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

TOKYO, Sept 27 Tue Sep 27, 2011 6:02am EDT

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TOKYO, Sept 27 (Reuters) - Japan's nuclear disaster minister confirmed on Tuesday that the government soon wants to lift an advisory for some areas near the quake- and tsunami-hit Fukushima Daiichi nuclear plant, in a sign that the operator was making progress with its cleanup work.

Operator Tokyo Electric Power (Tepco) last week brought forward its goal of bringing crippled reactors at the plant to a cold shutdown.

Minister Goshi Hosono said in parliament that the government at the end of this week wants to lift an advisory placed within a 20 to 30 km (12 to 18 miles) radius of the plant, which had required residents to stay indoors or evacuate during emergencies.

The Japanese government and Tepco said at a monthly review of the Daiichi plant's cleanup timetable that they are now aiming to bring the plants to a cold shutdown within this year, instead of by January as initially planned, with their cleanup work proceeding steadily.

WHAT IS COLD SHUTDOWN AND HOW DOES TEPCO DEFINE IT?

A cold shutdown is when water used to cool nuclear fuel rods remains below 100 degrees Celsius, preventing the fuel from reheating. But even when the temperature at the last remaining reactor falls below 100 degrees, Tepco said it would not automatically declare that a cold shutdown has been reached.

Under its definition of a cold shutdown, Tepco also said that radiation leakage from the reactors has to be under control and that the public's exposure to radiation should be largely minimised.

Declaring a cold shutdown will have repercussions well beyond the plant as it is one of the criteria the government said must be met before it begins allowing residents evacuated from the area around the facility to return home.

Tepco said that the Daiichi reactors were emitting about 200 million becquerels of radiation per hour as of mid-September, about one four-millionths of the amount seen in the days after the March 11 disaster.

It said this translates to about 0.4 millisievert per year of radiation measured at the fringes of the plant, below the 1 millisievert legal limit.

To further limit the spread of radiation Tepco has been building a giant structure to cover the No 1 reactor. It will also equip all three reactors with devices that would filter out radioactive substances from the gasses they emit.

HOW HAS TEPCO GOT TO THIS STAGE?

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

Immediately after the disaster Tepco tried to cool the reactors by pouring in tens of thousands of tonnes of water, much of it from the sea. But this left a vast pool of tainted runoff, some stored in huge tanks and some in the basements of the reactor buildings, that threatened to leak into the ocean.

It alleviated this problem by building a cooling system that decontaminate the tainted water and then reuses some of it to cool the reactors and spent fuel pools. The system has repeatedly stalled but, as of Sept. 20, Tepco had treated about 97,000 tonnes of water. It estimates that 120,000 tonnes of highly radiated water has accumulated at the plant.

Temperatures at all four of its spent fuel pools had fallen to levels considered stable by August. As of Tuesday temperatures at all the spent fuel pools were below 40 degrees.

The temperature at the No. 1 reactor dropped below 100 degrees in July and that of the No. 3 reactor fell below the threshold at the start of September, leaving only the No. 2 reactor above boiling point. As of Tuesday the temperature at the No 2 reactor was 101.4 degrees.

WHAT IS HAMPERING TEPCO?

The decontamination system was built in a hurry from a patchwork of technologies from France, the United States and Japan 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.

Tepco also has to divert resources to other expensive and labour intensive tasks, such as building a wall underground to stop contaminated water from leaking into the ocean.

Providing a measure of how long the cleanup could take, Tepco recently said it wanted to remove fuel stored at spent fuel pools within three years and fuel from reactors within 10 years.

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 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 prompted then Prime Minister Naoto Kan to say Japan should wean itself off nuclear power, and the parliament passed a feed-in-tariff bill in August that promotes the use of renewable energy. (Editing by Chris Gallagher)

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