Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Critical Mass - Questions

  1. Nov 17, 2009 #1
    The term critical refers to an equilibrium fission reaction (steady-state or continuous chain reaction); this is where there is no increase or decrease in power, temperature, or neutron population.(Ref: http://en.wikipedia.org/wiki/Critical_mass)

    1. How can be the neutron population stable when every fission in Uranium 235 releases 2 neutrons?
  2. jcsd
  3. Nov 18, 2009 #2
    Hi there,

    By flying over the wikipedia article you make reference, I believe you did not truly understand the idea behind critical mass.

    Let's start from the beginning, when a heavy nucleus (U-235) fissions, it releases many different particles: two new nuclei, two or three neutrons and energy (kinetic or photonic). You probably knew that right!!!

    You probably also know that the neutrons freed from this fission can induce new reactions, in the idea of chain reactions. But the problem is that the growth is exponential.

    Ok, but to keep these reactions going, you need a to have a nuclei of U-235 to catch the free neutrons. Otherwise, the reactions will stop. Therefore, you need a certain "critical mass" of U-235 to insure that at least one neutron per fission will be catch and produce a chain reaction.

  4. Nov 18, 2009 #3
    That was great explanation!!!! Thanks alot!!!
  5. Nov 18, 2009 #4
    But then again, what is the need for a certain mass of fuel when any amount of fuel can trigger a chain reaction, i mean the longer the fuel lasts the more the energy?
  6. Nov 19, 2009 #5
    Sorry that I did not go into more details.

    Let's start where I left off, in my last post.

    In a block of uranium, you will have desintegration that emits a few neutrons. These neutrons, of high energy, travel a certain distance before being caught by a nucleus. The neutrons emitted near the surface of the block will naturally drift out. Therefore, if you have too much of a small block, most of the neutrons will drift out, before being able to interact with another nucleus. The chain reaction can only be sustained with a block that is big enough to ensure neutron catching.

    Hope this clarifies your problem???

  7. Nov 19, 2009 #6
    Thanks for that information!!!:smile:
  8. Nov 19, 2009 #7
    Also note that the critical mass depends on the SHAPE of the body. For a spehere it is minimal, for a stick it is much much bigger.
  9. Nov 19, 2009 #8
    Does that mean enriched uranium should not be touched with bare hands? Sorry if that question is silly!
  10. Nov 19, 2009 #9
    Hi there,

    It depends!!!

    I will call "enriched uranium" = nuclear fuel, since this is the fuel used in many of the NPP in this world.

    If the fuel is new (never been inside a reactor core), you can touch it, put it in your pockets and walk around with it. The emission of this type of fuel is practically null. I would strongly suggest not to eat it, but that for other reasons than radioactivity. This is not true for MOX fuel, which is a mix of nuclear fuel containing plutonium.

    If your are talking about spent fuel (has been inside a reactor, bombarded by neutrons), then I would turn the other way and run as fast as you can.

    Is this clear in anyway???

  11. Nov 19, 2009 #10
    It does not. Natural radiation level of uranium is very low (on the order of 10 neutrons per kilogram per second, depending on isotopic composition). It is only through exponential multiplication that any significant amounts can be collected. You need to be exposed to 10^15 energetic neutrons in a short period of time to develop radiation sickness symptoms.

    Plutonium-239 is thousands of times more radioactive than uranium, and you can be around plutonium relatively safely, as long as you don't consume or inhale plutonium dust (because dust can end up stuck in your lungs and seriously mess them up).

    That is fairly nasty stuff, you can really get radiation burns from handling it. Not to mention that ingestion or inhalation of as little of 10 mg could be fatal. But its intensity declines rapidly over time. By the time it spent a couple of years out of the reactor, all the most radioactive elements have decayed, and its radiation level is down to perhaps 1% of the initial.
    Last edited: Nov 19, 2009
  12. Nov 19, 2009 #11
    Not only from the Shape, also from the surroundings. If there in the surroundings there are neutron reflecters or other neutron emitters in change drastically.
    In fact the system used to reach the very critical mass back in los-alamos was to surround the ball of plutoniom NEAR to the critical mass with Pb bricks, one by one.

    Questions are never silly.

    It better to does not, not for radioactivy (in fact Uranium is less radioactive then many of common-uses materials) but for the same reason is better to not touch lead with bare hands: hands get dirty with Uranium (or Pb) dust, that you will inhalate or eat. The heavy material then get on the lungs or generically in the body and is difficoult for the body to expel it. Then it inflames he cells nearby and increase the probability that these cells will react becoming cancer cells.
    Anyway the Uranium the chance is lesser than with the lead because the Pb is softer.
  13. Nov 19, 2009 #12


    User Avatar
    Staff Emeritus
    Science Advisor

    On average, at least one of the two neutrons released must be absorbed and cause fission to continue the chain reaction. As soon as the average number falls below 1, the mass becomes subcritical. In actuality, either 2 or 3 neutrons are released during fission, and some fission products also release 'delayed' neutrons, and it is this piece of physics that enables us to more easily control the fission process.

    In a nuclear explosion, the mass becomes prompt supercritical for a few microseconds.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook