Factbox: Japan's hidden nightmare scenario for Fukushima

TOKYO (Reuters) - Nearly a year after a huge quake and tsunami sparked Japan's Fukushima nuclear crisis, then-premier Naoto Kan is haunted by the specter of an even bigger disaster forcing tens of millions of people to flee Tokyo and threatening the nation's existence.

Two weeks after the crisis in March, the head of Japan's Atomic Energy Commission drew up a worst-case scenario. It was presented to Kan, but never officially released to the public.

Below are key points from the scenario document, obtained by Reuters, that was compiled by commission chairman Shunsuke Kondo and entitled "Sketches of Scenarios of Contingencies at Fukushima Daiichi Nuclear Power plant."

MULTIPLE REACTOR EXPLOSIONS, MELTDOWNS AT FUEL POOL

Multiple vapor and hydrogen explosions and a loss of cooling functions at the six reactors at Tokyo Electric Power Co's Fukushima Daiichi nuclear plant lead to radiation leaks and reactor failures.

Thousands of spent fuel rods, crammed into cooling pools at the plant, melt and mix with concrete, then fall to the lower level of the buildings.

CHAIN OF EVENTS

In a possible domino effect, a hydrogen explosion at one reactor forces workers to evacuate due to high levels of radiation, halting cooling operations at all reactors and spent fuel pools. Reactors and cooling pools suffer serious damage and radiation leaks.

TOKYO EVACUATION

Massive radioactive contamination forces residents in a 170-km radius or further to evacuate while those in a 250-km radius or further may voluntarily evacuate.

Tokyo, Japan's capital, is located about 240 km (150 miles) southwest of the plant and the greater metropolitan area is home to some 35 million people.

Radiation levels take several decades to fall.

WHAT ACTUALLY HAPPENED

The 9.0 magnitude earthquake and a tsunami exceeding 15 meters knocked out cooling systems at the six-reactor plant and meltdowns are believed to have occurred at Nos. 1, 2 and 3.

Hydrogen explosions occurred at the No. 1 and No. 3 reactor buildings a few days after the quake. Radiation leaks forced some 80,000 residents to evacuate from near the plant and more fled voluntarily, while radioactive materials have been found in food including fish and vegetable and water .

Reactor No. 4 was under maintenance and 550 fuel rods had been transferred to its spent fuel pool, which already had about 1,000 fuel rods. The pool caught fire and caused an explosion.

Reactors No. 5 and 6 reached cold shutdown -- meaning water used to cool fuel rods is below boiling point -- nearly 10 days after the tsunami but it took more than nine months to achieve that state at Nos. 1-3.

Decommissioning the reactors will take 30 to 40 years and some nearby areas will be uninhabitable for decades.

(Reporting by Yoko Kubota; Editing by Linda Sieg and Jonathan Thatcher)