Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!zaphod.mps.ohio-state.edu!pacific.mps.ohio-state.edu!linac!att!cbnews!cbnews!military From: seeger@thedon.cis.ufl.edu (F. L. Charles Seeger III) Newsgroups: sci.military Subject: Re: Thermonuclear Trigger Message-ID: <1990Dec11.020358.27627@cbnews.att.com> Date: 11 Dec 90 02:03:58 GMT References: <1990Dec4.002646.10188@cbnews.att.com> <1990Dec7.012233.2025@cbnews.att.com> <1990Dec8.223807.29796@cbnews.att.com> Sender: military@cbnews.att.com (William B. Thacker) Organization: UF CIS Dept Lines: 146 Approved: military@att.att.com From: seeger@thedon.cis.ufl.edu (F. L. Charles Seeger III) In article <1990Dec8.223807.29796@cbnews.att.com> henry@zoo.toronto.edu (Henry Spencer) writes: |From: henry@zoo.toronto.edu (Henry Spencer) |>From: karish@mindcraft.com (Chuck Karish) |>What about the polonium/beryllium igniters that were used in the |>first fission bombs? Are they still in use? | |It is said that modern fission bombs generally use a different method: |a burst of neutrons is fired into the core from outside at the appropriate |time. My guess would be that the major advantage is delaying nuclear |ignition until a bit later in the implosion process. Correct, though it is better to describe it as initiating the neutronic phase of the explosion with a large initial neutron population (not just when it starts). This gives the largest possible final neutron population before the bomb 'disassembles'. Of course, the fission yeild will closely correlate with the size of the last generation of neutrons. So, maximizing the number of neutron generations before disassembly, maximizing the excess criticality of the assembly during this neutronic phase, and maximizing the size of the initial neutron population all contribute to an 'efficient' use of the fissile material in the bomb core. |(How they get the neutrons is no mystery: an electron tube filled with |low-density gas will accelerate gas ions into the negative electrode if |a high voltage is applied. Make the gas deuterium, use a negative-electrode |material containing tritium, and apply a sufficiently high voltage, and you |will get a sprinkling of D-T fusion reactions as the ions hit. The result |is a spray of neutrons out the end of the tube. Such tubes are used for |certain civilian applications, e.g. neutron radiography -- although rumor |hath it that the bomb-detonator tubes are specially designed for the job, |which is not surprising.) It is my understanding that the dueterons are actually formed into a beam, and that the neutron burst is accurately timed by sweeping this beam across the tritium target at the 'correct' time. The startup of a gaseous discharge is not sufficiently predictable to give the accurate timing required, so this beam sweeping technique is used. Accurate timing is critical to optimal operation, so extremely stable, accurate and reliable oscillators are necessary for this application. (In another article Henry answered a question about H-3 (tritium) decay, but failed to answer a question about the stability of H-2 (dueterium). Deuterium is stable, though it will undergo a neutron emission reaction if exposed to gamma radiation. The neutrons so generated give a 'minimum' power level to reactors moderated by heavy water. Oh, Henry's answer was correct, i.e. tritium decays to helium-3 by soft beta decay with a 12 year half life, and helium-3 is stable. BTW, tritium has been used in watch dials, in place of radium, which is no longer used.) I believe that these 'neutron triggers' are actually produced at a DOE site run by GE in Pinellas County, Florida (north of St. Petersburg, my hometown, and west of Tampa). Recently, GE has told the U.S. government to find a new operator for this facility, because they are unwilling to accept the legal liabilities for operation of this site that the Feds are trying to foist off on them. GE is giving 18 months notice. Perhaps, similar situations will develop (or already have) at other such sites. Anyway, the local press has published reports in the past that 'significant' amounts of tritium are located on this site. I see this as confirmation of the use of these DT neutron triggers. I have heard some engineers dismiss this possiblity because of the trouble of generating 25 kV or so from batteries within the bomb. I don't have any guesses as to how this is done, but the dueterium nuclei will have to accelerated to a high potential to give a substantial reaction rate with the tritium target. This problem is probably why earlier designs used a beryllium neutron source (beryllium emits a neutron when hit by an alpha particle). Perhaps, the beryllium was isolated from plutonium (or polonium) by paper, which the initial chemical blast would destroy, allowing the alphas to reach the beryllium nuclei. Note that Pu-Be (or similar) nuetron sources are normally used to help bring a reactor up to power. |For those who aren't up on this, fissionable materials will ignite |spontaneously when compressed into a critical mass, but ignition is more |predictable and better yields can be obtained if you give the stuff a |positive kick at a well-chosen time. Spontaneous ignition then becomes |undesirable, and attention has to be paid to materials and design to |prevent it. For example, this is why you don't see the gun-type bomb |design used for plutonium, which has a greater tendency to spontaneous |ignition than U-235. It's also why building a bomb out of plutonium |reprocessed from civilian reactor fuel is a poor idea, because such |plutonium will have significant amounts of Pu-240 in addition to the |desired Pu-239, and Pu-240 makes spontaneous ignition very likely -- |military plutonium-production reactors remove the breeding rods from |the reactor relatively early to avoid Pu-240 buildup. If spontaneous |ignition occurs early enough, the bomb goes "splut" instead of "boom", |blowing itself apart before the fission reaction really gets going. Again, Henry is right, though I could quibble that he should have used the word "fissile" rather than "fissionable". The former refers to isotopes that will undergo fission after absorbing a neutron of any energy, while the latter refers to isotopes that will fission only after absorbing a neutron above a certain energy threshold. (Actually, "fissionable" can refer to both types of nuclei, but it is usually easier to just say "fissionable" rather than "fissionable, but not fissile".) Examples of fissile isotopes are U-233, U-235, U-239, Pu-239 and Pu-241. U-238 is an example of fissionable isotope. U-238 and Thorium-232 are examples of "fertile" isotopes, which can be bred into fissile isotopes. Sorry, if I am being too pendantic. Also, let me expand on Henry's point about Pu-240 contamination of fuel. The problem with Pu-240 is that it has a relatively high probability of decay by spontaneous fission. During detonation this is undesirable because the neutrons so released will tend to lead to early disassembly (and a "splut") by initiating the neutronic phase in an early, slow and far suboptimal manner. Hence, the use of high burnup civilian fuel for extracting Plutonium puts you back in a similar boat that you are with Uranium (i.e. isotope separation). However, the use of low burnup fuel, as Henry points out, largely obviates this problem. This is an interesting non-proliferation issue, in that it implies the need for monitoring of nuclear power plants in third-world countries to prevent diversion. It is also interesting that the Canadian CANDU reactor (based on unenriched Uranium and heavy water) is a proliferation problem because it produces low burnup spent fuel. Concerns over proliferation caused the Carter administration to abandon fuel reprocessing. As Henry suggests, this concern is less persuasive when we are dealing with high burnup fuel with high Pu-240 contents. However, as India showed (reportedly using reprocessed fuel from a CANDU reactor to build their bomb) there is legitimate concern for low burnup fuel reprocessing. Can anyone update us on the CANDU program or other details on this issue? (Also, if my memory fails about the diversion of CANDU fuel by India, please gently correct me.) Before anyone asks, American nuclear power plants generate high burnup spent fuel. (There is a body of opinion that Thorium-232/U-233 breeding is safer from a non-proliferation standpoint than U-238/Pu-239. However, U-233 should be better bomb material than U-235, though not nearly as good as Pu-239. I don't recall enough about the U-233 fuel cycles to state whether there is a situation similar to Pu-240. I would be interested to hear from someone that does, or can take the time to research it.) |Just in case anyone is getting edgey about this... The above is all |published information. See John McPhee's "The Curve Of Binding Energy" |and Howard Morland's "The Secret That Exploded" in particular. Thanks for the pointers to the books, Henry. Chuck -- Charles Seeger E301 CSE Building Office: +1 904 392 1508 CIS Department University of Florida Fax: +1 904 392 1220 seeger@ufl.edu Gainesville, FL 32611-2024