Assessment of Risk Studies and Severe Nuclear Accidents - The Risks of Nuclear Energy Technology - page 204

melt, after having molten through the gridplate (Fig. 10.3 ), would flow out in a
molten jet of about 0.2 m 2 cross section. There would be premixing with a water
volume fraction of 0.5-0.6. The maximum content of thermal energy in the
larger melt droplet as premixing zone amounts to roughly 3 GJ . (On the average
of all possible cases, the thermal energy content would be only 0.5-2.0 GJ with
the corresponding water volume fractions of 0.2-0.5.)
In the further course of the analysis it was postulated that fine fragmentation to
0.2-0.3 mm, size as measured in experiments, would occur and a steam explosion
would be initiated. After careful inspection and assessment of all experimental
findings and theoretical analyses against theoretical models available internation-
ally, a conservative value of 15 % was selected as the efficiency of conversion of
thermal into mechanical energy. This results in a maximum mechanical energy
release by the steam explosion of
3GJ 0 : 15 ¼ 0:45 GJ
(as the average of all possible cases, the result would only be 0.075-0.3 GJ).
10.3.1.4 Dynamic Mechanical Analysis of the Pressure Vessel
These results about the release of mechanical energy in the course of a steam
explosion in the lower plenum (bottom hemispherical head) of the reactor pressure
vessel were used to conduct dynamic mechanical stability analyses accompanied by
1:10 scale experiments (BERDA experiments) [ 36 - 39 ]. Theoretical models of
similarity theory and accompanying strength analyses allowed the results of the
1:10 scale experiments to be transferred to the dimensions of the reactor pressure
vessel.
As the steam explosion is initiated already while the melt jet is discharged, some
80 ton of core melt would still be present on the baseplate. After rupture of the
mechanical anchorage (break) of the gridplate, this volume must be accelerated
upward together in the reactor pressure vessel. The upper part of the reactor
pressure vessel of a PWR contains the internal structures with the guide tubes for
the control and shutdown rods and for the in-core instrumentation (Fig. 10.6 ). These
must be compressed by the core melt, accelerated upward so as to be able to transfer
the dynamic forces of the core melt to the head structures and head bolts. These
internal structures and the head structures were simulated in great detail in the 1:10-
scale experiments (BERDA) [ 37 , 40 ].
This is the overall finding of all Karlsruhe BERDA experiments and theoretical
analyses:
- Acceleration of the gridplate, with the remaining core melt resting on it, up to the
top internal structures of the head requires at least approx. 2 GJ.
- Another 0.8 GJ would be necessary to compress the internal structures and
elongate the head bolts by a few mm. The compression of the upper internal