Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
For pressurized water reactors, the likelihood of creep failure of steam generator tubes for
some severe accidents at high pressure of the reactor coolant is not negligible, with the
possible consequence of a containment bypass. To minimize the potential for containment
failure or containment bypass in severe accidents at high pressures of the reactor coolant,
the plant features may be enhanced, if necessary, to depressurize the reactor coolant
system reliably so as to prevent this process from occurring.
16.4.4 The TMI‐2 and Chernobyl accidents [7]
16.4.4.1 The TMI‐2 Accident
The core melt accident in the TMI‐2 reactor near Harrisburg Pennsylvania, USA occurred
in March 1979. The accident started with the loss of feed water to the steam generators
and coupled with equipment failure such as failure to automatic closing of the pilot‐
operated relief valve (PORV) without operators’ recognition for long time. In addition,
there were faulty operator actions such as shutting off the ECCS injection to the core. At
least, half of the core had melted. Later, it was found that some (20 tonnes) of the melted
core had reached the lower head.
The Zircaloy‐steam exothermic reaction produced hydrogen which was released to the
containment through the open PORV. It burned several hours after the start of the
accident and produced a pressure spike of 2 bars in the containment, which did not
sustain any damage. During the first 16 hours, only approximately 10 Ci was released to
the environment and ~70 curies of I was released over the next 30 days. The TMI‐2
accident did not cause any injuries or deaths. It, however, was a shock to the nuclear
establishments all over the world.
16.4.4.2 The Chernobyl Accident
A core melt accident happened in a RBMK reactor situated in Chernobyl in Ukraine,
Soviet Union. The RBMK are water‐ cooled channel type reactors in which the water
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