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The purpose of nuclear reactors and something about plutonium

Dear Reader,

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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