Assessment of Risk Studies and Severe Nuclear Accidents - The Risks of Nuclear Energy Technology - page 219

Fig. 10.19 EPR double containment system with reactor pressure vessel, cooling systems and
core catchers [ 86 ]
in-containment refueling water storage tank (IRWST) and be pumped through a
heat exchanger to the spray system (Chap. 3 ) [ 86 ].
The Core Melt of Present LWRs Penetrating into the Subsoil Underneath
the Reactor Building
Further penetration of the core melt into the ground below the outer reactor
containment was neither studied in the WASH-1400 [ 9 ] nor in the German Risk
Studies [ 10 , 64 ]. In the Three Mile Island accident the core melt did not penetrate
through the bottom head of the reactor vessel, probably due to the fact that enough
water was available timely enough for cooling. In the Chernobyl accident, dropping
sand and lead from helicopters on the destroyed reactor core created a molten mass
which ultimately did not melt through the bottom foundation of the reactor building
despite fears that this might happen. In the Fukushima accident the core melt caused
some small holes in the lower part of the reactor pressure vessel but the core melt
did probably not penetrate further, because cooling could be provided early enough.
All experimental and theoretical investigations culminate in the conclusion that,
in a PWR for instance, the core melt—after having molten through the bottom head
of the reactor vessel and through the concrete base plate—would move further into
the subsoil below the foundation of the reactor building [ 79 , 87 , 88 ]. In a matter of
roughly 200 days, it would expand to a radius of approx. 12 m (Fig. 10.20 ). It would
comprise a volume of 1,000 m 3 and consist of UO 2 , ZrO 2 , CrO 2 , FeO 2 , SiO 2 ,Al 2 O 3
and CaO. The SiO 2 fraction would amount to roughly 75 %. Ground-water would
cool this enlarged molten mass and slow down and stop eventually its further
penetration into the subsoil [ 87 ].