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A girl (above) sticks out her tongue at her mother as they are scanned for signs of radiation at the Azuma gymnastic hall in Fukushima, northern Japan, two Fridays ago. Radiation has been released into the atmosphere from the Fukushima nuclear plant on the country's north-east coast, which was badly damaged after a massive earthquake and tsunami on March 11. As part of attempts to prevent fuel from the plant from overheating and emitting dangerous levels of radiation, military fire trucks sprayed Unit 3 of the plant for the second day running last Friday. Unit 4 is in the background. The image above was taken from footage released by the Japan Defence Ministry
The nuclear disaster in Japan's Fukushima prefecture has thrown up many questions that range from the risks of radiation to food and travel safety. The Sunday Times asked readers to send their queries via our Facebook and Twitter pages. Feng Zengkun finds answers to their questions.
Q Why were the nuclear plants so vulnerable?
The short answer is: Japan was incredibly unlucky. The nuclear plants have three sources of electricity that operate their vital cooling systems: power lines, a back-up system that runs on diesel, and a second back-up system made of physical batteries.
The earthquake destroyed the power lines, and the tsunami that followed wiped out the diesel back-up system. The battery system is designed to run for only a few hours while the first two systems are being fixed, but the unprecedented scale of the disaster meant there was no time.
Q Why were the nuclear reactors not shut down after the earthquake? Are they still generating electricity now, despite the danger of overheating?
The nuclear reactors were shut down. The problem is that radio-active fuel at the reactors remains very hot even when the plants are shut down, and requires constant cooling to prevent a meltdown. But the cooling systems do not work now, because there is no electricity.
This is also why workers at the plant have been dumping sea water into the reactors - to cool the radioactive material.
Q What is the worst-case scenario?
The containment structures that hold the nuclear fuel explode, and the reactors go into total meltdown and keep burning. This could make the disaster equal to the Chernobyl nuclear disaster of 1986 in terms of lives lost and the extent of radio-active contamination.
Even in Chernobyl, however, significant radiation did not spread beyond 800km from the reactors.
Q Who is monitoring the impact of the disaster in Singapore? What are they doing?
The National Environment Agency is monitoring the radiation levels in Singapore's atmosphere.
The Agri-Food and Veterinary Authority is testing food from Japan imported into Singapore.
National water agency PUB is testing the water supply here.
The Civil Aviation Authority of Singapore is checking on the potential impact on flights to and from Singapore.
Changi Airport also has contingency plans to deal with potential radiation at the airport.
Q How far can a radiation cloud travel? Can it reach travel destinations like the west coast of the United States, Hong Kong, Taiwan, North and South Korea, Thailand and Malaysia?
In the Chernobyl incident, radio-active clouds wafted across to Finland, Sweden, Poland and Austria - as far as 800km away.
All the countries listed above are more than 1,000km away. South Korea - which is the closest to Japan at 1,200km - has reported no change in its radiation levels. Singapore, at 5,300km from Japan, is extremely unlikely to be affected.
In any case, the radiation levels at Chernobyl were about 50 times worse than those at Fukushima.
Q I have heard there are two radioactive substances that have leaked - iodine and caesium. Radioactive iodine has a short half-life and dissipates quickly, but what about caesium? Exactly how does exposure to radiation cause cancer?
Iodine dissipates in about three to four months, but if the amount is small, it can dissipate within a month. Caesium survives at least 30 years. Radiation - whether through caesium or iodine - damages cells in the body. This causes mutations that can lead to cancer, especially if the cell repairs itself wrongly.
The threshold for cancer risk is 1,000 millisieverts, when radiation starts to affect the immune system. At 2,000 millisieverts, the digestive system is damaged and hair starts falling out in clumps. Death occurs at levels above 5,000 millisieverts.
So far, the highest radiation levels recorded at Fukushima have been about 400 millisieverts, much lower than the 10,000 millisieverts recorded at Chernobyl.
Elsewhere in Japan, readings are negligible, measuring a harmless 0.03 to 0.04 microsieverts an hour in Hokkaido, Kyoto and Osaka. 1,000 microsieverts equals 1 millisievert.
This is a fraction of the radiation a patient is exposed to during a medical X-ray (0.2 millisieverts) or a CT scan (12 millisieverts). Exposure to the sun over a year results in 2 millisieverts of radiation.
Q Is there existing technology that absorbs radiation? Is it worth buying potassium iodide, iodine solution, table salt or any other substance that is supposedly anti-radiation?
Iodine pills that contain potassium iodide can be taken to prevent
radioactive iodine from being absorbed. These work by saturating the thyroid gland with harmless iodine so it does not absorb dangerous, radioactive iodine.
There is absolutely no truth to the myth that eating table salt blocks radiation. People are mistaken that table salt contains potassium iodide, which blocks radiation, but it does not.
There is no technology to prevent the absorption of radioactive caesium.
However, people who are not in Fukushima and its immediate surrounding areas are extremely unlikely to be affected by either radioactive iodine or caesium.
There is no need for people in Singapore to stock up on or consume iodine pills, because Singapore is extremely unlikely to be affected by radiation from Japan.
Potassium iodide only works in the short term and is not a preventive treatment, as many hoarders in other countries seem to think. It can also cause allergic reactions, stomach upsets and swellings.
Q Is radiation 'contagious'?
Yes. People who are externally contaminated with radioactive material can contaminate other people or surfaces they touch. People who have radioactive dust on their clothing may spread the dust when they sit on chairs or hug other people.
People internally contaminated can expose those near them to radiation from the radioactive material in their bodies. The bodily fluids of an internally contaminated individual can contain radioactive materials. Coming into contact with these fluids can result in contamination.
Q Should I therefore avoid everyone who has just returned from Japan?
It depends where they were in Japan. People within a 20km zone of Fukushima should be tested for radiation at any public hospital, but people outside the zone are unlikely to be infected.
So far, in Singapore, about 12 returning travellers from Japan have tested negative for radiation.
Q Will there be acid rain? Is it corrosive?
Acid rain cannot be caused by Japan's nuclear fallout. Acid rain is caused by sulphur dioxide and nitrogen oxides, whereas the radio-active material from the nuclear plants is primarily radioactive iodine and caesium.
But the nuclear fallout can cause radioactive rain in the area near the reactor sites, which is also dangerous because it may cause cancer.
Radioactive rain is likely to be confined to Fukushima and its surrounding areas. There is too little radioactive material from the plants - even if the clouds were to travel they would lose their radio-activity quickly.
Q Is flying over the Pacific Ocean risky, especially with wind blowing radiation over it?
No. A plane's body and computer systems are radiation-resistant.
Also, a cloud with enough radioactive material to affect passengers in planes would be visible as a black cloud due to the concentration of particles, and planes would not fly through it.
Q What about the ocean? Can the radiation blowing out into the Pacific Ocean fall as rain and pollute fish and other produce from the sea?
No. The radiation currently produced by the reactors is too small. Any amount that falls into the ocean would be so diluted as to become harmless.
Q What does 'contaminated land' mean? Is the water contaminated? What about livestock and vegetables? I have heard that in the Chernobyl disaster, many people got sick from drinking tainted milk. Should I avoid all Japanese dairy products?
Contaminated land is land infected by radioactive material. Anything that radioactive particles fall on - including water, livestock, grass and fruit - becomes dangerous and can cause cancer if eaten or drunk in large quantities.
Experts say, however, that the current radiation from the nuclear plants is too small to affect food supplies throughout Japan. Singapore also conducts checks on every shipment of food imported from Japan.
Q This seems to be earthquake season. Will there be more earthquakes? Where are the fault lines and should I avoid them?
It is impossible to forecast earthquakes, but experts say there are several areas where high-magnitude earthquakes may occur in the next decade. These are:
Some experts believe earthquakes occur in clusters because of chain reactions in the earth, but this has not been scientifically proven.
Q When will it be safe to visit Japan again, and Tokyo in particular?
Radioactive iodine can survive for four months and radioactive caesium for 30 years, but any long-lasting damage is likely to be confined to Fukushima if the disaster does not worsen.
Readings taken outside Fukushima - in Tokyo, Hokkaido, Kyoto and Osaka, for example - have registered safe levels of radiation so far.
Q What can we do to help the tsunami and earthquake victims? In the light of reports that they are very short of fuel and food, how can fuel and food donations reach them?
There are collection tins at all FairPrice supermarkets and stores such as Cold Storage, Shop N Save, Giant hypermart and 7-Eleven, as well as Guardian Health and Beauty stores.
Cash donations can be made to the Singapore Red Cross Society in Penang Lane from Mondays to Fridays from 9.30am to 9pm, and on Saturdays and Sundays from 9.30am to 6pm.
Cheque donations can be made payable to the Singapore Red Cross Society and sent to 15 Penang Lane, Singapore 238486. You should indicate at the back of the cheque the following: Japan Disaster 2011, the donor's name, contact number and address.
SMS donations can be made by sending a text message to 75772. For every text message, $50 will be donated to the fund. Each SMS carries a charge of 30 cents.
From today to April 13, customers of OCBC Bank, DBS Bank and United Overseas Bank can donate via ATM, mobile and Internet banking. Donors can find out more by logging on to the banks' websites.
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