Assessment of Risk Studies and Severe Nuclear Accidents - The Risks of Nuclear Energy Technology - page 223

10.3.8 Direct Heating Problem
As the reactor core is melting through the bottom hemispherical head of the reactor
pressure vessel under high pressure, the core melt, in the form of a water vapor—
melt spray, is driven into other compartments and into the outer reactor contain-
ment, respectively. Depending on the design of the inner and outer containments of
a pressurized water reactor, this makes it possible for fine particles of the core melt
to be distributed over large volumes of the reactor containment. As the droplets and
particles of the melt at the same time carry the residual heat, this accident sequence
in present PWRs is also referred to as direct containment heating. The Zr-particles
can interact with steam and generate hydrogen. The hot fuel particles heat the
containment atmosphere and increase the pressure.
These phenomena were studied in experimental programs which, in turn, allow
the appropriate design proposals to be made for the reactor cavity and compart-
ments below the reactor pressure vessel [ 97 , 98 ].
In this way, problems of direct containment heating can be excluded for future
reactors, such as the EPR, by a properly enforced flow of water vapor and melt
spray into specific compartments.
10.3.9 Summary of Safety Research Findings About
the KHE Safety Concept
The findings of recent safety research as outlined in this chapter differ decisively
from the results of the early WASH-1400 [ 9 ] and German Reactor Risk Study
[ 10 ]. The accident phenomena,
- steam explosion,
- hydrogen detonation,
- high-pressure core meltdown,
- containment bypass in the annulus and uncontrolled steam generator tube failure
associated with radioactivity release through the main steam relief valves
which had the most severe accident consequences (Figs. 10.1 and 10.2 ), are either
partially controlled in present PWR designs (KWU PWR-1,300) or can be elimi-
nated or managed in future LWRs by appropriate design measures. Examples of
such future LWRs are the EPR and the SWR-1,000 (KERENA) (Chap. 3 ) . Both
reactor lines were designed on the basis of the research findings reported in this
chapter and along safety recommendations laid down by the French and German
reactor safety commissions [ 16 , 99 , 100 ]. They still need to undergo the licensing
procedures required in those countries in which they will be built and operated.
The results of the German Risk Study Phase A [ 10 ] for earlier German pressur-
ized water reactors, which findings are outlined individually in Fig. 10.1 , are not