Light Water Reactor Design Against External Events - The Risks of Nuclear Energy Technology - page 154

Fig. 7.6 Example of a pipe
section equipped with
restraints and damping units
protecting against
dislocations caused by
seismic events [ 28 ]

7.1.3 Comparison Between Seismic Design and Seismic
Damage in Existing Nuclear Power Plants

Nuclear power plants are equipped with acceleration sensors which trigger a reactor
scram in earthquakes of sufficiently high intensity levels (below the design basis
earthquake), and then automatically change into the residual heat removal mode.
Nuclear power plants are inspected after an earthquake to compare any damage
with the design features laid down for the design basis earthquake.
In Japan, Armenia, and the United States of America, the reactor scram system
(RESA) of nuclear power plants has always functioned successfully in earthquakes.
The highest seismic loads so far have occurred in the Kashiwazaki-Kariwa nuclear
power plant caused by the Niigata-Chuetsu-Oki earthquake (November 15, 2007)
with a magnitude of M

6.2. The epicenter was only 16 km away from the nuclear
power plant. The Kashiwazaki-Kariwa nuclear power plant comprised seven boil-
ing water reactors designed for a safe shutdown earthquake with maximum seismic
accelerations of 1.67-2.73 m/s 2 . In all reactors, scram and transition to the residual
heat removal mode worked according to the rules. The maximum accelerations
measured during the earthquake in the bottom slabs of the seven BWR nuclear
generating units, however, were 3.22-6.80 m/s 2 , which is roughly twice the level of
the design basis earthquake. Nevertheless, all safety-related components remained
functional. Because of other damage to the building structures, these nuclear power
plants were down for repair for 3 years after the seismic event [ 21 ].

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