Q+A: What's going on at Japan's crippled nuclear power plant?

TOKYO (Reuters) - Tokyo Electric Power (Tepco), the owner of the quake and tsunami-hit Fukushima Daiichi plant, said on Monday it had successfully begun to cool one of two spent fuel pools that were still considered unstable.

That brings it another step closer to its aim of bringing the plant's reactors to a state of cold shutdown and stabilizing the spent fuel pools by January.

PROGRESS SO FAR?

After cooling systems were knocked out on March 11, causing meltdowns of nuclear fuel rods at three of the plant's six reactors and triggering the world's worst nuclear crisis in 25 years, Tepco has been trying to cool the plant's reactors and four of its spent fuel pools.

In July it completed the first stage of its plan to bring the plant under control, bringing down temperatures at its reactors and at its No. 2 and No.3 spent fuel pools to levels considered stable with the help of newly built cooling systems.

On Monday, Tepco said it had also successfully reduced the temperature of the No. 4 spent fuel pool by more than 20 degrees Celsius to 63 degrees, a day after it activated a circulatory cooling system. It aims to bring down the pool's temperature to between 30 and 40 degrees.

Before installing the new system, Tepco had cooled the No. 4 reactor spent fuel pool by injecting water from giant pump trucks.

The spent fuel pool at the No. 1 reactor, the only remaining one without a circulatory cooling system, remains relatively unstable but Tepco is aiming to install a cooling system later this month.

CONTAMINATED WATER CLEAN-UP STILL NEEDED

Tepco is working to bring the reactors to a cold shutdown by cleaning contaminated water that has accumulated at the plant and then running it through new cooling systems.

As an emergency measure early in the crisis, Tepco cooled the reactors by pumping in tens of thousands of tonnes of water, much of it drawn directly from the sea. Some of that water was stored in huge tanks and some in the basements of the reactor buildings and threatened to leak into the ocean.

Drawing on technology from French, U.S. and Japanese companies, Tepco completed a system to decontaminate the accumulated water and pump it back to cool the reactors.

The system started on June 17 and has repeatedly stalled but, as of July 31, Tepco had treated about 33,600 tonnes of water. It estimates that 120,000 tonnes of highly radiated water has accumulated at the plant.

WHAT IS HAMPERING TEPCO?

The decontamination system was built in a hurry from a patchwork of technologies and its very complexity -- it has to remove oil and radioactive substances and desalinate the water in different steps -- has left it prone to breaking down.

The cooling system uses 4 kilometers (2.5 miles) of plastic piping that snakes through the compound, which is a headache to maintain.

The summer heat is another challenge for the workers, with the threat of dehydration and heat stroke that much greater due to the protective suits and masks they have to wear.

Even after the plant is under control, clean-up work at the site is expected to continue for years if not decades.

WHAT ARE SOME OF THE CONSEQUENCES OF THE DISASTER?

Nearly 80,000 people have been forced to evacuate their homes, most of them from a 20-km (12-mile) radius around the plant.

Living in fear of radiation has become part of life for residents both near and far from the plant.

The crisis has prompted Japanese Prime Minister Naoto Kan to say he believes Japan should wean itself off nuclear power and to call for a bigger role for renewable sources such as solar power.

It has also hampered efforts to restart reactors idled for regular checks, raising the possibility of power outages during peak demand periods.

(Reporting by Shinichi Saoshiro; Editing by Edwina Gibbs)