A recent issue-revival of the radioactive water build-up at Fukushima Daiichi has brought fear of radioactive Cesium back to the forefront of the Japanese Press. The most recent analysis of the decontaminated waters at F. Daiichi reveals a Cesium concentration of 2.4 Becquerels per cubic centimeter. This is about the same radioactive level found in health spas all over the world (~3 Bq/cc, average). Health spas contain the radioactive “daughters” of naturally-occurring Radium, Thorium and Uranium: the decontaminated waters in storage at F. Daiichi contain radioactive Cesium – specifically the longest-lasting of its isotopes, Cesium-137 (Cs-137). While the radioactive concentrations in spa waters cause no concern and are touted by health-enthusiasts as being good for you, similar radioactive levels of Cs-137 result in widespread fears. Just how hazardous is Cs-137?
Cesium is often assumed to be a “bone-seeker” which will keep it in the skeleton for long periods of time, continually irradiating and eventually causing bone cancer. However, there is no evidence to support the “bone-seeker” notion relative to Cesium itself. In most cases, the bone-seeking notion comes from the misconception that Cesium is chemically similar to Calcium, which is the major element in our bones. This is simply not true. A quick look at chemistry’s Periodic Chart of the Elements shows it is not in group II, like Calcium, but is actually located in group I with Sodium. We also find Potassium in the same chemical group with Sodium and Cesium. All three are all chemically similar, but not akin to group II elements. Group II elements, like Calcium, have chemical (and biological) properties critically different from group I elements.
At this point, it should be mentioned that one of the radioactive elements found in reactor fission products is Strontium (Sr-90). Strontium is in the same chemical group II as Calcium, with similar bio-chemical properties. Thus, Sr-90 can honestly be called a “bone seeker”. However, the levels of Sr-90 around F. Daiichi are barely detectable and pose no real risk to anyone, even if it were ingested.
Back to the topic at hand…our bodies cannot store Sodium and Potassium, which is why we must continually keep replenishing these necessary nutritional minerals through the foods we eat. All group I elements have what’s called a “biological half-life”, which is used to estimate how long they will stay in our systems before being removed with body wastes. For Potassium, the half-life is about thirty days. That means half of the Potassium we ingest today will be gone in a month. Half of the remainder (or 25% of the originally-ingested) will be purged after two months…and so on. The rule-of-thumb is that after 10 half-lives (300 days, in the case of Potassium), it will essentially be gone from the body.
The biological processing of Cs-137 through our systems is similar to what happens with K-40, but with a biological half-life of about 100 days. It is totally purged about 2.5 years after ingestion if one does nothing at all. The biological half-life can be accelerated through medication (Prussian Blue) and reduced to 30 days, just like Potassium. In other words, the comparison between Cs-137 and K-40 is bio-chemically strong…not identical, but strong.
However, some people might say that Potassium isn’t as radioactive as Cesium, so there’s no realistic comparison to be made. This is another misconception. Potassium that we ingest necessarily contains isotope 40 (K-40) which is naturally radioactive. It doesn’t come from nuclear power plants: it is not a fission by-product. It comes from primordial supernovas billions of years ago. Its half-life is a little over a billion years, which means it’s been here a long time and not going away any time soon. But, how much radioactivity does it actually produce since it’s only about .01% of all the Potassium found in nature? Well, there’s a LOT of Potassium in the world…one of the most abundant non-gaseous elements on the Earth’s surface, in fact. It’s found everywhere in the soils of the world and is thereby contained in many of the foods we eat; broccoli, peas, beans, potatoes, some fruit (like bananas), and nuts, just to name a few. It’s safe to say that we all ingest some K-40 every day, have ingested it since birth, and will continue to do so for the rest of our lives. How radioactive does K-40 make the soil? A kilogram of dirt typically contains ~10 Becquerels per kilogram. Because we are constantly ingesting K-40 in our food, it is an ever-present radioactive constituent inside everyone’s bodies. How radioactive? At any given moment there are about 3,400 Becquerels of K-40 inside each person on our planet. In comparison, each of us is about as radioactive as 1.5 liters of the decontaminated water stored at F. Daiichi.
At this point, some might object that K-40 and Cs-137 produce different kinds of radiation. That’s partially true. To begin, both emit Beta radiation. A Beta is a high-speed electron. When it comes in contact with anything (even air) the Beta almost immediately gets absorbed into the outer electron shell of another atom, ionizing the new atom. Betas cannot penetrate…they are totally absorbed by a thin piece of tissue paper. It happens that fast. Your outer layers of skin (which are dead) are a perfect Beta radiation shield. If Betas are released inside living tissue, the ionization occurs in the tissue-itself and can cause localized harm. The energy level of the Cs-137 Beta is 1.17 MeV, while the Beta from K-40 is a bit higher at 1.31 MeV. In other words, their Betas are essentially equal energy-wise.
One difference between the two is that K-40 also emits a Gamma ray of 1.46 MeV about 11% of the time: Cs-137 only emits Beta. Does this mean K-40 is more hazardous than Cs-137? No. Here’s why…a Beta particle comes from the nucleus of the atom. A neutron in the nucleus suddenly becomes unstable, expunges a high energy electron (Beta) and turns into a proton. When this happens to a Cs-137 nucleus, it is no longer Cesium. It instantly becomes Barium-137. Ba-137 is about 12% of all the naturally-occurring Barium found on our planet, and it is not radioactive…unless it has been freshly formed by Cs-137 Beta decay…then it is radioactive. It releases a Gamma ray at an energy level of 0.7 MeV. It has a radioactive half-life of 2.6 minutes, so it will be gone in no more than 26 minutes after it is formed. It doesn’t stick around very long. Barium is a group II element…chemically similar to Calcium. Thus, it can be called a “bone-seeker”, per se. However, it usually takes longer than 26 minutes for radioactive Ba-137 spawned by Cs-137 to “find” some bone in our systems. Bodily-retained Cesium tends to concentrate in muscle tissue, which is why it has a longer biological half-life than Potassium. Thus, the Ba-137 is primarily released into muscle and not bone. Although biologically lodged in muscle when it is transmuted from Cs-137, there is a small but finite probability that some of the Cesium-spawned radioactive Ba-137 will find bone to irradiate. If this happens, bone exposure will occur for a very short time due to Ba-137’s very short half-life.
Because of the Cesium-Barium connection and Cesium’s longer biological half-life than Potassium, the bodily-retained limit for Cesium-137 is many times lower than the recommended (but not regulated) bodily-retained limit for K-40. We would have to continually eat a LOT of Potassium-rich food to reach the recommended K-40 limit…more than 10 bananas or roughly one 5lb. bag of potatoes every day. By the same token, we would have to eat at least 100 grams of Fukushima Prefecture-level, Cs-137 contaminated food every day to reach its regulatory limit in Japan, which is 10 times lower than the rest of the world.
Taking all of the above into consideration, two questions should be posed. First, are the stored decontaminated waters at F. Daiichi really “highly radioactive”, as is always stated in the news media reports? Second, is Cs-137 as hazardous as the Press and hard-core nuclear critics make it out to be?
References:
- Situation of storage and treatment of accumulating water including highly concentrated radioactive materials at Fukushima Daiichi Nuclear Power Station; Tepco News; October 31, 2012; http://www.tepco.co.jp/en/press/corp-com/release/2012/10/index-e.html
- Potassium; Argonne National Laboratory; http://www.ead.anl.gov/pub/doc/potassium.pdf
- Cesium; Argonne National Laboratory; http://www.evs.anl.gov/pub/doc/cesium.pdf
- General Information about K-40; Oak Ridge Associated Universities; January, 2009; http://www.orau.org/ptp/collection/consumer%20products/potassiumgeneralinfo.htm
- Periodic Chart of the Elements; Web Elements; http://www.webelements.com/
- Interactive Chart of the Nuclides; National Nuclear Data Center; Brookhaven National Laboratory; http://www.nndc.bnl.gov/chart/reCenter.jsp?z=19&n=21
