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A H-BOMB ''castle bravo'' -- Why was the yield so high?

  1. Sep 21, 2017 #1
    in the design of the hydrogen bomb ''castle bravo'' they usd lithium deuteride
    for the secondary part of the bomb.
    the yeld of the bomb was supposed to be 5 Mega Tons but it was 15
    i have 2 questions
    1) why they usd LiD as a fussin full instead of normal deuterium
    2) why did the lithium acted as fussion full
     
  2. jcsd
  3. Sep 21, 2017 #2
    LiD is a solid at room temperature, deuterium boils at −250 C.
    LiD is about 10 times denser than liquid deuterium.
     
  4. Sep 22, 2017 #3
    ok and how did the lithium increase the yeld so much ?
     
  5. Sep 22, 2017 #4
    The primary energy producing reaction is between deuterium and tritium. Tritium isn't used because it is an isotope of hydrogen, and it's really low density. It's also hard to produce in quantity.
    Lithium contains both Li-6 and Li-7. The lithium 6 was expected to produce tritium when hit by a neutron from either the fission bomb or from the fusion reactions, but Li-7 struck by a fast neutron also produces tritium. This produced more fusion reactions, those produced even more neutrons, wich produced even more tritium as well as more fission in the uranium tamper of the bomb.
     
  6. Sep 22, 2017 #5
    ok thank you man but why shuld lithium 6 go to tritium and not just become lithium 7 ? and why shuld the uranium tamper go fission ? (the tamper is U 238) i think !
     
  7. Sep 22, 2017 #6

    mfb

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    Li-6+n -> Li-7 is a very unlikely reaction - where would the excess energy go to? The nucleus can emit a photon, but splitting into helium plus tritium is much more likely.
    Why not? Fast neutrons can split U-238.
     
  8. Sep 22, 2017 #7
    wow i dident know that U-238 can get split from a Neutron are you sure abut that ?
     
  9. Sep 22, 2017 #8

    1oldman2

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    This is a useful site, many interesting links. The beginning mentions what was known as the "shrimp device" which is the test I believe your mentioning. This particular test was an education for the designers concerning the "Tritium bonus". The story is a very interesting read.
    http://nuclearweaponarchive.org/Usa/Tests/Castle.html
     
  10. Sep 22, 2017 #9

    mfb

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    It is the main source of the explosive yield of most high-yield thermonuclear weapons. U-235 fission produces the energy to start the fusion reaction, that leads to many high-energy neutrons and they fission U-238 put around the fusion stage.

    It doesn't play a role in nuclear reactors as fission doesn't release many neutrons with sufficient energy to fission U-238.
     
  11. Sep 23, 2017 #10
    so in a beryllium oxide reflector or graphite reflector there is lest yeld right ?
     
  12. Sep 23, 2017 #11
    (Mods - I hope this is not too off-topic for either the thread or the forum; the content is non-technical but relates to the historical question of yield calculation for Castle Bravo.)

    @asaf154 - You probably have already realized this, but just to clarify: When you ask "The yield of the [Castle Bravo] bomb was supposed to be 5 Mt but it was 15," there is more to that question than just the technical issues to do with lithium deuteride. In popular accounts, this difference in predicted vs. actual yield is represented as one of the worst technological mistakes (w/ resulting bad consequences) in modern history; the link posted by @1oldman2 has a brief but informative description of just how bad these consequences were. (Minor point, the actual estimate was 6 Mt, not 5 Mt, with a range of 4-8 Mt.)

    Myself, I read about all this in a book about the uses & abuses of shared information, The Knowledge Illusion, published 2017, by cognitive scientists Steven Sloman and Philip Fernbach. They use Castle Bravo as an example of how even the best human efforts at prediction and control of technology can fail in the face of complexity; here is part of their account:

    Two hours after the blast, a cloud of fallout blew over the Daigo Fukuryū Maru [a Japanese fishing boat] and rained radioactive debris on the fishermen for several hours. Almost immediately the crew exhibited symptoms of acute radiation sickness—bleeding gums, nausea, burns—and one of them died a few days later in a Tokyo hospital. Before the blast, the U.S. Navy had escorted several fishing vessels beyond the danger zone. But the Daigo Fukuryū Maru was already outside the area the Navy considered dangerous. Most distressing of all, a few hours later, the fallout cloud passed over the inhabited atolls Rongelap and Utirik, irradiating the native populations. Those people have never been the same. They were evacuated three days later after suffering acute radiation sickness and temporarily moved to another island. They were returned to the atoll three years later but were evacuated again after rates of cancer spiked. The children got the worst of it. They are still waiting to go home.

    The explanation for all this horror is that the blast force was much larger than expected . . . The scientists behind Shrimp expected it to have a blast force of about six megatons, around three hundred times as powerful as Little Boy. But Shrimp exploded with a force of fifteen megatons, nearly a thousand times as powerful as Little Boy. The scientists knew the explosion would be big, but they were off by a factor of about 3.

    The error was due to a misunderstanding of the properties of one of the major components of the bomb, an element called lithium-7. Before Castle Bravo, lithium-7 was believed to be relatively inert. In fact, lithium-7 reacts strongly when bombarded with neutrons, often decaying into an unstable isotope of hydrogen, which fuses with other hydrogen atoms, giving off more neutrons and releasing a great deal of energy. Compounding the error, the teams in charge of evaluating the wind patterns failed to predict the easterly direction of winds at higher altitudes that pushed the fallout cloud over the inhabited atolls.​

    So according to this sort of account, mistakes were made in two different categories: yield prediction and weather patterns; but it was the mistake w/ yield that mattered most, with mistakes in predicting wind patterns only "compounding" the initial mistake.

    However . . . popular accounts are not always entirely accurate; sometimes they get over-simplified by repetition. In searching the web to find more information, I came across a 2013 entry on a blog, "Nuclear Secrecy," titled "Castle Bravo Revisited"; the writer's concern is mainly to do with the weather patterns and location of the test. So far so mundane; however he mentions that in researching the topic, he was sent "a PDF of a recent report (January 2013) by the Defense Threat Reduction Information Analysis Center (DTRIAC) that looked back on the history of BRAVO." Link to that blog post here and link to the PDF here.

    What's so interesting about the DTRIAC report when it comes to yield? Simply that it asserts that it is only a "legend" that the design team had completely mangled yield estimates. The more accurate version, the report says, is that while designers underestimated the probable yield for Castle Bravo, they made a good estimate of the maximum yield. This might seem a difference that makes no difference - except that it is also asserted that the designers specifically requested that this possible maximum be allowed for in planning the test. This further implies that the most significant mistake had to do with estimating weather patterns (and possibly choice of location) rather than than yield calculation per se.

    Here is the section of the report in question, from p. 77 (which in the PDF file is p. 89); I have bolded parts of the text that seem especially relevant:

    3.4 Yield Effects

    The yield of the BRAVO device was much larger than the best preshot estimates. Legend has it that this came as a complete surprise to both the weapon designers at Los Alamos and to the task force, and it was this unexpectedly large yield that caused the fallout on the inhabited atolls.

    The 15 Mt yield was not a total surprise. The nuclear device designers and task force scientists thought the maximum possible yield of BRAVO could be this large. This upper yield limit, “which is not expected but should be allowed for in safety considerations,” had been calculated by the Los Alamos nuclear device designers at the specific request of the task force scientific staff, and was communicated to Dr. Graves and Dr. William Ogle, commander of the Scientific Task Group (Figure 3-18), in a radiotelegram sent on 18 February 1954 (JF-2130 [1954]). This same message gives the final estimate of the most probable yield to be substantially less than the maximum possible yield. Throughout Operation CASTLE, it was the policy of the task force that personnel protection measures were to be predicated on the maximum possible yields, while protection measures for “things” were to be based on the most probable yields. In December 1953, even as he pressed the Los Alamos device design team for an official yield statement, Dr. Ogle instructed that shot-time aircraft separations (JF-3069 [1954]) and other personnel safety measures for BRAVO be predicated on his personal perception of an upper-yield limit larger than 15 Mt. His appreciation for the uncertainties inherent in the nuclear device may have prevented prompt loss of life in the detonation.

    I realize this is not relevant to your questions about lithium-7 deuteride; however it does relate to the nature of the technical mistakes that resulted in disaster, which may be of some small interest.
     
    Last edited: Sep 23, 2017
  13. Sep 23, 2017 #12
    wow thank you man
    the Japanese boat name was ''the lucky dragon''
     
  14. Sep 23, 2017 #13

    mfb

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    No reflector leads to an even lower yield.
     
  15. Sep 23, 2017 #14
    yah but because its not a U238 tamper it means that it well not go fussin
     
  16. Sep 26, 2017 #15
    Commander Scott: Now that I've re-tuned the dilithium 6 and 7 crystals we should get a better tritium yield in the auxiliary fusion reactors -- maybe we can reach warp 15 if we can keep her from breaking up or going into a time shift ...
    Captain Kirk: Mister Sulu, ahead warp factor 13.
    Captain Picard: Make it so.
    Lieutenant Crusher: Aye Aye, Sir
    Commander Spock: Belay that order.
     
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