Nuclear power plant near Harrisburg, Pennsylvania, USA, with two pressurized water reactors. On 28th March 1979, a serious accident with partial core melt-down occurred in unit 2. The fission products were almost completely retained in the reactor pressure vessel and in the containment. As the retention function of the containment functioned as designed, only activity releases of xenon-133 and very low portions of I-131 into the environment occurred, resulting in the statistical maximum dose of 0.85 mSv.