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List of Neutron Moderators

  1. Mar 3, 2015 #1
    Hello everyone...
    I've been trying to make a list about moderator materials used in nuclear reactors. So far I've found some but still wondering if there exists more than I learned and also wondering if they are correct.

    Please see the list below and feel free to correct my mistakes or to add new materials.

    - Deuterium
    - Oxygen
    - Hydrogen
    - Carbon (Graphite)
    - Beryllium
    - Lithium
    - Boron-10
    - Boron-11
    - Vanadium
    - H20 (Water)
    - D20 (Heavy Water)
    - BeO (Beryllium Oxide)
    - ZrH0.9, ZrH1.5, ZrH1.6, ZrH1.65, ZrH1.7, ZrH1.85
    - ZrH2 (Zirconium Hydride)
    - YH2 (Yttrium (II) Hydride)
    - YH3 (Yttrium (III) Hydride)
    - LiH (Lithium Hydride)
    - ZrD2 (Zirconium Deuteride)
    - UZrH (Uranium Zirconium Hydride)
    - UH3 (Uranium Hydride)
    - TiH2 (Titanium Hydride)
    - C19H16 (Triphenylmethane)
    - FLiBe (Mixture of LiF-Lithium Floride- and BeF2-Beryllium Floride)

    And please do not hesitate to share your knowledge about the materials.

    Best regards...
     
  2. jcsd
  3. Mar 3, 2015 #2
  4. Mar 3, 2015 #3
    I saw Helium in the forum and than learned that its moderation ratio is so small. For that and due to the other issues mentioned in the page, I can not decide about it.
     
  5. Mar 3, 2015 #4

    QuantumPion

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    I think you will have trouble trying to use boron as a moderator.
     
  6. Mar 3, 2015 #5
    Honestly, I do not know. If you say such thing, you probably know something. The reason of adding Boron to the list was due to the Wikipedia page.
    You said that since boron is a highly effective neutron absorber, right?
     
  7. Mar 3, 2015 #6

    mathman

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  8. Mar 3, 2015 #7

    Astronuc

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    10B is a highly effective neutron absorber (σα ~ 5000b), which is why it is used as a soluble neutron absorber (buffered with LiOH or KOH) in PWRs, as an absorber on the surface of fuel pellets (ZrB2), as a solid absorber in wet-annular burnable absorbers (WABAs) or as pyrex in burnable absorber assemblies (BPAs), and in PWR and BWR control elements. Boron is often enriched above 50% 10B.

    6Li has a strong σα as well, so it and B would be consumed rather quickly and compete for neutrons, so they are used in low concentrations in PWRs. 7Li would be better.

    Vanadium (A=51) is not much of a moderator, but perhaps like Fe, has a good fast removal cross-section.

    He is a gas, so in a high temperature, graphite moderated reactor, it is as a coolant.

    Be is best used with a stabilizing element like Y.
     
    Last edited: Mar 3, 2015
  9. Mar 4, 2015 #8
    In here, sodium indicated as coolant I think, not as moderator. But the there also exists a sentence "In contrast, sodium atoms are much heavier than both the oxygen and hydrogen atoms found in water, and therefore the neutrons lose less energy in collisions with sodium atoms." Is this the reason that you add the sodium to the list as moderator ?
     
  10. Mar 4, 2015 #9
    Thanks for your explanations. So removing Vanadium, Boron and He will give me a more accurate list I assume.
     
  11. Mar 4, 2015 #10

    QuantumPion

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    A more accurate list would simply be: hydrogen, deuterium, beryllium and carbon; and any low-absorption compounds containing those elements.

    I've never heard of lithium being used as a moderator but I guess depleted Li7 would work, although I don't know much about it.
     
  12. Mar 4, 2015 #11
    Thanks a lot QuantumPion. I also would like to know your opinions about ZrH2 ,YH2, YH3, UH3, TiH2 basically metal hydrides.
     
  13. Mar 4, 2015 #12

    mathman

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    The reactor described in the article uses sodium as the moderator.
     
  14. Mar 4, 2015 #13

    Astronuc

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    I'm not so sure. The reference to moderator in the article on the sodium-cooled fast reactor is "An advantage of liquid metal coolants is high heat capacity which provides thermal inertia against overheating. Water is difficult to use as a coolant for a fast reactor because water acts as a neutron moderator that slows the fast neutrons into thermal neutrons." [my bold]

    Fast reactors do not employ moderators, otherwise, they would not be fast reactors. Lithium, particularly 7Li, would a better moderator. It's all about the atomic mass and atomic density, and how many collisions it takes to slow fast neutrons to epithermal or thermal range.
     
  15. Mar 4, 2015 #14

    Astronuc

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    I've seen some fuel designs that incorporate metal hydrides, e.g., the TRIGA reactor at the university where I studied nuclear engineering use U-Zr hydride fuel.

    http://www.ga.com/triga-fuels

    Thermal stability of metal hydrides is important. The hydrogen needs to stay put.

    The form of the moderator is important. Water is circulated, while Be/BeO and C are solid. In the case of FLiBe, the salt is liquid at operating temperature, but moderation is facilitated by graphite (C). Changes in moderator density are important to consider since this affects the neutron energy spectrum.

    Voids due to boiling of water hardens the spectrum. In the case of Na, voiding adds positive reactivity since the voids remove Na that otherwise 'absorbs' neutrons.
     
  16. Mar 4, 2015 #15

    mheslep

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    ZrH1.6 is the moderator chosen for the Transatomic molten salt reactor proposal, enabling 86 MWth/m3, a very high power density.
     
  17. Apr 9, 2015 #16
    Listing the minimum cross-sections of elements:
    H 0,519 mb D; 332,6 mb
    He 0 4; 7,47 mb
    Li 45,4 mb 7; 70,5 b
    Be 7,6 mb 9; M
    B 5,5 mb 11; 768 b
    C 1,37 mb 13; 3,5 mb
    N 0,024 mb 15; 1,9 b
    O 0,1 mb 16; 0,19 mb
    F 9,6 mb 19; M
    Ne 36 mb 20; 39 mb
    Na 530 mb 23; M
    Mg 38,2 mb 26; 63 mb
    Al 231 mb 27; M
    Si 101 mb 29; 171 mb
    P 172 mb 31; M
    S 150 mb 36; 530 mb
    Cl 433 mb 37; 33,5 b
    Ar 660 mb 40; 675 mb
    K 1,46 b 41; 2,1 b
    Ca 410 mb 40; 430 mb
    Sc 27,5 b 45; M
    Ti 179 mb 50; 6,09 b
    V 4,9 b 51; 5,08 b
    Cr 360 mb 54; 3,05 b
    Mn 13,3 b 53; M
    Fe 1,28 b 58; 2,56 b
    Co 37,18 b 59; M
    Ni 1,52 b 64; 4,49 b
    Cu 2,17 b 65; 3,78 b
    Zn 92 mb 70; 1,11 b
    Ga 2,18 b 69; 2,75 b
    Ge 160 mb 76; 2,2 b
    As 4,5 b 75; M
    Se 44 mb 82; 11,7 b
    Br 2,7 b 81; 6,9 b
    Kr 3 mb 86; 25 b
    Rb 120 mb 87; 380 mb
    Sr 58 mb 88; 1,28 b
    Y 1,28 b 89; M
    Zr 11 mb 90; 185 mb
    Nb 1,15 b 93; M
    Mo 15 mb 94; 2,48 b
    Ru 280 mb 96; 2,56 b
    Rh 144,8 b 103; M
    Pd 226 mb 110; 6,9 b
    Ag 37,6 b 107; 63,3 b
    Cd 75 mb 116; 2520 b
    In 12 b 113; 194 b
    Sn 114 mb 114; 626 mb
    Sb 3,8 b 123; 4,91 b
    Te 215 mb 128; 4,7 b
    I 6,15 b 127; M
    Xe 260 mb 136; 24 b
    Cs 29 b 133; M
    Ba 270 mb 138; 1,1 b
     
  18. Apr 9, 2015 #17

    Astronuc

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    Minimum with respect to what?

    What is one attempting to indicate with these data?
     
  19. Apr 10, 2015 #18
    Minimum absorption of all stable isotopes, and which isotope it is. If an element has a single isotope, it is marked with M; otherwise the absorption of natural composition is given for comparison.

    Note that boron is actually a pretty good moderator - better than beryllium!

    Another relevant feature, though, would be scattering cross-section.
    D has scattering cross-section 7,64 barns. Which means the number of scatterings per capture is 7,64 b/0,519mb=14 700
    Whereas H has extremely high cross-section for scattering: 82,03 b (probably thanks to virtual deuterons). So the number of scatterings per capture is 82,03/0,3326=246,6. Still fairly good despite high capture cross-section. And H is a very good neutron reflector even at low densities, which D is not.

    He-4 scattering cross-section is 1,34 b. So even though absorption is zero and He-4 is a perfect mirror, it has to be pretty thick and dense to prevent transmission - much thicker than H and even thicker than D.
    Natural He has absorption 7,47 mb (all due to He-3), scattering 1,34 b - thus 179 scatterings per capture. Worse than H.

    Furthermore, another relevant quality factor is energy loss per scattering.
    It depends on the nuclear mass, but how to quantify it?
     
    Last edited: Apr 10, 2015
  20. Apr 11, 2015 #19
    Li-7: scattering 1,4 b, absorption 45,4 mb, so 31 scatterings per absorption - not a good moderator. Not to mention natural Li has scattering 1,37 b, absorption 70,5 b, so 0,02 scatterings per absorption.
    Be-9: scattering 7,63 b, absorption 7,6 mb, so 1004 scatterings per absorption
    B-11: scattering 5,77 b, absorption 5,5 mb, so 1050 scatterings per absorption. Only slightly better than Be, and considering the lower mass of Be, Be is probably the better moderator after all. Of course, natural B has scattering 5,24 b, absorption 768 b, thus 0,007 scatterings per absorption!
    C-13: scattering 4,84 b, absorption 1,37 mb, so 3530 scatterings per absorption. For natural C, scattering 5,55 b, absorption 3,5 mb, so 1600 scatterings per absorption.
    N-15: scattering 5,21 b, absorption 0,024 mb, so 220 000 scatterings per absorption. For natural N, scattering 11,51 b, absorption 1,9 b, so 6 scatterings per absorption.
    O-16: scattering 4,232 b, absorption 0,1 mb, so 40 000 scatterings per absorption. For natural O, scattering 4,232 b, absorption 0,19 mb, so 22 000 scatterings per absorption.
    F-19: scattering 4,018 b, absorption 9,6 mb, so 420 scatterings per absorption.
    Ne-20: scattering 2,965 b, absorption 36 mb, so 75 scatterings per absorption. For natural Ne, scattering 2,628 b, absorption 39 mb, so 57 scatterings per absorption.
    Na-23: scattering 3,28 b, absorption 530 mb, so 6,2 scatterings per absorption.
    Mg-26: scattering 3 b, absorption 38,2 mb, so 80 scatterings per absorption. For natural Mg, scattering 3,71 b, absorption 63 mb, so 59 scatterings per absorption.
    Al-27: scattering 1,503 b, absorption 231 mb, so 6,5 scatterings per absorption.
    Si-29: scattering 2,78 b, absorption 101 mb, so 27,5 scatterings per absorption. For natural Si (popular construction material component, O in silicates is much less absorbing!), scattering 2,167 b, absorption 171 mb, so just 12,7 scatterings per absorption.
    ...
    K-41: scattering 1,2 b, absorption 1,46 b, so 0,8 scatterings per absorption. For natural K, scattering 1,96 b, absorption 2,1 b, so 0,93 scatterings per absorption.
    ...
    Ti-50: scattering 4,8 b, absorption 179 mb, so 27 scatterings per absorption. For natural Ti, scattering 4,35 b, absorption 6,09 b, so 0,7 scatterings per absorption.
    V-51: scattering 5,09 b, absorption 4,9 b, so 1,04 scatterings per absorption. For natural V, scattering 5,1 b, absorption 5,08 b, so 1,00 scatterings per absorption.
    Cr-54: scattering 2,6 b, absorption 360 mb, so 7,2 scatterings per absorption. For natural Cr, scattering 3,49 b, absorption 3,05 b, so 1,14 scatterings per absorption.
    ...
    Fe-58: scattering 28 b, absorption 1,28 b, so 22 scatterings per absorption. For natural Fe, scattering 11,62 b, absorption 2,56 b, so 4,54 scatterings per absorption. Note how good reflector even natural Fe is compared to the other common construction elements like Mg, Al or Si - but still, Fe-58 (0,3% in natural Fe) is much more useful!
    ...
    Zn-70: scattering 4,5 b, absorption 92 mb, so 50 scatterings per absorption. For natural Zn, scattering 4,131 b, absorption 1,11 b, so 3,7 scatterings per absorption.
    Ga-69: scattering 7,89 b, absorption 2,18 b, so 3,62 scatterings per absorption. For natural Ga, scattering 6,83 b, absorption 2,75 b, so 2,48 scatterings per absorption.
    ...
    Kr-86: scattering 8,2 b, absorption 3 mb, so 2700 scatterings per absorption. For natural Kr, scattering 7,68 b, absorption 25 b, so 0,3 scatterings per absorption.
    Rb-87: scattering 7,1 b, absorption 120 mb, so 59 scatterings per absorption. For natural Rb, scattering 6,8 b, absorption 380 mb, so still 18 scatterings per absorption. Compare K above. Na and Rb do form an eutectic, with -5 Celsius melting at 82 atom % Rb, so even natural NaRb should be much more useful than NaK!
    ...
    Y-89: scattering 7,7 b, absorption 1,28 b, so 6 scatterings per absorption.
    Zr-90: scattering 5,1 b, absorption 11 mb, so 460 scatterings per absorption. For natural Zr, scattering 6,46 b, absorption 185 mb, so 35 scatterings per absorption.
    ...
    Mo-94: scattering 5,81 b, absorption 15 mb, so 390 scatterings per absorption. For natural Mo, scattering 5,71 b, absorption 2,48 b, so 2,3 scatterings per absorption.
    ...
    Cd-116: scattering 5 b, absorption 75 mb, so 67 scatterings per absorption. For natural Cd, scattering 6,5 b, absorption 2520 b, so 0,0025 scatterings per absorption.
    ...
    Sn-114: scattering 4,8 b, absorption 114 mb, so 42 scatterings per absorption. For natural Sn, scattering 4,892 b, absorption 626 mb, so 7,81 scatterings per absorption. Note that Sn-114 (0,7 % natural) is a middle isotope.
    ...
    Hg-204: scattering unknown (as of 1999), absorption 430 mb. For natural Hg, scattering 26,8 b, absorption 372 b, so 0,07 scatterings per absorption, yet was successfully used as coolant of a reactor (Clementine).
    Tl-205: scattering 11,4 b, absorption 104 mb, so 110 scatterings per absorption. For natural Tl, scattering 9,89 b, absorption 3,43 b, so 2,88 scatterings per absorption.
     
    Last edited: Apr 11, 2015
  21. Apr 11, 2015 #20
    Pb-208: scattering 11,34 b, absorption 0,48 mb, so 24 000 scatterings per absorption. For "natural" Pb, scattering 11,1 b, absorption 171 mb, so 65 scatterings per absorption.
    Note that Pb-208 is natural and does not need isotope separation - just choose ore which on formation included Th-232 to the exclusion of any preexisting Pb and U.
    Bi-209: scattering 9,156 b, absorption 33,8 mb, so 271 scatterings per absorption.
     
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