Two closely related radioactive nuclides found in agricultural products are Cesium-137 (Cs-137) and potassium-40 (K-40). The former does not occur naturally except in very tiny amounts in uranium ores from the spontaneous fission of uranium. Observed quantities of Cs-137 now found worldwide in almost all of the biosphere are all anthropogenic. Most of it resulted from the age of open air nuclear testing, beginning in the 1940's and lasting through the 1960's.
The nuclear accident at Chernobyl however, famously distributed a new pulse of this radionuclide in large areas of Europe, as far away as Scotland, and this pulse is still easily traceable and detectable today. It is incumbent upon me, as a nuclear power advocate, to discuss this point.
Potassium-40, by contrast, is naturally occuring and has been present in living things - albeit in ever decreasing amounts - since the dawn of life on this planet. It is an artifact of the fact that with the exception of hydrogen, almost all the mass of living things was created in the interiors of extinct, exploded stars.
Potassium and cesium are closely related in their chemical, and thus, biological behavior, since along with lithium, sodium, and rubidium - which is also naturally radioactive, like potassium - they are all in the same chemical group, group I of the periodic table.
Francium is also in this group, and it is the most radioactive of all naturally occuring elements; its only natural isotope has a half-life of just 22 minutes. Relatively rare natural decays of uranium-235, the less common isotope of natural uranium, forms small amounts of francium, which almost immediately decays. The equilibrium quantity of all the francium on earth is thought to be on the order of a few grams, in extremely low concentrations. Its existence is mostly a laboratory curiousity, of no practical consequence.
For more than a half of a century, scientists around the world - at first in connection with radioactive fallout from nuclear testing and then in the interest of tracer analysis things like the erosion of soils, and finally to understand the effects of Chernobyl - have been investigating the behavior of cesium-137.
I covered the topic of cesium-137 as a tracer in soils in this space in a diary called Every Cloud Has A Silver Lining, Even Mushroom Clouds: Cs-137 and Watching the Soil Die.
The nuclear properties of Cs-137 are as follows according to the Table of Nuclides maintained by the Korean Atomic Energy Research Institute:
The half-life is 30.07 years. It decays to an unstable isomer of Barium-137, Ba-137m - which has a half-life of just 2.552 minutes, which in turn decays to give the other isomer, stable non-radioactive Ba-137.
The radioactive decay law thus indicates that 57.5% of the cesium-137 released by Chernobyl still exists, and 42.5% of it has already decayed.
The decay energy of cesium-137 is 1.175630 MeV (million electron-volts), mostly in the form of low penetrating beta particles, whereas the decay of Ba-137m, which is always present with Cs-137 in very small quantities, gives highly penetrating gamma rays with an energy of 0.662 MeV. The latter are more dangerous than the former, because of the nature of the energy, but the former are still dangerous internally in tissue because beta particles deposit their energy in the tissue within a few centimeters.
Cesium-137 is generally considered to be, because of its chemical and nuclear properties to be the most problematic of all fission products. I am actually fond of cesium-137, but I agree that in the environment it clearly is the most dangerous fission product.
We may now compare Cs-137 to K-40, the naturally occurring radioisotope of potassium.
The nuclear properties of K-40 are as follows: Its half-life is 1.277 billion years. It decays by two means: beta decay and by electron capture, with its "branching ratio" indicating that 89.28% of the time it decays by the former mechanism, decaying by the latter the rest of the time. The energy of the two decays are not equal. For beta decay, the energy is 1.311 MeV, and for electron capture, 1.505 MeV, all of it released as highly penetrating gamma and x rays.
You cannot be alive without being exposed to K-40's radioactivity. All of the potassium on earth - which is essential to life - contains potassium-40. In percentage terms, 0.0117% of earth's potassium is radioactive K-40.
The earth is thought to be 4.5 billion years old. Thus it is easy to calculate from the radioactive decay law what fraction of the earth's potassium-40 remains since the formation of the earth: About 92.1% of it has decayed and about 7.9% of it remains. Life on earth may have arisen about 3.7 billion years ago, it is believed. If this is true, life evolved - again by direct calculation - in an environment in which potassium was about 8 times as radioactive as it is now.
So much for the introduction for the paper from the primary scientific literature that I will discuss today, written by two Austrian scientists, Herbert Rabitsch, and Elke Pichl. The reference, with abstract, is Journal of Environmental Radioactivity 99 (2008) 1846–1852.
The title is Lifetime accumulation of 137Cs and 40K in the ribs and sternum of an Austrian "mountain pasture" cow.
Since we are discussing, um, radionuclides, it might be relevant to discuss how the cow, um, died. No it was not from cancer. The life and death of the cow are discussed in detail in the paper. Here's what it says:
The calf under investigation was born in a highly contaminated region of Styria, Austria, at the time of the fallout following the Chernobyl accident. During the fallout and the first week after the deposition, dam and calf were kept in the barn. In the first three months of life, the calf ingested highly contaminated milk from its suckling cow. After this time the animals were alternately fed over the course of seasons on contaminated mountain pastures and by contaminated hay in the barn. Therefore, the growing calf ingested the artificial radionuclide 137Cs (physical half-life: 30.1 y) by high contaminated forage, which was mainly due to the Chernobyl accident above all in the first years, and less contaminated forage in the following years until the end of its life. The time course of 137 Cs-intake was not pursued. There was also a fairly continuous ingestion of 40K (physical half-life: 1.28 X 109 y). The continuous ingestion of potassium leads to an approximately stationary activity level in the adult body. Thus, the activity levels of 137Cs and 40K were caused by chronic ingestion of contaminated feed starting from suckling during the first months and thereafter by consuming common cows diet up to the day of slaughter. At the time of slaughtering in November 1992, the cow was 6.5 years old. During its lifetime the cow had born three calves.
There's no comment on the three calves, whether they had twenty five or more eyes of if they grew up to be as tall as the Empire State Building or as tiny as a boll weevil.
That's a shame.
What happened to the cow after death - don't be squeamish, especially if you eat cows (I don't) - is described in the experimental section:
...Samples of the ribs and sternum are originating from an adult cow which was slaughtered in November 1992. Materials under investigation were deep frozen after slaughtering and had to be thawed before sample preparation and measurement. Preparation procedures were made mainly by hand with a scalpel or chisel, but also a combined circular saw-blade machine and milling cutter was used. Most of the various components of a rib pair were prepared separately and then measured as paired left and corresponding right specimens. Some samples of low mass had to be pooled appropriate to their physiological function...
The exact date the poor cow, a mother of three, was, um, executed is given in another part of the experimental section:
All data for activities are related to November 14, 1992 (day of slaughter) and include corrections for self-attenuation of the photons within the different sample materials and also for moisture losses during freezing, thawing and sample preparation. Corrections due to moisture losses of sample materials came up to 15% and were shared according to the masses of those samples that were involved during the preparation. Activity concentrations are related to fresh weight and all results for activity concentrations and activities are listed in the tables with one combined standard uncertainty. These uncertainties include all identified standard uncertainties from random and systematic effects. Statistical uncertainties of 40 K-activities are greater than the corresponding values of 137Cs because of the 40K-background effect. Measured values of activities and their calculated concentrations were rounded up or down according to scientific rules. Nevertheless, values for activity ratios of 137Cs and 40K are presented with two decimals.
The paper is not about risk from eating cows contaminated by Cs-137 from cows. The chief point that the paper makes is that internal bone contamination by the isotope is not homogenous. It is, instead, unevenly distributed between various bones.
So how "hot" are the bones of the executed cow?
The unit of radiactivity is the Bequerel, which is one decay per second. A nuclide with a short half-life, like cesium-137, will have a lot more decays per second, than a nucleus with a long half-life like potassium-40. Thus something with a short half-life is way more radioactive, mole for mole - a mole being 6.023 X 1023 atoms - than something with a long half-life.
So again, how hot are the bones of the executed, dismembered cow?
Here are some figures from table 4 in the paper:
Cortical bone Cs-137: 35.9 Beq/kg K-40: 25 Beq/kg
Trabecular bone Cs-137: 53 Beq/kg K-40: 31 Beq/kg
Cartilage matrix Cs-137: 105.8 Beq/kg K-40: 55.5 Beq/kg
Articular cartilage Cs-137: Beq/kg 259 K-40: 133 Beq/kg
Periosteum Cs-137: 127.10 Beq/kg K-40: 47.6 Beq/kg
Costal pleura and periosteum, Cs-137: 95 Beq/kg K-40: 48 Beq/kg
Pure intercostal muscle tissue: Cs-137: 219 Beq/kg K-40: 80 Beq/kg
Fat: Cs-137: 40 K-40: 29
Cortical bone Cs-137: 26.8 Beq/kg K-40: 28.0 Beq/kg
Trabecular bone Cs-137: 45.9 Beq/kg K-40: 27.7 Beq/kg
and articular cartilageCs-137: 167 Beq/kg, K-40: 98 Beq/kg
I have omitted, for editorial convenience, the uncertainties in these measurements.
Depending on the tissue, the "contaminated cow" had quantities, measured in decays per kg, of cesium-137 that were 1 to 5 times that of natural potassium-40.
Risk coefficients for cesium-137 are given here. The units of risk are in pCi. A picocurie is 1 trillionth of 3.7 X 1010 Beq, or roughly 0.037 decays every second or put in the inverse, 1 Beq has 27 pCi.
We would expect in a human population that about 20% of the people who are alive today will die of a fatal cancer. Put another way, if you have 100,000 people in a stadium, about 20,000 will statistically die from cancer. This of course, is a lifetime risk. Included in these cancers are heritary factors, and environmental factors, including air pollution, heavy metal contamination, etc, occupational factors, such as being an airline flight attendant, as well as radiological factors that occur naturally, including the necessity of having some K-40 in your flesh, without which you would immediately die. There are many other types of cancer etiology, of course.
If all of the above is true, we can define the number of extra cancers, beyond 20,000 that would result from eating one kilogram of the most contaminated tissue of the contaminated cow, specifically the intercostal muscle tissue.
The risk is 8.1 X 10-12 cancers per pCi, and we have 27pCi/Beq X 218 Beq X 8.1 X 10-12 cancers per pCi = 0.004 extra cancers per 100,000 people, from the seriously contaminated cow. It follows that if you ate 210 kg of the most seriously contaminated tissue in the 6 year old Austrian cow raised on contaminated grass, you would increase your cancer risk by 1 in 100,000.
We may note, that there are people who eat hundreds of kilos of meat per year. I haven't had a kilo of meat in decades, but I do understand that many people do eat meat.
If one eats 210 kg of the most seriously contaminated cow meat for 20 years, ignoring nuclear decay and the decreasing absorption of cesium-137 into grass owing to adsorption into, say illitic clay soils, the risk would be about 20 extra cancers in 200,000.
This is non-trivial. Let me go further: As someone who lost two parents to cancer, I can tell you that one cancer death is non-trivial, but, that said, this is really the wrong question.
I oppose the car culture. I want it, and the dangerous fossil fuels that support it, phased out. Thus if I wish to be disingenuous, I could point to the Yugo and announce that its properties demonstrate that cars are unsafe. Of course, I would be being dishonest. A Yugo is a very different car than a Mercedes Benz or even a Ford Escort.
Chernobyl was never a typical type of nuclear reactor. It was a very, very, very, very poor design, and it was operated in a completely reckless way when it failed.
Chernobyl was the worst case, a reactor that was at the end of a full fuel cycle, and thus had the maximum radioactivity that a reactor can have, and which then, because it had a flammable core, was able to burn for weeks distributing fuel particles all over Europe.
It is relevant to ask if Austria - which is an anti-nuclear state and has refused to operate the Zwentendorf reactor it built, thus starting a Czech humorous campaign to "Start Zentendorf" - has seen a huge blip in cancers post-Chernobyl.
I frankly don't know.
Since 1986, life expectancy in Austria has risen from from 74 years in 1986 to 80 years in 2008. Of course this is not because of Chernobyl and may, in fact, be in spite of it.
The authors of the paper did prove one thing that surprised them. From executing the cow and dismembering the cow they did show that the distribution of the radioactive isotopes Cs-137 and related K-40 is not uniform.
In any case, the question I really want to raise is whether the worst case ever observed with nuclear energy, Chernobyl, is better than the best case with nuclear energy's only alternative, dangerous fossil fuels.
My contention is that nuclear energy need not be perfect to better than everything else. It merely needs to be better than everything else, which, happily it is.
I say that all the time.
(This diary has been crossposted at Charles Barton's here, along with a famous picture of a cow's husband.)