March 15th, 2011 – Congressional Research Service – Fukushima Nuclear Crisis

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Summary of the Crisis

The earthquake on March 11, 2011, off the east coast of Honshu, Japan’s largest island, reportedly caused an automatic shutdown (called a “scram”) of eleven of Japan’s fifty-five operating nuclear power plants. Most of the shutdowns proceeded without incident. The plants closest to the epicenter, Fukushima and Onagawa, were damaged by the earthquake and resulting tsunami.

TEPCO immediately began to experience problems with the Daiichi units, as temperatures began to rise in the reactors. With the primary and secondary cooling systems for the Daiichi reactors offline, TEPCO began trying to cool the reactor cores with seawater. Boron6 has been added to the seawater to help slow down the nuclear reactions and cool down the reactor cores. Pressure began building in Daiichi reactor 1, resulting in an explosion on March 13, 2011, and radiation leak possibly from a build-up of hydrogen gas. Falling water levels in the reactor core are thought to have exposed fuel rods, leading to oxidation of the zirconium cladding resulting in the formation of hydrogen gas.

An explosion was reported at reactor 3 on March 14, 2011, with an associated release of radiation. At this time, while the containment structures at reactors 1 and 3 were breached, the reactor vessels themselves were thought to be undamaged. Falling water levels in reactor 2 and increasing pressure eventually led to another explosion on March 15, 2011, resulting in damage to the roof of the building above the reactor vessel and a release of radiation. It was unclear at that time whether the reactor vessel itself was damaged in the explosion. Fires were also reported at reactor 4, with the loss of water levels in the spent fuel pool. Elevated radiation levels measured around reactor 4 caused the temporary suspension of reactor control room operations on March 16, 2011. The spent fuel pool of reactor 3 was also reported to be boiling, with the reported release of radioactive steam. Water is also being introduced to the non-operational reactors 5 and 6 at the Daiichi station. The Japanese military may be enlisted to pump water into reactor 3 and the spent fuel pool in reactor 4.

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