Q&A: what is the radiation risk and how far will the danger spread?

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This was published 13 years ago

Q&A: what is the radiation risk and how far will the danger spread?

Nearby residents are protecting themselves as engineers work hard to cool the reactors at Fukushima, writes Ben Cubby.

By Ben Cubby

What levels of radiation have been detected?

Dangerously high radiation levels were present immediately outside the Fukushima plant.

Trying to stay safe ... a one-year-old boy is rechecked for radiation at Nihonmatsu, Fukushima.

Trying to stay safe ... a one-year-old boy is rechecked for radiation at Nihonmatsu, Fukushima.Credit: AP Photo/Asahi Shimbun

Radiation exposure is measured in units called ''sieverts'' and people immediately around the power station were exposed to about 400 millisieverts in an hour yesterday. The immediate effects of exposure at this level can be nausea and loss of white blood cells, but it is unlikely to kill people unless they were exposed for a long time. Normal background radiation is about three millisieverts a year, so the dose of 400 an hour represents a highly concentrated blast of radioactivity.

A standard CT scan in a hospital is the equivalent of about seven millisieverts.

An official ushers people through a radiation scanning centre in Koriyama.

An official ushers people through a radiation scanning centre in Koriyama.Credit: AP Photo/Mark Baker

How does it affect people?

The immediate effects of the high dose from the nuclear plant on people close by could include symptoms of radiation sickness - nausea, weariness and vomiting. A radiation dose of about 100 millisieverts a year, about a quarter of the level detected in an hour yesterday morning in Fukushima, is known to raise the chances of a person contracting cancer at some point in their life. Iodine-131, which has been detected in small amounts, can contaminate water and milk. It lodges in the thyroid gland and can cause cancer. It lasts in the environment for about three months. Cesium-137 can also cause cancer. It lasts for about 30 years in the environment and can accumulate in the food chain.

What is the danger close to the plant?

The Japanese government conceded for the first time yesterday that radiation was posing a serious health risk for workers at the Fukushima plant. Of the 800 workers, all but 50 were evacuated, and those remaining are undertaking rotating shifts and wearing charcoal-lined suits that offer fairly good protection. Three people have been treated for radiation sickness and about 200 people have been exposed to unsafe levels and taken to hospital.

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What is the danger in Tokyo and other areas of Japan?

The risk in Tokyo, and most other centres outside the 20-kilometre exclusion zone, remains very low at this stage. Radiation levels about 30 times the normal background level were detected yesterday morning and last night, as a plume of slightly contaminated air was carried away from the Fukushima plant by the wind. But this is nowhere near enough to cause health problems - it would not be noticed by most people unless they were exposed for a very long time.

How can people protect themselves?

People outside the exclusion zone but within 30 kilometres of the plant are being told to stay indoors, shut windows and turn off any functioning airconditioning systems, to stop the movement of air carrying radioactive particles into their homes. They are being warned not to drink water or eat food that might have been exposed to the outside air. If people have to go outside, they are being advised to wear face masks, and an outer layer of clothes that can then be taken off before they come back inside, so that any radioactive particles are left outside.

Is there a risk of contamination spreading outside Japan?

At this stage there is no likelihood of serious radioactive contamination outside Japan, and no immediate danger to other countries. If the reactor cores in the Fukushima powerplants remain intact, the serious effects can be confined mainly to the 20-kilometre radius around the nuclear plant.

If one or more reactors melt down, this will change. It would then be possible that plumes of radioactive dust could move offshore and affect other countries around the North Pacific, including Russia, China, Canada and the US.

How long will the danger last?

If the reactor cores are not damaged, the main risk is expected to pass by the end of this week. However, steam containing radioactivity from inside the cooling system may have to be released into the air for weeks to come. This steam would quickly dissipate in the air, but there would be a continuing risk of exposure to high radiation levels close to the plant. Spent fuel rods stored near the reactors pose a separate, serious risk, and are capable of releasing high levels of radiation. Smoke from the fire at the fuel rod storage area is likely to be highly radioactive and some particles of smoke and ash could be transported for long distances in the wind.

What caused the explosions?

The first two explosions at the Daiichi No. 1 and No. 3 reactors were caused by hydrogen collecting inside the buildings. The hydrogen was created as the reactors heated up, and it was released along with steam to stop too much pressure building up inside the reactor itself. The explosions appear not to have cracked the steel reactor cores, even though they damaged the buildings around them. A third explosion yesterday morning at the No. 2 reactor may have been caused by hydrogen, however this has not been confirmed. It is thought to have ruptured the cooling system of the second reactor, complicating efforts to stop the fuel rods inside from heating up.

What is a ''partial meltdown'' ?

This is when the fuel rods inside a nuclear reactor, which are usually covered in deep water, overheat and start to melt, emitting large amounts of harmful radiation. A full meltdown takes place when they disintegrate and generate enough heat to melt the metal walls of the reactor itself, releasing big quantities of radiation outside the power plant. Experts believe a full meltdown of one or more of the reactors is now highly unlikely, because the fuel rods have now been cooling down for five days.

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What is happening now at the Japanese reactors?

Engineers in the armoured control bunker at the Daiichi plant are adjusting water levels to try to keep the nuclear fuel rods cool enough to stop a total meltdown. They are using improvised pumps to direct seawater into the reactor chambers. They will also continue to vent radioactive steam into the air, to stop too much pressure building up inside. Fire trucks have been deployed to help pump enough water. Most of the staff have been evacuated because of the high levels of radiation detected yesterday.

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