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Home arrow Environment arrow Radiation Monitoring and Dose Estimation of the Fukushima Nuclear Accident

Basic Information

The Fukushima Daiichi NPP is located in the towns of Okuma and Futaba, which are in the county of Futaba in Fukushima Prefecture. This NPP consists of six boiling water reactors (BWR) installed, Units 1 through 6, with a total generating capacity of 4,696 MWe (Table 3.1). The reactor model of Unit 1 is BWR3, that of Unit 2 through 4 is BWR4, and that of Unit 5 and 6 is BWR5. The Primary Containment Vessel (PCV) model of Unit 1 through 5 is Mark-1 and that of Unit 6 is Mark-2, respectively. Before the earthquake on March 11, Units 1 through 3 were under operation and Units 4 through 6 were undergoing periodic inspection. Unit 4 was undergoing a major construction for renovations, with all the nuclear fuel in the

Table 3.1 Major characteristics of Fukushima Daiichi Nuclear Power Plant

Unit

1

2

3

4

5

6

Electric output (MWe)

460

784

784

784

784

1,100

Commercial operation

1971/3

1974/7

1976/3

1978/4

1978/4

1979/10

Reactor model

BWR3

BWR4

BWR4

BWR4

BWR

BWR5

PCV model

Mark-1

Mark-1

Mark-1

Mark-1

Mark-1

Mark-2

No. of fuel assemblies

400

548

548

548

548

764

in core

reactor pressure vessel (RPV) having already been transferred to the spent fuel pool. More details on the plant specifications and initial and boundary conditions are described in 2].

Accident Sequences

Before the Tsunami Attack

The Pacific Coast area of eastern Japan was struck off the Tohoku District by the Pacific Ocean Earthquake, which occurred at 14:46 on March 11, 2011. This earthquake occurred in an area where the Pacific plate sinks beneath the North American plate. The magnitude of the earthquake was 9.0, the greatest in Japan's recorded history. Within seconds of the earthquake, the reactor was shut down in all three operating units with the insertion of control rods. The turbo-generators also tripped, and main steam isolation valves closed. All power supplied from a total of six external power supply lines connected to the power plant stopped as a result of damage to the breakers and collapse of the power transmission line tower caused by the earthquake. The earthquake thus disrupted the electrical supply from the grid, which resulted in a loss of offsite power for all six units. As designed, the emergency diesel generators (EDGs) started providing essential power for all safety systems, including the residual heat removal system. Up to the present time, major damage to the reactor facilities that are important for safety functions has not yet been identified [1–4].

 
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