Repères - N°48 / March 2021 - The IRSN magazine - 5

JAPAN 2011
The Fukushima Daiichi nuclear power plant
© 2020 Maxar Technologies

Reactor units 1-4 at Fukushima Daiichi on March 18, 2011.

5

6

Marine contamination
The marine environment suffered significant
radioactive contamination. This was primarily due
to contaminated water being discharged from
the plant, which continued up to April 8.
To a lesser extent, the sea was also contaminated
by fallout from some of the radionuclides
(e.g. cesium-137) released to the atmosphere
between March 12 and 22.

Units 5 and 6
These reactors were built more
recently and as they are situated
about ten meters higher up than the
other four, they were consequently
able to better withstand the
external load. One of the emergency
generators for unit 6 was used
to cool the spent fuel storage pools
of units 5 and 6, which were shut
down for maintenance.

Units in operation
Units shut down for maintenance

Different reactor technologies

146,520
residents of Fukushima
Prefecture were evacuated
following the accident in 2011,
as ordered by the government.
Source: The Fukushima Nuclear Accident
Independent Investigation Commission
official report (2012).

Although all nuclear power reactors make
use of fission energy* to generate electricity,
their design differs. The units at Fukushima
Daiichi plant are equipped with boiling
water reactors (BWR), whereas the nuclear
power plants in the French fleet house
pressurized water reactors (PWR). The
main difference between these technologies
lies in the type of cooling system:
* In a BWR, the water used to cool the reactor core turns to steam at the top of the
reactor vessel before being channeled to the
turbine. The steam is then condensed to
liquid state in a condenser (cooled by seawater at the Fukushima Daiichi plant) before

© Hervé Bouilly/Art Presse/ABC Communication/IRSN Media Library/Repères magazine

Atmospheric contamination
The series of decompressions and
explosions resulted in significant
releases of radioactive fission products
to the atmosphere, such as iodine-131
and cesium-137. In the hours
following the accident, the Japanese
authorities decided to evacuate
80,000 people within a 20 km radius
of the site and ordered those living
within a 20-30 km radius to shelter
indoors. The most hazardous releases
occurred over a period of three weeks.

The Fukushima Daiichi nuclear
power plant (Daiichi means
" Number One " ) is made up of
six American-designed Boiling Water
Reactors (BWRs). Built in the 1970s,
it is operated by Tokyo Electric
Power Company (TEPCO). Unit gross
power capacity ranges between
460 and 1100 MWe depending
on the reactor.

being recirculated back into the core.
* The core of a PWR is cooled by a system
called the primary circuit in which the
water is pressurized so that it remains in
liquid form. The heat from this circuit is
transferred to the secondary circuit, an
independent system in which the water
evaporates and drives the turbine.
Both of these technologies are equipped
with various backup emergency systems
which, in the event of an accident, are
designed to inject water to cool the reactor core.
* During fission, the nucleus of an atom splits,
releasing a very large quantity of energy.

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Repères - N°48 / March 2021 - The IRSN magazine

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