Civil Engineering Reference
In-Depth Information
6.4 Releases of Fission Products from a Reactor Building
Following a Core Meltdown Accident
6.4.1
Initiating Events
Initiating events controlled by the safety system will not contribute to risk. Accord-
ingly, major contributions to the overall risk must be expected to arise only with
large scale failure of fuel rod claddings and from those events in which the reactor
core will melt down partially or completely because of an extensive failure of the
safety systems. Event tree studies show that the occurrence of a major leak in a
main coolant pipe followed by a failure of the respective safety systems (emergency
cooling systems and afterheat removal systems) will cause the reactor core to melt
down; within a few hours the molten core can even penetrate the reactor pressure
vessel. In an early superheated phase of the reactor core, hydrogen will be generated
in a reaction between water and the zirconium in the fuel claddings.
After having penetrated through the reactor pressure vessel, the hot core material
will contact the concrete foundation slab of the reactor building and gradually melt
into the concrete. This will cause water bound in the concrete to be released and
react with the melt, which will generate hydrogen. Depending on the type of
concrete used, also CO, may be released. For the further sequence of accident
events it was assumed in a pessimistic estimate in the US and German risk studies
[
1
,
2
,
7
,
9
] that the molten core contacts and evaporates the sump water, thus
increasing the pressure in the containment.
6.4.2 Failure of the Containment
A number of penetrations through the containment building for locks, pipes and
cables may develop leaks with a certain failure rate. In this case, radioactivity could
escape to the outside. If, on the other hand, the containment is assumed to remain
tight, i.e., preserve its integrity, the core meltdown accident described above would
generate vapor, H
2
, CO and CO
2
, and raise the pressure in the containment so that
the permissible design pressure of the outer containment could be exceeded. After
failure of the outer containment integrity, radioactivity could be released into the
environment.
In the US Reactor Safety Study, WASH-1400 [
1
], and the German Risk Study
[
2
,
9
], also the case of large scale hydrogen detonation and of a potential steam
explosion resulting from a contact between molten hot core material and water was
discussed. This was assumed to occur with a certain probability in the bottom part
of the reactor pressure vessel.
