Civil Engineering Reference
In-Depth Information
pool. It is likely that no damage has occurred to the spent fuel. The cooling water
temperature had been maintained
35
C[
21
].
<
9.3.1.4 Unit 4
After the upper building structure had been destroyed by the hydrogen detonation,
initially, water was sprayed by water cannons and concrete pumps. The structures
supporting the spent fuel pool of unit 4 were improved by steel support pillars to
provide protection against damage that might result from additional seismic events.
Photographs showed that the fuel racks of the spent fuel pool of unit 4 were intact.
A cooling system for the spent fuel pool was put in service. The coolant temperature
of the spent fuel pool had been maintained
40
C[
21
].
<
9.3.1.5 Units 5 and 6
No damage occurred to the fuel in the spent fuel storage pools 5 and 6. One Diesel
generator of the units 5 and 6 could be restored soon enough such that emergency
electric power was available for the cooling of the spent fuel [
21
].
9.3.2 Measurement of the Radioactivity Released
Large parts of the radioactivity released were initially carried out to the sea by the
prevailing winds. Measurements of the radioactivity released over land (radioactive
noble gases, radioactive I-131 (halflife 8 days) and radioactive aerosols, such as
Cs-134 (halflife 2 years) and Cs-137 (halflife 30 years)), were carried out by
specially equipped aircraft of the American National Nuclear Security Agency
(NNSA). These measurements (Fig.
9.11
) showed a particularly pronounced dis-
tribution of radioactivity towards the northwest of Fukushima. The measurements
were evaluated in order to determine the radioactive exposure, which must be
known for decisions about evacuation of the public. The Japanese government
then evacuated the population (roughly 200,000 persons) in the vicinity of the
nuclear power plants. The initial evacuation zone was soon expanded to a radius
of 20 km. Afterwards, also some places situated beyond the zone of 20 km were
evacuated because the annual dose to the population there had been estimated to run
up too high [
18
,
19
].
