Civil Engineering Reference
In-Depth Information
Fig. 10.10
Buoyancy force on pressure vessel lower head because of core melt through under
operational pressure of 15.5 MPa [
66
]
frequency of occurrence roughly one order of magnitude lower per reactor year than
for core meltdown under high pressure [
64
].
Within the framework of the KHE Safety Concept, the RELAP-MOD-3 code
[
30
,
31
] was used to determine the buoyancy forces arising from failure of the
bottom hemispherical head at
the full primary pressure of 15.5 MPa in the
RPV [
66
].
This buoyancy force is shown in Fig.
10.10
as a function of time. It starts with
300 MN and decreases down within about 250 ms. A technical concept was
proposed for reinforced anchorage of the reactor pressure vessel [
5
-
7
]. This stron-
ger mechanical anchorage of the reactor pressure vessel—as an ultimate technical
safety solution—would prevent the vessel from moving upwards even in the case of
the failure of depressurization of the primary coolant system (opening of the
pressure relief valves). The integrity of the containment would be preserved even
in this hypothetical accident (core meltdown under high primary pressure). In this
way it provides high flexibility in safety design to avoid the core melt down under
high primary pressure.
install two or three blowdown valves, with very high discharge capacities of
900 ton/h (EPR) which is to ensure depressurization to at least 2 MPa (EPR) with
a high reliability within a short time period. These blowdown valves can be
actuated, e.g. from the control room (EPR) when the coolant outlet temperature
exceeds 650
C. Depressurization to
2 MPa avoids significant melt dispersal
(direct containment heating Sect.
10.3.8
) in case of core melt break through the
bottom of the reactor pressure vessel [
67
,
68
]. The three tandem pressure relief and
safety valves as well as the two blowdown valves (depressurization devices) for
EPR are shown in Fig.
10.11
. For AP1000 and US-APWR the depressurization
3.16
.
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