A Banana A Day More Rafdioactive Than Fukushima
by Ian R Thorpe
CREATIVE COMMONS: Attribute, non commercial, no derivs.
KEYWORDS: technology, nuclear, recactor, fukushima, japan, radio activity, radioactive, radiation, fear and panic, banana, potassium, nature satire, humour, humor
There's a lot of fear and panic being spread about the nuclear power station accident at Fukushima.
The thing to remember about radiation is it's not as dangerous as fear and panic. Everything in nature is a bit radioactive; a few examples of radioactive substances are coal, people, cheese, poo, television screens (and yes, your computer screen that you are looking at now) and bananas.
We are all being bombarded with radioactive material all the time.
Radioactivity is measured by simply by measuring how often an atom decays and transmutes to another kind of atom, freeing some energy. The international unit used to answer the question “how radioactive is it?” is the Becquerel (Bq), named for Henri Becquerel who with Marie and Pierre Curie pioneered research into radiation. A Becquerel is one radioactive decay per second.
The amount of radiation needed to harm humans cannot be measured in Bequerel however, there are defined units of “biological equivalent dose” (Sieverts, named after a Swedish physicist - don't let anyone tell you scientists are not egotistical,) that tell us the effect of a dose on a person. This is defined by using a “quality factor” or “weighting factor” for different kinds of radiation such as alpha, beta or gamma rays. To cut out a lot of very technical stuff what we are looking for is the amount of absorbed energy which is measured in another international unit, Gray which is defined by the amount of energy absorbed instead of the ionization produced. Multiply the absorbed energy in Gray by the weighing factor which for emissions from a commercial reactor is usually 1, so 1 Gray of absorbed energy equals 1 Sievert (Sv) of human dose.
Because radiation is not so much complicated as messy, with lots of units named after scientists who wanted a unit named after them to affirm their status in the scientific pantheon. One of these was Rontogen who was something too do with X rays. From him we get RAD (Rontagen absorbed dose) the amount of radioation absorbed by air and then Rem,(Rontagen equivalent in man) or how much it takes to harm us. 1 mSv/hr = 0.1 rem/hr
On the morning of March 20 in Japan the dose rate at the main gate of the Fukushima complex (i.e. pretty close to the source of the radio active leak) was was around 1 micro sievert per hour (mSv/hr) Some locations, where some amount of radioactive material released appears to have landed and concentrated, it’s as high as 14 mSv/hr. A few places inside the plant were measured as high as 400 mSv/hr,
The normal background dose rate — the amount of radiation we absorb from the world around us — is 2.5-3.5 micro - sieverts a year (mSv/year) and that goes way higher in areas where there are nuclear power stations, bananas or granite bedrock. In Rajasthan, India the natural background dose rate is 260 mSv/yr — 100 times the “normal background” as I’ve defined it although I'm not sure whether that is down to granite, bananas or elephant poo.
And 2.5 mSv/yr is about 0.0003 mSv/hr, so clearly at even 1 mSv/hr the dose is considerably stronger than normal background radiation.
But it’s still not that strong; by comparison, a normal chest X-ray is about 0.1 mSv, a mammogram up to 3 mSv, a CT scan can be as much as 13-15 mSv: several times the normal yearly does in a few minutes. And these have to be well within the safety limit.
Now here's the part that confuses mainstream meeja science writers: that’s total dose, not dose rate. It’s the difference between setting the oven to 350, and baking at 350 for an hour. Think in terms of how long it would take to have the radiation affect your health. So at 1 mSv/hr, it would take 15 hours to accumulate the dose from one CT scan, and several weeks to absorb enough radiation to make much difference to health.
There is an interesting way of comparing different amounts of radiation, now we have established a few basics. Nuclear physicists and engineers sometimes use a unit called the “banana equivalent dose.” This is basically how it’s calculated.
Take an average sized banana.
Like pretty well everything in nature, including cheese and poo, bananas are slightly radioactive. Because bananas concentrate potassium, they are more radioactive than a many other foods — natural potassium includes a small proportion of the radioactive isotope potassium-40. That means eating a banana, and thereby ingesting an element vital to our nerve function, potassium, which adds a measurable radiation dose from the radioactive potassium-40.
If you are the type of person who listens to the fear and panic merchants you will now be swearing never to even look at another banana in case you die or grow two heads. Bananas aren’t the only food rich in potassium however. Potatoes are another very popular food that concentrates potassium and it is present in most fruits. If you eliminated potassium from your diet you would die a lot more quickly than radiation from a nuclear meltdown would kill you unless you were sitting on top of the reactor.
Working with The Banana Equivalent Dose not only takes some of the menace out of radiation, it means we can get some perspective on nuclear energy risks by comparing the total dose we get from a banana with other small amounts of radiation.
According to the CRC Handbook on Radiation Management and Protection (pg 620), a “reference banana” is listed at about 130 Bq per kilogram.
- A “standard banana” weight about 150 grams
- So we get about 20 Bq per banana.
When you eat your banana thus destroying evidence that you might be part of an Al Qaeda plot to explode a dirty bomb, the radioactive potassium enters your body; if you have enough potassium in your system the kidneys will flush it out within about a day, so for a short time you are a little bit more radioactive.
To really work out the dose-rate from being exposed to 20 Bq of Potassium-40, we’d need to calculate it based on distance, the specific energy of the emitted radiation, and so forth.
You will, I’m sure, be relieved that I’m not going to drag you through that.
Instead, I’ll refer to the RadProCalculator (you didn't think I did all this stuff myself did you? I hate mathematics), which tells us that one banana’s worth of radioactive potassium gives a dose rate of about 0.0037 µSv/hr. If the banana’s potassium stays in the body for about 24 hours, the total dose is 0.09 µSv.
- the banana equivalent dose rate is about 0.04 µrem/hr
- the banana equivalent dose is about 1 µrem
- eating one banana a day for a year is about 365 µrem/year
Looked at another way, one banana dose for a year is 365 µrem/300 mrem — or just over 0.001, one tenth of one percent, of the normal background dose.
20 µrem/hr, over the course of a year (8760 hours) is about 1762 mrem — or about 6 times normal background dose. Which is about one-tenth a single abdominal CT scan.
So a visit to your hospital for a scan is more likely to harm you than a year's radiation from an accident in a commercial reactor. And there are no records of anyone dying from a CT scan. Radiation from nuclear weapons is on a different scale altogether. There aren't enough bananas in the Caribbean archipelago to run a comparison.
There's an interesting graphic at this New Statesman page showing how dangerous nuclear power is per terawatt compared to coal or oil.
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