prm@ecn.purdue.edu (11/20/90)
From: prm@ecn.purdue.edu >From: v064lnev@ubvmsd.cc.buffalo.edu (Zerxes Bhagalia) > >Could someone please point out the distinctions between a thermonuclear >device and a plain old run-of-the-mill nuclear device. Thanks in advance 8-). > >Zerxes Nuclear refers to fission weapons. Thermonuclear refers to fusion weapons. I'll explain the difference here. FISSION WEAPON PRINCIPLES Fisson weapons (atomic bombs) use either Uranium (Isotope 235) or Plutonium (Isotope 239) as the nuclear explosive. U-235 is also known as Oralloy, and U-238 is also known as Tuballoy. Bomb grade U-235 must be at least 93.5% pure. Lithium (primarily Li-6) can also undergo fission; see the discussion below under the Fusion Weapon firing sequence. The general principle of fission weapons is "spheres within spheres"; it's built like a very hot onion. :-) The outermost layer is a series of high-explosive lenses, designed to direct all their force inwards. The next layer is a tamper, usually made of gold, beryllium or one other element that I don't remember offhand :-(. A small airspace separates the tamper from the nuclear explosive to allow the tamper material to gain momentum before it compresses the core. The next layer is the core, made of either Oralloy or Plutonium. In older weapons and devices, those older than about 1956, have a neutron initiator in the center. Oftentimes this was made of Beryllium; it was nicknamed the golf ball or the urchin. In more modern weapons, the center is hollow to allow gaseous Deuterium (an isotope of hydrogen, see below) injection just after compression. Injecting Deuterium into the core causes a small amount of fusion, and a resultant spray of neutrons, which "boosts" the fission weapon's yield. The yield is the amount of energy released. Boosted fission devices all use external neutron generators, since the core must be hollow to allow the Deuterium injection. There is a measure of the effectiveness of the weapon or device that is called Alpha. It represents the number of fission generations before the pit (or core) disassembles. Unboosted fission weapons have alphas between 50 and 200. Boosted fission weapons have alphas of about 1000, with the resulting yield increase. Most fission weapons in the US inventory are boosted. Additionally, most (if not all) fusion weapons use boosted fission primary stages (see below). Firing sequence: High explosives fire, compressing the core. Compressed core is bombarded with neutrons from the external neutron generator, initiating chain reaction. Gaseous deuterium is injected into the core. Deuterium in core undergoes fusion, releasing more neutrons. Momentum of Beryllium tamper and core materials prevents core from blowing apart for a short time (< 1 microsecond), allowing core elements to fission more effectively. Beryllium tamper reflects escaping neutrons back into core. Energy release overcomes inward momentum and inertia of core elements and the weapon or device disassembles (explodes). FUSION WEAPON PRINCIPLES Fusion weapons are built in stages, rather then concentric spheres. The first stage, called the Primary, is a boosted fission device. The primary provides the thermal heat, the radiation pressure and the neutrons to initiate the fusion reaction in the second stage, called the Secondary. The secondary is a cylindrical fuel capsule. The stages have a very specific structure. On the "top", facing the primary and a few inches from it, is a radiation shield of Tuballoy (U-238). There is a hole in the radiation shield called a neutron channel. The radiation shield is several inches thick. The rest of the stage is a concentric series of cylinders. The outermost cylinder, which lines up with the outer edges of the radiation shield such that the shield "plugs" the stage, is a reactive jacket made of either Tuballoy or, for high-yield weapons, Oralloy (U-235). This is a thin cylinder, perhaps 1/2 inch at most. This jacket is called the "pusher". Inside the pusher is the thermonuclear fuel. This is usually lithium deuteride, made with Li-6, an isotope of lithium, and deuterium. The fuel capsule is made by placing lithium salt in a vacuum canister, heating it, and bathing it in gaseous deuterium. The deuterium bonds with the lithium, creating powdered lithium deuteride. This powder is then compressed into a ceramic and milled to the correct shape and size. Inside the fuel capsule is a Oralloy "sparkplug". On the bottom of the stage is another shield of Tuballoy. If this is to be a three (or greater) stage weapon, there will be a neutron channel in the bottom radiation shield of the second stage. Each stage, including the primary, is supported in a dense plastic foam. This is how the "styrofoam H-bomb" story got started. Outside the plastic is the bomb casing, which must be very strong. Firing sequence: Primary stage (boosted fission device) fires, releasing X-ray burst and fission particles. The X-rays travel an order of magnitude faster through the bomb casing than the particles and fragments from the primary. The inner lining of the bomb casing is designed to reflect the X-rays back onto the secondary. The radiation inside the bomb casing turns the plastic foam to a plasma with about the same density as lead. The radiation pressure compresses the secondary, packing the atoms in the pusher, fuel and sparkplug closer together, which increases both the probability and efficiency of the upcoming fission/fusion reaction. This is the "linear compression" fusion reaction some people have wondered about. Neutrons from the primary arrive at the secondary and travel down the neutron channel in the radiation plug, initiating a chain reaction in the sparkplug, releasing a spray of neutrons and additional heat and compression. The thermonuclear fuel, compressed from both the inside (by the fissioning sparkplug) and the outside (by the now-fissioning pusher) begins to react. The lithium isotope (usually li-6) in the fuel fissions, producing Helium isotopes, more neutrons, and Tritium (the third, heaviest and last isotope of Hydrogen). The Tritium from the Lithium fission fuses with the Deuterium already present in the stage, releasing additional energy energy. If the device has more than two stages, X-rays from the secondary compress the Tertiary stage, and neutrons from the fissioning sparkplug in the secondary initiate a chain reaction in the sparkplug in the Tertiary, and the cycle begins again. Eventually, the bomb casing can no longer contain the energy, and the weapon or device disassembles. In theory, it would be possible to create a fusion device using a high- explosive compression around a cylindrical fuel stage that would remove the need for the fission primary. If this design has progressed beyond the theory phase, details have not been declassified. In theory, at least, the "linear compression" thermonuclear device is possible. I will provide additional details and references if there is enough interest. Also, if anyone from Lawrence Livermore or Los Alamos can provide corrections or clarifications, I would be most appreciative. Cheers, Phil "For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." - Richard P. Feynman
henry@zoo.toronto.edu (Henry Spencer) (11/22/90)
From: henry@zoo.toronto.edu (Henry Spencer) >From: prm@ecn.purdue.edu >... Inside the fuel capsule is a Oralloy "sparkplug"... People in a position to know have been heard to state that modern US thermonuclear weapons no longer need "sparkplugs" to ignite the fusion reaction, although older ones did use them. >Also, if anyone from Lawrence Livermore or Los Alamos can provide corrections >or clarifications, I would be most appreciative. I think it's unlikely that any of them will speak up, given the almost obsessive secrecy about a lot of this stuff. If you have a security clearance, you think twice about even repeating published material, as the security bureaucrats may claim that you've thus "confirmed" it. -- "I don't *want* to be normal!" | Henry Spencer at U of Toronto Zoology "Not to worry." | henry@zoo.toronto.edu utzoo!henry