Civil Engineering Reference
In-Depth Information
- The frequency of occurrence of core meltdown is determined by the diversity
and number of redundancies of the cooling circuits, safety systems, emergency
core cooling systems, emergency power supply systems. The overall frequency
of occurrence for a core meltdown should be around 10
6
per reactor year. It can
hardly be reduced below this level for practical reasons (common mode failures
etc.).
- Probabilistic safety analyses only can serve for the optimization of safety
systems and for demonstrating that the overall frequency of occurrence is in
the range of 10
6
per reactor year. However, probabilistic safety analyses no
longer shall support the argument that risk—as a product of frequency of
occurrence multiplied by amount of damage—is so low that it can be accepted.
The results of the research program conducted at the Karlsruhe Research Center
on the basis of this KHE safety concept are presented in the following sections of
this chapter.
This KHE safety concept was essentially accepted by the German and the French
Reactor Safety Commissions for future LWRs after thorough discussions [
15
,
16
].
10.3 New Findings in Safety Research
This section will contain the new findings of safety research into the accident
phenomena, described in the section above, which, in WASH-1400 [
9
] and the
German Reactor Risk Study Phase A [
10
], still resulted in major accident
consequences.
10.3.1 Steam Explosion (Molten Fuel/Water Interaction)
A steam explosion is the explosion-like evaporation of a cool liquid, such as water,
in contact with very hot liquid fuel [
17
,
18
]. This contact must allow a very fast heat
input from the molten fuel to the coolant (water) in a very short time. Peak pressures
may be in excess of 10 MPa. Steam explosions can also occur due to interaction of
other liquids, e.g. contacts between
- hot oil and water,
- molten aluminum and water,
- water and liquefied gas at cryogenic temperatures,
- hot lava (volcanic eruption) and water.
In an LWR, an accident sequence can include mainly two contact modes for a
steam explosion:
