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Stable atoms and radiation

  1. Jul 18, 2005 #1
    Hi,

    i understand that fission breaks apart radioactive atoms into smaller 'slightly more stable' atoms.. and that this process continues until the atoms break down into stable atoms..

    i have been wondering if a radioactive atom/isotope once broken down into a stable atom/isotope would still be considered radioactive?


    additionally; in toxic waste matterial, if a random radioactive atom/isotope were pulled from the waste and undergone a controlled fission process --- would this atom/isotope (and it's children atoms/isotopes) be capable of breaking down into purely non-radioactive atoms/isotopes?
    --OR--
    would they always contain some form of radioactivity regardless of how far they are broken down through fission?


    thanks in advance for any light you could shed on the questions!

    Rich
     
  2. jcsd
  3. Jul 18, 2005 #2
    Let me elucidate a bit. There are radioactive-nuclei which consist of atoms, and these are the nuclei which undergo fission . Fission is 'generally' initiated using slow moving nuetrons, in this process , radioactive nuclei is bombarded with slow moving nuetrons, as a result a reaction is initiated . products in this reaction are still radioactive . The products in the nuclear fission consists of lower atomic-mass nuclei along with some more nuetrons as side-products. Now theseemitted neutron again bombard with the products and there is a chain reaction initiated leading to a chain-fission reaction which is also the concept behing atomic Hydrogen Bombs.

    Radioactive waste consists of used radioactive-instruments like the carbon-rods used in Nuclear Power Plants . These 'used' instruments are no more capable of producing same energy as needed by the Power Plant and hence are disposed off. So they still contain some radioactive nuclei which can further give a reaction giving partially radioactive materials

    BJ
     
  4. Jul 18, 2005 #3
    i see.. your explaination makes think..

    on the spent rods.. i imagine that they are very highly radioactive at first but as they become 'spent' their radioactiveness decreases considerably but still remain at leat somewhat radioactive.. do i have that part right?


    sooo, if i were to take any of the radioactive nuclei from the waste, could it still undergo its own fission until it breaks down into stable atoms/isotops?

    i guess im wondering.. can 'any' radioactive atom/isotope actually undergo fission and break down into non-radioactive atoms/isotopes? or are there some radioactive atoms/isotopes that could not break down through fission into non-radiactive stable atoms/isotopes?
     
    Last edited: Jul 18, 2005
  5. Jul 19, 2005 #4

    sorry if im a little dense, physics can get tricky for me at times it seems..


    so in the example; used fuel-rods would still contain some radioactive nuclei..

    if we took all of the product atoms/isotopes (and all future products) with radioactive nuclei and applied a controlled fission to them until they broke down into atoms/isotopes where none of the atoms/isotopes had any radioactive nuclei at all; would we then have a purely non-radioactive collection of stable atoms/isotopes?

    or will some of the final broken down atoms/isotopes still be radioactive and/or unstable?

    i guess what im trying to ask is, is it possible through this extended fission process (that you and i are talking about) to completely remove all radioactivity from the toxic waste? and if so then would the final atoms/isotopes be completely stable and non-radioactive? or could we even do this at all?
     
    Last edited: Jul 19, 2005
  6. Jul 20, 2005 #5
    A clear answer to ur question no, they can still be radioactive, but probably not enough energy to undergo another fission reaction...(if we r talking about controlled reactions)

    An example in the cancer therapy the radioactive nuclei undergoes a chain reaction emitting gamma rays that are the purpose of the whole process, but after 3 or 4 more fissions, it stops, cause this is the amount or gamma rays needed, but the next product is still radio active but not excited enough to undergo another fission..

    Oh i have a little note, we do have sources of uranium and plutonium in college but they r no longer radioactive, so it happens, but if we were able to control that i don't think there would be a problem gettin rid of nculear waste, it takes 100s of years for a radioactive source to fade...
     
    Last edited: Jul 20, 2005
  7. Jul 20, 2005 #6

    Astronuc

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    Staff: Mentor

    Try these links -

    Radioactivity
    Natural decay series
    Nuclear binding energy and fission
    Fission concepts

    There are a few nuclei which readily fission with low energy (thermal) neutrons: U-233, U-235 and Pu-239 - these are called fissionable.

    Other nuclei will fission with fast neutrons (usually E > 1 MeV) such as Th-232 and U-238, which can also absorb neutrons (neutron capture) becoming Th-233 and U-239 respectively, which then undergo decay and become U-233 and Pu-239, respectively. These two nuclei are consider fertile.

    Other heavier nuclei, e.g. Californium-252 (half-life 2.645 years, SF branch ratio 3.09%) is an isotope that undergoes spontaneous fission. Cf-254 and Fm-256 convert almost exclusively by spontaneous fission. These isotopes have to be created with accelerators.

    In a nuclear reactor, the heaviest isotopes are those of Am and Cm, although isotopes of Bk might form, they have short half-lives so would decay after shutdown.

    See also Chart of the Nuclides2004

    In the fission process, the nucleus fissions into two 'light' nuclei and 2 or 3, and sometimes 4 neutrons (although a few isotopes release 'delayed' neutrons). The 'lighter' daughter products of fission are themselves radioactive, but are not fissionable. These products will decay until a stable isotope is achieved, after which they become inert in the nuclear sense.

    Any stable (i.e. nonradioactive) isotope can be transformed into a radioactive isotope by the absorption of a neutron.
     
  8. Jul 20, 2005 #7
    you guys keep me thinking, i like that :biggrin:



    so in order to fission an atom/isotope into smaller products, they must have enough energy inside to do so...

    i take it that's the reason why some atoms with +/- neutrons in them can still be a stable atom which we call stable isotopes.. such as deuterium; H2 ( 1N and 1P )



    and some daughter products could reach a point in the process of becoming smaller isotopes that will not break down any further with another fission process due to lack of necessary energy.. but will still be radioactive such as tritium; H3 ( 2N and 1P )?

    at that point, they would still decay at a normal rate for the isotope until they break-down into non-radioactive isotopes that are essentially inert and stable.. but this process takes a very long time to occur 'naturally'..

    i understand that introducing a neutron causes a fission process but...... what else would cause a fission process?
     
  9. Jul 20, 2005 #8

    Astronuc

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    Staff: Mentor

    That is the point of this forum.

    Correct. However, other decay processes may compete. Only certain radionuclides (atoms) fission.

    Correct - look at the trend in the chart of nuclides.

    H3 decays into He3. Atoms with an 'excess' of neutrons decay primarily by beta-emission. The heavier nuclides (Rn) and heavier may decay by alpha emission. Radionuclides with a 'deficiency' of neutrons may decay by positron emission or electron capture (in which a K shell electron is captured by the nucleus).

    Each radionuclide has its own decay rate, quite independent of other isotopes.

    Fission can be induced by ion (nuclear) bombardment (including anti-protons), but that is impractical for power generation. Nevertheless, there are some concepts to use accelerators to induce fission of transactinide elements (actually transuranic elements such as Am, Cm).

    Nuclear reactors are based on using neutrons from the fission process to keep the reaction going - this is the definition of criticality. However, as the fission process continues, the fission products build-up and begin to compete with the fissionable atoms (U-235 and Pu-239), which are slowly being consumed (in a controlled system, i.e. nuclear reactor). At some point, the amount of U-235 and Pu-239 reaches a concentration at which criticality cannot be maintained. The reactor is then shutdown and the fuel with the greatest burnup (or greatest depletion of U-235) is removed and new fuel is added. Then the fission process is resumed.

    See also the Nuclear Engineering forum where these matters are frequently discussed.
     
    Last edited: Jul 20, 2005
  10. Jul 20, 2005 #9
    i've been reading a lot of interesting threads here; thanks for the link to the Nuclear Engineering forum, just started going through there


    i was thinking after my last post.. fusion.. in fusion, will the resulting atom/isotope always be the exact same and posess the exact same amount of radioactivity, energy, half-life, decay rate, and properties ( both checmical and physical ) as the same atom/isotope found in nature?
    i.e.: if we performed a fusion several times on several atom/isotopes until u235 were produced; would that u235 be absolutely identical in every way to a u235 found in nature?
     
  11. Jul 20, 2005 #10

    Astronuc

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    Staff: Mentor

    Stars are basically giant (and natural) fusion reactors. There is the p-p fusion reaction and the CNO cycle - the latter being more complicated.

    On earth, we are looking at D-D or D-T fusion and a few other reactions. We have yet to achieve a feasible system based on fusion. The major hurdles are temperature and pressure, and the ability to reduce energy losses. Stars obviously do not face those 'engineering' constraints.

    To get to higher mass atoms, the temperature/pressure required and the energy losses increase substantially - well beyond our ability to construct as system that would allow it.

    I would recommend you look at the topics on fusion or fusion reactions. You might have to search PF Nuclear Engineering or Physics forums over the past year.

    A U-235 atom is a U-235 atom regardless of how it is produced. The natural source of heavy elements is believed to be supernovae.
     
  12. Jul 20, 2005 #11
    Here's a hint when 2 nuclii fuse together, the mass of the new nuclei is less than the total mass of the parents...And u know the rest is transformed to energy.

    Fusion reactions need a lot of energy, happens at very high temperatures, it's a different case though, the fusion reaction results a new nuclei but the numbers aren't the same for sure..
     
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