Japan's Nuclear Emergency Tied to Tsunami

There are 55 nuclear reactors generating power throughout Japan. They produce about 30 percent of the electricity needed by the country. Because space is limited in this island nation, the reactors are clustered in 18 sites.

The Japanese government has declared a state of emergency after cooling system failures at reactors at two sites in Fukushima Prefecture. At the Fukushima Daiichi site, two reactors are involved in the emergency. At the Fukushima Daini site, three reactors are involved. Both sites are on the east coast of the island of Honshu, with the Daiichi site being to the north of the Daini site on the North Pacific shore.

All the reactors involved are classified as "light water reactors." Water is used to cool the reactor and to generate steam. The steam powers turbines that generate electricity.

Both sites lost power due to a combination of the 8.9 magnitude earthquake that shook the region in the late afternoon of March 11 and the several tsunami waves that struck the coast after the quake. In the event that cooling pumps lose power in these reactors, a backup system of diesel powered generators is supposed to take over. It appears at this time the backup generators were flooded and unusable due to the tsunami.

Without proper cooling, the nuclear material continues to generate heat and also turns what water remains into superheated steam. The pressures involved are meant to be contained by the structure of the reactor, the containment vessel. Hydrogen is created at high temperatures, which is potentially explosive. The core itself could melt and fall to the floor of the container and possibly melt through the floor.

If the containment vessel is breached in some way, an explosion of steam and radioactive material may be released to the atmosphere. The combination of intense heat and steam pressure could cause a rupture, the core could melt and slump to the floor of the container or containment failure could be due to damage from the earthquake and the tsunami.

The Japanese government has evacuated citizens in a six-mile circle around each site. Various reports suggest radioactive steam may be vented from one or more reactors to reduce the pressure in the containment vessel. The government and the operators of the reactors are trying to cool them with pumps powered by battery, generators that have been brought in to the area and by using other, secondary, cooling systems.

The weather in Fukushima is expected to be chilly for the next few days with winds mainly from the west at 5-10 mph. Any brief release of radioactive material could be expected to be carried out to sea. An explosive release could create a situation similar to the Chernobyl disaster, where significant radiation was carried for hundreds or thousands of miles by winds at altitude.