Reactor containment of steel

Except for two blocks at Gundremmingen nuclear power plant, all containments in Germany are made of steel. The containments of the more recent German PWR plants (Convoy and pre-Convoy plants) consist of a steel sphere 56 m in diameter with walls 30-40 mm thick. These dimensions are based on a design pressure in the range 4-5 bar overpressure at a design temperature of approx. 150 °C. The guideline values for the maximum permitted leakage rate are 0.25-0.50% per day.

Steel containments are designed in accordance with KTA rules. KTA 3401 [59-62] covers materials, design conditions, design, production and testing. The material that KTA 3401.1 [59] requires is 15 MnNi 63 steel, whose mechanical characteristics, with a yield point of 330-370 N/mm2 and a tensile strength of 490-630 N/mm2 are comparable with those of construction steel S355.

Design is governed by KTA 3401.2 [60], and is based on permitted stresses, departing from the partial safety concept. Permitted stresses are defined for four stress levels and the various stress categories, allowing for how steel characteristics change at high temperatures. A loss of coolant accident as the dominant verification demand of the containment is put in the operating stress level and hence not regarded as a failure case.

Stability studies are also required to cover the possibility of a partial vacuum arising in the containment. The pressure tests here assume a partial vacuum of 45 mbar and a partial vacuum of 5-30 mbar in normal operation.

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