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Appendix E Physics of Nuclear Weapons Design FISSION WEAPONS To obtain a pure fission explosion, it is necessary to have assembled a highly supercritical masse mass of fissile material several times larger than the cnucal mass, considering the particular reflector and density that may apply (see Appendix C). The fissile material and reflector should be in the metal form, so that the neutrons are not slowed down and the chain reaction can build up an extremely high energy density before the forces driving a disassembly can take effect. The reaction will be stopped because of the drop in the density and consequent drop in the criticality (see Appendix C) of the fissile material as it explodes away. Before it is fired, the fissile material in the weapon must be in a subcritical configuration, and it will require some time to move the material to make the highly supercritical assembly. The final part of this timeafter the material has first become critical, but before it has reached the desired fully assembled state- is the "supercritical time," during which the fissile material is capable of sustaining a chain reaction. Because of the extreme speed with which such a reaction can build up, and because of the fact that once the fissile material is vaporized which sets in when the energy density is only about half a kilogram of high explosive equivalent per kilogram of fissile materialthe further progress of the assembly would be halted by the pressures being developed. The result would be that the total energy then generated would be smaller than that intended by a large factor 10, or a 100, or more. The chance of experiencing such a "predetonation,' is just the chance that a background neutron may initiate a chain in the time 123

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124 APPENDIX E interval through which the system is supercritical during assembly. This depends on both the duration of the interval and the strength of the background neutron source. Neutron Sources Cosmic rays provide a universal source of neutrons, but the number of these is smaller than other sources that will be present, being only one neutron per cm2 every few minutes. This is at sea level: at an altitude of 20 km this source would be about 103 times larger. The dominant sources are those originating from the weapons materials uranium and plutonium. Each of these is radioactive by alpha decay, and each undergoes some spontaneous fission. The source of neutrons from spontaneous fission is inherent in the material and dependent on the isotopic composition, and nothing can be done about it once the material is chosen. The rate at which alpha particles are released is also inherent in the material and depends on the isotopic composition; but the rate at which neutrons are produced by the alpha particles is directly proportional to the amount of light element (lithium through fluorine, with the exception of nitrogen) impurities in the matenal. Efforts to reduce the light element content by chemical purification would be worthwhile at least to the extent that the neutrons produced by alphas colliding with impurities (the alpha-e reaction) not exceed the neutrons produced by spontaneous fission. The boron content may serve as a gauge in this connection because, although it may not be possible to reduce the mass fractions of carbon or oxygen to as low a level as boron, the neutron production from carbon or oxygen at a given mass fraction is a few hundred times smaller than for boron. Also, although beryllium would produce about three times as many neutrons as boron per unit mass, its mass fraction can be reduced by an order of magnitude or more below that of boron. In highly enriched uranium the neutron source from spontaneous fission is close to 2 neutrons/kg-s, as is the source from alpha-e reactions in such material having 10 parts per million ~pm) boron content. It requires care to reduce the boron content to this level, but it is not extremely difficult to reduce it to a level a few times smaller. Because the rates for alpha decay of the plutonium isotopes are very much higher than for uranium, the alpha-e source in weapons-grade plutonium is about a 1, 000 times larger than that in highly enriched uranium at the same purification level. The difference does not matter, however, because the neutron source from spontaneous fission in weapons-grade plutonium is larger than that in highly enriched uranium by a factor of more than 10,000. The chemical processing of fissile materials in the weapons complex is directed at obtaining very high purity fissile material, and some of the specifications originated in considerations outlined above concerning the alpha-e neuron source.

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APPENDIX E 125 Assembly Methods With respect to obtaining a rapid assembly, which is to say a short supercritical interval during assembly, two approaches were used in 1945. One was the `'gun- assembly" method in which a subcritical piece of active material (called the projectile) was Fred down a gun barrel to mate with a subcritical piece of material (called the target), so that when the projectile was seated in the target the resulting configuration would constitute several critical masses. With a feasible projectile velocity of about 1,000 feet per second (fps), the time interval from first criticality to final assembly was only a few hundred microseconds (a few inches of motion at 1,000 fps). The second was the "implosion method" in which a near-critical assembly of fissile material is surrounded by a layer of high explosive. When the explosive is detonated on its outer surface in such a way that a spherically converging shock wave is imposed on the fissile material compressing it by a factor between about four thirds and two, the assembly is highly supercritical by the time the shock wave reaches the center. With this method of assembly, the supercntical time may be only a few microseconds, as compared with the few hundred microseconds required in the case of the gun method. It will be obvious that with compressions of twofold or so available, some fraction of a critical mass at normal density may also be made quite supercritical by implosion. In either cas~gun method or implosion a modulated initiator, some structure containing separated layers of beryllium and polonium, for example, that is crushed by the projectile or the shock wave can be used to provide a strong source of neutrons to initiate a chain reaction when the assembly is complete. Predetonation In an assembly of a few tens of kilograms of highly enriched uranium (as would be required for a uranium weapon using the gun method, which provides no compression) and a neutron source in the fissile material of 2 or more neutrons/ kg-s, the neutron source in the system would be on the order of 102 neutrons/ second. With a supercritical assembly time interval of several times 10 - s, the chance that a background neutron would appear during this interval would be several times 1~2. The likelihood that a single neutron will initiate a chain in a mildly supercritical assembly is not very close to unity (cf. the discussion in Appendix C showing that, in a system that is nearly critical, more than 50 percent of the neutrons escape without causing a fission). It follows that the probability of predetonation of a gun-assembly of uranium can be reduced to 1 percent or so. Plutonium-239 made in a reactor is unavoidably accompanied by some plutonium-240. The fraction plutonium-240/plutcnium-239 increases with the integrated neutron flux to which the uranium-238 source material for plutonium-

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126 APPENDIX E 239 has been exposed. The separation of plutonium from uranium is an expensive process, and it is not considered practical in obtaining plutonium in bulk quantities to exact it at an earlier stage than one at which the plutonium-240 content has reached a level of a few percent. Indeed, "weapons-grade" plutonium is defined as material having no more than 7 percent plutonium-240. The neutron source from spontaneous fission in plutonium-240 is 103 neutrons/gm-sec. Thus the neutron source In plutonium with only 1 percent plutonium-240 is 10 neutrons/ gm-sec, and with a few kilograms of material having several percent plutonium- 240, the neutron source will be somewhat larger than 105 neutrons/s. With such a source, and a supercntical assembly interval of several microseconds as in an implosion assembly, on the average several tenths of a neutron will appear during the interval. It requires about 10 neutrons ~ provide a 99 percent probability of initiating a chain in a mildly supercritical system, so the predetonation probability in such an assembly can be seen to be about a few percent. In a gun-type system, with no compression, two or three times as much plutonium must be used as in an unplosion-type. The supercntical interval is about a hundred times longer, so several hundred neutrons will appear during the supercntical interval. This is enough to assure predetonation quite early in the assembly process, and, for this reason, plutonium cannot be used effectively in the gun method. BOOSTED WEAPONS A booster is a fission device containing a small amount of deuterium-tritium (D-T) gas at the center. As the chain reaction proceeds, heating the fissile material, it can get to the stage at which the temperature of the fissile material, and the adjacent gas in the middle, is in the neighborhood of a kilovolt (10 millionC). At about this point, a thermonuclear reaction (deuteron plus tritium combining to yield a neutron plus an alpha particle plus 17 MeV of energy) will be initiated, which, once it is stmed, proceeds extremely rapidly. The energy released will be of little consequence, being overshadowed by the energy already released by fissions; but the number of neutrons produced may exceed the number otherwise present in the system. Being introduced quite independently of the progress of the chain reaction, and in a near-instan~neous pulse, the neutrons increase the rate of fissions very sharply, with the result that the yield ultimately realized may be several-fold larger than it would have been without the "boosting." As a consequence of employing this technique, it has been possible to obtain a larger yield from a device of a given size and cost in fissile matenal, to obtain the same yield from a smaller amount of fissile material, and most importantly to obtain a desired yield from devices reduced ilk size and weight. Almost all weapons produced since about 1960 have been boosted. Apart from the advantages in weapon size and weight and the direct importance of that with respect to delivery systems, the wide-scale use of boosting has had several other consequences.

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APPENDIX E 127 Hydrogen reacts readily with uranium or plutonium to form the solid hydride, so the D-T gas is stored at high pressure in a steel reservoir and released into the pit only at the time the weapon is to be fired. Because of the radioactive decay of Iridium (a half-life of 12.3 years) these reservoirs have to be returned every few years to be recharged. Also, because of the tntium decay, it is necessary to produce new mtium to maintain the stockpile at a constant level. The polonium-beryllium modulated initiator previously referred to in connection with a pure fission device had a short shelf life because of the 138-day half-life of polonium-210, and the replacement of Hose was once a major activity at the Mound facility. In boosted weapons the initiation function has been taken over by electrically powered neutron generators obtained from the Pinellas Plant. Also, in the early pure fission implosion devices, the fissile material was kept outside the high explosive and only installed in place in preparation for firing. At least partly because of the geometrical complexity of the gas reservoir and transfer system, the active material in boosters is stored in place in the high explosive. This leads to the requirement for "one-point safety"the requirement, that is, that should the high explosive be accidentally detonated at any one point (as a result, for example, of being dropped from a height, or struck by a projectile, or exposed to a fire) there must be an extremely low probability of generating any appreciable nuclear yield. While it would be possible to design a new pure fission device and, by a combination of non-nuclear experiments and calculation, predict the yield win very high confidence without resorting to full-scale test, this does not seem to be possible with respect to a new booster design. The booster yield depends very strongly on the state of the D-T gas at the time it may burn and on the extent of its burning; and since these conditions develop only in the course of the nuclear explosion, they are not subject to observation or confirmation by any non-nuclear experiment. For different reasons, the need for testing with actual fissile material would also apply to confirming the one-point safety of a new booster design. THERMONUCLEAR WEAPONS Weapons with yields much larger than 10 kilotons, or so, would probably make use of a thermonuclear design, or [I-bomb. Such devices have been described as having two separate nuclear components mounted inside a case. One component, designated as the "primary," would be a fission device, most probably of the booster type because of the invariable interest in reducing the overall weight and size to the smallest feasible level consistent with the objective specified for the weapon. The other component the "thermonuclear capsule" or "secondary" is designed to provide almost all the total yield specified for the weapon. It consists of a mass of solid Li6D enclosed in a layer, or capsule, of some heavy metal. Since the fast (14 MeV) neutrons produced in the burning of Li6D can cause fission in uranium-238 (the most common isotope in uranium), the capsule would most probably be made of uranium, or even depleted uranium.

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128 APPENDIX E When the primary, with a yield of a few kilotons, say, is fired, its energy will distribute itself very rapidly throughout the volume inside the outer case and surrounding the secondary capsule. Since the case will have to fit inside some delivery vehicle, its volume is unlikely to be much larger than 1 m3, and it may be considerably less than that. At least for a short timeuntil the case can be swept away Me energy density surrounding the secondary capsule will be of the order of a kiloton high explosive equivalent per cubic meter, or most probably more. The density of chemical high explosive is about 1.6 g/cm3, so 1,000 tons of high explosive occupies a volume of about 600 ma, and the energy provided by chemical high explosive will be about 1 kiloton per 600 ma. The energy density, and pressure, that the explosion of the primary provides inside the case may be seen, then, to be 1,000, or so, times larger than those provided by chemical high explosives. The secondary is consequently subjected to an extremely violent implosion which will result in compressions and densities of the thermonuclear and wall materials very much larger than chemical high explosive could impose. Such conditions are favorable for a rapid thermonuclear burning of the Li6D; and the energy from this, along with that from the fissions induced in the wall, determines the yield of the weapon. Apart from calculating the progress of processes just referred to, a main problem for the designer will be to conform to the shape and dimensional constraints imposed by the characteristics of the delivery vehicle in question, while at the same time striving to meet the conflicting desires of the military customer that the weight be reduced and the yield increased as much as possible. Subsequently, the fabricator will have to meet unusually stringent requirements on dimensional tolerances as well as on the composition, purity, and uniformity of the materials.