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Which is more efficient: Nuclear Q

  1. Aug 26, 2006 #1
    Which is more efficient, nuclear power using enriched uranium and light water or unenriched uranium and heavy water?

    and by efficient I mean uses the least materials and energy for maximum energy output.

    And I'm only asking out of pure curiosity.
    Last edited: Aug 26, 2006
  2. jcsd
  3. Aug 28, 2006 #2


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    The answer will depend upon the LWR enrichment, and the average burnups achieved by both fuels, and the thermal-to-electrical conversion efficiency (which is temperature dependent) of the power generation system.

    I'll have to get back on this, since it requires some calcs.
  4. Aug 28, 2006 #3


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    I'm afraid your question is ill-posed.

    In regard to your definition of efficiency being with respect to "maximum energy output";
    you have to realize that one you get the reactor critical - you can operate it at ANY
    power level you desire. The energy output of a reactor is limited by its cooling
    system, and not by the fuel it uses and the enrichment thereof.

    One could ask the question as to which reactor type uses the least amount of
    material to achieve criticality; but even here one has to be specific - least amount of
    what material? Total Uranium? Total U-235?

    Because the concentration of U-235 is higher for a reactor using enriched uranium;
    the core of an enriched uranium reactor can be made physically smaller than that
    for an unenriched uranium reactor. However, the fuel is seldom the sole limiting factor
    in the design of the reactor core. Cooling consideration play a much bigger part.

    If a country has access to enriched uranium fuel, they usually opt for enriched uranium
    reactors, while others like Canada, forego enrichment and use unenriched uranium in
    their heavy water moderated CANDU reactors.

    Both reactors work equally well as heat sources to drive a steam cycle power plant.

    Dr. Gregory Greenman
  5. Aug 28, 2006 #4


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    This statement is hyperbole, yes? It implies infinite energies. If this statement were litereally true as stated, one power plant of arbitrarily small size could power the whole world without operating at its maximum capcity.

    I think the OP was asking which type of reactor produces the most power for the least overall cost investment of energy & fuel.
  6. Aug 28, 2006 #5


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    There is a practical limit (not a theoretical one) in that at some point the reactor would explode or meltdown.
  7. Aug 28, 2006 #6


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    Not hyperbole. It doesn't imply infinite energy - but infinite power. The transport
    equation is linear and homogeneous; thus for any solution, any arbitrary multiple of
    that solution is also a solution. If there were no feedbacks, the reactor could put out
    infinite power.

    The practical limit is if the same material were put into a weapon configuration, not a
    reactor configuration; how much energy would be delivered then.

    That's the question I'm saying doesn't make sense. For a given investment of
    energy and fuel, you can get the reactor critical - thus it produces energy. If the
    cooling system can handle it - this same reactor can operate at whatever power
    level your cooling system can handle.

    In practice, the power level of the reactor is not limited by the fuel; it is limited by
    your capacity to get the heat energy generated out of the core.

    Dr. Gregory Greenman
  8. Aug 29, 2006 #7


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    It would be useful to constrain the problem to say a 1000 MWe and an equilibrium cycle for the respective plants. Then it is a matter of energy production per unit mass of fuel and the cost associated with producing and utilizing the fuel. Enriched fuel has the cost of enrichment and U-ore compared to using natural uranium. But then one has to look at the burnup (energy produced per unit mass, e.g. GWd/tU, or MWh/kgU). CANDUs use less ore input per unit mass, but they also have lower burnups, so they might use 5 or 6 times the mass of fuel.
  9. Aug 29, 2006 #8


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    Exactly. That's why I said the problem as originally stated is ill-posed; it is under
    constrained in its definition.

    One also has the cost of the isotopic separation for making the heavy water.
    Although not as difficult, nor energy intensivie as enriching uranium; it still
    represents a significant factor.

    In the end, the decision seems to hinge on whether a country has access to
    enrichment technology. If a country has enrichment technology, or can purchase
    fuel from a country that does have enrichment technology; then light water reactors
    are usually used.

    Canada on the other hand, does not have enrichment technology, but does seek to
    be self-sufficient in their nuclear fuel supply. Hence, they developed / use the
    CANDU system.

    Dr. Gregory Greenman
  10. Aug 29, 2006 #9


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    Yep, that's another part of the problem. Then one also has to consider the backend costs, which historically are one of the biggest unknowns.

    If we stick to mass of ore/fuel and energy required for enrichment and heavy water, then we'd have the simplest comparison. Trying to determine actual costs is much more difficult, especially when one figures in discount rate and currency conversion rates.
  11. Aug 30, 2006 #10


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    Generally speaking, light water reactors are safer, more reliable and easier to maintain, IMO. Albeit I think both designs are obsolete. PBR's and fast gas reactors will, again IMO, be the wave of the future. The safety factor alone is overwhelmingly attractive.
  12. Aug 30, 2006 #11


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    The gas-cooled fast reactor [GCFR ] is probably one of the LEAST forgiving in terms of

    One has the high power density that is the hallmark of a fast reactor, coupled with the
    low thermal inertial [ heat capacity ] of a gas-cooled reactor. One needs a lot of good
    engineering to resolve those problems.

    At this stage of the game; I wouldn't say that any reactor types are "obsolete" or that
    any other design is "overwhelmingly attractive".

    Dr. Gregory Greenman
  13. Oct 20, 2006 #12
    Outside of +ve void co-eff. I don't think there is much argument to say that light water is safer than CANDU.

    As far as fuel burnup is concerned, I could be wrong but I thought that the CANDU online refuelling made it so that we had higher fuel burnup, since we never run with extra poison due to a new fresh fuel load. I guess this is likely offset due to the lower Bundle Power of the natural Uranium. That said, I would assume Maximum bundle power in a PWR must be about the same since it is center line melting and dryout that are the limiting factors. I guess the enriched bundle could just stay incore for more days.

    Any comments??
    Last edited: Oct 20, 2006
  14. Oct 28, 2006 #13


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    It's not hyperbole at all - it is simply the physics and mathematics of neutron transport.

    By what "logic" did you conclude that the multiplicative indetermism of the power
    necessarily implies "infinite energy"? Power is the derivative of the energy with
    respect to time. An infinite derivative DOES NOT imply an infinite value of the function.

    Take for instance, an electrical "square wave" - a voltage that jumps from 0 to 1 then
    back down to zero, then later jumps back up to 1.....

    Although one can't make a perfect square wave in practice; an "ideal" square wave
    has the voltage jump from 0 to 1 instantaneously. The value of the derivative is

    Using your flawed reasoning; one would conclude that the infinite derivative would
    result in an infinite value of the voltage. But that isn't true. The voltage stays finite.

    The neutron transport equation is linear and homogeneous. IF, and that is a big IF;
    one is able to cool the reactor adequately - one can take the energy out as fast as
    one likes.

    That does NOT imply infinite energy. The total amount of energy in the finite mass of
    fuel is FINITE.

    This is essentially what allows nuclear weapons to work. There is a finite amount of
    energy; even a nuclear weapon is not an infinite source of energy; and that energy
    can be produced in a very small amount of time. However, that time is finite; so the
    power of a nuclear weapon is not infinite.

    However, the mathematics and physics allows energy to be released at very fast rates.
    We are limited in how fast we can extract that energy by limits on how well we can
    cool the system or hold it together if we don't adequately cool it.

    Dr. Gregory Greenman
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