Assessment of Risk Studies and Severe Nuclear Accidents - The Risks of Nuclear Energy Technology - page 213

Fig. 10.12 Cooling of AP-1000 vessel from the outside in case of core melting by flooding with
water [ 73 ]
10.3.6.1 Possibility of Cooling the Molten Reactor Core by Flooding
the Reactor Pressure Vessel with Water from the Outside
In this variant applied, e.g. for AP600 and AP1000 the reactor pressure vessel is to
be flooded with water from the outside (Fig. 10.12 ) as the reactor core is melting
[ 69 - 73 ]. There are a number of theoretical and experimental investigations in the
wake of the Three Mile Island accident in the USA which make this accident
management measure appear successful. The heat fluxes from the melt to the wall
of the bottom hemispherical head, heat conduction through the wall of the pressure
vessel, the temperatures in the wall of the pressure vessel, and the stability of steel
as a function of the wall temperature are taken into account.
These research investigations in the USA demonstrated that the molten core will
not melt through the wall when the reactor pressure vessel is flooded with water
from the outside. This is valid also for the case that the thermal insulation of the
pressure vessel remains intact on the outside of the reactor pressure vessel. How-
ever, for PWR's with a power output of 1,400 MW(e) or more this variant of
flooding the reactor pressure vessel with water from the outside needs more
research efforts [ 74 ].
Molten core cooling by flooding the reactor pressure vessel from the outside is
also proposed for the SWR-1000 (KERENA) design (Fig. 10.13 )[ 75 ]. The
SWR-1000 bottom head of the pressure vessel has many penetrations (welded
tubes for control rods etc.). Therefore, BWRs have a higher surface to volume
ratio in this bottom part than PWRs. In addition, BWRs have a lower power density