Assessment of Risk Studies and Severe Nuclear Accidents - The Risks of Nuclear Energy Technology - page 212

Fig. 10.11 Safety valves and dedicated depressurization devices of EPR [ 67 ]
This changes the core meltdown accident sequence under high pressure into a
core meltdown under low pressure [ 67 , 68 ].
10.3.6 Core Melt Down Under Low Coolant Pressure
Even if no steam explosion occurs at low pressure in the bottom hemispherical head
of the reactor pressure vessel (Sect. 10.3.1.3 ) during the influx of molten fuel, the
residual water remaining there will evaporate very quickly and the core will melt
down. There are then two possibilities for the progress of the accident evolution and
two safety design options:
- Cooling the reactor pressure vessel by flooding with water from the outside as an
accident management measure [ 69 - 73 ],
- No cooling of the reactor vessel by flooding with water from the outside because
of the hazard of a steam explosion when the molten core would melt through.
Instead installation of a molten core fuel retention and cooling device (core
catcher), accommodating and retaining the core melt after core meltdown [ 68 ].
The molten core will always remain subcritical. This was explained by Fig.
2.10 in Sect. 2.10.2 . As the water in the core is evaporated and the lattice
structure of the fuel elements is destroyed the molten core becomes subcritical
to a k eff of less than 0.9. Even the meltdown and displacement of the absorber
material of the control and shutdown rods (silver, indium, cadmium or boron
carbide) cannot jeopardize this subcriticality of the molten core.