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Some basic nuclear chemistry to help understand Japan radiological releases

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  • Some basic nuclear chemistry to help understand Japan radiological releases

    At present I have seen no evidence of worrying levels of radio isotopes or poisons outside of the immediate site area, but if this does become a problem I thought it might be helpful to write something to help sort out some of the weirder stories that are likely to circulate. As it may have been a while since some of us took physical chemistry I am starting basic, just skip ahead until you find something you did not already know, or skip altogether..

    Basic nuclear chemistry.
    Atoms have a nucleus comprising a number of protons and some neutrons. The proton number determines which element it is and the neutron number is variable but is usually about the same as the number of protons. If there are ‘too many’ neutrons the isotope tends to be unstable and split into parts releasing some free neutrons and the ‘parts’ are fission products, the total mass of the fission products is slightly less than the original atom and the difference is converted into energy (heat) according to Einstein’s instructions E=MC2 . These fission products are often unstable as well and break down further until some stable, or semi stable products are left. How often this spontaneous fission occurs is the isotopes half life (the time after which half the atoms in a lump will have decayed). This rate is fairly constant in that temperature, pressure etc., which effect chemical reactions, do not effect nuclear reactions.

    Uranium
    All the reactors onsite, apart from 3 which burns MOX (Uranium with about 7% Plutonium), use LEU (low enriched uranium) as a fuel. All Uranium (U) atoms have 92 protons, by definition, but are usually referred to by their atomic weight which is the combined proton & neutron numbers. Naturally occurring Uranium is a mix of isotopes with over 99% being U238 (92+146 neutrons) and U235 making up most of the rest. This naturally occurring mix is ‘enriched’ meaning the <1% U235 is brought up to about 5% for this type of reactor. The spontaneous fission of a U235 atom releases 3 neutrons with high energies. These radiate out and they can be absorbed, or their energy reduced, by the medium they pass through. If one, of the right energy, impacts another U235 atom it induces fission in that atom. Neutron energies can be too high, as well as too low, and water is an important moderator in bringing neutrons down to the right energy to sustain a chain reaction. If we now consider a fuel pellet in a single fuel rod there are U235 atoms releasing neutrons in random directions, if they happen to pass along the length of the rod then there is a good chance they will hit another unstable U235, but most directions will shoot out of the rod in to the surroundings. As with an infectious disease’s reproductive number (Ro) what counts is does each neutron initiate one other release or more or less? If we start packing more rods around the original rod then the chance of sustaining the reaction goes up, too close and we release energy faster than we can control. The energy is largely in the form of heat and it should be clear from the above that even if rods are not packed tightly the spontaneous decay alone generates some heat and at 5% enrichment there is always going to be neutron mediated decay even if it is not at a sustained chain reaction level. To finish this section we need to mention the pellets are in a Zirconium sheath which, if the pellets are not cooled, can react giving off hydrogen which has occurred in reactors 1, 2, and 3 - causing explosions - and probably in 4 and possibly in 5.

    U235 fission cascade
    As mentioned before the U235 produces many fission products most of which are short lived and probably would remain in and around the reactor, unless there is a major explosion or fire capable of lifting them high enough to be distributed over a large area in quantity. If we consider those that are most likely to be a problem, and why. Caesium 137 and Strontium 90 are regretfully both long lived solids produced by secondary decay from noble gases. Although these intermediate gases only have fairly short half-lives it does mean they can be transported away from the source. This linked to an excellent table of Isotopes ( Anne) and their radiation types (alpha radiation has very poor penetration can be stopped by a sheet of paper) but that is of little help if the body concentrates it as in the case of Plutonium (Pu). When reading the linked table the biological half life is a measure of how quickly it is excreted from the body so in the case of U235 this is 15days but in the case of Pu239 this is 200years. Thinking back to the introductory explanation of neutron release in fission if you recall 95% of the fuel pellet is initially ‘inert’ U238 but if impacted by a neutron of the right energy this can undergo a capture process which results (with some other releases) in the production of Pu239. A spent fuel rod may have about 1% Pu239 plus many other capture isotopes, unused U235 and all the other fission by-products so some of the concerns regarding the fact that new MOX includes Plutonium, as if it is something that would not be around anyway, are misplaced. This link to Wikipedia’s page on Nuclear Fission Products has a section on ‘Countermeasures against the worst fission products found in accident fallout’ subsection with a good explanation of the behaviour of Caesium, Strontium and Iodine in the environment.

    I hope some of it was useful
    Last edited by JJackson; May 18th, 2015, 06:56 AM.
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