Controlled depressurization of the containment

The late overpressure failure of the containment steel shell can be prevented by a controlled depressurization, in the course of which the gas-steam mixture escaping from the containment is directed to an additional system in which the radionuclides are retained. The purpose here is to further reduce the possibility of release of radionuclides to the environment as a consequence of a severe reactor accident. Practical application of this idea can be based on various principles an overview of the design of different systems was given by Schlueter and Schmitz (1990). 

In the Swedish BWR plant Barseback, a so-called vented containment was employed for the first time, consisting of a 10,000 m gravel bed filter to which the gas-steam mixture from the containment would be directed in the event of a severe accident. The steam will condense on the initially cold pebble surfaces, resulting in an effective retention of aerosols and iodine. However, for various reasons this solution proved not to be optimal. 

Volatile iodine species are not efficiently retained by the steel fiber filter. For this reason, an additional filter was proposed containing a silver-impregnated 

The retention systems which are installed in the Canadian Candu plants show a similar design (McArthur and Salaff, 1988). They consist of a combination of an aerosol filter with different pore sizes and an impregnated charcoal filter with an iodine retention efficiency of better than 99.9%. 

When compared to the solid state filters, liquid-phase retention systems prove to have several advantages. The heat introduced by the venting flow as well as by the decay of the absorbed radionuclides is passively removed by boiling of the water phase, the load capacity for aerosols is virtually unlimited, and the retention of volatile iodine compounds can be guaranteed by appropriate chemical conditioning of the water phase. An important prerequisite for high retention efficiency, besides the thermal and radiation stability of the chemicals applied, is a very intimate contact between the gas-steam flow and the liquid phase of the retention system, in order to obtain a fast and effective exchange of matter. 

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Tightness of the containment over a longer period of time is an important factor in the retention of radionuclides in support of natural deposition processes and engineered safety features. To this end, facilities for controlled depressurization of the containment have been installed in many plants (see Section 7.3.4.4.). 

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.). 

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