I Understanding the Fukushima Daiichi Accident and Its Consequences

Event Sequence of the Fukushima Daiichi Accident

Shinya Mizokami and Yuji Kumagai

Abstract On March 11, 2011, the Great East Japan Earthquake and subsequent tsunami hit Fukushima Daiichi Nuclear Power Station. Flooding by the tsunami induced loss of AC and/or DC power for reactor cooling, hence the reactor water level decreased and fuel was exposed. Water reacting with high temperature fuel metal covering resulted in hydrogen generation and hydrogen explosion of reactor buildings. This accident caused radioactive release to the environment. In this chapter, an attempt has been made to understand in detail the mechanism of the accident progression for Units 1–3 that were in operation by utilizing results of computer simulations. It should be noted that, due to limited information and capability of the state-of-the-art severe-accident simulation tools, there are still unanswered questions, which should be tackled by academic research for improving and enhancing safety for the nuclear industry now and in the future.

Keywords Fukushima Daiichi nuclear power station • Severe accident • Accident progression • Great East Japan earthquake • MAAP simulation

Overview of the Accident

The Tohoku-Chiho--Taiheiyo--Oki Earthquake^[1] (the Earthquake, hereafter) and ensuing tsunami, which occurred on March 11, 2011, led the Fukushima Daiichi Nuclear Power Station (NPS) to a situation far beyond design basis accidents and was even further exacerbated by multiple failures assumed in developing accident management measures. Consequently, Units 1–3 ultimately experienced severe accidents; although they were successfully shut down, they lost functions related to cooling.

On March 11, 2011, Units 1–3 of Fukushima Daiichi NPS were in operation, while Units 4–6 had been shut down for periodic inspection outage. Due to the shock of the Earthquake that occurred at 14:46, the safety function of Units 1–3 was actuated by the seismic over-speed trip signal, which resulted in automatic shutdown of all reactors in operation at the time.

Due to the collapse of the electric tower connection to off-site, all power supply from off-site to Fukushima Daiichi NPS was lost, but the emergency diesel generators (EDGs) started up as expected, and the electric power necessary to maintain safety of the reactors was acquired.

Later, the tsunami hit the Futaba area of Fukushima Prefecture where Fukushima Daiichi NPS is located. It was one of the largest in history. Many of the power panels were inundated, and the EDGs, except for Unit 6, stopped, resulting in the loss of all alternating-current (AC) power and, consequently, loss of all the cooling functions using AC power at the site. As a consequence, corecooling functions not utilizing AC power were put into operation, or, alternatively, attempts were made to put them into operation. These were the operation of the reactor core isolation cooling system (RCIC) in Unit 2, and the operation of the RCIC and the high-pressure injection system (HPCI) in Unit 3.

Units 1–3 had a different process, but in the end the loss of direct-current (DC) power resulted in the sequential shut down of core cooling functions that were designed to be operated without AC power supply. Then, due to water evaporation by decay heat and depressurization boiling, the reactor coolant in the reactor pressure vessel gradually decreased, which caused boil-dry of the fuel. Accordingly, water injection was attempted through an alternative water path by joining fi engines with the fire protection system and make up water condensate system (MUWC), but water could not be injected into the reactor vessels in Units 1–3 for a certain period of time.

Due to exothermic chemical reaction between steam and zirconium (Zr) included in the fuel cladding tube, Zr + 2H2O → ZrO2 + 2H2, massive heat was generated, causing the fuel to melt and the generation of a substantial amount of hydrogen.

Subsequently, in Units 1 and 3, explosions, which appeared to be caused by hydrogen leakage from the primary containment vessel (PCV), destroyed the upper structure of their respective reactor buildings.^[2]

• [1] The earthquake is also often referred to in Japan as the Great East Japan Earthquake. In the Press Conference by Prime Minister Naoto Kan on April 1, 2011, it was announced that the Cabinet decided to officially name the disaster the Great East Japan Earthquake.
• [2] Japanese BWR was designed to replace gas inside PCV with nitrogen to prevent hydrogen explosion inside PCV.