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Big bad boom !

Dear Reader,

Well back to hot particles, I have read the big review on hot particles and it has been rather interesting.

The Chernobyl hot particles were formed by two processes, both of which appear to be thankfully absent from the Fukushima accident. The first process was the mechanical smashing up of the fuel, during the first day of the Chernobyl event the fuel was subjected to vast mechanical stresses during the big bad steam explosion.

The second process was oxidation of the metallic particles in the fuel by the oxygen of the air, we will get onto that another day. But back to the big bad bang !

The Chernobyl steam explosion took the lid off the reactor and smashed a hole in the roof; it was followed by a hydrogen air explosion. But I think that the steam explosion did the most damage. The term steam explosion might make the event sound quite benign to some readers, but I can tell you that in some ways a steam explosion is worse than a fuel/air explosion.

The classic steam explosion is an example of a BLEVE, what happens in a BLEVE is that a sealed container has a liquid inside it. The liquid is heated and thus creates a vapour pressure. The vapour pressure can be estimated using the boiling point at one bar (760 mmHg), the temperature of the liquid and the molar enthalpy of vaporisation using the Clausius–Clapeyron equation which is in the first year class on thermodynamics which is normally given to chemistry and chemical engineering students.

Ln (P/Po) = (ΔHvap / R) . ((1/To)-(1/T))

In the case of the propane tank or beer keg the pressure inside the container will increase more than the typical increase per °C which you might expect for a permanent gas like nitrogen or hydrogen. For these gas cylinders which do not have liquid inside (the gas at room temperature is above the critical temperature) the pressure is given by the equation.

PV = nRT

Which rearranges to

P = nRT / V

The heat can be supplied from outside the tank; this would occur if some stupid frat boys put a metal beer keg on a camp fire or if a propane tank is exposed to the heat of a fire. A more insidious heat source is a chemical reaction inside the tank, I know that a nasty exotherm occurred in the methyl isocyanate tank at Bhopal but this tank did not burst in a BLEVE instead the off gases from the tank escaped via some pipes to the environment.

One chemical which scares me in terms of heat generation is trimethyl phosphite {(CH3O)3P:}, if a large volume of this chemical was to be placed in a tank and heated then a reaction called the Arbuzov reaction could occur. This is an exothermic (heat releasing) reaction which could suddenly cause the temperature of the liquid in the tank to increase exponentially to some horrible high temperature. What happens is that methylphosphonic acid dimethylester forms from the trimethyl phosphite, the safety data sheets issued by the makers of this chemical normally include a warning that in the event of a fire avoid the ends of any tank. Also it includes the advice that if you hear funny noises from the tank then run !

So our tank heats up, it does not matter if it is an external heat source, a chemical or even a nuclear reaction inside but as T goes up things get a bit ‘hairy’ !

As the vapour pressure in the vessel increases or if the tank metal which is not in contact with liquid (the upper part of the tank) loses its strength as it is heated up. We move towards the big nasty bang.

If the container has some form of pressure release valve then it may delay the blast, but each time a recloseable valve shuts the tank will have a larger head space with no liquid to cool the metal. So if you ever see a propane tank venting gas during a fire, then I would advise you to retreat quickly as this is a sign that you may be heading towards the big bang.

If the pressure spikes and breaks the tank or if the tank weakens and tears then the same thing happens next. As the tank opens the pressure in the tank starts to drop. The hot liquid in the tank now starts to boil violently which maintains the high pressure. This boiling of the liquid will help to convert heat energy in the mechanical energy of the explosion.

The tank will tear further and in some cases the ends fly off at a great speed as both liquid and gas spews out of the tank. This is why the BLEVE is called a Boiling Liquid Expanding Vapour Explosion, I have read that some fire-fighters have some silly names for this class of explosion but you lot are too mature and sensible to want to use the silly joke names.

After the BLEVE you need to consider what happens to the gas and liquid which has been released by the breaking of the container. In the case of a propane tank I suspect that the original fire which overheating the tank will then ignite the fuel and cause a fuel/air explosion which I think will be mechanically more gentle but thermally harsher than the BLEVE. I have been told that the heat radiated from the cloud of burning gas can ignite fuels (clothing, grass, wood, trees etc) at great distances.

In the case of the trimethyl phosphite tank things could get very ugly. Trimethyl phosphite is toxic and I am sure it will also burn. So you should expect to be both gassed out with trimethyl phosphite and scorched when such a tank bursts in a BLEVE. My advice with a trimethyl phosphite tank in a fire is to escape from the scene !

The Chernobyl BLEVE would have released a large amount of steam, the steam would have boiled alive anyone who was near the pile cap (top of the reactor). The bursting container would have also released the hydrogen which was formed by the exothermic reaction of zirconium metal with the water.

I am not sure if the hot graphite would have started to react with the water to form carbon monoxide and hydrogen, this is an endothermic reaction but it would have been handicapped by the fact that the graphite has a low surface area when compared with the coal/coke which was normally used for this reaction in UK gas works back in the day when town gas was used rather than methane gas.

So lets ignore the reaction between the carbon and the water

I think that the most significant thing at Chernobyl would have been how the rush of steam and water up through the fuel channels would have been exerting large forces on the fuel elements. These forces would have been tearing open the fuel from its cladding and smashing pellets into smaller particles. I suspect that this accounted for much of the release of fuel which occurred at Chernobyl.

As no explosion occurred inside the reactor pressure vessels at Fukushima, I think we can expect that the dire rain of fuel particles which occurred during the Chernobyl accident did not occur at Fukushima.


2 Responses

  1. You keep referring to a steam explosion, however recent studies indicate a different senario, and that a steam explosion drove elements of the reactor into a criticality explosion. A steam explosion or hydrogen explosion would not have been adequate to blow the roof off the NPP building. Isotope studies at a distance are consistant with a fission explosion. How can I send you an article on this subject?

  2. Thanks Brian for comment,

    I would be glad to read a paper. If you want to send me a paper then please feel free to either post the reference in a comment or email me at Chalmers in Sweden.

    Regarding the isotope signature, assuming that we are talking about the chernobyl event then we have the problem that things may be very complex. I know that the fission product signature of a bomb looks different to a reactor which is run at constant power. But if we were to take a reactor fuel and run it at constant power for a long time, and then suddenly increase the power output to a very high level then we could get a fission product signature which would be somewhere between that of the bomb and the normal reactor fuel.

    My worry is that the radioactivity from the normal operation of the reactor would be likely to mask whatever happened in the short time when the power output went sky high.

    I know that in systems where the neutron flux is very high, such as during H bomb detonations and supernovas that the process by which the transuranium actinides are formed changes from the normal slow process to the rapid process where there is no time for beta decay between the neutron captures. As a result some new actinides can start to form when the neutron flux becomes very high.

    Was the study you read one where the authors were able to look for these rare minor actinides or did they do it a different way ?

    Regarding the Chernobyl blast which caused the big release of radioactivity, I hold the view that while the positive void coefficient would have allowed some overlap between the steam explosion and the spike in the fission rate that the steam explosion was caused by the surge in fission rate.

    I think the order of events are

    1. The reactivity transient which is known as a RIA (Reactivity Initiated Accident) suddenly increases the thermal power output of the fuel in the core. I blame this on the sudden consumption of the xenon in the fuel.
    2. The boiling rate for the water would have increased vastly during the steam explosion which starts to break the container open. During this phase of the accident the cladding would start to react with both water and the uranium dioxide fuel. Both of these reactions are exothermic
    3. The loss of water would have increased the fission rate thus increasing the rate of nuclear heat production
    4. The steam explosion has now occurred; the hydrogen ignites giving the second of the two blasts.
    5. Something terminates the fission process, this could be the loss of some of the fuel from the reactor, or it could have been the fact that the geometry of the fuel has now changed. I know that during the accident the core did melt, mix with building materials and relocate downwards into the basement. But I do not know how long it was after the pile cap flew off before the fuel formed the lava which went into the basement.

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