[Summary: Much has been reported about the presence of radioactive Iodine-131 and Caesium-137 following the problems at the Fukushima nuclear plant. However, nothing has been said about the arguably more dangerous isotope, Strontium-90, which accumulates in the body's bones. I have been puzzled by this for the last month. Strontium-90 is produced by nuclear plants at about double the volume of Iodine-131. As Iodine-131 levels have been reported as very high around Fukushima, I was sure that Strontium-90 must be present as well. You could say that I had a feeling in my bones! I have been following up the issue with the IAEA and ARPANSA. Today, they got back to me confirming that elevated levels of Strontium-90 have been detected around the Fukushima region.]
Walking around Tokyo you cannot help but notice that a significant number of people are wearing face masks. At a guess, I would say about 10% of people but that is just a guess. I had seen this in other Asian countries and knew that it was because the wearer of the mask:
had a cold and didn't want to spread it;
suffered hay fever and didn't want to breath in pollen;
didn't want to catch other people's germs; or
all of the above.
Still, there were more masks than I expected. So I asked a few mask wearers why they were wearing them. They said that it was for the above reasons (particularly (2) since it's Spring time in Tokyo) but also because they were concerned about radioactive material in the air from the Fukushima nuclear plant.
I was surprised by this. Despite some sensational reports, I always thought the risk of background radiation was minimal. The background radiation levels increased in Tokyo a few weeks ago before falling away again. Even at their peak, they were still far less than the background radiation levels in many other parts of the world and definitely beneath harmful levels. From my calculations, the background radiation in Tokyo peaked at around 0.95mSV per year
i. In parts of India and Brazil, the background radiation is 35mSV per year while in parts of Iran it is 260mSV per year. The average background radiation worldwide is around 3mSV per year
ii. So at its peak, Tokyo's background radiation was less than a third of what the average person is exposed to.
One thing which has been puzzling me is the presence of Strontium-90 in Japan. There has been a lot said about the presence of two other isotopes
iii, Iodine-131 and Caesium-137, around the Fukushima region but nothing about Strontium-90. I found this strange because Strontium-90 is in fact more dangerous than both Iodine-131 and Caesium-137. Please bear with me while I explain why.
There are three factors to consider when determining how dangerous a particular isotope is:
its half-life;
where it accumulates in the human body; and
whether we can stop it accumulating in the human body.
Half-life
Iodine-131 has a “half-life” of around 8 days. This means that in 8 days approximately half of a sample of Iodine-131 will convert itself into another element and in the process emit a set amount of radiation. This conversion is called “decay”. This decay process is continuous. So after 16 days, 1/4th of a sample of Iodine-131 will remain and after a further 8 days 1/8th will remain. After a few weeks, a sample of Iodine-131 will have largely disappeared. Caesium-137, on the other hand, has a half-life of around 30 years.
So what does this mean? In short, a sample of Caesium-137 will remain in the environment emitting radiation for generations while a sample of Iodine-131 will have largely vanished after a few months. The longer an isotope remains in the environment, the greater the probability of it being consumed by a human. But what would happen if you consumed Iodine-131 or Caesium-137? This leads us to the second factor mentioned above: Where does an isotope accumulate in the human body?
Where does an isotope accumulate in the human body?
Different elements accumulate in different parts of the human body. Where they accumulate depends on their chemical properties rather than their nuclear properties (please see the explanation of the difference between chemical and nuclear properties at foot note ii at the end of this post). All of the isotopes of a particular element will accumulate in the same part of the body because they all have the same chemical properties.
Iodine-131 accumulates in the thyroid gland where it can increase the risk of developing thyroid cancer.
The human body mistakes Caesium-137 for Potassium and absorbs it into the electrolyte system. From there, it spreads throughout the body. It remains in the body for a relatively short time as it is excreted through urine.
Can we stop an isotope accumulating in the human body?
We have all probably read about Iodine tablets which prevent Iodine-131 from accumulating in the thyroid gland.
The thyroid gland can only absorb so much Iodine. If it is “saturated” with a non-radioactive isotope of Iodine, there is simply no room for it to absorb the radioactive Iodine-137. Iodine tablets contain a non-radioactive isotope of Iodine, being Iodine-127. Provided sufficient iodine tablets are taken, the thyroid gland will be saturated with this non-radioactive isotope and there will be no room for Iodine-131. This is particularly helpful for protecting human health because Iodine-131 is very dangerous once it is absorbed into the thyroid gland. Because Iodine-131 decays at a fast rate (remember that it only has a half-life of 8 days) it will emit a large amount of radiation in a short period of time. This high rate of emission can significantly increase your risk of developing thyroid cancer.
Alternatively, Caesium-137 decays slowly (remember that is has a half life of 30 years) and will be largely excreted before it emits much radiation. That said, there is no equivalent to iodine tablets for Caesium-137.
So what does this all mean for Strontium-90?
It's taken a while but I'm now finally getting to what this all has to do with Strontium-90. If you're still with me, thank you for reading all the background information. I think this information is important to understanding the dangers of Strontium-90.
Strontium-90 has a half-life of around 29 years. So it remains in the environment for generations. What is particularly concerning about Strontium-90 is that the human body mistakes it for calcium and theref
ore absorbs it into your bones where it will stay increasing your risk of bone cancer and leukaemia. About 20-30% of Strontium-90 which is ingested will be retained in your bones while the remainder will pass out of your bodyiv. As far as I am aware, there is no equivalent to iodine tablets for preventing Strontium-90 accumulating in the human body.
So why have we heard nothing about Strontium-90 levels following the Fukushima nuclear plant incident? It is certainly dangerous enough. In fact, it is regarded as one of the most dangerous isotopes due to the fact it is absorbed into your bones. After the Chernobyl incident, it was one of the most problematic isotopes.
Further, Strontium-90 is very likely to be present in Japan right now. Of all the isotopes produced by a nuclear plant, the percentage concentrations of Iodine-131, Caesium-137 and Strontium-90 are as follows:
1. Iodine-131 – 2.8%
2. Caesium-137 – 6.1%
3. Strontium-90 – 5.8%
As you can see, the amount of Strontium-90 produced is over double that of Iodine-131. So every time you read a report about an elevated level of Iodine-131, just think that the level of Strontium-90 may well be double that. Yet we still hear nothing about Strontium-90.
The silence regarding Strontium-90 has been puzzling me for the last month. I have written to the International Atomic Energy Agency (IAEA) and the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) to get answers. This morning I received a response from the IAEA (which took some weeks) and a response from ARPANSA.
The IAEA agreed with my concerns and agreed that Strontium-90 has “an important (radiological significance for) impact on human health”. The IAEA also said that the reason there has been no reports on Strontium-90 is that the laboratory techniques required to detect it take several weeks to complete and for that reason Strontium-90 could not be detected … that is until now.
Yes, the IAEA told me that they have detected Strontium-90 levels in Japan. Apparently, they took soil and plant samples from the Fukushima region on March 16. On April 12, the lab results came through and detected Strontium-90 levels up to 32 times the normal level in soil. The IAEA gave no indication as to whether these levels are dangerous. They simply said that they are “up to now” less significant than the levels for Iodine-131 and Caesium-137 which had been detected at much higher levels.
After receiving this information from the IAEA, I noticed that they had released information on their website about the detection of Strontium-90. Some foreign news agencies have also started reporting Strontium-90 today.
As for ARPANSA, they agreed with me that Strontium-90 is a high yield product of nuclear plants. ARPANSA said the reason Strontium-90 has not been detected until now is:
(1) it is less volatile than Caesium-137 and Iodine-131 and for that reason more likely to be retained in the fuel rods of the Fukushima plant rather than released to the environment; and
(2) it is harder to detect than Caesium-137 and Iodine-131.
What does this mean going forward?
As far as I can tell, this means that there is about a one month delay in detecting Strontium-90 levels. Until recently, radioactive water was being dumped into the ocean from the Fukushima plant by the Tokyo Electric Power Company. Given this one month delay time, we may have to sit tight and see what levels of Strontium-90 are actually out there.
So what will I do in the mean time? Simple, live life as usual. If you read my previous post on earthquakes, you probably realised that I'm not a particularly alarmist individual. Ever since arriving in Tokyo I've been drinking the water, drinking the milk and eating whatever I please. If this stuff does turn out to be radioactive, hey, I might gain radioactive superpowers! I've always wanted to be able to see in the dark!
Seriously, although Strontium-90 is a dangerous isotope, I am not presently concerned. I trust the Japanese government. I trust them to impose adequate restrictions on produce from the Fukushima region and generally watch out for me. Still, the whole Strontium-90 issue (which has gone largely ignored until now) intrigues me and I'll keep monitoring it.
Take care
Our Man in Japan
iii. For those of us who avoided high school and university physics, “isotopes” are different variants of a particular element. For example, the element Uranium has several isotopes, the most famous being Uranium-238, Uranium-235 and Uranium-236. The numbers after the name of the element indicate which isotope we are dealing with.
Isotopes of the same element all have the same chemical properties. Chemical properties include things such boiling temperature, melting temperature and solubility in water. However, different isotopes of the same element have different nuclear properties. Nuclear properties include things such as whether the isotope emits radiation and its half-life. Because different isotopes can have very different nuclear properties, we must be careful to distinguish between different isotopes of the same element when discussing radiation.
iv. http://www.epa.gov/rpdweb00/radionuclides/strontium.html#properties
