Assessment of Risk Studies and Severe Nuclear Accidents - The Risks of Nuclear Energy Technology - page 202

Fig. 10.4 Melt jet injected
into water with different
zones for premixing and
fragmentation [ 17 , 18 ]
experiments was 2-3 % [ 22 , 26 , 27 ]. Only experiments with iron—aluminum—
thermite and water resulted in efficiencies roughly a factor of 2 higher.
Estimates of the damage resulting in the SL-1 accident led to an efficiency of
10-15 % [ 19 ]. However, the SL-1 accident is to a certain extent in the category of
the first mode of contact between the molten fuel and water, which is initiated by a
(superprompt critical) power transient with the fuel rods rupturing.
10.3.1.2 Description of a Steam Explosion Sequence
One of the most important preconditions of a steam explosion is fragmentation of
the melt into many particles of about 0.1 mm size to create the area necessary for
fast heat transfer.
Estimates show that this kind of fragmentation from a large molten core mass
into many particles of roughly 0.1 mm in size is not possible in one single step, as
the energy input would be too large. The melt first can be split up only into larger
droplets of cm in size (premixing). In this process, Rayleigh-Taylor or Kelvin-
Helmholtz instabilities play a major role when the molten jet flows into the water
(Fig. 10.4 ). These larger melt droplets roughly cm in size are surrounded by a vapor
film.
A pressure pulse is required as the trigger initiating a steam explosion. The
pressure pulse causes the vapor film to break down and very fine melt particles
(Fig. 10.5 ) to be produced in a process of fine fragmentation. As a consequence,
there is very fast evaporation and a steam explosion, respectively [ 17 , 18 , 23 ,
24 , 28 ].