Civil Engineering Reference
In-Depth Information
from the outer containment after the onset of an accident and the fractions of fission
products released can be determined. These fission product fractions refer to the
total radioactive inventory of the PWR or BWR core.
In the US Reactor Risk Study [
1
] and in the German Risk Study Phase A [
2
,
9
]a
very conservative approach was applied, e.g. it was assumed that a core meltdown
accident is followed by a steam explosion with a certain probability of occurrence.
For such a steam explosion it must be assumed that the molten core is mixing with
water and fragmenting into very small particles which transfer heat very quickly to
the water, thus rapidly producing a large amount of steam. This release category
includes the highest radioactivity release which would occur at about 1 h after the
initiating event with subsequent core meltdown. More recent studies described in
Sect.
10.1
, however show that after a large scale steam explosions a subsequent
failure of the pressure vessel and of the outer containment can be considered to be
impossible.
Other release categories of e.g. the German Study Phase A [
1
,
2
,
10
] did comprise
core meltdown accidents with a subsequent large scale hydrogen-air detonation
and failure of the outer containment (Sect.
10.2
) or so-called containment bypass
ment of 25-300 mm equivalent diameter were assumed. Lower release categories of
the Risk Studies [
1
,
2
,
10
] represented core meltdown with late containment
overpressure failure. The lowest release categories cover loss-of-coolant accidents
controlled by the emergency cooling systems. Since, in these cases, the reactor core
is cooled sufficiently by the emergency core cooling system, the fuel element
claddings will be damaged only partially and the reactor core will not melt.
6.9 Accident Consequences in Reactor Risk Studies
The US Reactor Safety Study, WASH-1400 [
1
], was performed for 100 reactor
plants (PWRs and BWRs) in the United States. It was published in 1975, thus
preceding the similar German Risk Study [
2
,
10
] which was performed in 1979 for
25 German reactor plants. Compared with the results of the German Risk Study, and
aside from slightly different safety designs of German LWRs, it was mainly the
meteorological data, the population density, the purely linear dose-risk relationship
as well as protective measures and countermeasures, which differed in the two
studies.
It must also be emphasized that in these reactor risk studies the results are
averaged over 68 different sites (WASH-1400 [
1
]) or 19 different sites (German
Reactor Risk Study, Phase A [
2
]) with several hundred different weather
conditions.
The frequency of core meltdown accidents was determined to be 5
10
5
per
reactor year in the US Reactor Safety Study [
1
]. The largest number of early
fatalities was approx. 3,300, with a probability of occurrence of 10
7
per reactor
year. The cases of early illness are 45,000, with a frequency of occurrence of 10
9
per reactor year.
