The Fukushima nuclear accident refers to the events in the Fukushima prefecture of Japan on March 11th, 2011 and the days following. Different articles discuss the radiation release from the Fukushima nuclear accident and the health effects of the Fukushima nuclear accident.
Many of Japan's nuclear reactors are boiling water reactors (BWR) (see figure 1). These types of reactors use light water as both the coolant and moderator in the reactor core. The fuel, enriched uranium dioxide, is kept in the reactor pressure vessel filled with water that is moved from the bottom to the top of the reactor pressure vessel, removing heat as it goes. The separator located on top of the reactor pressure vessel removes the steam which is at about 290°C and over 1000 PSI, and feeds the turbine housing to spin the generators that produce the electricity. The steam exiting the turbines is then condensed back to a liquid form, then reheated and sent back into the reactor core as depicted in the image. The two re-circulation pumps are key in maintaining the reactor core temperatures down at the normal operating temperature. The control rods that are inserted from the bottom of the core also help regulate the fission events.
On March 11th, 2011 a magnitude 9 earthquake struck the coast of Japan, followed by a tsunami that was reported to be as high as 15 meters at some coastal regions. At the time there were 3 reactors (Units 1-3) operating in Daiichi, while 3 other reactors (Units 4-6) were in shutdown. However reactor 4 was shut down not long prior to the event, thus was still in the cooling process and had not achieved the cold shutdown state. Figure 2 illustrates the Fukushima site before the accident.
Sensors on reactors 1 through 3 immediately sensed the earthquake and the control rods were automatically inserted into the core to shutdown the reactors. This was successfully done, however the reactor needed cooling to remain in operation so that the fission products can cool down to a cold shutdown state in which the water in the reactor core is no longer evaporating. However the AC power line running to all 6 reactors was disturbed during the earthquake. This prompted the backup diesel generators to start up so that the pumps would allow recirculation of the coolant in the reactors. The tsunami that arrived 50 minutes after the earthquake caused the backup diesel generators that were housed in the turbine building to flood and thus interrupting the power supply to the coolant pumps. This led to the Loss of coolant accident (LOCA) that caused the temperature and pressure to rise in the reactor core.
The pressure started to increase and exceeded the design limit. At the same time the fuel rods were no longer submerged in coolant and started to heat to temperatures that resulted in the meltdown of the fuel rods and its components. This melting down produced a magma called Corium. This corium melted its way down to the reactor vessel, and then pierced through to the concrete base mat. At the same time the melting of the zirconium fuel rods reacted with the water producing hydrogen gas denoted by this equation: Zr + 2H2O → ZrO2 + 2 H2 . In the containment vessel the hydrogen gas did not react due to the presence of nitrogen, since nitrogen is an inert gas that acts as an oxygen suppressant. The pressure kept rising to dangerous levels in the reactor vessel, this prompt the opening of the vent valves, allowing the pressurized gas to vent to the suppression pool that was filled with water. The pool would act as a filter by trapping much of the radioactive elements. However due to the high temperature and pressure of the gas, the water started to boil in the ventilation pool losing its filtering properties. The pressure continued to rise, causing the containment vessel go into an over pressure situation, in which the operators had to release the gas into the atmosphere. The gas contained hydrogen and had leaked into the upper level of the reactor building. At this point the gas mixed with oxygen causing the upper level of the building to explode violently. The hydrogen explosion was thought to have been caused due to a back flow of vent gas into the reactor building due to the malfunctioning of the anti-flow valve. According to Japanese officials, the containment vessel had not been breached. On March 12th following the explosion of Unit 1, the core cooling was done via external pumps and fire hydrant lines using sea water as an emergency measure. Later the sea water was replaced with fresh water to avoid corrosion. On March 14th Unit 3 also blew off the reactor building upper level in the same manner. Following this, On March 15th Unit 2 did not blow up, but a noise was heard near the pressure suppressant chamber, followed by a drop in pressure. At this point it was believed that the gas had also leaked into the atmosphere even though the reactor did not explode.
These reactor units were shutdown for scheduled maintenance during the incident. However the unit 4 reactor was not in cold shutdown state. This is the state at which the water stops evaporating in the reactor core following a shutdown of a reactor. Until this point is achieved, a reactor must have coolant flowing through the core at a constant pace. The fuel rods of reactor 4 were already in the spent fuel rod pool. However the reactor building also exploded because gas had leaked from unit 3 to unit 4 through a conduit that connected the buildings. Units 5 and 6 were already in a cold shutdown state prior to the earthquake and hence were not affected during the incident.