Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!att!cbnews!cbnews!military From: henry@zoo.toronto.edu (Henry Spencer) Newsgroups: sci.military Subject: Re: Thermonuclear Trigger Message-ID: <1990Dec8.223807.29796@cbnews.att.com> Date: 8 Dec 90 22:38:07 GMT References: <1990Dec3.050137.1908@cbnews.att.com> <1990Dec4.002646.10188@cbnews.att.com> <1990Dec7.012233.2025@cbnews.att.com> Sender: military@cbnews.att.com (William B. Thacker) Organization: U of Toronto Zoology Lines: 47 Approved: military@att.att.com From: henry@zoo.toronto.edu (Henry Spencer) >From: karish@mindcraft.com (Chuck Karish) >|...reported that US fusion bombs used to use sparkplugs, but that modern >|ones do not need them. > >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. (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.) 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. 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. -- "The average pointer, statistically, |Henry Spencer at U of Toronto Zoology points somewhere in X." -Hugh Redelmeier| henry@zoo.toronto.edu utzoo!henry