behavior of deposited caesium iodide particles in the RCS and to assess the effect of gaseous boric acid with the deposited particles. Caesium iodide particles were generated by nebulization of a concentrated caesium iodide aqueous solution and then passed through a Thermal Gradient Tube (TGT). The gaseous boric acid was generated by vaporization from a crucible placed inside the furnace. Aerosols and gaseous species were sampled at 150 degrees C on filters and liquid scrubbers and analysed with HRICP-MS. Aerosol number size distributions were measured with TSI Scanning Mobility Particle Sizer (SMPS), with series 3080 platform, series 3081 Differential Mobility Analyzer (DMA) and series 3775 Condensation Particle Counter (CPC). The aerosol mass concentration was monitored by Tapered Element Oscillating Microbalance Series 1400a (TEOM). The study showed that the deposited caesium iodide particles were subject to revaporization process in Ar/H-2 atmosphere; gaseous iodine was released from the deposits. When gaseous boric acid was present in the carrier gas (Ar/H-2), the percentage of gaseous iodine released was higher. The interpretation of the SMPS and TEOM results also suggested that the presence of gaseous boric acid would then have an influence on the transport of the deposited caesium iodide particles as it showed different behaviour than during revaporisation/resuspension under Ar/H-2.