Radionuclide retention in BWR pressure suppression pools

The retention of fission products by scrubbing the steam flow in a BWR pressure suppression pool is based on the exchange of matter between gas and liquid phases. Similar scrubbing processes are also at work in other accident situations, such as in a PWR steam generator tube rupture event (see Section 6.2.3.), as well as in the Venturi scrubbing process during controlled depressurization of the containment after a core melt accident (see Section 7.3.4.4.). 

From the Marviken large-scale aerosol tests (see Section 7.3.2.2.) it was reported that the aerosols remaining after passage of the simulated primary system were retained to at least 99% in the water-filled scrubber located at the back end of the test facility at a submergence depth of about 1 meter (Rahn et al., 1988). 

Test Temperature pool, °C Submergence sparger, m Substance Decontamination factor 

The decontamination factors mentioned above may be affected over time by mechanisms which lead to a revolatilization of fission products from the pool water. When the bubbles break up at the pool surface, they produce new aerosol droplets by which non-volatile fission products can be carried to the containment atmosphere. Calculations using the Sparc code (Owczarski and Burk, 1991), however, showed that radionuclide entrainment caused by this mechanism is very small and, thus, can be ignored in source term analyses. Iodine, on the other hand, can be revolatilized if the pH of the aqueous phase is decreased as a consequence of water radiolysis. In the presence of alkalizing substances such as CsOH, such a decrease in pH leading to a lower I2 partition coefficient is not to be expected. The existing codes (such as Sparc, Supra, Busca) do not take chemical reactions into consideration and, thus, do not provide any information on chemical iodine revolatilization from the pool. 

According to calculations with the Supra code (Wassel et al., 1984), condensation of steam in the eompartment above the pool can additionally result in high system decontamination factors. Calculations showed that for hot pools the system decontamination factor would be higher by about a factor of 10 than that of the pool itself. 

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