Vorgänge im Reaktorschutzgebäude eines mittelgrossen Hochtemperaturreaktors beim Kernaufheizstörfall unter besonderer Berücksichtigung der Bildung von Gasschichten

Accidents are conceivable for medium- and large-sized HTR's equipped with a prestressed concrete reactor pressure vessel (PCRV) leading not only to the escape of hot helium but also of combustible gases into the containment. Whether and at what time the containment is endangered by the formation of ignitable mixtures depends, amongst other factors, on the formation of gas and temperature layers. Studies using a computer model adapted to HTR conditions have shown that during depressurization of the primary circuit helium and temperature layers arise in the containment which, however, are then rapidly reduced after the end of depressurization so that at the start of combustible gas release, beginning later, the containment atmosphere is homogeneously mixed. The combustible gas jet only causes slight gas layering since continuous convective mixing takes place due to cooling at the containment structures. Under the selected conditions, ignitable mixtures are formed temporarily in the pressure-resistant, closed containment over the entire space. This can be ruled out with a ventilated containment (confinement), if the intake of air from outside due to negative pressure after the depressurization can be prevented. Without additional steam feeding, the atmospheric humidity isnot sufficient to saturate the containment atmosphere. Although the addition of water vapor leads to saturation the quantity ofcondensed water remains so slight that in the case of a gas ignition its vaporization heat can bind practically no combustion energy resulting in a reduction of the pressure build-up in the containment. This could only be achieved by spraying water in the course of the ignition phase