La troisième barrière
The third barrier is provided by the enclosure of the reactor building. This external containment structure is maintained in depression, which avoids that the radioactivity eventually present in the reactor building escapes to the outside. It must be taken to isolate the bushings. This is the good performance of the containment structure that has prevented the dispersion of radioactive materials in the case of serious accident at Three Mile Island in 1979. The drawing on the right shows the inside of the building.
AREVA / EDF/Electra