Assessment of Risk Studies and Severe Nuclear Accidents - The Risks of Nuclear Energy Technology - page 215

Fig. 10.14 Steel support
plate acting as a back-up
core catcher [ 75 ]
In most presently, operating PWRs the reactor pressure vessel is not flooded with
water from the outside. Then this would be the further course of accident events:
The molten mass of the reactor core will melt through the bottom of the reactor
pressure vessel and drop into the dry reactor cavity. This is where the molten fuel
will react with the concrete and its water of crystallization, giving rise to water
vapor, hydrogen, carbon monoxide, and carbon dioxide. While these gases flow into
the compartments above the reactor cavity, fuel melt moves downward into the
concrete foundation (Figs. 10.15 and 10.16 ). The fuel melt also spreads in radial
direction and is able (as in the containment design of the Konvoi-PWR of
Kraftwerk Union) to melt through the lateral biological shield. In this process, the
core melt can contact the water of the building sump (Fig. 10.16 ). The sump water
will then evaporate. This will cause the pressure in the outer reactor containment to
rise gradually. After roughly 5-6 days, the fuel melt can eventually penetrate the
concrete foundation of approximately 6 m [ 64 , 78 - 80 ]. These theoretical predic-
tions are supported by experiments and theoretical analyses (WECHSL code
[ 79 , 80 , 103 - 105 ]). After several days, the steam pressure can reach the design
pressure of the outer reactor containment of 0.6 MPa [ 9 , 10 , 64 ].
In the German Risk Study, Phase A [ 10 ] it is concluded that the outer contain-
ment would develop a leak when exceeding this design pressure of 0.6 MPa. The
radioactive aerosols and radioactive gases would escape into the environment.
More recent investigations [ 81 , 82 ] showed, however, that the outer containment
of KWU-1,300 PWRs would develop a leak at the materials transfer lock not below
some 1.2 MPa. In addition it was shown that dangerous overpressures in the reactor
containment cannot be reached if a pressure reduction by a so-called ex-venting