Late overpressure failure of the containment steel shell

3 Late overpressure failure of the containment steel shell 

The heat transport from the core melt into the sump water, as well as the production of permanent gases in the core melt - concrete interaction, will result in a pressure increase inside the closed containment. Provided that no measures for controlled depressurization are undertaken (see next section), then, according to the results of thermodynamic calculations, the steam-gas pressure within the containment would reach the postulated failure value of the containment steel shell of about 0.9 MPa after 5 to 10 days, depending on the accident sequence. Such an overpressure failure will not be a catastrophic burst of the shell, but rather the enlargement of the operational leaks to a size permitting the escape of gas and steam at a rate high enough to keep the pressure inside the containment at a 

Due to its different chemical forms present in the containment atmosphere, fission product iodine shows a complex behavior in these areas as well. Csl carried by water droplets or by aerosol particles will behave in the same manner as do the other aerosols. The volatile iodine species I2 and CHjI will be distributed between the newly formed sump water and the atmosphere of the annuli, as is schematically shown in Fig. 7.45. Experiments have demonstrated that the extent of I2 plate-out depends on the degree of steam condensation, amounting to about 90% at a condensation temperature of about 40 °C (see Section 7.3.3.4.2.). The newly formed sumps in the annuli and in the auxiliary building show a pH of about 7 (since they consist solely of condensed steam), and temperatures between 50 and 80 °C this means that I2 plated out into these liquid phases is not only instantaneously hydrolyzed to I and HOI, but will also disproportionate rather quickly under formation of lOs . As can be seen from Fig. 7.24., under such conditions the disproportionation equilibrium will be reached within a comparatively short time. If one assumes very conservatively an initial I2 concentration in the sump water of about 1 mg/1 and a temperature of 100 C, then it can be derived from Fig. 7.24. that the 

Unlike the containment, the annuli do not represent a closed system. This means that equilibrium considerations cannot be applied here, since the buildup of a partition equilibrium is continuously disturbed by the flowing atmosphere which consists of steam and permanent gases and contains traces of I2. Consequently, the processes which take place inside the annuli are very complex, and a simple calculation of the extent and the rate of iodine revolatilization from the liquid phases is not possible. Assuming an instantaneous establishment of the equilibrium state in each volume element of the flowing atmosphere would result in an overconservative approach, leading to the result that iodine would be only temporarily retained in the annuli. Therefore, the thermodynamic approach has to be complemented by kinetic considerations which include the rate of formation of I2 from 1 and lOa by the Dushman reaction, as well as the kinetics of the diffusion-controlled transport of I2 from the liquid to the gas phase. In addition, revolatilized I2 can be temporarily or even permanently trapped by the paint on the walls of the annuli and the auxiliary building, which would additionally diminish or at least delay iodine release from the plant. 

In an open system with a gas phase moving parallel to the surface of the shallow water pool formed, the attainment of an equilibrium characterized by the I2 partition coefficient R applies only for a boundary layer that is limited by the flow characteristics, and not for the whole moving gas phase. This means, then, that there is a greatly reduced transport of I2 from the liquid to the gas phase, as compared to the assumption of an instantaneous attainment of equilibrium for the entire gas phase. It can be theoretically derived that the fraction of I2 released (X) in time t is given by 

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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). 

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