Both the Fukushima Daiichi Nuclear Power Plant (FNPP) accident in 2011 and the Chernobyl NPP Unit 4 (CNPP) accident in 1986 are classified as Level 7, which is the highest level on the International Nuclear and Radiological Event Scale (INES) as defined by the International Atomic Energy Agency (IAEA) [1, 2]. In both accidents, a large amount of radionuclides were released into the environment from the damaged reactors and large areas of land were heavily contaminated to the extent that many people have to be evacuated for a long period [3,4,5]. Although it is clear that each accident had a big impact on society, they were different in various aspects, including the accident process, composition of radioactive contamination and geological conditions [6, 7].
FNPP was the first nuclear power plant built by the Tokyo Electric Power Company (TEPCO) and its first unit (Unit 1: 460 MWe) began operation in 1971. By 2011, FNPP had six boiling water reactor (BWR) units (Units 2–5: 780 MWe; Unit 6: 1100 MWe) developed by General Electric (GE, USA). Units 4, 5 and 6 were out of operation at the time of the earthquake (14:46 March 11, 2011) due to annual maintenance work, while Units 1, 2 and 3 were operating at full power [8, 9].
The epicenter of the Great East Japan Earthquake was approximately 180 km away from FNPP. At 14:47, the three operating reactors were automatically shut down due to a large seismic acceleration, and emergency diesel generators (EDGs) were then activated to provide necessary electricity to the station. The tsunami waves, at over 10 m high, arrived at FNPP around 15:36 and flooded the basement of the turbine buildings where EDGs were located. EDG failure resulted in power loss for the pumps providing coolant water to remove decay heat from the reactor cores, which was the real emergency that led to the FNPP accident.
Several emergency cooling systems that do not require electric power were installed at each BWR in the event of a power outage. These cooling systems included isolation condenser (IC) systems, reactor core isolation cooling (RCIC) systems and high-pressure coolant injection (HPCI) systems. Unit 1 was equipped with IC and HPCI, while Units 2 and 3 were equipped with RCIC and HPCI. These emergency cooling systems were not designed to work for a long period, and consequently three FNPP reactors operating at the time of the tsunami became damaged one by one. The sequence of reactor damage is summarized below :
Unit 1: After EDG power failure, both IC and HPCI systems lost function. Without emergency cooling, the reactor core began meltdown, and fuel melted through the reactor pressure vessel (RPV) in the evening of March 11. At 02:30 March 12, the drywell (DW) inner pressure was measured to be 840 kPa, about twice the maximum design pressure of 427 kPa. To avoid rupture of DW, the operator tried to vent the pressure, successfully releasing pressure to the acceptable level at 14:30. At 15:36 March 12, however, a hydrogen explosion occurred at the roof of the reactor building, which was strong enough to destroy the roof and the wall on the highest floor of the reactor building.
Unit 3: After power loss, RCIC remained functional until 11:36 March 12, and then HPCI was automatically actuated. In the early morning on March 13, the Unit 3 operator decided to switch the cooling system from HPCI to the line using fire engine water prepared outside the building. However, the cooling systems were not switched quickly, which left the reactor without a cooling supply for about 7 h. The meltdown and melt-through process of Unit 3 began in the morning on March 13 and subsequently worsened. A hydrogen explosion occurred at 11:01 March 14.
Unit 2: After power loss, RCIC remained functional until 13:25 March 14. The Unit 2 operator attempted a change of cooling system to fire engines, but was unable to switch systems quickly, leaving the reactor without a cooling supply for about 2 h. The meltdown and melt-through process of Unit 2 began in the evening on March 14. A high DW pressure was observed that night, and an attempted venting operation was unsuccessful. In the morning on March 15, a sudden drop of DW pressure was observed, which suggested containment rupture and massive release of radioactivity into the atmosphere.
In this paper, we compare the radiological consequences between the FNPP accident and the CNPP accident with respect to the amount of radioactivity released into the atmosphere and the radioactive contamination on land.