On March 11, 2011 an earthquake and tsunami severely damaged four nuclear reactors at the Fukushima Number 1 power plant on the northeast coast of the Japanese island of Honshu.
The Unit Two reactor is a boiling water design fueled with about eighty tons of uranium dioxide in zirconium alloy fuel rods. The primary concrete containment vessel surrounds the core of the reactor and the secondary concrete containment vessel included upper levels which contained pools for storing fuel rods and irradiated equipment.
Around 3 PM Unit Two was shut down in response to the earthquake which shook the reactor and broke pipes. Around 5:30 PM all the electrical power generated by the reactor stopped. Emergency batteries were supposed to take over to provide power for monitoring and control systems but Unit 2's backup batteries were damaged when the tsunami struck and could not provide emergency power. Fifteen minutes later, TEPCO, the company that managed Fukushima Number 1, declared a Nuclear Emergency Situation because they could not confirm that emergency cooling systems were injecting coolant into the core of Unit 2. Radioactive steam was released into the secondary containment vessel to reduce pressure in the primary vessel.
At first, after the quake, TEPCO used the isolation condenser system to cool Unit 2 but after ten minutes, they shut down the isolation condenser and turned on the emergency cooling injection system which sprayed coolant into the reactor core. After a half hour, the loss of electrical power to the reactor disabled the spray cooling system but the operators were able to manually activate the cooling system. At 5 PM on March 12th, the cooling system shut down and restarted again at 9 AM on March 13th. Some building pressure was vented around midnight on March 12th. The operators worked problems with the fuel rod storage pool of Unit 2.
Around noon on March 13th there was an explosion in the Unit 3 reactor building next to Unit 2. The explosion blew holes in the wall of Unit 2 and damaged four of the five cooling pumps in Unit 2. The fifth pump shut down when its fuel was exhausted. By 9 PM on March 14th, the cooling system was still operating and power had been restored from a mobile generator.
The emergency cooling system was shut down by dropping pressure in the primary reactor vessel around 7:30 PM on March 14th. The fuel rods were exposed by dropping coolant levels and there were concerns about a possible core meltdown. The reactor was partly filled with water but rods were still exposed. The fuel rods were exposed because a monitoring gauge had been accidentally shut off preventing flow of coolant into the reactor. Seawater was pumped into the reactor on March 14th.
There was an explosion in the Unit 2 reactor around 6 AM on March 15 but TEPCO and the Japanese government continued to report no significant breach of the reactor vessel though temperature and radiation levels were dangerously elevated. It was revealed by TEMPCO in May that the fuel rods in Unit 2 had melted down around March 15th.
By March 26th external electrical power had been restored to Unit 2. Water was moved from the condensers in the reactor building to trenches and waste water treatment facilities. In May it was revealed that much of the cooling water injected into Unit 2 had leaked out of the containment vessel.
Work continued through the summer of 2011 to reduce the temperature in Unit 2 with mixed results. In September of 2011was still too high at 114 degrees Celsius and more water was pumped into the reactor.
In November, the detection of xenon-133 and 135 indicated that fission reactions were still occurring in the Unit 2 reactor. Huge amounts of boric acid were injected into the core to prevent the fissions reactions from achieving a self-sustaining criticality.
In February of 2012, the temperature began fluctuating again. Temperature monitoring was hampered by damaged thermometers which could not be replaced due to high radiation in the reactor building of Unit 2. However, the temperatures did not exceed the 100 degree Celsius limit for considering a reactor to be in cold shutdown status.
