Assessment of Risk Studies and Severe Nuclear Accidents - The Risks of Nuclear Energy Technology - page 198

- a leak in the annulus between the outer containment and the concrete shell, or an
uncontrolled steam generator accident with steam blowdown valves getting
stuck in the open position, release only somewhat smaller amounts of airborne
radioactivity;
The different contributions of these most severe accident sequences to the
overall damage consequences, i.e. early deaths (fatalities), are shown in Fig. 10.1
as the result of the German Safety Study, Phase A [ 10 - 12 ]. The results are
normalized to a 1.2 GW(e) PWR plant. These results are similar in terms of early
deaths to the results of the US Reactor Safety Study WASH 1400 [ 9 ] as shown in
Fig. 6.6 for 100 LWRs in the USA.
Figure 10.2 shows the areas which would have to be evacuated as a function of
the different release categories during a severe core melt accident with subsequent
containment failure. The abscissa of Fig. 10.2 shows the fraction of radioactive
aerosols released and also the probability of occurrence for the radioactivity release
categories (steam explosion, high pressure core melt through, hydrogen detonations
etc.) as already presented in Chap. 6 . On the ordinate the areas for evacuation are
given, which become necessary as a function of the different weather conditions
which are possible in the surroundings of the nuclear plant. The curves shown
account for 50 % (dotted line) and 95 % (full line) of all weather conditions
occurring [ 11 - 14 ].
The reasoning of the KHE safety concept is as follows:
If it can be demonstrated that the above accident phenomena (and their conse-
quences detailed in Figs. 10.1 and 10.2 ) occurring after a severe core meltdown
accident result in neither early nor late failure of the outer containment i.e. the outer
containment retains its integrity, then the
- radiological consequences to the population,
- need for evacuation and resettlement of the population,
- hazard of contamination of large areas
are reduced to a minimum, i.e. limited to about the area of reactor plant itself. The
curves shown by Fig. 10.1 are shifted to the left (close to the ordinate) [ 8 ].
If in addition the reactor core can be prevented from melting through the
concrete baseplate into the ground below the reactor containment, there would be
no danger of contamination of the groundwater over long periods of time.
The safety requirements of the outer reactor containment as outlined in the
Karlsruhe KHE safety concept then are as follows:
- The consequences of severe core meltdown accidents must be managed by the
structures of the inner and outer reactor containments. The inner reactor con-
tainment should retain the leaktightness of smaller than 0.3-1 % leakage rate
even after a severe core meltdown accident. The molten reactor core must not
melt through the bottom of the outer containment.