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The purpose of nuclear reactors and something about plutonium

Dear Reader,

It has come to my attention that the myth that the civil nuclear power industry is part of the military nuclear sector has shown itself again. I would like to point out the folly of this idea. One blogger has repeated this claim recently, so that I can not be accused of quoting him out of context. I am going to make a length quotation of his text. He claimed recently that

If fission technology did not have military application, if the fission of uranium did not produce plutonium for use in nuclear weapons, there would be very few nuclear reactors on the planet.

The production of boiling water due to waste heat from isotope creation in nuclear reactors is not the reason for their existence.

The very first nuclear reactors were built in order to produce the bomb which killed Nagasaki.

Every reactor in the world is dual use. The primary use is military.

Now I think what is happening is that Paul is mixing a small amount of truth (yes the first reactors were part of the US nuclear bomb project) with a lot of his personal opinion. Then he is posting it in a way which makes it look like a series of facts. It is important to distinguish between fact and opinion.

Many reactors are needed for isotope production for medical and industrial purposes. A world without reactors would mean that radiotherapy would become much more expensive in some parts of the world. Thus if we get rid of reactors we will make this life saving treatment less available to the poor, frankly the idea of the rich getting medical treatment while the poor suffer and die of curable things is as morally offensive as it gets.

Next while the idea of wicked nuclear plant companies supplying plutonium to an equally wicked bunch of bomb makers might be something which troubles many people. I can tell you that there is very little to worry about as the problem has been solved.

I have been inside a range of civil nuclear sites in different parts of Europe, I can tell you that a lot of security features exist in these sites which prevent the illicit movement of nuclear materials. One of the safeguards are cameras which are watching you, I never quite know when I am being filmed in a such a place so I make a policy not to have a silly look on my face. Frankly I do not want some bunch of UN inspectors to end up looking at pictures of me with a silly face on my face.

The UN make sure that nobody sneaks plutonium or used fuel from the civil sector into the military sector.

I have also had my workplace inspected by the UN, unlike some rogue states I was cooperative. A few polite but firm men from the UN visited my lab, they wanted to know what I was doing. I told them (truthfully) that there was close to no radioactivity in the lab, but they still collected a gamma spectrum in my lab. I think that they were using a BGO detector and they found nothing interesting in the spectrum. I imagine that if I had been cooking some illegal nuclear brew in the corner that the inspection would have been rather disagreeable for me.

For many decades the civil nuclear sector has been quite rightly very separated from the small islands of the military nuclear sector.

Also the plutonium which is made in the civil sector is frankly no good for the bomb markers, if anything plutonium has been crossing from the military sector into the civil sector. I have seen reports explaining how Soviet made bomb grade plutonium is to be converted into MOX and then sold to civilian nuclear power companies. Now to my mind that is a great example of beating swords into ploughs and converting spears into pruning hocks. This is because when the plutonium comes out of the civil power reactors it will no longer bomb grade, as far as bomb makers are concerned it will be a rather disagreeable grade. The great redeeming feature of this used plutonium will be the plutonium-240.

Now in the interests of world peace I am not going to give out any details which have misuse potential but I feel that I can tell you that to build an atom bomb which works the device must do the following three things.

  1. Change from sub critical to super-prompt critical
  2. Make change 1 in less time than the typical time between the random appearance of neutrons in the fissile material
  3. Inject a pulse of neutrons into the fissile material at the right moment to power up the bomb

Now requirement three in a plutonium based bomb is already quite hard to do, but there are ways to do it. I think that the main barrier against would be wicked nuclear hooligans is requirement two.

The spontaneous fission of plutonium-240 is the key to stopping bomb makers. If we consider for a moment the plutonium signature in the fuel of unit 2 at Fukushima then we will see that the fuel has the following isotope signature (all in atom %). I got the data from Z.D. Thome et. al. in Nuclear Engineering and Design, 2012, volume 247, pages 123-127. 

0.69 % Pu-238, 65 % Pu-239, 21 % Pu-240, 11 % Pu-241 and 2.5 % Pu-242.

Now this 21 % Pu-240 will be a major head ache for a bomb designer, it will raise the spontaneous fission rate for the plutonium by a factor of 27 from the grade of plutonium which was used for the first atomic bomb test.

It is also important to bear in mind that even for fast neutrons the fission to activation ratio is worse for Pu-240 than it is for Pu-239. As a result the addition of a large amount of Pu-240 to the fissile material in a bomb would require the mass of plutonium to be made larger. In general the more plutonium in the bomb the higher the rate of spontaneous fission.

This will mean that the bomb designer working in his den will need to design something which works more than 27 times faster than the first American design had to. Now while technology may have improved, but I am sure that given the choice a bomb maker would far rather use a bomb grade plutonium with far less Pu-240.

Now imagine some evil gremlin of a bomb maker has built a nefarious bomb and I imagine that the gremlin wants to threaten the world and hold it to ransom with the threat of an A-bomb detonation somewhere. I imagine the wicked gremlin wants his long and dire reign of evil, and he knows that he needs a bomb which can be left on the shelf for a long time and still be trusted to function. As soon as his bomb has gone past its “best before date” the gremlin will lose his means to threaten the international community.

The plutonium-241 will shorten the shelf life of the bomb, this isotope of plutonium undergoes a beta decay to form americium-241 which has a far higher decay heat and emits gamma rays. As a result the bomb will be plagued by an increasingly intense heat source at its core which also is becoming a bigger and bigger radiation threat to the gremlin each time it tries to service the bomb. I have done some calculations (using A-level physics) and I have been able to confirm that the Fukushima grade of plutonium will emit much more heat than a bomb grade plutonium. The heat output will skyrocket as more and more americium-241 forms.

Now some of my readers will agree with me, that is fine with me but some of my readers may not agree with me. If you do not agree with me then feel free to comment and we will discuss the matter like adults.

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The truth about the purpose of nuclear reactors and something about plutonium

Dear Reader,

It has come to my attention that the myth that the civil nuclear power industry is part of the military nuclear sector has shown itself again. I would like to point out the folly of this idea. One blogger has repeated this claim recently, so that I can not be accused of quoting him out of context. I am going to make a length quotation of his text. He claimed recently that

If fission technology did not have military application, if the fission of uranium did not produce plutonium for use in nuclear weapons, there would be very few nuclear reactors on the planet.

The production of boiling water due to waste heat from isotope creation in nuclear reactors is not the reason for their existence.

The very first nuclear reactors were built in order to produce the bomb which killed Nagasaki.

Every reactor in the world is dual use. The primary use is military.

I hold the view that this blogger is mixing a small amount of truth (Yes the first nuclear reactors were there to support the USA’s nuclear bomb program) with his own opinions which have been dressed up as facts.

Many reactors are needed for isotope production for medical and industrial purposes. A world without reactors would mean that radiotherapy would become much more expensive in some parts of the world. Thus if we get rid of reactors we will make this life saving treatment less available to the poor, frankly the idea of the rich getting medical treatment while the poor suffer and die of curable things is as morally offensive as it gets.

Next while the idea of wicked nuclear plant companies supplying plutonium to an equally wicked bunch of bomb makers might be something which troubles many people. I can tell you that there is very little to worry about. The problem has already been solved.

I have been inside a range of civil nuclear sites in different parts of Europe, I can tell you that a lot of security features exist in these sites which prevent the illicit movement of nuclear materials. One of the safeguards are cameras which are watching you, I never quite know when I am being filmed in a place like CLAB so I make a policy not to have a silly look on my face. Frankly I do not want some bunch of UN inspectors to end up looking at a picture of me with a silly face on me.

Also the UN do make inspections at short notice of any site where they think nuclear activities either do occur, or might be going on.

I have also had my workplace inspected by the UN, unlike some rogue states I was cooperative. A few polite but firm men from the UN visited my lab, they wanted to know what I was doing. I told them (truthfully) that there was close to no radioactivity in the lab, but they still collected a gamma spectrum in my lab. I think that they were using a BGO detector and they found nothing interesting in the spectrum. I imagine that if I had been cooking some illegal nuclear brew in the corner that the inspection would have been rather disagreeable for me. To my mind the fact that the UN can catch the bad guys with inspections is another thing which reduces the chance of people being able to bad stuff.

For many decades the civil nuclear sector has been very separated from the small islands of the military nuclear sector. The degree of separation is quite rightly strict.

Also the plutonium which is made in the civil sector is frankly no good for the bomb markers, if anything plutonium has been crossing from the military sector into the civil sector. I have seen reports explaining how Soviet made bomb grade plutonium should be converted into MOX and then sold to civilian nuclear power companies. Now to my mind that is a great example of beating swords into ploughs and converting spears into pruning hocks. This is because when the plutonium comes out of the civil power reactors it will no longer bomb grade, as far as bomb makers are concerned it will be a rather disagreeable grade. The great redeeming feature of this used plutonium will be the plutonium-240.

Now in the interests of world peace I am not going to give out any details which have misuse potential but I feel that I can tell you that to build an atom bomb which works the device must do the following three things.

  1. Change from sub critical to super-prompt critical
  2. Make change 1 in less time than the typical time between the random appearance of neutrons in the fissile material
  3. Inject a pulse of neutrons into the fissile material at the right moment to power up the bomb

Now requirement three in a plutonium based bomb is already quite hard to do, but there are ways to do it. I think that the main barrier against would be wicked nuclear hooligans is requirement two.

The spontaneous fission of plutonium-240 is the key to stopping bomb makers. If we consider for a moment the plutonium signature in the fuel of unit 2 at Fukushima then we will see that the fuel has the following isotope signature (all in atom %). I got this data from Z.D. Thome et. al. in Nuclear Engineering and Design (2012, volume 247, pages 123 to 127)

0.69 % Pu-238, 65 % Pu-239, 21 % Pu-240, 11 % Pu-241 and 2.5 % Pu-242.

Now this 21 % Pu-240 will be a major head ache for a bomb designer, it will raise the spontaneous fission rate for the plutonium by a factor of 27 from the grade of plutonium which was used for the first atomic bomb test. A typical bomb grade plutonium contains less than 8 % of plutonium-240. While the trinity test used a very good quaility bomb grade (less than 1 % Pu-240) according to P.P. Parekh et. al. in Journal of Environmental Radioactivity, 2006, volume 85, pages 103-120.

If you look at the history of the American bomb project you will see that when the first atom bomb was being got ready for testing a great concern existed that it would fail due to the tiny trace of plutonium-240 in the fissile material. I suspect that a master bomb making team which have already built many designs of bombs that they could cope with 8 % Pu-240, but for a first timerI think that this level of Pu-240 would be a great barrier.

It is also important to bear in mind that even for fast neutrons the fission to activation ratio is worse for Pu-240 than it is for Pu-239. As a result the addition of a large amount of Pu-240 to the fissile material in a bomb would require the mass of plutonium to be made larger. In general the more plutonium in the bomb the higher the rate of spontaneous fission.

This will mean that the bomb designer working in his den will need to design something which works more than 27 times faster than the first American design had to. Now while technology may have improved, but I am sure that given the choice a bomb maker would far rather use a bomb grade plutonium with far less Pu-240.

Now imagine you are some evil gremlin of a bomb maker, you have built your nefarious bomb and I imagine that the gremlin wants to threaten the world and hold it to ransom with the threat of an A-bomb detonation somewhere. I imagine the wicked gremlin wants his long and dire reign of evil, and he knows that he needs a bomb which can be left on the shelf for a long time and still be trusted to function. As soon as his bomb has gone past its “best before date” the gremlin will lose his means to threaten the international community.

The plutonium-241 will shorten the shelf life of the bomb, this isotope of plutonium undergoes a beta decay to form americium-241 which has a far higher decay heat and emits gamma rays. As a result the bomb will be plagued by an increasingly intense heat source at its core which also is becoming a bigger and bigger radiation threat to the gremlin each time he tries to service his bomb. I have calculated the heat output of ten kilos of plutonium with the same isotope signature as the Fukushima plutonium, and the heat output of this reactor grade plutonium will be far higher than a bomb grade plutonium.

While some of my readers might agree with me, that is fine with me. However some of you might not agree with me, that is fine with me as long as you do not allow your disagreement to lead you to misbehave. If you do not agree with me then please leave a comment and we can discuss the matter like adults.

The horrors of polonium

Dear Reader,

I read recently how it has been suspected that Yasser Arafat may have been murdered using polonium-210. This claim that polonium-210 has been used for another murder made me think for a moment about radium-226 and its daughters. I can tell you that polonium-210 is one of the radioisotopes which most radiation workers love to hate. Polonium-210 and radon-222 are both alpha emitters which are able to diffuse through rubber and plastics. This makes them more mobile than plutonium is. When you write plutonium you have to be careful to understand that not all plutonium is born equal.

Plutonium-239 is a long lived alpha emitter which has a moderate activity per gram (and emits few gamma photons and neutrons), pellets of plutonium dioxide which have been sintered are solids which are clean to handle inside a glove box. I used to work with a nuclear fuel chemist (John Pecket) who used to make plutonium dioxide fuel, MOX and some very werido fuels. He told me how Pu-239 was a nice radioisotope to work with, while plutonium-238 was a nightmare in comparison. Pellets of plutonium-238 dioxide emit so much heat that they tend to glow red hot, they also tend to emit plenty of radioactive dust. He told me that if you place a Pu-238 pellet in a glove box then within days every surface in the box will be crawling with radioactivity. But even plutonium-238 will not pass through a neoprene glove. Thus at least it will stay inside the glove box, the worst radioisotopes I know are alpha emitters which are very mobile.

You can think of low LET radiations (beta / gamma) as being a bit like a goblin with a big stick. This horrible little monster will chase you around the house before trying to hit you with the stick, it can do you some harm but in some ways there is something worse. For alpha imagine a big bath filled with boiling hot jam, while the bath might not be able to chase you, if you fall in then you are going to get a far worse injury. The alpha is very short ranged but if it does get you then it can go a lot more harm than the beta or gamma.

The super mobile alpha emitters are like a evil goblin armed with a steel bucket of boiling hot jam, this evil goblin is also equipped with running shoes or roller blades so it is able to chase you before throwing the boiling hot jam on you. In short this ones combine the some of worst features of alpha and the more long ranged nasties. The only way to stay safe from this wicked goblin is to lock all the windows and doors of the house and keep him sealed outside, with some luck he will die of old age (become weaker when he decays away) before you have to go outside to mow the grass.

OK time to return from analogyland back to reality

With these mobile alpha emitters special extra precautions are needed to keep them contained, for example with polonium-210 some people have been known to put a glove box inside another glove box to increase the thickness of plastic through which the polonium needs to diffuse. Also for radon-222 some people trap the radon on an absorbent material rather than allow it to wander freely around their glove box.

I feel that many members of the general public have a great misunderstanding of what it is like to work with radioactivity.

I have heard of radiochemists being asked “do you glow in the dark”, the short answer is “no”. While the long answer is that is close to impossible to get sufficient contamination on you to make you glow, the only creditable cases I have heard are of some of the radium dial painters who painted their bodies with the radium based glow in the dark paint.

A paper on the legal battle for compensation can be seen here.

I think that the worst aspect of the radium dial industry was the fact that many workers would lick their brush to get a better shape tip. If you look at this document you will see that radium-228 may have been the real villain rather than radium-226. I hold that the pre 1926 two radium dial painting industry could well be the worst part of the radioactivity sector. It is interesting that it appears that only 20 % of the radium which is taken orally is retained in the human body.

Also according to Norris et. al. as cited in this report the human body is quite good at eliminating radium from its self. Some years ago in America as part of a crazy attempt at curing mental patients some people at Elgin state hospital were injected with radium. Using the data from these medical treatments it was possible in the 1950s to work out a mathematical model for the retention of radium in a human. I have rearranged this equation and used a standard bit of maths which allowed me to calculate a biological half life for radium of only 1.33 days.

This value is rather shocking to me, as a chemist I have always been taught and held the view that radium, strontium and lead are calcium mimics which have very long biological half lives because they become part of the bones. I suspect that if radium is injected or swallowed that only part of the radium which enters the blood stream will end up in the bones. While the biological half life of the radium in the bones may be very long, the biological half life of the radium in other parts of the body will be much shorter.

As a result the half life will appear to change if you consider the whole body after a single intake of radium, I suspected that the biological half life will appear to become longer with increasing time after the intake of the radium. Reading more of the report I found that Norris in 1955 published a mathematical equation which predicts how radium is slowly lost from a human. An article in Nature (March 1969, 221, 1059) suggests that the biological half life for radium in humans (long after the intake) is between 10 and 36 years.

The review of radium in humans points out that Dudley in the 1960s suggested an experiment using short lived radiotracers, the experiment was done using humans and it was found that only 20 % of the radium in a mock dial paint was absorbed when it was taken by mouth while only 0.02 % of the thorium in the dial paint was absorbed.

Another interesting point from the review is the fact that if radium-226 sulfate is deposited in the lungs then 25 % of the radon formed can be exhaled, while if radium-226 is deposited in the bones of a person then 60 to 70 % of the radon can be exhaled. This is an interesting difference, which I suspect is due to the difference in the mobility of radon in bone tissue and radium/barium sulfate. I may well get back to this point later.

Gamma spectrum of Fukushima soil

Dear Reader,

In case you want to look at a gamma spectrum for the soil from about 20 km from the Fukushima site then I suggest you look at the paper published by Keiko Tagami, Shigeo Uchida, Yukio Uchihori, Nobuyoshi Ishii, Hisashi Kitamura and Yoshiyuki Shirakawa, Science of the Total Environment, 2011, volume 409, pages 4885 to 4888.

The paper concludes that the only isotopes released from the plant were noble gases and volatile elements such as I, Te and Cs. The isotopes detected in large amounts were I-131, Te-129m, Cs-134, Cs-136 and Cs-137. Very small traces of Nb-95, Ag-110m and La-140 were detected but these levels were too low for measurement.

No Zr-95, Ru-103 or Ru-106 was detected which suggests that it is very unlikely that a large scale release of plutonium has occurred. If anyone suggests to you that plutonium has been released in large amounts then bear in mind that Zr-95 is a good mimic for plutonium in nuclear fuel. It forms a dioxide and a Perovskite SrZrO3 both of which are very similar to the plutonium dioxide in their boiling points and water solubilities. If a large scale plutonium release was to occur from a nuclear power plant accident then I would expect zirconium-95 to be found in the same places as the plutonium. The zirconium-95 is much more easy to find as it is a strong gamma emitter while plutonium is only a weak gamma emitter.

Explosion at French Nuclear site

Dear Reader,

I have become aware that a fatal accident has occurred at a place in France called Marcoule. The Guardian has reported it as having occurred at a “nuclear waste processing site”, but I think that is a bit of a misleading comment. Marcoule is more than just waste processing. It also has reactors and research on site.

From my own experiences the Marcoule site is a place where many different nuclear activities occur. Yahoo news reported that the accident occurred at the Centraco site which is owned by EDF rather than being the Marcoule site.

The BBC has reported that the part of the site where the explosion has occured is a place where decommissioning waste is processed.

Some form of oven or furnace has exploded, the comments by the French nuclear authority that the oven was used for “metallic waste of a weak and very weak level of radioactivity” suggests to me that it was not the oven for processing nuclear fuel. It may have been some furnace for the processing of scrap metal. Based on the fact that the main waste processed at the site is from  decommissioning suggests to me that it may be steel from old reactors which is being melted in a furnace.

The CEA have reported that no radioactive release has occurred outside the plant, that the ventilation and containment systems of the plant are still working OK. THe CEA web site is currently down, I suspect it is due to the interest in the accident.

When I get more details I will tell you what I know, I may also explain to you the process by which nuclear fuel is made later when I have more time to type.

Cesium maps for Japanese farmland

Dear Reader,

The Japanese government have issued maps of cesium contamination on farmland in the areas near to the Fukushima reactor accident. The main map of that area of Japan is here. Based on a google translate examination of the text with the map the soil has been taken from paddy fields at up to 15 cm depth while for upland soils it is the top 30 cm of soil. If any of my readers can read Japanese then I would be very grateful if they could give me a translation of the text from the Japanese Agriculture, Forestry and Fisheries Research Information Technology Center.

The bad news is that the cesium level in some areas is high, but the good news is that there are things which normal farmers can do which will lower the transfer of cesium to the food crops. I think that farms are going to need to learn a few new skills to allow them to farm in a safe and healthy way using their contaminated land.

Peachs, cesium, fruit and fukuashima

Dear Reader,

I have read with interest the blog of a lady called Dr Susan Burton who teaches English at a Japanese University. She is quite reasonable to be concerned about the levels of radioactivity in her diet. In her blog she questioned the wisdom of selling fruit from the Fukuashima area in the supermarket, right now I can not say if they are safe or not to eat. This question is one which can be better answered by the radiation protection authority in Japan.

Susan lamented that the cesium has a half life which is greater than a decade, she was worried that the cesium would spoil her enjoyment of Japan for a very long time. This comment about cesium and peaches prompted me to check the literature on radioactivity and fruit.

Back in 2001 a person in Italy published a review paper on the transfer of radioactivity from soil to fruit (F. Carini, Journal of Environmental Radioactivity, 2001, volume 52, pages 237 to 279). I went through this review and I found some data for peach trees.

One of the most important things to know about farming in a radioactive area is the transfer factor. The transfer factor is a measure of how easy it is for radioactivity to get from the soil into the part of the plant which you eat. It is defined as the ratio of the radioactivity (Bq kg-1) of the food to the radioactivity level of the soil (Bq kg-1). It is important to bear in mind that the transfer factor depends on the species of the plant, the soil type and the element.

Element Soil type Transfer factor
Cs Not recorded 0.0131
Cs Sandy loam 0.009
Sr Not recorded 0.0218
Sr Sandy loam 0.07
Pu Not recorded 0.000163
Am Not recorded 0.000436
I Not recorded 0.0109
Ru Not recorded 0.00109
Ce Not recorded 0.000436
Cm Not recorded 0.000436

As the main radioisotopes released by the accident were noble gases, iodines and cesiums it should be clear from the table above that in future years the humble peach tree will have a filtering effect. While most people do not like eating mud, if you were to eat mud then you would get a greater intake of cesium than if you ate the same mass of fruit from a tree grown on the radioactive soil.

I predict that in future years that the radioactivity level in the fruit will be dictated by the absorption of radioactivity by the roots of the tree and the transport of the radioisotopes through the tree into the fruit. But this year due to the direct deposition of radioactivity onto the leaves of the plant we need to consider a different route.

Sadly I could not find any results for peach trees but for bean plants I could find some results. In an experiment a leaf of a bean plant was soaked in a solution of a radioisotope to simulate radioactive rain. Then the plant was grown further before the radioactivity levels in different parts of it was measured. It was found that most of the radioactivity absorbed remained in the leaves.

Only 13 % of the cesium, 0.06 % of the strontium, 0.2 % of the americium and 0.002 % of the plutonium was found in the bean pods.(P. Henner, C. Colle and M. Morello, Journal of Environmental Radioactivity, 2005, volume 83, pages 213 to 229) This suggests that while cesium might be mobile inside plants the other elements are not very mobile inside the plants. While this effect may protect the consumer from plutonium, strontium and americium (which have not been released from Fukuashima in large amounts) due to the fact that the cesium is mobile it is possible that fruit grown this year on trees may be contaminated by cesium which was absorbed directly into leaves and then transferred through the plant into the fruit. So this year great care is needed to check the contamination level of the fruit, in future years it is likely that the cesium contamination level in the fruit could be much lower than this years contamination level.

Now one of the wicked lies which some parts of society like to either spread about deliberately, imply or assume is that humans are powerless in the face of the evil radioactive atoms. This idea is clearly wrong in several ways.

  1. Atoms and radiation knows no morality, no matter how good or evil you are atoms / radiation will treat you the same way.
  2. Humans can take action to alter their exposure to radiation and radioactivity.

For example by changing farming methods the level of cesium in the crop can be lowered. I saw one paper (W.L. Robison, P.H. Brown, E.L. Stone, T.F. Hamilton, C.L. Conrado and S. Kehl, Journal of Environmental Radioactivity, 2009, volume 100, pages 76 to 83) which explained that by using potassium fertiliser on coconut trees which were growing on Bikini island (Where the Americans used to test H-bombs) that the cesium level in the edible parts of the coconuts can be greatly lowered.

My advice to any Japanese farmers who might be reading this blog is to do the following.

  1. Find out what sort of soil your farm has.
    1. Clay soil tends to bind cesium more than sandy soil
  2. Find out from a cesium map how contaminated your farm is likely to be
  3. Ask the farmer’s union, TEPCO and the state radiation protection authority for advice on how to lower the contamination level of your crops. I would suggest that you ask about the following
    1. Deep ploughing to prevent the transfer of cesium via grass to livestock
    2. Prussian blue to decontaminate livestock
    3. Potassium fertilizers to prevent plants taking up cesium
    4. Changing to a different crop which is less able to take up contamination. Oilseed rape might be a good plant. The oil pressed from the seeds is normally has very little contamination in it even if the rest of the plant is contaminated. Also onions may be a good crop to plant. Below is some data from 1989 in Finland ( A. Paasikallio, A. Rantavaara and J. Sippola, The Science of the Total Environment, 1994, volume 155, pages 109-124) which shows that some crops are better able to avoid taking up cesium from the soil.

Transfer factors for different crops in three different soil types

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