I. BACKGROUND

I.A. The Tohoku Earthquake and Tsunami

Japan is located along the Pacific Ring of Fire, which is an area that rings the Pacific Ocean and is characterized by mountains, volcanoes, and faults. Of the 16 largest earthquakes in the world recorded since 1900, 15 occurred in the Pacific Ring of Fire. On Friday, March 11, 2011, at 2:46 p.m. (Japan time),1 the largest earthquake in the recorded history of Japan (and one of the largest in the recorded history of the world) occurred on the east coast of northern Japan: the Tohoku earthquake (hereafter referred to simply as “the earthquake”). The earthquake was felt at Fukushima and in much of eastern Honshu, including the Tokyo-Yokohama area. The earthquake was felt from the island of Hokkaido to the island of Kyushu. Beyond Japan, the earthquake was felt in the Northern Mariana Islands, North Korea, Taiwan, northeastern China, and southeastern Russia (Fig. 1).

The earthquake generated a major tsunami, which was the catastrophic blow of a “one-two punch.” The majority of casualties and damage, which occurred in the prefectures of Iwate, Miyagi, and Fukushima, was caused by the tsunami. Along the entire east coast of Honshu from Chiba to Aomori, at least 15,700 people were killed, 4,650 went missing, 5,300 were injured, and 131,000 were displaced; moreover, at least 332,400 buildings; 2,100 roads; 56 bridges; and 26 railways were destroyed or damaged. The total economic loss in Japan is estimated to be about $500 billion (USD). Electricity, gas and water supplies, telecommunications, and railway service were disrupted. Such disruptions affected the Fukushima Daiichi nuclear power plant (NPP) reactors, which were severely damaged. This is where the nuclear story begins.

I.B. Light Water Reactors

Of the more than 400 NPPs currently operating throughout the world, accumulating ~16,000 years of reactor experience, >90% are light water reactors (LWRs), which produce heat by controlled nuclear fission and are cooled by water. In the United States, all 104 operating NPPs are LWR NPPs. There are two general LWR designs: boiling water reactors (BWRs) (Fig. 2) and pressurized water reactors (PWRs) (Fig. 3). In BWRs, the heat generated by fission turns the water into steam, which directly drives the power-generating turbines and the electrical generator connected to them. In PWRs, the heat generated by fission is transferred to a secondary loop via a heat exchanger (steam generator), where the steam is produced and drives the power-generating turbines. In both BWRs and PWRs, after flowing through the turbines, the steam turns back into water in the condenser. The water required to cool the condenser is taken from and returned to a nearby ocean, river, or water supply.

Our main focus in this report is BWRs, because the Japanese NPPs involved in the Fukushima Daiichi accident were BWR NPPs.

I.C. Boiling Water Reactors: General Description

In a BWR NPP, the nuclear reactions take place in the nuclear reactor core, which mainly consists of nuclear fuel and control elements. The nuclear fuel rods (each ~10 mm in diameter and 3.7 m in length) are grouped by the hundred into bundles called fuel assemblies (Fig. 4). Inside each fuel rod, pellets of uranium, or more commonly uranium oxide, are stacked end to end. The control elements (shown as red in cross section), called control rods, are filled with substances like boron carbide that readily capture neutrons. When the control rods are fully inserted into the core, they absorb neutrons, precluding a nuclear chain reaction. When the control rods are moved out of the core, enough neutrons are produced by fission and are absorbed by fissile uranium-235 or plutonium-239 nuclei in the fuel rods, causing further fissions, and more neutrons are produced. This chain reaction process becomes self-sustaining, and the reactor becomes critical, producing thermal energy (heat). The fuel and the control rods and the surrounding structures that make up the core are enclosed in a steel pressure vessel called the reactor pressure vessel (RPV) (Fig. 5).

When uranium (or any fissile fuel) is fissioned and energy is produced, fission products (atomic fragments left after a large atomic nuclear fission) remain radioactive even when the fission process halts, and heat is produced from their radioactive decay, i.e., decay heat. Although decay heat decreases quickly from a few percent to <1% of the rated NPP thermal power after a few hours, water must be circulated within the RPV to maintain adequate cooling. This cooling is provided by numerous systems. Some systems operate during normal conditions, and some systems, such as the emergency core cooling systems (ECCSs), respond to off-normal events. Normal reactor cooling systems maintain the RPV and temperature and a proper cooling water level, or if that is not possible, ECCSs directly flood the core with more water. More detail of BWR safety systems is provided below.

It is important to note that all of these systems require electricity for control and/or motive power for water systems to transfer the decay heat out of the fuel and reactor and into the environment. There are two particular systems in the BWR that require electricity only for control purposes: the isolation condenser system and the reactor core isolation cooling (RCIC) system. These systems play a key role in accident progression.

Because of the large amount of radioactivity that resides in the nuclear reactor core, regardless of the specific design, the defense-in-depth philosophy is used. This approach provides multiple, independent barriers to contain radioactive materials. In the BWR, the fuel rod itself and the RPV with its primary system act as the first two barriers. The containment system is designed around the RPV and its primary system to be the final barrier to prevent accidental release of radioactive materials to the environment.

The Mark I containment system design is the one that was challenged most severely at Fukushima Daiichi and is indicated in Fig. 6. It is important to note that the containment system is not only a physical boundary but also a series of systems and components that are designed to prevent the release of radioactivity. As Fig. 6 shows, the Mark I containment comprises a building (drywell) where the RPV and primary system reside. The drywell is connected to another water-filled suppression chamber (wetwell) (shown in Fig. 6 with a water pool called the suppression pool) that is designed to condense any steam that may be accidentally released in any reactor accident. Further, the wetwell can be cooled over long periods of time to maintain lower pressures and temperatures to maintain its integrity. If this cooling is lost, the wetwell can be vented under controlled conditions by operator action to the atmosphere, where the suppression water pool filters out radioactive material before the release of gases by the vent. In the Fukushima Daiichi accident, the containments were challenged by an extended loss of emergency power for cooling and by delay in initiating the venting process, thus contributing to the failure of the containment and venting system to provide their intended function. The spent-fuel pool (SFP) is also shown and resides outside of the containment in the reactor building.

I.D. Boiling Water Reactors: Safety Systems

All BWRs have control rod drive systems that can be inserted to shut the reactor down. As a backup there is also a standby liquid control system consisting of a neutron-absorbing water solution (borated) that can be injected to shut down the fission chain reaction. After shutdown, the reactor continues to produce reductive low-level decay heat—from a few percent at shutdown, reducing to a fraction of 1% after 1 day—that must be removed in order to prevent overheating of the nuclear fuel.

In the event that the normal heat-removal pathway to the main turbine/condenser is lost, BWRs have, as the first backup, systems to provide core safety by either adding water to the RPV or by an alternate heat removal path, or by both. BWR/3s have isolation condenser systems that both remove the decay heat by condensing the generated steam in the RPV through heat exchange with a water pool outside the drywell and return condensate to the reactor over a wide range of reactor pressures. No additional water is added, however, so if there are leaks in the primary pressure circuit, additional water is required from other sources. BWR/4s and BWR/5s use an RCIC system, which is a turbine-driven pump using reactor steam that can add water to the RPV over a wide range of reactor pressures. The RCIC system draws water from either a large pool inside the containment, the suppression pool, or from a tank located outside the containment, the condensate storage tank (CST). The RCIC system has the advantage that it can provide significantly more water than needed to make up for decay heat–generated steam, but it does not remove the heat. When the reactor becomes isolated from the main turbine/condenser, that heat is transported to the suppression pool via safety and relief valves (SRVs) that open and close to maintain the primary system pressure within safety limits. There is sufficient heat capacity in the suppression pool for many hours of decay heat storage before the heat must be removed from the containment using pumps and heat exchangers requiring electrical power. If this does not occur, the pressure and temperature in the containment will rise as time progresses.

If these first backup systems are not sufficient, then ECCSs are provided to both add water to the RPV and to remove decay heat either from the RPV or from the containment. With one exception, all these systems require alternating-current (AC) power that is supplied either by the NPP normal AC distribution system or by emergency diesel generators (EDGs) if the normal supply is lost. The exception is that as part of the ECCSs in BWR/3s and BWR/4s, there is a high-pressure coolant injection (HPCI) system that is a turbine-driven pump that uses reactor steam and that has about seven times the capacity of the RCIC system and can add water over a wide range of reactor pressures.

As we discuss below, because for many hours the Fukushima Daiichi nuclear power station (NPS)2 was without electrical power and long-term cooling to remove the decay heat to the environment, the aforementioned systems were not available to keep the reactor core from overheating and the fuel from being damaged.

1All times in this report are in Japan time.
2Nuclear power stations comprise the grounds, the buildings, and the reactors that generate electricity. NPSs can have one or more reactors, which are referred to as units with numbers. For example, the Fukushima Daiichi NPS comprises six reactors, i.e., six units: Unit 1, Unit 2, Unit, 3, Unit 4, Unit 5, and Unit 6.