Factbox: What happens when a reactor loses coolant

(Reuters) - Several nuclear power reactors in Japan shut as designed following the massive earthquake off the nation's northwest coast, but the failure of a back-up power generator at one unit risks a radioactive leak.

Operators at Tokyo Electric Power Co (Tepco) are still working to control the situation at one of their plants.

Tepco said at about 2:46 p.m. local time Units 1, 2 and 3 at its Fukushima Daiichi plant -- three of six at the plant -- shut following the earthquake. All are boiling water reactors.

Tepco said the reactors shut due to the loss of offsite power due to the malfunction of one of two off-site power systems. That triggered the emergency diesel generators to startup and provide backup power for plant systems.

About an hour after the plant shut down, however, the emergency diesel generators stopped, leaving Units 1, 2 and 3 with no power for important cooling functions.

Nuclear plants need power to operate motors, valves and instruments that control the systems that provide cooling water to the radioactive core.

Tepco declared an emergency and the government ordered thousands to evacuate the area, while engineers worked to restore power. The company is bringing in mobile generators to restore the power supply, but pressure inside the containment of Unit 1 continued to increase.

Early on Saturday Tepco said it had lost the ability to control pressure at the No. 1 and 2 reactors.

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NORMAL OPERATIONS

* In a reactor operating normally, pumps circulate water through the reactor core to keep the rods from overheating. The temperature inside a reactor operating normally is about 550 degrees F (285 C).

See www.nrc.gov/reactors/bwrs.html on the U.S. Nuclear Regulatory Commission site for diagrams on how water moves through a boiling water reactor operating normally.

ORDERLY SHUT DOWN

* When a reactor shuts, pumps continue to move water over the fuel rods. The electricity to run the pumps usually comes from off-site power supplies brought in by transmission lines.

But, if the power lines fail, the plants have redundant on-site power sources, including backup diesel generators and batteries.

BLACK-OUT

* When all sources of power fail like at the Fukushima Daiichi plant, coolant begins to boil off, exposing the fuel rods. It would likely take several hours to boil off enough coolant before the core is hot enough to damage it.

If the fuel rods became damaged they would release radioactive material into the remaining coolant.

- Hours beyond that the metal surrounding the ceramic

uranium fuel pellets could potentially start

melting if temperatures reach well beyond 1000 F as

occurred at Three Mile Island. The ceramic fuel

pellets would not melt until temperatures reached

about 2000 degrees F.

- Hours after the fuel rods started to melt, the heat

could potentially melt through the reactor vessel,

which is made of high tensile steel four to eight

inches thick

- It would take even more time before the containment

fails, which could result in radioactive releases

to the environment. The containment is an air tight

steel or reinforced concrete structure with walls

between four to eight feet thick.

As the Three Mile Island accident showed, operators can regain control of the situation if they can get the coolant flowing again.

(Reporting by Scott DiSavino; Editing by Jonathan Leff)