Assessment of Risk Studies and Severe Nuclear Accidents - The Risks of Nuclear Energy Technology - page 201

Fig. 10.3 Molten fuel pouring near the edge of a core ( left ) and molten fuel pouring through the
lower grid plate [ 17 , 18 ]
- During a very fast superprompt critical power transient leading far beyond the
nominal reactor power, the fuel rods may burst and the molten fuel will be
injected as very finely dispersed fuel (with very large heat transfer area) into the
cooling channel under high pressure and mixed with the cooling water. This
happened, e.g., to a certain extent in the so-called SL-1 accident (water-cooled
experimental reactor in USA) [ 19 - 21 ] and in the Chernobyl accident, Ukraine,
(Chap. 9 ) ;
- in the second mode of contact, molten fuel of the reactor core after a core
meltdown accident can come into contact with remaining water (Fig. 10.3 ) either
within the reactor pressure vessel, after melting through the grid plate, or outside
the reactor pressure vessel, after melting through the bottom hemispherical head
of the reactor pressure vessel [ 17 , 18 ].
10.3.1.1 Mechanically Released Energy in a Steam Explosion
The maximum mechanical energy which can be converted from the thermal energy
of the molten fuel in a steam explosion is obtained in the case of heat transfer at
constant volume and rising pressure and ensuing isentropic expansion of the steam
[ 22 ]. Heat transfer from the fuel melt to the water must occur roughly within 1 ms.
The ratio of volumes of the fuel melt and water in that case should be around
1. Theoretically, this would allow an efficiency of roughly 40 % to be attained for
the conversion of thermal into mechanical energy [ 23 - 25 ]. However, the efficiency
measured in experiments with a simulated core melt (corium) on average nearly
always is below about 1 %. The maximum efficiency of conversion in some