BNCT a great way to cure cancer

Dear Reader,

Recently I looked into the core of a nuclear reactor for the first time in my life, the closest I had been before then was looking through the door in the inner shielding at the top cap of a defunct reactor which had been shut down decades ago.

I was standing in the operating position above the water pool of a 250 kW reactor which is used for research, training and for treating cancer; the reactor was not running while I was visiting. Apparently in that location the dose rate is about 400 microsievert per hour when the reactor is running, while this dose rate is no where near the level which would cause an injury or death. As the 1970s LD₅₀ dose for radiation was about 3.5 grays, it would take 8750 hours there to reach this dose. The 365 days required to get this dose would mean that the self repair mechanisms in my body would reduce the baneful effect on my body. I do not think that it would make me fall down dead, but within a week or so I would be hitting my yearly limit, so that I would not want to linger in that spot while the reactor is running.

It was an interesting view looking down through a 6 meter pool of water at the core of the reactor; this reactor is a type which is not designed to make electric power. Instead it is designed to make neutrons for radioisotope production, for training and for the treatment of cancer. Almost twenty years ago the reactor was modified to allow it to be used as a neutron source for the treatment of cancer by the boron capture method.

Now I know that some elements in society are very antinuclear but I would ask even the most antinuclear people to stop, read this and think for a while. Frankly I would like it if you shared my point of view but even if you do not come away from reading this with my point of view I accept that people are free to choose what they like to believe.

Now if you have the misfortune to get cancer then one of the treatment methods is radiation, now the problem is that it is best to give the cancer cells one heck of a going over with one almighty dose of radiation while only giving the healthy tissue a very small dose. This is the ideal but sadly with many radiation treatments it is not quite possible to do this.

The most common method seems to be either X-rays or gamma rays delivered from a source outside the patient. The problem with these treatments is that the beam of radiation damages all tissue that it passes through. One solution to try to spare the healthy tissue is to aim beams of radiation into the person from different angles so that the paths of all the different directions converge on the spot where the nasty tumour is. This is not a perfect way of doing things, no matter how good the radiation expert is they will damage some healthy tissue.

The next step up in controlled and localised dose is to implant a source into the person; it is possible to implant a small but intensely radioactive source right at the spot where the cancer is. This can be used to treat a range of different cancers which include cancer of the cervix, breast and prostate. As radiation obeys an inverse square law this treatment is often very good at sparing the healthy tissues.

If you double the distance from the source you make the dose four times lower, while if you triple the distance from the source then the dose is nine times smaller. I hope that you can now see that the effect should be very well localised in one part of the body. With the right choice of photon and beta particle energy you can make the dose even shorter ranged thus allowing you to wipe the smile off the cancer’s ugly face, send it away with its tail between its legs while leaving the vast majority of the person undamaged.

Sounds great doesn’t it! But there is a fly in the ointment. Today for some applications some of the greens are yelling that we need to stop using nuclear reactors. The problem is that for the generation of the radioactive sources often the only thing which will do the job is a nuclear reactor which has been optimised for a high neutron flux. To do this you need to make the core nice and compact and run the reactor with a highly enriched fuel. Here is one of the best arguments for keeping radioisotope production reactors, while they might not fit in with some people’s idea of what is green they do provide an affordable and reliable supply of lifesaving diagnostic and curative medical products.

Now some people might be yelling at the screen that we should ditch the old fashioned radioactive sources for medical use and use modern particle accelerators like LINACs. I would like to point out that the treatment systems based on radioactive sources are simpler and there is much less to go wrong. Using no more than a sheet of graph paper it is possible to predict the strength of a radioactive source on day X, while accelerators are more complex. I am aware of radiotherapy accidents in both Poland and the USA where accelerators have failed to behave as expected. Both cases caused some ugly overexposures of people.

Also to deliver the radiation just where it is needed to some where like the cervix or the prostrate it is not possible to do it with a typical medical accelerator. The way that the LINACs typically work is by whipping up electrons to very high speeds and then slamming them into a metal target. The change in velocity (deceleration) of the electrons cause them to emit very high energy gamma rays. An alternative second method is to use a gadget called a betatron. Both the betatron and the LINAC are suitable as replacements for the cobalt-60 based teletheraphy units which used radioactive sources to make beams of gamma rays, but they are not able to replace the treatments based on radioactive sources which are placed right in the tumours.

Now while brachytherapy is all very well, there is something even better. One of the problems with cancer is the oxygen effect. If tissue is nice and well oxygenated then low LET radiation like gamma and X-rays are good at causing harm. But when the tissue is poorly oxygenated then it has much less effect. While the surface layers of a tumour are often well oxygenated, the core of a tumour is often poorly oxygenated. What can happen is that when a tumour is given a dose of radiation the inner less oxygenated cells survive and then continue to grow thus making the tumour reappear.

But a high LET radiation such as alpha particles still works even if the oxygen content of the tissue is low. If boron is subjected to neutron bombardment then it forms alpha particles which are able to then do immense damage to the cancer while leaving the healthy tissue alone. The reason that this works is that the person is given a dose of a boron containing drug which mainly absorbs into the cancer cells. The drug used in Finland for this treatment is L-para-Boronophenylalanine, this is an amino acid bearing a B(OH)₂ group.

The boron-10 reacts with the neutrons to form alpha particles and lithium-7.

¹⁰B + n → ⁴He + ⁵Li

The helium and lithium-7 ions then damage the cancer cells, as the boron concentration in the healthy cells are low the healthy tissue gets a far lower dose. Here is a picture of the boron compound which is used for the treatment.

A molecule of the amino acid which bears the B(OH)2 group required for the BNCT

Now some of those of you reading this blog might not be the greatest enthusiasts of the nuclear sector, but I would like to caution the “antinuclear brigade” against throwing the baby out with the bath water.

While I am well aware that it is possible to make bombs using some nuclear technology, I would like to point out that Patrick Moore pointed out that the fact that car bombs made with ANFO (Ammonium Nitrate Fuel Oil) are bad. Frankly I have to say I strongly agree that ANFO based car bombs are perfectly horrible.

But he wrote that the fact that you can make a nasty large bomb out of a car, some ammonium nitrate fertilizer and some diesel fuel is not a good reason to ban any of these three items.

I would like to also point out that the nuclear equipment in the form of a radiotherapy reactor is not in a form which is suitable for use as a weapon, I think that the only weapon I had access to at the reactor site were some lead bricks in the radiochemical lab and the bright yellow extra long tongs. I think I can get some weapons which are more suitable for mindless violence from a typical garden centre !

I have to explain something to you, it is possible to use many objects as weapons but the fact that it is possible to employ or adapt an object into a weapon is never a valid reason for banning an object. I will give you an example, in Mr Archer’s prison diary it explains how one person once took a toilet brush, they cut off the bristles and then sharpened it into a sword. The fact that someone managed to adapt a toilet brush into a sword should not be used as an excuse to ban the things !

Bob and his nuclear “facts”

Dear Reader,

It has come to my attention that a person calling themselves Bob Nichols is publishing “news” on a web site. Being a person with some knowledge and understanding of nuclear matters I thought I would take a look.

Bob as we will call him is making the bold claim that nothing is being done to mitigate the accident or clean up the site. I think that this claim is totally false, I am well aware that waste water on site is being contained, treated and then reused to greatly reduce the amount of radioactivity which is released into the ground and the sea.

Bob has claimed that even industrial robots can not cope with the radiation levels on site, I think that this is deeply wrong. A friend of mine has been on the site and he only got a small dose. I would like to know what location he is talking about. In a normal nuclear plant there are some areas which are off limits to humans for radiation safety reasons during normal operation. After shut down it is possible to enter some of these areas within minutes. There are areas at the Fukushima site (inside the reactor pressure vessels and in some areas of the containments) which might be off limits for humans for many years but I suspect that the vast majority of the plant buildings can be entered by either humans or robots.

He suggests putting the reactor cores under water, this is being done but as some of the reactors have leaks it is not a simple matter. His text suggests that no work has been done to fix the reactors is misleading, while fixing these reactors is not a simple matter the work to fix the site has already started.

He writes about the “evils of uranium”, but I would like to point out that small uranium particles are unlikely to stay in the human body for long. Uranium oxides tend to dissolve in water when oxygen and carbon dioxide are present. The uranium will then be lost via the urine. If he wants to think about any radioisotopes then he should be thinking of the shorter lived beta/gamma fission products which were released back in march 2011.

He also fails to note that the amount of radioactivity in the reactor site is now far less than it was back in march 2011, radioactivity in a nuclear reactor’s fuel tends to decay away greatly after the plant is shut down. He also makes some rather far fetched claims about chernobyl claiming that 30 % of the core was released, trust me only about 3.5 % of the fuel at Chernobyl was able to leave the plant. If Bob had read either an undergraduate text book on nuclear chemistry (I can name two books which would tell him this) or even (dare I saw it) wikipedia then he would have found that the release of radioactivity from a damaged nuclear plant is controlled by the boiling point of the main form of the element.

While iodine, tellurium and cesium are mobile, the real nasties such as plutonium and strontium are much less mobile (thank goodness for small mercies). His suggestion of using atomic bombs to cause a landslide to make the reactor site fall into the sea is very silly. I sincerely hope that nobody ever tries to do this !

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.

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.

Is fusion safe ? and beryllium chemistry

Dear Reader,

I imagine that you have seen the suggestions by fusion experts that nuclear fusion will give us a cheap, safe, clean and green source of energy which will provide power for the world’s needs. I am currently thinking about how green is fusion, right now I have contacted a fusion expert who I know and I am awaiting his views on the matter.

While we are waiting I think it is important to ask the question of what was is the typical cause of a nuclear accident. Is it a issue with management, an “act of god“ or was it a technical failure ?

In the case of the Windscale fire I have seen suggestions that it was human error, poor design of the reactor or mismanagement of the project. I know that before the 1999 Tokaimura that a criticality accident at the JCO site was considered to be a was considered to “be an unrealistic scenario” according to the UN report on the accident.

I have to ask the question, did a failure in a regulatory body (either the external state regulator or the companies own internal regulation) cause the first step to be taken which lead to the accident in 1999.

One model of how accidents occur is the Swiss cheese model, the idea is that a weakness in a system is like a void in a lump of cheese. Due to some event a void can appear in the organisation, this void can grow in size, shrink, vanish or move around. As long as some solid cheese exists which prevents a path existing from one side of the block of cheese to the other then everyone is “safe”.

But when a series of holes align themselves to create a path through the cheese block then an accident occurs and then the airplane crashes, the core melts or some other horrible outcome occurs. In some ways the most important step is for the plant owner or the management is to recognise that a given type of accident is possible.

This first step of admitting that a given accident type is at least a theoretical possibility enables the company to start to take steps to prevent it occurring. For example the understanding that someone could get a body part caught in the moving parts of a machine lead to the idea of the 19th century UK law which requires where possible all moving parts of machines to be fenced off or guarded.

While it is impossible to fence off some moving parts such as the chain of a chain saw or all the parts of a handheld electric drill, this law does improve safety by greatly reducing the number of moving parts which can cause injury to factory workers. In the same way if a fusion reactor is going to be built we need a good understanding of the possible threats which it poses.

One is the beryllium used in the heat transfer fluid in some designs, I was reading recently about fusion reactor safety and I saw that a mixed lithium / beryllium fluoride has been proposed as a tritium breeding layer and as a heat transfer layer. I can tell you that beryllium is a very nasty element, in some ways it is worse than some of the radioactive elements. As a result special care will be needed if beryllium or its compounds are used in fusion reactors.

I have looked at the crystal structure of Li2BeF4 (J.H. Burns and E.K. Gordon, Acta Crystallographica, 1967, 1, 1948-1923), this is an interesting looking 3D network. But before we get stuck into it we should look at some organic salts of “H2BeF4″. L.A.Gerrard and M.T.Weller (Acta Crystallogr.,Sect.C:Cryst.Struct.Commun., 2002, 58, m407) report a nice and simple tetrahedral BeF4 unit which has protonated DABCO as the counterion. Those of you who know VSEPR should have predicted that one OK. Here is a picture of the anion in the solid.

The tetrahedral BeF4 dianion

If we have less fluorides per beryllium centre (to make the Be:F ratio 2:7) then we need to use one of the fluorides as a bridging ligand to give us four electron pairs (eight electrons) around all the metal centres. Then we get the following dinuclear complex. See S. Aleonard and M.-F. Gorius (C.R.Seances Acad.Sci.,Ser.II, 1989, 309, 683)

The BeF7 trianion

If we go a little further and have a Be:F ratio of 1:3 then we will end up with a dinuclear complex which has two bridging flourides. This is shown below. (B. Neumuller and K. Dehnicke, Z.Anorg.Allg.Chem., 2005,631, 2535)

The Be2F6 dianion, note the SiF6 dianion in the right of the picture

And now for something completely different (sorry no monty python for you today) if we mix lithium and beryllium fluorides with an salt of an amine fluoride to give us Li2Be4F14 in the unit cell (L.A. Gerrard and M.T. Weller, Chem.Commun., 2003, 716 ). This network will have a charge of -4 and it will form long strips of metal atoms which are in a 1D coordination polymer. Here is the picture for you of the metal atoms and flouride anions in the unit cell.

The metal and flourine atoms in one unit cell

How here are two strips of metal atoms side by side.

Two strips side by side, note that there are no interconnections between the strips

Now here is four strips viewed from a different angle.

Four strips viewed from a different angle, note that they do not touch each other

Now if we look at Li2BeF4 we will see it is a complex solid, I have looked and all the metal atoms have tetrahedral environments, here are a series of views of the unit cell to show you what the solid looks like. This is going to be hard, it is a 3D coordination polymer. These 3D coordination polymers can turn out to be what I call “atomic fog” but this one is not too bad, I have seen much worse in my time.

Side view of Li2BeF4 cell showing the bonds going in one dirrection

Now after turning by 10 degrees

Now the end view.

One last thing in case any of my readers are thinking of doing beryllium chemistry, my short answer is “do not do it !“. Beryllium is the most toxic non radioactive element, some forms of it are almost as bad gram for gram as Pu-239. In some ways I would like John Hunt (the voice of the UK’s AIDS advert) to dispense advice to you about beryllium chemistry using the voice of doom, but you just have me right now.

I would suggest that if any chemistry students do not want to turn back and do something else then I suggest they talk to your local friendly radiochemist  and learn how to work with gram amounts of plutonium. Then do the beryllium chemistry in the same way using negative pressure boxes and all the other safety precautions which you would use for large scale Pu work.

Radiation free food… more likely common sense free food !

Dear Reader,

It has come to my attention that Greenpeace have told us that a major Japanese food seller is moving towards food which is free of radioactive contamination. While at first this might seem like a sensible and noble goal, when you look at it in more detail it may well be the start of a very silly thing.

Now before I get started I want to go on the record and say that I do not want to consume food / drink with high levels of radioactivity and I also do not want my fellowman to eat food with high levels of radioactivity, but the great question is how do we set the limit for what is “too high ?”. The great problem is that if we set the limit too low then we will end up throwing out plenty of good food and then we all starve to death !

I hold the view that we must not try to get a zero limit for radioactivity in food, it is impossible to reach this limit. Even before 1900 food contained radioactivity as a result of the fact that it contains potassium-40. A diet which is free of potassium-40 will be fatal ! This is because if you get no potassium in your diet you will die ! All potassium contains some K-40 so as a result it is impossible to have non radioactive food.

Also due to the fact that scientific equipment has advanced over the years it is possible to measure many “dangerous” things at concentrations far below the level at which they can cause harm. A classic example which I like is uranium in glass, now imagine that I am sitting back drinking a cool beer poured out of a glass bottle into a glass with my feet up. Am I at risk of uranium poisoning ?

It so happens that glass contains uranium, and the uranium will slowly appear in a sample of super pure nitric acid which is left in a normal glass bottle. This observation of uranium leaching out of the glass bottle does not mean that the uranium will reach the level at which it causes a health problem.

What we need are sensible laws dictated by sound science rather than the views of extremists from either end of the political spectrum. What we need to do regarding radioactivity in food is to make a judgement based on the risk per unit of activity and the benefits of eating the food. One method is to choose a dose which is thought to be an acceptable risk to the general public, from this dose and some assumptions about the amount of food eaten by a person in a year and the dose per Bq it is then possible to calculate a limit for radioactivity in food.

I would rather stay out of the debate as to what is a reasonable dose per year to the public for the purposes of radioactivity in your diet. But I would like to point out a few things, the photo which Greenpeace publish showing someone waving a gamma detector over a fish is a very misleading picture. It might look good for the general public but if you know something about radiometric work you will understand why it is not good.

The tool which greenpeace are using is a tool which is good for hunting for small and very intense sources such as dropped radiographic or cancer treatment sources, if I was required to locate a lost gamma source then while I could do it with a simple non-energy dispersive counter such as cheap NaI counter I know that I could be able to do the job under some conditions faster if I had the gamma spectrometer which greenpeace’s person was holding. A better use of the tool would be to use it to survey the most contaminated land to get a gamma spectrum from the released radioactivity, a good gamma spectrum shows an important part of the isotope signature. From the isotope signature it can be possible to work out what has happened, I find it interesting that despite me asking Greenpeace twice they have not been willing to share with me a gamma spectrum in the form of a table of counts vs energy. This data could have allowed me at an early date to make an independent analysis of some of the important isotopes which were emitted during the accident.

I hold the view that if the following group of three spectra were to be released in the form of tables of counts vs channel number it would allow independent scientists and some of the more technically minded members of the general public to check what the isotope signature of the accident is if soil is contaminated by radioactivity released by an accident.

1. A spectrum taken with the detector well shielded, I would suggest during an accident that an aluminium sheet, covered in turn by a copper sheet which in turn is covered with a large volume of a dense (and non radioactive solid) should be used to get a background for the detector. One suggestion I would have is to surround the detector with jerry cans of water if lead bricks can not be obtained. Another suggestion would be sacks of potting compost (bagged before the accident).

2. With the shielding in place a small radioactive source which has many gamma lines should be placed close to the detector. I would suggest an old radium containing watch would do nicely. If you use such a watch then it would be a good idea to record the geometry used and the thickness of the watch glass. This is used to allow the relative sensitivity of the detector to different energies of gamma rays to be tested, also it allows the energy calibration of the detector to be checked.

3. The repeat the measurement with no shielding around the detector. This will give a spectrum of the soil contamination.

4. If the levels of low energy gamma radiation are very high, then repeat stage three with the copper and aluminium shields in place, this will give a more simple spectrum.

It is important to publish the counting time, the dead time and all the other parameters/settings used. I hope soon to tell you about some of the pitfalls which can exist.

The measurement of low levels of radioactivity in food is not a trivial matter, I hold the view that the use of only gamma detector is a bad way as it will miss Sr-89, Sr-90 and moderate levels of plutonium. What is needed is for a large amount of food to be be dried out and burnt to ash, the ash then needs to be gamma counted with a well defined geometry. This would allow things like Cs-137, Cs-134 and Zr-95 to be measured.

This is because the distance from the object to the detector and a series of other things can alter the results. I would also like to point out burning kilos of fish to make a little ash allows the detection limit to be made lower.

I would also suggest that the ash should be leached with acid to extract the non gamma emitting isotopes, to allow these to be chemically separated to allow better counting. For example to count low levels of plutonium with an alpha spectrometer it needs to be deposited onto a metal disk in a super thin layer, while to measure low levels of plutonium-239 by ICPMS often you will want to separate all the plutonium from the uranium which is in the sample. This is becuase the UH peak at 239 will mask the Pu peak at 239.

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.

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.

Prussain blue

Dear Reader,

I have explained how cyanide can bind to metals such as iron to form complexes, these complexes have lone pairs poking out which can bind to other metals. Here is a picture of a unit cell of prussian blue.

A unit cell of a cesium nickel iron cyanide

The carbons are black, the nitrogens are blue, the irons are purple, the nickels are gold coloured and the green atoms are the cesium.

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.