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.

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.

Another interesting document

Dear Reader,

Here is another interesting document it is by a man who explored the basement under the Chernobyl reactor after the serious accident back in the 1980s.

What ever you think of the nuclear sector, I think you have to be impressed with this man for his brave work to establish what happened in the reactor building.

Silver and tellurium at Fukushima

Dear Reader,

I have been a little quiet recently as I have had a bit of writers block. I was not sure quite what you wanted to read, life would be better if my loyal readers would write in and say what they want to read more about but today my thoughts wandered onto the subject of silver chemistry.

Those of you who have been paying close attention to the Fukushima event will recall that some radioactive silver has been released by the accident, but to date no ruthenium has been released by the accident.

The boiling point of silver is about 2200 ^oC, while ruthenium metal has a melting point of about 4200 ^oC. As a result of this difference in a reducing environment the silver will boil out of the reactor and form aerosol particles at a much lower temperature than the ruthenium. But under oxidizing conditions (if air gets to the hot fuel) the ruthenium metal will be converted to ruthenium tetroxide (RuO₄) which is very volatile. It will enter the air even at room temperature.

What the RuO₄ will do is to coat the surfaces of dust particles which are formed from things like steel to form ruthenium rich hot particles which will then escape from the plant. The silver can not form a volatile oxide, above about 300 ^oC silver oxide will decompose to oxygen and silver metal.

These facts which we have from the radioisotope signature suggest to me that the fuel was at least 2200 ^oC but it was not exposed to air while it was hot. If we look at page 18 of the following set of slides, then you will see an Ellingham diagram which explains how tellurium is more noble than uranium. The lower down the diagram the more able an element is to react with oxygen, if the line for an oxide is higher than the yellow line for uranium dioxide then the element is likely to be in the zero valent state (elemental form).

Tellurium dioxide has a boiling point of 1245 °C, while tellurium has a boiling point of 988 °C. If the tellurium is in the form of the element then it will be able to diffuse and boil out of the fuel at a relatively low temperature. A paper was written by S.G. Prussin, D.R. Olander, W.K. Lau  and L. Hansson, Journal of Nuclear Materials, 1988, volume 154, pages 25 to 37 which is all about how the tellurium, iodine and other fission products can diffuse out of hot fuel.

Control rod chemistry

Dear Reader,

In the blogosphere I have noticed that one blogger claimed that the radioactive silver spread around by the Fukushima accident was due to the use of silver in control rods. While one paper I read suggested that the reactor one used boron carbide, I can not rule out that the reactors used silver control rods.

Control rods are used on almost all reactors to control the rate of reaction, the further out they are pulled from the core the faster the reaction occurs. You can think of the control rods as the accelerator pedal of the reactor.

Silver has a high cross section for neutrons and as a result would make a good control rod material, the ideal control rod would.

1. Last forever
2. Cost nothing to make
3. Not become radioactive while in service
4. Behave nicely even during a horrible accident

The first issue is an interesting one, designs for control rods vary from reactor to reactor. One common choice is to use boron; this is because one of the isotopes of boron has a very large cross section for neutrons.

The cross section for neutrons is expressed in barns; this is an old unit of measurement which dates back to the Manhattan project. The idea was that if everyone expressed the cross section areas in barns rather than square meters then if a spy saw a cross section then it would be just a meaningless number. I suspect that the term barn relates to barn door.

The choice of boron has a sting in the tale which can come back to bite you on the arse. The problem is that any isotope with a very high cross section for neutrons will not be needed in large atom numbers. Each time it catches a neutron then one less atom will be present, so this can cause a change in the effectiveness of the control rod. This idea is known as burning out the neutron poison.

While the slow weakening of a control rod’s effect is an undesirable effect, this effect can be used in a beneficial way. Some fuels which have very high fissile contents have a little boron blended in. The idea is that as the fissile atoms are used up the boron is also burnt up by the neutron bombardment. The overall idea is that the fuel keeps the same reactivity level throughout its whole life inside the reactor.

The reaction by which the boron works is

n + ¹⁰B –> ⁴He + ⁷Li

This reaction generates helium gas; one Russian design for a control rod uses a boron steel alloy. The problem with this design is that the life of the control rod is limited because the helium starts to form bubbles in the steel. These bubbles then harm the properties of the rod.

A common western design is to use boron carbide (B₄C), the rods are made of a steel and have holes into which are place pellets of boron carbide. As the steel is separated from the boron we do not have the helium bubble problem, but if the rod is overheated then a reaction can occur between the stainless steel and the boron carbide. This is an exothermic reaction which forms metal borides and some carbon. For example

B₄C + 4Fe –> 4FeB + C

One other disadvantage of boron carbide is that during an accident it can form methane; the methane can lead to the formation of methyl iodide during an accident.

An alternative is to use a cadmium-silver alloy; the nice thing about cadmium is that it is very selective. It has a very large cross section for slow thermal neutrons while for fast neutrons is has next to no cross section. As the thermal neutrons are more able to cause fission then the cadmium has the nice effect of selectively mopping up these neutrons thus altering the energy spectrum of the neutrons in the core.

As a result of the fact that cadmium is selective for slow neutrons, I think that a control rod based on only cadmium would be a poor choice for a fast reactor such as a sodium cooled fast breeder, for such a reactor I would be inclined to use boron as it has a simple broad graph of absorption cross section as a function of neutron energy.

Cadmium is a metal which has a series of non radioactive isotopes, so when the cadmium-113  (the isotope with the largest thermal cross section) swallows up a neutron it forms cadmium-114 which is non radioactive and has a small capture cross section for neutrons. In this way many of the cadmium atoms can swallow up a series of neutrons without forming much radioactivity. Also the radioactive isotopes of cadmium are mostly well behaved short lived isotopes.

¹⁰⁶Cd, 1.25 %

¹⁰⁷Cd, half life of 6.5 hours decays to ¹⁰⁷Ag

¹⁰⁸Cd, 0.89 %

¹⁰⁹Cd, half life of 463 days decays to ¹⁰⁹Ag

¹¹⁰Cd, 12.49 %

¹¹¹Cd, 12.80 %

¹¹²Cd, 24.13 %

¹¹³Cd, 12.22 %, very long half life (7.7 x 10¹⁵ years or 7.700000000000000 years)

¹¹⁴Cd, 28.73 %

¹¹⁵Cd, half life of 53.46 hours decays to ¹¹⁵In

¹¹⁶Cd, 7.49 %

¹¹⁷Cd half life of 2.49 hours decays to ¹¹⁷In

On the other hand silver has two stable isotopes, both of which form radioisotopes when they swallow up a neutron. This means that silver control rods will make more long (half life > 1 day) radioactivity per million neutrons which they absorb than a cadmium control rod will.

Some time ago I visited a disused nuclear power plant in Sweden; it was a small heavy water plant which produced only 10 MW of electric power and heat for district heating. In the reactor containment I saw the area where the used fuel used to be stored (the fuel had been taken away long ago) but the control rods remained locked inside the storage area. They were being left there to decay while everyone is waiting to decommission the reactor building. The thing about decommissioning is that the longer you wait the lower the levels of many irksome isotopes. For example the ⁶⁰Co which forms as a result of the cobalt impurities in stainless steel will become half as strong every five years, thus by waiting for 50 years this radiation source will become one thousand times weaker.

Due to the fact that the control rods are exposed to such high neutron fluxes when in use, and as they are intended to absorb neutrons they can become very active.

When an accident occurs and a core melt occurs, it is likely that silver containing control rods will melt and start to form fine silver rich particles. This is likely to be a good thing as iodine has a strong affinity for the silver; this aerosol of silver may help to trap out radioactive iodine inside the plant. On the other hand if the silver particles are of the “wrong size” then maybe they will assist the escape of the radioactive iodine. One of the key features of the Chernobyl accident was that ruthenium tetroxide (RuO₄) was formed; this is a very volatile and strongly
oxidizing metal oxide. The RuO₄  enabled the ¹⁰³Ru and ¹⁰⁶Ru to form a coating on steel surfaces. These steel surfaces included both parts of the plant and also fine steel particles which were then able to escape from the plant. This is likely to be the reason why ruthenium rich hot particles were observed after the Chernobyl accident.

Ruthenium is a nice metal which I have a deep love of; I picked up this liking for it when I worked for Tony Hill. He joked that he had a special attraction to [RuHCl(CO)(PPh₃)₃] which is a complex formed by heating ruthenium chloride in methoxyethanol with triphenyl phosphine. I think I can see why Tony likes this complex; it is a useful starting material for a series of other things. It is also possible to make an osmium version of this complex but I will save my views on osmium for another day. The formation of this rather interesting looking compound is related to the work of Vaska. Vaska is a chemist from Eastern Europe who is something of a genius, he did a lot of nice chemistry with elements such as iridium. But lets get back to control rods.

So sometimes formation of solids or particles are a good thing and sometimes a bad thing.

One alternative to boron carbide, boron steel and indium-silver-cadmium alloys is to use hafnium. This is an interesting element; while zirconium has a very low cross section for neutrons (it is close to transparent to neutrons) hafnium is a very strong absorber of neutrons. As the elements are so similar in chemistry hafnium is commonly found in zirconium minerals, the zirconium used for nuclear reactor applications is normally a special low hafnium grade.

The hafnium is used in the form of hafnium hydride in control rods; these control rods are unlikely to react violently during a reactor accident or to form troublesome gases.

Tellurium-129m maps

Dear Reader,

It has come to my attention that the Japanese government have published a map of the tellurium-129m contamination levels in the countryside near to Fukushima. Here is a link to the maps from Japan.

Now before we get going we might want to consider what tellurium is.

Tellurium is a heavy version of sulphur (sulfur), it is named after the earth while selenium (the element) above it is named after the moon. Tellurium has some rather interesting chemistry it is more than just a heavy version of sulphur.

Many organisms are able to transform it from one chemical form to another, one of the typical symptoms of tellurium poisoning in humans is that the unlucky person who is stricken with tellurium will start to stink to high heaven of garlic.

This is because inside the human body the tellurium is converted into dimethyl tellurium, while I do not think that the tellurium will leave the reactor in the form of dimethyl tellurium I think it is likely that bacteria or animals will convert the tellurium into this volatile form. It is possible that the ability of living things to form this volatile organometallic may make the environmental chemistry of the tellurium more complex.

Now I hope that it is OK with my readers but right now I do not want to get sucked into a discussion of the environmental chemistry of tellurium, but I am willing to comment on another aspect of tellurium chemistry.

One of my all time favourites in chemistry is VSEPR, or Valence Shell Electron Pair Repulsion theory. For those of you who are not in the know, this is a simple theory which predicts correctly the majority of covalent compounds which lack transition metals. Be careful of the transition metals the electrons in the d orbitals can throw you a bit of a weirdo ball, on the other hand the main group elements tend to just play nicely with a normal over the shoulder delivery of the cricket ball without trying to do things like spin bowling.

The core idea of VSEPR is that the central atom has a series of electron clouds (orbitals) which poke out of the atom to form sigma bonds. The sigma bonds would look like sausages if we could see them. Think it is a nice thing that s is for sausages and also for sigma bond.

These sausage like clouds of electron density will repel each other, this is a plain simple electrostatic effect much like the effects you get if you comb your hair and then wave the comb near some small scraps of paper.

If an atom has two such sausages (opps I mean bonds) then the furthest they can be from each other on a sphere is at 180 degrees (pi radians) from each other, an example of this would be the arrangement of atoms in acetylene.

If an atom has three sausages then the furthest they will be from each other is at 0, 120 and 240 degrees around the equator of a perfect sphere. An example of this type of arrangement would be the atoms in benzene or styrene. While we are at it be thankful for styrene, without styrene there would be no polystyrene or ABS plastic. The chances are that your computer screen has a case made of ABS.

If an atom has four sausages then these bonds will be arranged in a tetrahedral manner, this is the arrangement which makes them the furthest from each other.

If the atom has five sausages then the geometry which puts the bonds the furthest apart would be to stick one at each of the poles, and three spaced at 120 degrees apart on the equator. One alternative would be a square based pyramid but I have done the maths for that
yet.

If the atom has six sausages then the geometry which puts all the bonds in an octahedral arrangement.

When an atom has a lone pair this should be regarded as a sigma bond which goes to nowhere. Treat it as if it almost as it it was just another bond, as the electrons are on average closer to the atom the lone pair is more able to repel other other bonds than a normal sigma bond.

The fun thing about tellurium is that sometimes its lone pair is stereochemically active this means that it repels other electron pairs just like any other electron pair. But sometimes the tellurium lone pairs are stereochemically inactive which means that these lone pairs do not repel other electron pairs (bonds and lone pairs). The non-stereochemically active lone pair does not normally appear in light elements such as sulfur and oxygen, it tends to be something which only happens with heavy elements such as tellurium and lead.

Recriticaility in the Fukushima nuclear reactor

Dear Reader,

Those of you who are following the story in the press may have heard of claims that one of the reactors had gone critical again. Now I would like to state that at this point after the melt downs I think that criticality is less likely than it was before the reactors were damaged by overheating. The uranium dioxide is now in the form of a big water free lump at the bottom of the pressure vessels. As the geometry is such that water can not be mixed between the uranium dioxide pellets it will be harder to moderate the fuel thus the fuel is more likely to stay subcritical compared with a core of intact fuel elements plus water.

One of the things which had made people think that the reactor had gone critical again is the observation of xenon-135, this is a shortlived (half life 9.14 hours) fission product, the thing which is important to understand that a short lived fission is not perfect evidence of criticality. This might seem odd but I will explain.

The reason is spontaneous fission, if we assume that the fuel in unit two has the same isotope signature as the stricken chernobyl reactor had back in 1987 and that both reactors contain the same amount of fuel then I can estimate how much xenon-135 will be formed by the fuel.

The chernobyl reactor was thought to contain 26 PBq of Cm-242, according to the chart of the nuclides 0.00062 % of curium-242 decays occur via spontaneous fission.

So as 1 PBq = 1000 TBq then every second we have 161.2 x 10^9 fission events per second due to the random spontaneous fission of the curium.

As the fission yields of the different products change as the energy of the state which undergoes fission increases, then we have to choose which fission yield to use with case. The data which I have does not have a listing for spontaneous fission of curium, but I will make an educated guess and use the data for Pu-239 with fast neutrons for the curium case.

The fission the xenon isotopes are

Xe-133, 6.9 %

Xe-135, 7.4 %

So we will have 11.9 x 10 ^9 atoms of Xe-135 formed per second in the fuel, if we make a dire assumption that all the xenon will be mobile then as the decay constant of Xe-135 is

(Decay constant = ln 2 / half life = 0.69 / 32904 seconds = 2.097 x 10^-5 s-1)

As activity is = number of atoms x decay constant

Then the activity of this xenon isotope released per second by the curium decay will be 0.25 MBq of activity, now if we ignore the decay of the xenon and a charcoal filter absorbs one hour’s worth of xenon then the filter will have about 900 MBq of activity on it. The activity of the filter will be higher as I have ignored the other actinide isotopes, many of these all also undergo spontanious fission.

Why and how does Prussian Blue form

Dear Reader,

Welcome back and I have to warn you fine folk that I am still thinking about Prussian blue the wonder substance which helps us to manage the radioactive cesium from the Fukushima accident.

While on a boat crossing the north sea I asked myself the question of why does Prussian blue form and how. I think that I have come up with an answer. It is important for us to start with the unfriendly sounding molecule hydrogen cyanide. It goes backward and forwards. It is refined, very much maligned and misunderstood. Go easy on this fellow, he must never be abused. He gets the metals going and you find him fizzing in the corner in the bleach bin.

Some of you may have spotted the reference to 1980s culture, those of you who have not then do not worry. All will become clear soon. It is important to bear in mind that Prussian blue will not give you cyanide poisoning.

HCN is a very refined fellow, the modern and green way to make the dinitrile required for the production of the 1,6-diaminohexane required for nylon-6.6 production is to use hydrogen cyanide (with a nickel catalyst) rather than using sodium cyanide. So the next time some asks you to name a green reagent you can say “hydrogen cyanide” in a truthful way. While it is a toxic reagent it is more green than sodium cyanide as its use forms less toxic solid waste which is hard to deal with.

For a process to be truly green it must satisfy three things.

1. Be economically sustainable (Eg process for making aspirin at £ 10 per gram will not be OK)

2. Be environmentally sustainable, it must not guzzle resources or spew out vast amounts of waste for a small amount of product (Eg if I have to cut down a square mile of rainforest and kill five rare birds to make you an egg sandwich then this method is not an OK egg production system)

3. Be socially sustainable (Eg if a process requires small children to climb up chimneys then it will not be considered morally acceptable. As a result it will be impossible to sustain the process in today’s Soceity)

Next HCN is a very maligned and misunderstood substance; it is a toxic gas but if we want to base our vilification of gases on purely their toxicity then hydrogen sulphide beats it in the top ten worst ever gases. My own view is that carbon monoxide is more of a fright gas as CO has absolutely no smell and is much more common (check your when your gas appliances were last checked by a service engineer). But as a result of the fact that HCN was the poison gas used at some Nazi extermination camps, in the American gas chamber and in many detective stories hydrogen cyanide has acquired a super nasty reputation. It is interesting to note that carbon monoxide was also used by the Nazi murderers (the gas van), but why then has CO not become viewed with equal horror by the public ?

I would say that as a chemist or an industrial worker it is important to avoid breathing in or otherwise absorbing HCN, it is bad for your health. As well as the dire short term effects which are well known it can have some horrible long term effects which are sometimes seen in parts of Africa where people tend to live on a vegetable known as cassava. If you prepare this food wrongly then you will get a dose of cyanide in every meal, this can lead to chronic cyanide poisoning which causes among other things trouble with the nervous system. So my advice is to “go easy on your body” when working with cyanide. Do not abuse your body by forcing it to endure the stress of having to metabolize cyanide, take that bit of extra care to lower your occupational intake of cyanides.

The cyanide anion is a very strong ligand for many transition metals, indeed it does get the metals going. Sometimes in very much the wrong way, some time ago there was a large spill of cyanide waste in eastern Europe. It ended up in a river where it then killed the fish, one of the problems with cyanide it binds to an iron complex in mitochondria which then stops oxygen binding. As a result the fish could no longer use oxygen, as a result they died. But we need to understand why does cyanide bind to metals so well, the binding of cyanide to metals is much stronger than the binding of most simple monodentate ligands.

Monodentate ligands is a fancy term for a molecule or atom which binds through one atom onto a metal.

A snake which grabs you with its mouth is a monodentate animal

A crab which grabs you with both claws is a bidentate animal

A scorpion which grabs you with both claws and applies the stinger to you is a tridentate animal

The reason is the “backwards and forwards”, hydrogen cyanide when deprotonated forms the cyanide anion which uses a lone pair on the carbon to form a sigma bond to a metal. It also uses its empty pi* orbitals to suck electron density off of metals thus forming pi bonds to the metal.

Now we need to look at the orbitals of the hydrogen cyanide, the orbitals of the cyanide anion are almost identical.

Lets start with the HOMO, this is not a sexual term it means Highest Occupied Molecular Orbital in chemistry. Those of you who were expecting something sexual here, I am sorry but I am going to disappoint you, this blog is not about sexual matters. But feel free to carry on reading as you might find the chemistry interesting.

The HOMO of HCN

Here you should be able to see that on the nitrogen atom (blue atom) a lobe of the orbital pokes out into space away from the CH group, this part of the orbital will form the lone pair which allows the nitrogen to bind to things. Around the hydrogen atom is a big blue lobe. When the HCN loses a proton this will form a cloud of electron density which also pokes out into space. Here is another view which may make it more clear, the lone pair on the nitrogen and the blue blob on the carbon will allow it to form the sigma bonds which go to metal atoms.

Alternative view of HCN's HOMO

Here is a view of the HOMO of the cyanide anion, look at how similar it is to the HOMO of hydrogen cyanide.

Next here are two alternative views of the HOMO of the cyanide anion to allow you to have a better idea of the shapes of the orbitals.

The next thing to look at is the p orbitals of HCN, I have calculated these orbitals for the cyanide anion and they are the same shape so I will only show you one set. Here is one of the them.

One of the pi bonding orbitals of HCN

The hydrogen cyanide molecule has two occupied pi orbitals which look like a pair of sausages arranged parallel with the line between the carbon and the nitrogen. Here is a view of the other one.

A view of the other pi orbital

Next we have the pi* antibonding orbitals.

LUMO of HCN

HCN LUMO +1

I guess they looked the same to you, so here is the end view. Note that they are at ninety degrees to each other.

HCN LUMO

HCN LUMO +1

Now to understand antibonding, I want you to think of a nice person. How about St Francis of Assisi, after a wayward youth he grew up to be a man known for being kind to poor people and taking care of animals.

The anti-St Francis would be a nasty man who steals bread from staving single mothers and homeless men, for fun he throws
animals down the well.

The anti-St Francis is the total opposite of St-Francis, everything good about St-Francis has been turned into something horrible in the anti version. In the same way all the energy lowering effects of the bonding orbitals are turned into energy increasing effects in an antibonding orbital. Typically an antibonding orbital is more antibonding than the bonding orbital is bonding. So if you fill up both orbitals with electrons then overall the sum of the two orbitals is antibonding.

In case you want to see some of the other orbitals of HCN then here they are.

LUMO +3

LUMO +2

 

LUMO +1

LUMO

HOMO

HOMO -1

HOMO -2

HCN HOMO -3

HCN HOMO -4

I hope to bring you some more about our new friend (Prussian Blue) soon.

Nuclear fuel

Dear Reader,

When I read about the accident in France which turned out to be at a radioactive metal treatment site, I had said that I would write and explain some of the mysteries of how nuclear fuel is made. Now I suspect that you either think one of the following.

1. Nuclear fuel is some mysterious material which is made by some modern day version of a high priest.

2. You think that you know everything about nuclear fuel and how it is formed.

I hold the view that statement 1 is wrong, and many people who think statement 2 is correct are not so well informed as they think they are. I will be limiting what I write about to oxide fuel, the problem is that nuclear fuels can take many different forms and it is impossible for me to write about all fuels because no man (or woman) alive can pack their brain with everything about all forms of nuclear fuel.

The first thing to understand is that uranium dioxide (UOX) and plutonium dioxide (part of MOX) nuclear fuel are hard ceramics. These do not dissolve in water. But there is some important news to consider still about Greenpeace’s reaction to the French scrap metal accident.

A Greenpeace press release has said that this event is a “a tragic reminder of the dangers of nuclear power“, while I hold the view that the vast majority of deaths due to industrial accidents are very preventable and sad. I think that this statement is yet another example of Greenpeace’s irresponsible behaviour regarding nuclear matters, the accident in France has not caused a release of radioactivity, nor did it involve radiation nor did it occur because of the radioactive / nuclear properties of the materials being handled. To claim that the event is a “reminder of the dangers of nuclear power” is wrong, how does Greenpeace know that the accident was anything other than a normal industrial accident which happened to occur in an area where radioactive materials have been handled.

From what I know the French accident was a non-nuclear accident which happened to occur on a site where radioactive scrap metal is processed. I think it is wrong for Greenpeace to try to score political points through the death of a plant worker whose death is likely to be nothing to do with nuclear or radioactive materials.

I recently learnt about the circumstances of the death of a 36 year old man named Neil Cannon, he was working at height as a demolition worker taking down the stack of a Windscale pile. His safety line was damaged during an event which made him slip and fall. He then fell almost 100 meters, while this accident occurred in a contaminated area it is not a nuclear accident. This accident is special to me, in my youth I used to sometimes work on aerial farms doing things which I would not do today. Nowadays when I climb a radio mast I normally use PPE. I wear a builder’s hard hat and a harness which is fixed to a strong point to prevent me falling. While I am not as skilled a steeplejack as Neil Cannon or Fred Dibnah I know something of the dangers of working high above the ground. By the way I am a devote of the two line method of climbing.

You fix two lines to your safety harness, you start with one line fixed onto the tower. You then reach up and fix on the second line. Then you remove the first line and then climb a little further up the tower before fixing the original line a bit higher. This way no matter what happens you are unable to fall very far from the tower. You also make a point of not allowing your safety lines to come into contact with sharp edges or anything which could cut them.

What happened to Mr Cannon was very sad, but to dress such an accident up as anything other than a tragic building site accident is not right, Mr Cannon died because the system of work required him to leave the safety of a platform and work on a ledge. Something happened which both made him fall and also cut his safety line. To dress up that accident as a nuclear accident is not right, in the same way Greenpeace should not rush to label the French scrap metal accident as a nuclear accident.

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.