Dose = total amount of radioactivity absorbed by the body over a certain period.
− microSievert (µSv)
− milliSievert (1 mSv = 1000 uSv)
− Sievert (1 Sv = 1000 mSv).
Dose rate = the amount of radioactivity absorbed per hour, expressed in
− micro Sievert per hour (µSv/h)
− milli Sievert per hour (mSv/h =1000 µSv/h)
Assessment of potential radiological dispersion
As of March 18th, the situation at the Fukushima 1/Daiichi nuclear power plant was still unfolding and could still worsen; the exact radiological impacts are as yet unknown, but more information on radioactive fallout and contamination of food and water is continuing to emerge. Here we describe the potential impacts, based on available information. We advise populations in potentially affected areas to take appropriate measures to ensure exposure to radiation is kept as low as possible.
Radioactive materials have been released into the environment, with a continuing risk that much more will be released in the coming days. The direct radiation from the damaged plant was also reportedly high - potentially resulting in radiation dose rate levels up to 10 microSv/hr at a distance of 10 km from the plant.
The radioactive materials released into the air from the plant are spreading over a large area in the form of radioactive ‘clouds’. The precise source of the releases, the distance, the altitude of the cloud, the wind direction and the weather conditions determine the dispersion of the radioactive cloud and how and where it deposits radioactive particles on the ground. This is very difficult to predict, but impacts have already been detected up to Tokyo, 250km south of the Fukushima plant.
The total amount of radioactivity that could theoretically be released over the course of the crisis is linked to the total amount of radioactive material in the damaged reactor cores and spent fuel pools. Radioactivity can be released in separate ‘batches’ depending on the developments in each part of the power plant. This would result in separate release- events of radioactive clouds that could be produced over several days or weeks.
Each release event has the potential of a radioactive cloud being transported in different directions depending on weather conditions. Greenpeace continues to monitor the weathered forecast to get a better indication of the dispersion and direction of such radioactive clouds.
Dose rates of a few hundred milliSieverts per hour (mSv/h) have been reported close to the reactors in Fukushima. Dose rates in the order of 150 microSieverts per hour (µSv/h) have been measured 30km from the plant. Areas up to several hundred kilometres from the Fukushima plant have already been contaminated with radioactive material.
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Potential radiation risks
The risk of radiation exposure can be divided into two categories:
- External radiation from radioactive elements outside of the body;
- Internal contamination of the body with radioactive elements that enter the body.
Exposure to radioactivity has been linked to genetic mutations, birth defects, cancer, leukaemia and disorders of the reproductive, immune, cardiovascular and endocrine systems. High doses of radiation (> 1 Sievert) can result in immediate health effects and even death. Health effects of low doses of radiation will only become visible over the longer term. Internal contamination with radioactive materials often results in significant radiation exposure because the substances can stay trapped in the body for long periods of time and continue to emit radiation.
In the vicinity close to the plant (<10km), direct radiation from the Fukushima site is significant. The closer to the plant, the higher the radiation levels. People who stay in this area for longer periods (for days, weeks or months) can receive an accumulated dose that can result in medium to longer-term health risks.
A radioactive cloud can cause direct and indirect radiation exposure. People who encounter a radioactive cloud will directly be at risk from (1) the external radiation dose of the cloud and (2) the inhalation of radioactive particles. Indirectly, there is a risk of inhalation of radioactive particles that were deposited and re-suspended, and of ingestion of radioactivity spreading in the food chain.
The exposure to radiation and the resulting health risks depend on many factors. Currently the magnitude of the radioactive releases from Fukushima nuclear plant are largely unknown, but experts (IRSN in France, ZAMG in Austria) have estimated that the releases up to now add up to 10 – 20% of the total radioactivity that was released during the Chernobyl accident. The contamination is includes long-lived isotopes like Caesium-137, which remains radioactive for 300 years.
On 23 March, the Tokyo Metropolitan Government announced that iodine that exceeded that allowable amount for infants had been detected in Tokyo tap water. Tests conducted by the Tokyo Metropolitan Government identified 210 becquerels of iodine-131 per 1 litre of tap water in the city, more than twice the limit of 100 becquerels considered safe for infants.
The IAEA reportedly found radioactive surface contamination (fallout) of 900,000 MBq/km2 in the area 35-68km from the damaged Fukushima nuclear plant. In the Ibaraki prefecture, about 150km south of the plant, 93,000 MBq/km2 (million Becquerel per square kilometer) iodine-131 and 13,000 MBq/km2 caesium-137 was reported. Similar contamination levels were found in large parts of Europe and Russia after the Chernobyl nuclear accident in 1986. The contamination levels close to the plant are consistent with the estimated radioactive releases from Fukushima thus far, being estimated by the French radiation protection and nuclear safety institute IRSN to be about 10% of the total radioactivity that was released during the Chernobyl accident.
The first official analyses of soil contamination published by the authorities show 163,000 Bq/kg of caesium-137 and 1,170,000 Bq/kg of iodine in soil 40km from Fukushima nuclear plant. The Japanese Nuclear and Industrial Safety Agency NISA confirmed that the soil might have to be cleaned up. However, when large areas show similar of higher levels of contamination, clean-up is difficult, if not impossible, and some areas could remain unsuitable for future cultivation.
Japanese authorities have started reporting on the contamination levels found in 11 different vegetables. In many vegetables such as broccoli and cabbage, from the Fukushima prefecture -, the most contaminated area, radioactivity levels exceeded safety limits set by the Ministery of Food and Safety. In Motomiya, 50 km East of the plant, the Caesium-137 concentration in ''kukitachina'' leaves was detected to be 82,000 becquerels per kilogram, 164 times the limit. The government asked consumers not to eat the 11 vegetables, and food exports from the contaminated areas have been banned.
It is clear that more special measures will need to be taken, potentially for a long time, to protect the local population in the area around the Fukushima plant and in other contaminated areas. The information available at this moment is insufficient to determine if people will be able to return to the area.
For children (especially at young age) and pregnant women, the potential health risks are higher because of the sensitivity of the reproducing human cells in their bodies, it is of paramount importance that this group takes all necessary precautions.
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Radiation protection advice
In order to keep radiation exposure as low as possible:
- People should stay away from the area around the Fukushima nuclear plant and other contaminated areas. All people in a 30km zone around the Fukushima plant have been evacuated, and the US government advises American citizens to evacuate from an 80km zone around the plant, after an assessment by the US nuclear safety authority.
- Pay close attention to national radio/tv for possible announcements on the risk of a radioactive cloud nearing your area. Listen to official announcements on radiation protection measures.
- In case a radioactive cloud is coming to your area, stay indoors with windows, doors and air inlets closed. Do not go outside unless it is absolutely necessary. If you have been outside, wash thoroughly.
- In areas where elevated radiation is reported, avoid drinking locally produced milk and wash vegetables carefully. The UK Embassay in Japan has additional advice on food, milk and water.
- When instructed by officials (and only at that time!), you can take iodine pills to prevent radioactive iodine from accumulating in the thyroid. Follow instructions for use, and seek medical advice in case of pregnancy, allergies and certain medical conditions and for young babies (the pills may have significant side effects).
Remember that iodine pills will only protect against radioactive iodine, NOT against other radioactive isotopes. Hence it is still important to stay away from contaminated areas and stay indoors in case of radioactive clouds even after taking iodine pills.
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BASICS OF RADIATION
1. What is nuclear radiation?
There are several types of radiation: heat, light, microwave and nuclear. Nuclear radiation can occur in different forms (see 5). The common characteristic of all types of nuclear radiation is that it is so energetic that it can destroy molecules. Heat and sunlight are unable to do such damage.
2. Why is nuclear radiation dangerous?
Radiation (from here, we will use the word ‘radiation’, to mean nuclear radiation) can destroy molecules, including the molecules in our bodies. When DNA-molecules in our cells are destroyed, this creates a run a risk of developing cancer. Radiation is therefore called carcinogenic: it causes cancer. The specific problem with radiation, compared to other carcinogenic substances (i.e. chemical etc.) is that there is no 'safe dose' below which there is no effect. (see 9)
3. What is the origin of radiation?
Radioactive substances emit radiation. These substances are made of non-stable atoms that disintegrate into stable atoms. During disintegration the atom emits energy in the form of radiation. This process is also called radioactive decay.
4. What is radioactivity?
The radioactivity of a substance is the indicator for the amount of disintegrations of atoms in the substance. When there is a lot of radioactivity in a substance, the substance is called very radioactive, and there can be two reasons for this:
1.The substance has many of radioactive atoms within it
2.The radioactive atoms in this substance decay very quickly.
A combination of both makes for the most radioactive substances.
5. What kind of radioactive atoms exist?
Three categories of radioactive atoms exist: gamma-emitting atoms, beta emitting atoms, and alpha emitting atoms, resulting in three types of radiation: gamma, beta and alpha radiation. Most radioactive atoms send out a combination of these three types of radiation, with one type being the most dominant.
6. What's the difference between gamma, beta and alpha radiation?
Gamma radiation is a 'wave', just like light, but with a more energy. Beta radiation consists of small particles (electrons) travelling at an incredible speed, Alpha radiation consists of big particles (two protons, two neutrons altogether), also travelling with incredible speed.
More important is the difference in 'destructive power'; alpha radiation has the most 'destructive power' and gamma-radiation the least. Fortunately, radiation with the greatest 'destructive power' does not travel far, because it loses energy rapidly. Alpha particles, for instance, do not penetrate through skin, or even through a piece of paper.
7. What is radiation dose and how is it influenced by different radiation-types?
The radiation dose is the amount of energy that the radiation gives to the body. The dose is the indicator of the risk, thus a high dose is a high risk. A radiation dose is received when radiation travels through the body and hits cells and organs. This explains why alpha-emitting atoms are not dangerous outside of the body; the radiation coming from them doesn't even enter the body. However, if for instance a plutonium-atom is taken up in an organ - the alpha-radiation it gives off, in the form of large particles, will create a significant damage in the body. The radiation can have an impact on everything (DNA, cell membranes) it encounters and therefore result in a relatively high dose.
Gamma radiation travels right through your body, but the chances of it 'hitting' something are quite small and therefore does not deliver such high doses. However all gamma- emitting atoms, whether inside or outside of the body, add to the radiation risk.
8. What is radioactive contamination?
Something/somebody becomes contaminated when radioactive elements (particles varying in size, but sometimes as small as a few atoms) are present on it (external contamination) or in it (internal contamination). Remember that radioactive elements behave like all elements: they can end up anywhere: dust, food, furniture, humans etc. Like normal 'dirt' most of the contamination is washable, after which the person/material is called decontaminated. Internal contamination is a bigger problem: both because the risk from alpha-emitting atoms is present and because it’s more difficult to remove.
9. What is a safe dose-limit?
As stated in the beginning: there is no safe dose of radiation. Next to the need for keeping the radiation dose as low as possible, internationally accepted limits are set for members of the public for additional doses, excluding, background or natural radioation, is set at 1 milliSievert per year. For nuclear workers this is 20 milliSievert per year. To compare, the global average for natural radiation doses is 2.4 milliSievert per year.
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UPDATES AND MORE INFORMATION