Civil Engineering Reference
In-Depth Information
10.3.7 Molten Core Retention and Cooling Device (Core
Catcher)
The countermeasures listed above for the period after melting of the reactor core
through the concrete baseplate of the reactor containment, and the radioactive
contamination of the groundwater and rivers in the vicinity, can be rendered
superfluous by
- flooding the reactor pressure vessel on the outside vessel with water in case the
core is going to melt down (accident management measures Sect.
10.3.6.1
)
- a molten core cooling and retention device (core catcher) underneath the reactor
pressure vessel. This cooling device for molten core masses is part of the EPR
concept (Fig.
10.21
), but not of any other PWR safety design concepts known up
to now.
In the EPR design, the core melt is first kept in the reactor cavity for a short
period of time so that core masses dropping slightly later than the initial bulk of
molten fuel can also be collected. After penetrating a melt plug (steel plate covered
by a layer of concrete), the core melt flows through an inclined canal onto a
dispersion area of approx. 170 m
2
. The core melt is allowed to spread there evenly
to a thickness of roughly 30 cm. Then flooding of the melt with water is initiated
passively by water flow from the IRWST. In this way, the melt is cooled from the
top and solidifies in part. From the bottom, the melt is cooled by active bottom
cooling and stabilized in this way (Fig.
10.21
).
10.3.7.1 Other Core Catcher Designs
As a result of research programs, a number of other concepts were developed to
cool core melts [
89
-
94
]. At this point, only the COMET concept developed at
Karlsruhe will be explained briefly [
90
,
93
-
95
]. In the COMET concept, the melt is
to be flooded with water from below after erosion of a sacrificial layer. The rapidly
evaporating water disrupts the melt, cooling its interior as a water-steam mix.
Figure
10.22
shows the COMET concept. The melt is collected below the reactor
pressure vessel and then first erodes a dry sacrificial layer of concrete 15 cm high.
Afterwards the melt can spread completely and melt any cooling channels in the
concrete layer from the top. This allows water to enter under the pressure of an
overflow tank located high up. This is followed by an effective phase of melt
cooling and fragmentation. The melt solidifies within a short period of time and
can be flooded fully and cooled. The steam produced is cooled in a heat exchanger
and condensed.
Both the EPR core catcher and the COMET concept were tested at Karlsruhe
and developed in many years of pilot experiments (KAPOOL, KATS, COMET)
[
93
-
96
].
