Q+A-What's going on at Japan's crippled nuclear power plant?
TOKYO Oct 18 (Reuters) - The operator of Japan's tsunami-hit Fukushima nuclear power plant said on Monday that the amount of radiation being emitted from the complex has halved from a month ago, in the latest sign that efforts to bring the facility under control are progressing.
Operator Tokyo Electric Power (Tepco) said at a monthly review that the damaged reactors at the Daiichi plant, 240 kilometres (150 miles) northeast of Tokyo, were emitting about 100 million becquerels of radiation per hour, about one eight-millionth of the amount seen in the days after the March 11 disaster and half of what it announced a month ago.
It said the latest figure translates to about 0.2 millisievert per year of radiation measured at the fringes of the plant, below the 1 millisievert safety limit set out in government guidelines.
HOW HAS TEPCO GOT TO THIS STAGE?
The decline in emitted radiation follows a drop in temperature at the plant's three damaged reactors, which suffered nuclear fuel meltdowns in the first days of the crisis as the water meant to cool them evaporated after the tsunami knocked out their cooling systems.
"We see a relationship between the drop in temperature and the decline in the amount of radiation the plant emits. When temperatures at the reactors are high, it means that substances spread into the air along with evaporating water," said a spokeswoman for Tepco.
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 that 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.
The cooling progressed after Tepco alleviated this problem by building a system that decontaminates the tainted water and then reuses some of it to cool the reactors and spent fuel pools.
Temperatures at all three damaged reactors dropped below 100 degrees Celsius at the end of September, a state technically defined as cold shutdown as water used to cool nuclear fuel rods remains below boiling point and prevents the fuel from reheating.
Tepco has also nearly completed the construction of a giant barn-like structure to cover one of the three reactors, which it says will further limit the spread of radiation.
Tepco and the government have held off from declaring that cold shutdown has been reached although temperatures at all three reactors are below 100 degrees.
"We still need to proceed with care. We need to continue monitoring whether the temperatures of the reactors and radiation levels being emitted remain stable going forward," Yoshinori Moriyama, deputy director-general of the government watchdog Nuclear Industrial and Safety Agency, said to reporters on Monday.
Tepco has, however, brought forward its cleanup plan, saying it wants to declare cold shutdown has been achieved this year instead of by January as initially planned.
Declaring 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 the 80,000 residents evacuated from within a 20 km radius of the facility to return home.
Even if a cold shutdown is declared, Tepco has acknowledged that it may not be able to remove the fuel from the reactors for another 10 years and experts say cleanup at the plant could take several decades.