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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 time—after 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 material—the 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
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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
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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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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 million°C).
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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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 time—until 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.
Representative terms from entire chapter:
neutron source
