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Neutron to start the chain reaction

  1. Feb 8, 2007 #1

    I can't find :confused: where do we get the starting neutron(s) for the chain reaction in the reactor. I'm aware that then the splitted atom emits enough(in fact even too much?) neutrons to selfsustain the reaction, which are then slowed down to be able to split uranium and so on.

    So where do one get the triggering neutron, to start the chain reaction?

  2. jcsd
  3. Feb 8, 2007 #2
    There is a small probability that the Uranium nucleus can spotantiously fission, this releases neutrons. So uranium triggers its own chain reaction.

    In nuclear weapons there is a source of neutrons included to trigger the reaction, something like a beryllium and polonium mix. When beryllium gets hit by alpha particles from the polonium it emits neutrons. This to ensure that there is indeed enough neutrons around exactly when needed.
  4. Feb 8, 2007 #3


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    Nuclear reactors contain special 'startup' sources to provide sufficient neutron activity, so that the activity in the core can be monitored. Otherwise, one would approach criticality without sufficient knowledge of knowing how close one is. The objective is to maintain control of the reactor.

    While the (alpha, Be) sources are possible, e.g. Po-Be, Ra-Be or Pu-Be, they are not typically used. Rather the startup sources use Sb124-Be, which is a photoneutron source in which a high energy gamma photon knocks out a neutron from Be-9, which becomes Be-8, which promptly decomposes to 2 alpha particles. Sb-124 (which is formed from neutron absorption (activation) by Sb-123) emits a 1.6 MeV gamma.

    See pages 37 & 38 of this document - http://lrs.web.psi.ch/educational/courses/2006_EPFL_DOCTORAL_PSI_COURSE/week3/Week_3_Lecture_5.pdf

    Once the reactor has completed a few cycles of operation, it might have sufficient transuranics in the twice burned fuel to enable a 'sourceless' startup. Otherwise, the source stay in the reactor where they are reactivated by the neutron flux.
  5. Feb 8, 2007 #4
    Thanks for that information. It never occured to me that a reactor needs a startup source aswell.:blushing:
  6. Feb 8, 2007 #5


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    Actually after the reactor has been run; you don't even "need" the startup source, since
    there will be enough radionuclides present as a result of fission that can provide the
    initial neutrons.

    However, as a matter of a condition on the operating license; the operator is required to
    have a startup source in the core when it is sub-critical in any case. The instrumentation
    for monitoring the core and its criticality are based on detecting neutrons.

    Although with a fresh core, there are probably enough stray neutrons around to start the
    reaction; you can't count on that. So the operator is required to have a source in the
    reactor when it is subcritical, as Astronuc points out; so that one can monitor the
    condition of the core.

    Dr. Gregory Greenman
  7. Feb 9, 2007 #6
    Thank you all for the info.
  8. Feb 14, 2007 #7
    Further to the point that a extraneous neutron source is not always required, consider the natural fission reactor that is believed to have existed at Oklo, Africa a couple of billion years ago.
  9. Feb 14, 2007 #8


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    This is true. It depends on kinf and size, which in part determines keff. Composition and homogeneity are also factors.
  10. Feb 14, 2007 #9


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    Without the neutron source; the Oklo could have just sat there in a critical state; but with
    no neutrons in it. However, given that there are extraneous neutrons; it is only a matter
    of time before one finds its way to the reactor, and we get a self-sustaining chain reaction.

    But there DOES need to be an initial neutron - provided by cosmic rays, radioactive decay...

    In a manufactured reactor, that has been operating for even a short period of time; the
    neutrons to restart the reactor can come from the fission products; some of which are
    neutron emitters.

    For a brand-new manufactured reactor; then you need a source. You don't want to be
    trying to take the reactor critical and not have any neutrons in it. The instrumentation
    as to the criticality of the reactor responds to neutrons. You can put the reactor into
    a critical state - or even a super-critical state - without neutrons; and without those
    neutrons, you wouldn't know that you had reached the critical or super-critical state.

    That's why reactor operators are REQUIRED to have a source in the reactor at low
    power and sub-critical conditions.

    Dr. Gregory Greenman
  11. Feb 20, 2007 #10
    What the...?

    That would be akin to suggesting that I could start-up, run, and accelerate an internal combustion engine without fuel.

    No neutrons, no criticality. No criticality, no super-criticality.

    I'm not sure who you're generalizing this to, but operating power reactors that aren't brand new most certainly do not have this requirement. There is more than enough neutron activity coming from the spent fuel going through multiple cycles for the nuclear instrumentation to pick up.
    Last edited: Feb 20, 2007
  12. Feb 20, 2007 #11
    I think Mobius is talking about the reactor being critical as in its size is big enough to sustain a reaction if there are neutrons. For example, 10 kg of Pu is supercritical, but not much will happen until there are incident neutrons.
  13. Feb 20, 2007 #12


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    Then you don't understand the concept of "criticality".

    The car is NOT a critical system - the analogy doesn't hold.

    Criticality is a property of the geometry and the materials; NOT the neutrons.

    Theoretically, you could have a system that is "critical" even when neutrons
    are not present.

    [ A critical system will be "steady-state" only if the neutron distribution that is
    present is the fundamental eigenfunction of the critical system. ]

    You can assemble a system that is super-critical; even without neutrons.

    That's why nuclear weapons have "initiators":


    The nuclear weapon has a device to put neutrons into the system; in case there aren't
    any there to start the reaction when the device assembles. You wouldn't call an
    assembled nuclear weapon "sub-critical" would you?

    Of course not! You can assemble a system that is critical, or even super-critical; but
    nothing will happen if there are no neutrons present.

    That's because the concept of "criticality" is NOT a property of the neutron population;
    it is a property of the materials and the geometry that they are in. Criticality tells you
    what the system would do if neutrons are present; whether or not those neutrons are
    actually present.

    The word "critical" comes from mathematics. If the eigenvalue of the transport equation
    for the sytem is exactly unity; mathematically the system is "critical". Note that the
    solution of the transport equation is only dependent on the materials and geometry;
    NOT the neutron population. In fact, it CAN'T be - because the eigenvalue system is
    singular - being both linear and homegeneous - any constant multiple of a solution is also
    a solution; including a multiple of zero. The criticality doesn't depend on the neutron

    The "reactivity" on the other hand, DOES depend on the neutron population. A system
    could be exactly critical; but with a neutron population that isn't the fundamental
    eigenfunction. Such a system would not be steady-state; the reactivity would be a
    finite non-zero value until the system evolved into one where the neutron population
    is the fundamental eigenfunction.

    It's really the matematical system that is "critical"; and that just depends on materials
    and geometry, and NOT the neutrons.

    Dr. Gregory Greenman
    Last edited: Feb 20, 2007
  14. Feb 21, 2007 #13
    Good lord... why is it always the book-bound guys that come up with this stuff? If you want to talk in the purely theoretical realm, then by all means, feel free to talk right out of a reactor dynamics book all day. However if you want to talk about the world of operating power reactors (which you did), then I suggest you step away from the chalkboard and try not using the most mathematically bound definition of criticality that you can find in place of the concept of a critical mass or critical geometry.

    I passed all my reactor theory & neutron transport classes without problem. I understand full well what you're talking about, and then some. However I, just like every other nuclear professional who isn't holed up in a university, office, or other paper-bound world, recognize that the zero solution is always immediately discarded as trivial and that nobody ever talks about criticality exclusively in terms of critical geometry. It's always in the context of neutron population and 'k'.

    That's why this statement:
    ... is false. The zero solution to the transport equation means nothing to us.

    I'm sorry if this seems a bit curt, but in the world of operating power reactors, or any related work or study function, critical is not taken in the literal (but wondefully and uselessly abstract) mathematical definition. In the working nuclear world, critical has EVERYTHING to do with your neutron population and how it behaves. The reactor is real, the mathematical model is not. One is reality, the other abstractly attempts to model it.
    Last edited: Feb 21, 2007
  15. Feb 21, 2007 #14
    if i may: LOL...are you serious? is speaking about nuclear physics in mathematical terms silly?

    Homer Simpson works at a power plant too...i think that you are way over your head in debating Morbius on these kinds of issues - why don't you search the PF archives...
  16. Feb 21, 2007 #15


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    The use of the word "critical" comes from the mathematics. It is YOU that is using it
    in a non-standard fashion

    You may have passed; but this is one part of transport theory that you have

    Asserting facts not in evidence. I see no evidence of "..and then some".

    Yes - 'k"!! What is "k". It is an eigenvalue of the transport equation. [There are
    others.] The existence and value of "k" exists INDEPENDENT of the value of the eigenfunction;
    whether that eigenfunction is the trivial zero, or a non-zero eigenfunction.

    Look at any expression for "k". Do you see anything that has to do with neutrons like a
    flux? No - you will see only material and geometric properties.

    The above shows that you really DON'T understand some very important features about
    the operation of a nuclear reactor. I would defy you to calculate reactor transients,
    and do reactor "noise" analysis with your concept of what criticality is.

    It is actually VERY IMPORTANT to the transport computer codes that are used in the
    design and safety analysis of the reactor. You wouldn't know about that - because all
    that work and analyis is done BEFORE you get your hands on the reactor.

    Your concept of "reality" being divorced from the mathematics; what I call the
    "seat of the pants" reality is really quite limiting. Without the mathematics, which you
    seem to disdain; the reactors wouldn't work as well or as safe as they do.

    The term "criticality" comes from the mathematics. The concept of "k" is an
    eigenvalue; which comes from the mathematics.

    It is the mathematics that keeps us well grounded in the physics; NOT the "seat of the
    pants" understanding.

    You might find it illuminating to consider the problem I posed in my previous post.
    Let's consider the detonation of "Little Boy". The fissile material in Little Boy has
    assembled, but the initiators haven't fired yet. Let's suppose additionally that there
    were no stray neutrons around.

    Little Boy has assembled its mass of U-235; but for an extremely brief period of time,
    there are no neutrons present, as of yet. Consider:

    Is Little Boy critical?

    What is "k" of Little Boy? Does it exist? If so; can you say something about its value?

    Or consider the following. I have an EXACTLY critical reactor with the fundamental
    mode flux therein. I send a pulse of neutrons into the reactor.

    Have I changed the criticality of the reactor by adding the neutron pulse?

    Have I changed "k" for the reactor by adding the neutron pulse?

    Have I changed the reactivity of the reactor by adding the neutron pulse?

    Dr. Gregory Greenman
    Last edited: Feb 21, 2007
  17. Feb 21, 2007 #16


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    Yes - it was nuclear power plant operators like "Homer Simpson" working
    at Three Mile Island Unit 2 that needlessly plunged the nuclear industry
    into a tail-spin for more than a quarter century; because they didn't
    understand the physics of a reactor as well as they thought they did.

    That was the subject of a pevious discussion here, not long ago.

    It's like the old adage from the aerospace industry; "Planes are designed
    by people with PhD degrees, built by people with Master degrees..."

    There's a REASON they have the PhDs design the planes.

    Dr. Gregory Greenman
  18. Feb 22, 2007 #17
    I was going to say I meant source in the "conventional sense" of a neutron source, but that is actually incorrect & lazy. I still don't think the initial neutron would have to literally be a neutron external to the assembly. Could it have been the initial neutron was from a spontaneous fission (SF) inside the assembly that happened to "get lucky" at a time when the reflecting & moderating properties of the assembly were also just right? If so, then your inclusion of radioactive decay as a source of initial neutron could be said to include SF.

    Btw, I would be interested to read your input on the questions you posed on the example scenarios.
  19. Feb 22, 2007 #18


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    I'll give Emfuser a chance to answer.

    If not - then I will give a full explanation to each of those questions.

    Dr. Gregory Greenman
  20. Feb 22, 2007 #19
    Not really

    Morbius is the typical paper-bound scientist. Completely stuck in a paper world with a very narrow, rigid view of things. He reminds me of my neutronics teacher who was a physics guy. There's no hate, just an amusement at the unnecessary rigidity when talking specifically about actual nuclear power plant operations.

    Just to clarify what I am and what I do, I'm a reactor engineer with experience as an operator. I actually work at a nuclear power plant, participate in the design of cores, participate in and supervise plant startups, and regularly write power transient plans. I'm not exactly talking out of my behind here.

    So to us, it's fine and dandy that, in the world of universities, research labs, and the rest of the paper bound world, the single word "critical" is used broadly to incorporate what the functional nuclear power world calls "critical mass" and "critical geometry." Yes, we all understand the theoretical basis for these things because we went to school and learned them and apply said education every day in our work.

    Even the companies that write the codes, like Westinghouse, Areva, GE, and some now gone or absorbed, older vendors, don't ever use criticality in the fashion that Morbius is so stuck on. We always specify "critical mass" or "critical geometry" if you want to talk about those specific aspects of core design. In the world of nuclear power, which I have restricted my comments to, criticality is always in terms of neutron population.

    To talk about who is using what terminology in a "non-standard" fashion is needless. To a physicist, it's purely in the mathematics. They will use "critical" in the abstract mathematical sense, which considers only the mathematical model. In the functional nuclear power world, "critical" is always in regards to actual neutron population behavior.

    This statement
    ... is laughable. I understand it just fine. I just happen to be in the position of having to deal with the reality of nuclear power operations, rather than just spouting theory and mathematics at people.

    Maybe if I go to the nearest nuclear engineering grad program, and get my PhD, my true-to-life experiences will stop meaning anything? Maybe it has to be a physics PhD...
  21. Feb 22, 2007 #20

    I couldn't agree more.

    As someone currently doing their PhD dissertation in theoretical chemistry, I know what you mean. I worked in industry for several years before going to grad school, so I am well familar with the false assumptions behind the attitude that Emfuser is espousing. I've even been referred to once as "too academic", as if that were an insult :rofl:

    Despite being a "paper-bound scientist" I have managed to design a few systems at my computer-bound desk that have worked out just fine in the non-paper-bound "real world" (as if the entire theoretical physics established by humanity were somehow "not real") :rolleyes: My experimental collaborators seem happy enough at these abstract, and hence clearly false, results. :tongue:
  22. Feb 22, 2007 #21
    No, your experiences of course mean a great deal and there is no substitute for that. But you may also realize that folks who have a PhD in physics have worked very hard to understand their subject matter extremely well.

    It was the endeavors of academic physicists that resulted in the development of nuclear science (over 90% of all cutting edge science is still done at universities and government labs) - in a setting of peer review, not supply and demand. Do you respect that even a tiny bit?
  23. Feb 22, 2007 #22
    I believe you misinterpret my words.

    I have a real problem with people stuck in the theoretical world who go around beating people over the head with their degrees. I have an even bigger problem with the people who not only beat people over the head with their degrees, but they do so to those who are actually knee deep in the applied world, and not sitting at a desk or in a lab.

    It is pretentiousness such as what I see in this thread that causes the very disrespect that you describe. The failure of high-horse-riding scientists to acknowledge the realities of how something is actually done vs. the conceptual world is the very reason that the idea of being "too academic" exists. Any engineer recognizes full well the value of the theory and methodology we are taught, but it's not until you take that engineer, slap a hardhat and some PCs on him, and drag him into containment while we're offline does he or she gain the understanding of what is actually going on when you have to take those ideas you came up with on paper, and try to fit them with reality.

    Thus, I will reiterate that I do not come into this thread with any agenda to disrespect, but I have little tolerance for people who don't actually do the job I do, telling other readers that I have a gross misunderstanding of the concepts at hand while simultaneously making needlessly obnoxious comments about academic degrees, and figuratively thumping their chests, with authority.

    That turbine sitting a few hundred feet away from me is still spinning. That reactor is critical. No, we don't have any silly requirement about having a neutron source in the core all the time.

    Now, can we agree to acknowledge that there are two schools of equally valuable thought here? One is the purely theoretical, one is applied. They each have their own nuances, and it would probably be a good idea for those involved in each to be representing their respective schools of thought, without one trying to put on the hat of the other, and speak for them.

    Is such a suggestion agreeable to you academic folks? You can talk in the realm of theory all you want, but instead of trying to stifle and browbeat people who come from the applied side, actually consider their viewpoints.

  24. Feb 22, 2007 #23


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    WRONG!!! I have worked on design and design methods for REAL devices; real
    nuclear reactors, and other real nuclear systems.

    These reactors and systems have been BUILT and TESTED!!!

    NO - you can't paint me as someone who is in an ivory tower somewhere.

    My experience is AT LEAST as "real" as yours!!!

    Could have fooled me!!!

    WRONG!!!! My thesis supervisor was the late Prof. Allan F. Henry - who used to be
    the head of codes and methods development at Westinghouse. Prof. Henry was a
    stickler for the proper use of these terms. In his textbook, which is one of the seminal
    texts in nuclear reactor theory; Prof Henry made a point of the fact that "critical" is a
    property of the geometry and materials, and does NOT mean neutrons!!!

    And you are WRONG!!! It is those "in the industry" that have BASTARDIZED the term;
    and use it incorrectly.

    I note that you sidestepped my challenge to answer the questions I posed in my last
    post. It would have been illustrative for you to have attempted them.

    However, let's consider a super-critical assembly - like the example I gave of the
    core of Little Boy before it exploded. Lets assume that we have this assembly sitting
    there without any neutrons present.

    Now suppose I introduce a single neutron heading into the assembly. [ This is the type
    of problem nuclear weapons designers have to consider.] The interactions of neutrons
    with the material is stochastic - that is it is probabilistic - NOT deterministic. There is
    a finite probability that the neutron thus introduced will be absorbed without starting a
    chain reaction. That's because the radiative capture cross-section of the Uranium is

    All we can say is that there is a certain probability that the neutron so introduced will
    lead to a chain reaction - it is not guaranteed. So how do we determine that probability?

    That probability is a VERY REAL physical property of the assembled nuclear system.

    It turns out - that the probability is a function of the "k"; or "criticality" of the assembled
    system - something Emfuser claims doesn't exist because there are no neutrons.

    There's a whole inter-related body of physics and knowledge here that requires that
    "k" and "criticality" exist as a function of the geometry and materials independent of
    whether there are neutrons present or not.

    Emfuser, you are analogous to the child that is playing a toy piano wherein all the black
    keys are just painted on. The only notes you have at your disposal are those in the key
    of C-Major. The problem is you don't see any use for the other notes.

    Well - there is a WHOLE WORLD of music and music theory beyond the key of C-Major.

    There's the key of D, E,....as well as the existence of "minor keys", and "diminished keys"
    ... ALL of which you see no use for because you are only familar with the notes in the
    key of C-Major.

    Then you would have the audacity to tell Mozart that he doesn't need any notes beyond
    those of C-Major; when Mozart has actually composed symphonies, that you HAVEN'T!!

    You keep proving that you DON'T understand just fine.

    I doubt that you have done say a reactor noise analysis? Have you ACTUALLY done
    reactor noise analysis?

    OK - you're a startup engineer. Did you DESIGN the reactor?

    A friend of mine was a startup engineer. I'm familiar with the job. You've essentially
    been given the reactor - you DIDN'T design it. You didn't deal with all the physics that
    require the mathematics and concepts that I'm talking about.

    If you do go study at a nuclear engineering grad program; you would find out that
    "criticality" is a property of the geometry and materials.

    Dr. Gregory Greenman
    Last edited: Feb 22, 2007
  25. Feb 22, 2007 #24


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    Since Emfuser took a pass at answering the questions i posed; I'll do so for you.

    In regards to the core of the Little Boy device sitting there without any neutrons:

    Is Little Boy critical? Yes - in fact the assembled Little Boy core is super-critical.
    Even though there are no neutrons, the "k" of Little Boy is defined; and it is greater than
    unity. The "k" and the "criticality" are properties of the geometry and materials;
    irrespective of whether there are neutrons in the device at the time.

    As I explained in my previous response to Emfuser, one of the physical properties
    of the super-critical assembly is the probability that a neutron introduced at a given
    position, with a given energy, and going in a given direction will lead to a chain reaction,
    as opposed to immediately being absorbed, or having the chain reaction start and then
    die out. It turns out that the probability that the neutron will cause a runaway chain
    reaction is a function of the "k" the assembly [ in addition to the properties of the added

    Under Emfuser's understanding [ or in actuality lack thereof ]; there would be no "k"
    defined because his "understanding" requires neutrons to be present. The super-critical
    Little Boy assembly has a physical property every bit as REAL as any other physical
    property - namely this probability - but we can't characterize it without the value of "k".

    Evidently Emfuser is also ignorant of the concept of "neutron importance". You need
    the concept of neutron importance when you define reactivity. Neutron importance
    will properly account for the fact that a given number of neutron introduced into the
    core will have differing effects on the reactivity based on the position, energy, and
    direction of the incident neutrons.

    The neutron importance is the solution to the adjoint transport equation. However,
    unlike the forward transport equation which solves for a physical quantity - namely
    the distribution of neutrons in phase space - there are no "adjoint neutrons". So
    Emfuser would contend that it isn't "real".

    As another analogy, consider the suspension system on your car. This suspension
    system can either be underdamped, overdamped, or "critically damped". [ If your
    "shocks" are worn - it will be underdamped. ] This property of the car's suspension
    system EXISTS and is WELL DEFINED - independent of whether the suspension
    system is actually moving at the time. The car's suspension is either underdamped,
    overdamped, or critically dampded; even if it is not moving. Likewise, a reactor is
    sub-critical, super-critical, or exactly critical; independent of whether there are actually
    neutrons in the reactor multiplying or attenuating.

    Emfuser knows only basic Reactor Theory 101. Fortunately, I would assume the
    people that actually designed the reactor he works on have a more comprehensive
    knowledge of Nuclear Reactor Theory.

    The question about putting a pulse into an exactly critical reactor:

    Putting a pulse of neutrons into the reactor DOESN'T CHANGE the criticality.

    Likewise, putting a pulse of neutrons in the reactor DOESN'T CHANGE the "k" of the
    reacor. [ Assuming no heating that would alter the material properties. There will be
    an insignificant amount of heating - depending on the magnitude of the pulse ].

    The reactivity is different. The reactivity DOES DEPEND on the distribution of neutrons
    in the reactor.

    Before the pulse. the exactly critical reactor had a reactivity of ZERO. The reactivity
    vanishes because the geometry and materials are in an exactly critical configuration,
    AND I also premised the question with the fact that the neutron population present was
    the fundamental mode. Under those conditions, the reactivity vanishes identically.

    However, when I add the pulse of neutrons; I'm augmenting the neutron population with
    a bunch of neutrons that are NOT in the same distribution as the fundamental mode.

    With the addition of this neutron pulse; the reactor will go through a transient on its way
    to a new steady-state. During this transition the reactivity is NOT ZERO!!

    When the reactor reaches a new steady state with an increased neutron population; that
    new population will be a multiple of the fundamental mode; so that the neutron population
    is once again in the fundamental mode and the reactivity vanishes exactly.

    [This is all modulo the assumption of constant material properties; i.e. insignificant heating
    or the assumption that the cooling system removes any excess heat ]

    This is EXACTLY the type of questions one would expect in a graduate program in
    nuclear engineering; in a reactor theory or transport theory course.

    Dr. Gregory Greenman
    Last edited: Feb 22, 2007
  26. Feb 22, 2007 #25
    i have a copy of Arfken's "mathematical methods for physicist" from a grad course i took.

    in one chapter it gives what is called the neutron diffusion equation as:

    [tex]-D \grad^2 \phi(r) + k^2 D \phi(r) = Q \delta(r)[/tex]

    where [tex]\phi(r)[/tex] is the neutron flux, [tex]Q\delta(r)[/tex] is the source, k and D are constants (I assume D is the diffusion constant).

    (the solution involves fourier transforming, solving, then back-transforming).

    i am curious, is the "k" in this expression the same "k" that you guys are talking about? (i know virtually nothing about nuclear science).
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