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4.3 Fission-Fusion Hybrid Weapons

The first designs proposed for fusion bombs in the U.S. assumed that the heat from the fission trigger would ignite a self-sustaining fusion reaction in a mass of liquid deuterium adjacent to it. In the late 40s and early 50s improved calculations showed that this was impossible. The only fusion reaction achievable by simply heating the fuel with a fission bomb is the D- T reaction:

   D + T -> He-4 (3.5 MeV) + n (14.1 MeV)

The naive approach to using this reaction - making a large explosion by igniting a large mass of D-T fuel mixture with a fission trigger - is prohibitively costly. Plutonium is a factor of 10 times cheaper per unit of energy released compared to D-T fuel, and HEU is 3-5 times cheaper still. Furthermore due to radioactive decay the tritium continuously disappears at a rate of 5.5% annually and must be replaced.

A number of weapon designs have been developed that use the D-T reaction in a variety of ways however. All of them depend on the highly energetic neutrons produced by the D-T reaction. Some of these designs use the neutrons to achieve significant fission yield enhancement, thus reducing the expenditure of fissile material for a given yield. Others exploit the neutrons directly as a weapon.

The fusion boosting and Alarm Clock/Layer Cake designs were pioneered by the US and USSR in the early 1950s. Neutron bombs were apparently not developed by either nation until the late 1960s or early 1970s.

4.3.1 Fusion Boosted Fission Weapons

Fusion boosting is a technique for increasing the efficiency of a small light weight fission bomb by introducing a modest amount of deuterium- tritium mixture (typically containing 2-3 g of tritium) inside the fission core. As the fission chain reaction proceeds and the core temperature rises at some point the fusion reaction begins to occur at a significant rate. This reaction injects fusion neutrons into the core, causing the neutron population to rise faster than it would from fission alone (that is, the effective value of alpha increases).

The fusion neutrons are extremely energetic, seven times more energetic than an average fission neutron, which causes them to boost the overall alpha far out of proportion to their numbers. Is this due to several reasons:
1. Their high velocity creates the opposite of time absorption - time magnification.
2. When these energetic neutrons strike a fissile nucleus a much larger number of secondary neutrons are released (e.g. 4.6 vs 2.9 for Pu-239).
3. The fission cross section is larger in both absolute terms, and in proportion to scattering and capture cross sections.

Taking these factors into account, the maximum alpha value for plutonium (density 19.8) is some 8 times higher than for an average fission neutron (2.5x10^9 vs 3x10^8).

A sense of the potential contribution of fusion boosting can be gained by observing at 1.5 g of tritium (half an atom mole) will produce sufficient neutrons to fission 120 g of plutonium directly, and 660 g when the secondary neutrons are taken into account. This would release 11.6 kt of energy, and would by itself result in a 14.7% overall efficiency for a bomb containing 4.5 kg of plutonium (a typical small fission trigger). The fusion energy release is just 0.20 kt, less than 2% of the overall yield. Larger total yields and higher efficiency is possible of course, since this neglects the fission-only chain reaction required to ignite the fusion reaction in the first place and that fission multiplication would continue significantly beyond the fissions caused by the fusion induced secondaries.

The fusion reaction rate is proportional to the square of the density at a given temperature, so it is important for the fusion fuel density to be as high as possible. The higher the density achieved, the lower the temperature required to initiate boosting. Lower boosting initiation temperatures mean that less pre-boost fission is required, allowing lower alpha cores to be used.

High fusion fuel densities can be achieved by using fuel with a high initial density (highly compressed gas, liquid hydrogen, or lithium hydride), by efficient compression during implosion, or most likely by both.

Although liquid D-T was used in the first US boosting test (Greenhouse Item), this is not a practical approach due to the difficulty in achieving and maintaining cryogenic temperatures (especially considering that 3 g of tritium constitutes a heat source of approximately 1 watt).

US nuclear weapons are known to incorporate tritium as a high pressure gas, that is kept in a reservoir external to the core (probably a deuterium - tritium mixture). The gas is vented into the weapon core shortly before detonation as part of the arming sequence. Initial densities with a room- temperature gas (even a very high pressure one) are substantially lower than liquid density. The external gas reservoir has the important advantage though that it allows the use of "sealed pit", a sealed plutonium core that does not need servicing. The tritium reservoir can be easily removed for repurification and replenishment (removing the He-3 decay product, and adding tritium to make up for the decay loss) without disturbing the weapon core.

A possible alternative the use of a high pressure gas reservoir is to store the gas in the form of a metal hydride powder, uranium hydride (UH3) for example. The hydrogen can be rapidly and efficiently released by heating the hydride to a high temperature - with a pyrotechnic or electrical heat source perhaps.

A problem with using hydrogen gas is that it reacts very rapidly with both uranium and plutonium to form solid hydrides (especially plutonium, the Pu-H reaction rate is hundreds of times higher than that of any other metal). The formation of hydrides is very undesirable for the boosting process since it dilutes the gas with high-Z material. This can be prevented by lining boost gas cavity with an impermeable material. Thin copper shells have been used for this purpose. Alternatively the injection of fusion fuel could simply be conducted immediately before detonation, reducing contact between the core and the hydrogen isotope mixture to no more than a few seconds.

Lithium hydrides achieve an atomic density of hydrogen that is about 50% higher than in the liquid state, and since the hydride is a (relatively) stable inert solid it is also easy to handle. A key disadvantage is that the hydride must be permanently incorporated into the core requiring complete core removal and disassembly to replenish and purify the tritium.

The ideal location for the boosting gas would seem to be in a cavity in the very center of the fissile mass, since this would maximize the probability of neutron capture, and the core temperature is also highest there. In a levitated core design, this would make the levitated core into a hollow sphere. This is not desirable from the viewpoint of efficient fissile material compression however since a rarefaction wave would be generated as soon as the shock reached the cavity wall.

An alternative is to place the boosting gas between the outer shell and the levitated pit. Here the collapsing thin shell would create multiple reflected shocks that would efficiently compress the gas to a thin very high density layer. There is evidence that US boosted primaries actually contain the boosting gas within the external shell rather than an inner levitated shell. The W-47 primary used a neutron absorbing safing wire that was withdrawn from the core during weapon arming, but still kept its end flush with the shell to form a gas-tight seal.

The conditions created by compressing the gas between the collapsing shell and levitated core are reminiscent of a recently reported shock compression experiment conducted at Lawrence Livermore in which liquid hydrogen was compressed the metallic state by the impact of a 7 km/sec gas gun driven plate. This experiment generated pressures of 1.4 megabars, and hydrogen densities nine times higher than liquid. The velocity of an imploding shell is more like 3 km/sec and the boost gas is at a lower initial density, still, the pressures that can be expected are at least as high, so a similar hydrogen density (around 0.75 atom-moles/cm^3) may be achievable.

It is also possible to dispense with a levitated pit entirely and simply collapse a hollow sphere filled with boosting gas. Since the fissile shell would return to normal density early in the collapse, there does not seem to be any advantage in doing this.

Fusion boosting can also be used in gun-type weapons. The South Africans considered adding it to their fission bombs, which would have increased yield five-fold (from 20 kt to 100 kt). Since implosion does not occur in gun devices, it cannot contribute to fusion fuel compression. Instead some sort of piston arrangement might be used in which the kinetic energy of the bullet is harnessed by striking a static capsule.

The fusion fuel becomes completely ionized early in the fission process. Subsequent heating of the hydrogen ions then occurs as a two step process - thermal photons emitted by the core transfer energy to electrons in the boost plasma, which then transfer energy to the ions by repeated collisions. As long as this heating process dominates, the fusion fuel remains in thermal equilibrium with the core. As the temperature rises, the fusion fuel becomes increasingly transparent to the thermal radiation. The coupling is efficient up to around 10^7 K, after which the fuel intercepts a dwindling fraction of the photon flux (which is should still keep it in temperature equilibrium given the greatly increasing flux intensity).

The fusion process releases 80% of its energy as neutron kinetic energy, which immediately escapes from the fuel. The remaining 20% is deposited as kinetic energy carried by a helium-4 ion. This energy remains in the gas, and can potentially cause significant heating of the fuel. The question arises then whether the fusion fuel continues to remain in equilibrium with the core once thermonuclear burn becomes significant, or whether self- heating can boost the fuel to higher temperatures. This process could, in principal, cause the fusion fuel temperature to "run away" from the core temperature leading to much faster fuel burnup.

I have not resolved this question satisfactorily at present, but it may be that the fusion fuel will remain in equilibrium, rather than undergo a runaway burn. Most of the helium ion energy is actually transferred to the electrons in the plasma (80-90%), which then redistribute it to the deuterium and tritium ions, and to bremsstrahlung photons. The energy must be transferred to the ions before it is available for accelerating the fusion reaction, a process which must compete with photon emission. If the photon-electron coupling is sufficiently weak then the boost gas can still runaway from the core temperature, otherwise it will remain in thermal equilibrium.

Boosting effectively begins when the ions are hot enough to produce neutrons at a rate that is significant compared to the neutron production rate through fission alone. This causes the effective value of alpha in the core to increase leading to faster energy production and neutron multiplication. In the temperature range where boosting occurs, the D-T fusion rate increases very rapidly with temperature (modelled as an exponential or high order polynomial function), so the boosting effect quickly becomes stronger as the core temperature climbs.

At any particular moment the contribution to alpha enhancement from boosting is determined by the ratio between the rate of neutron increase due to fission spectrum neutron secondaries, and the rate of increase due to fusion neutron secondaries. The fission spectrum contribution is determined in turn by the unboosted fission spectrum value of alpha, and the fission spectrum neutron population in the core. The fusion contribution is determined by the fusion reaction rate, and the fusion neutron alpha value. To optimize yield this enhancement should be at a maximum just as disassembly begins.

The fusion reaction rate typically becomes significant at 20-30 million degrees K. This temperature is reached at very low efficiencies, when less than 1% of the fissile material has fissioned (corresponding to a yield in the range of hundreds of tons). Since implosion weapons can be designed that will achieve yields in this range even if neutrons are present a the moment of criticality, fusion boosting allows the manufacture of efficient weapons that are immune to predetonation. Elimination of this hazard is a very important advantage in using boosting. It appears that every weapon now in the U.S. arsenal is a boosted design.

4.3.2 Neutron Bombs ("Enhanced Radiation Weapons")

The design objective of the tactical neutron bombs developed in the 1960s and 70s was to create a low-yield, compact weapon that produced a lethal burst of neutrons. These neutrons can penetrate steel armor with relative ease, enabling the weapons to be effective against tanks and other armored vehicles which are otherwise highly resistant to the effects of nuclear weapons. A flux of several thousand rems were desired so that incapacitation of armored crews would be relatively rapid, with in several hours to a couple of days at most. In this exposure range death is inevitable. To minimize the effects of collateral damage, the effect of thermal radiation and blast outside the neutron kill radius, it was also very desirable to minimize the energy released in forms other than the neutron flux.

The means for generating this intense neutron flux is to ignite a quantity of deuterium-tritium fuel with a low yield fission explosion. It is essential however to avoid the absorption of those neutrons within the bomb, and especially to *prevent* the fusion-boosting effect on the trigger. The weapon must also fit inside an 8" diameter artillery shell.

An example of such a weapon is the US Mk 79-0 warhead for the XM-753 8" AFAP (artillery fired atomic projectile). This shell was 44 inches long and weighed 214 lb. The W-79-0 component was only about 37 cm long. The maximum yield of the W-70-0 was 1 kt, of which 0.75 kt was due to fusion, and 0.25 kt to fission.

It has been suggested by some that a neutron bomb is simply a variation of a boosted fission bomb, e.g. the fusion fuel is in the center of the fissile mass. Elementary analysis shows that this idea is impossible. The 3:1 fusion:fission yield ratio of the W-79-0 indicates that there must be 31 fusion reactions releasing 540 MeV (and 31 fusion neutrons) for each fission (which release 180 MeV). This means more than 97% of the fusion neutrons must escape the core without causing fission. Since a critical mass is by definition one in which a neutron has less than a 35-40% chance of escaping without causing fission, the fusion reaction cannot occur there. Consequently the fusion reaction must take place in a location outside the fissile core.

Simulations show that at the temperatures reached by a 250 ton fission explosion, and at normal densities (gas highly compressed to near liquid density, or in lithium hydrides) even deuterium-tritium fuel does not fuse fast enough for efficient combustion before the expanding fissile mass would cause disassembly. The fuel must be compressed by a factor of 10 or so for the reaction to be sufficiently fast.

Computations also show that care must be taken to heat the fuel symmetrically. The radiation pressure and ablation forces during heating are so large that if significant asymmetry occurs, the fuel will be dispersed before much fusion takes place.

Taken together, these considerations make it evident that neutron bombs are miniaturized variants of staged radiation implosion fusion bombs (see Sections on Thermonuclear Weapons below). The fissile mass is separated from the fusion fuel, which is compressed and heated by the thermal radiation flux from the fissile core. Due to the small mass of the fusion fuel, and the low temperature of ignition, a fission spark plug internal to the fusion capsule is not necessary to ignite the reaction. The ignition probably occurs when the thermal radiation diffuses through the pusher/tamper wall of the fusion capsule. It is also possible that the localized region of intense heating that develops when the shock in the fuel capsule converges at the center may be responsible for, or contribute to, the ignition of the fusion reaction (this is similar to the ignition process in inertial confinement fusion experiments).

The W-79 fissile core is plutonium and is assembled through linear implosion. It is known to contain tungsten and uranium alloys. The likely use of the tungsten is to provide a high-Z material for providing the radiation case, and for the fuel capsule pusher/tamper. Uranium may be used simply to provide inertial mass around the core compression system, it may also serve in part as a neutron reflector.

A notional sketch of the W-79 is given below. The dimensions in centimeters are given along the left hand and lower border of the design. Typical screen formatting will tend to stretch the graphic vertically since line width:character width ratios are usually something like 5:3.

5CCEEEEEfffffffEEEEE                RRC
4CCEEEfffffffffffEEERRRR            RRC
3CCEEfffffffffffffEERRR      HH      RC
2CCEEfffffffffffffEERR      HHHH      RC  Ogive End ->
1CCEEfffffffffffffEERR     HHHHHH     RC (pointy end)
0CCEEfffffffffffffEERR     HHHHHH     RC
9CCEEfffffffffffffEERR      HHHH      RC
8CCEEfffffffffffffEERRR      HH      RC
7CCEEEfffffffffffEEERRRR            RRC
6CCEEEEffffffffffEEE                RRC

C - casing (steel and uranium?)
E - explosive
f - fissile material (plutonium)
R - radiation shield/radiation case (tungsten)
H - hydrogen fuel capsule, made of tungsten, filled with D-T gas

The fissile material mass in this design would be something like 10 kg. The 750 ton fusion yield indicates at least 10 g of D-T mixture for the fusion fuel. Under high static pressure hydrogen can reach densities of around 0.1 mole/cc (0.25 g/cm^3 for DT). This indicates a fuel capsule volume of at least 40 cm^3, or a spherical radius of 2.5-3 cm including wall thickness.

4.3.3 The Alarm Clock/Layer Cake Design The earliest and most obvious idea for using fusion reactions in weapons is to surround the fission core with a fusion fuel. The radiation dominated shock wave from the expanding fission core would compress the fusion fuel 7- 16 fold, and heat it nearly to the same temperature as the bomb core. In this compressed and heated state a significant amount of fusion fuel might burn.

Calculations quickly showed that only one reaction ignited with sufficient ease to make this useful - the deuterium-tritium reaction. The cost of manufacturing tritium relative to the energy produced from the fusion reaction made this unattractive.

Two ideas were later added to this concept to make a practical weapon design: The first: use lithium-6 deuteride as the fuel. The excess neutrons released by the fission bomb will breed tritium directly in the fuel blanket through the Li-6 + n -> T + He-4 + 4.78 MeV reaction. A layer at least 12 cm thick is necessary to catch most of emitted neutrons. This reaction also helps heat the fuel to fusion temperatures. The capture of all of the neutrons escaping ahead of the shock wave generates about 2.5% as much energy as the entire fission trigger release, all of it deposited directly in the fusion fuel.

The second: encase the fusion fuel blanket in a fusion tamper made of uranium. This tamper helps confine the high temperatures in the fusion blanket. Without this tamper the low-Z fusion fuel, which readily becomes completely ionized and transparent when heated, would not be heated efficiently, and would permit much of the energy of the fission trigger to escape. The opaque fusion tamper absorbs this energy, and radiates it back into the fuel blanket. The high density of the fusion tamper also enhances the compression of the fuel by resisting the expansion and escape of the fusion fuel.

In addition the uranium undergoes fast fission from the fusion neutrons. This fast fission process releases far more energy than the fusion reactions themselves and is essential for making the whole scheme practical.

This idea predates the invention of staged radiation implosion designs, and was apparently invented independently at least three times. In each case the evolution of the design seems to have followed the same general lines. It was first devised by Edward Teller in the United States (who called the design "Alarm Clock"), then by Andrei Sakharov and Vitalii Ginzburg in the Soviet Union (who called it the "Layer Cake"), and finally by the British (inventor unknown). Each of these weapons research programs hit upon this idea before ultimately arriving at the more difficult, but more powerful, staged thermonuclear approach.

There is room for significant variation in how this overall scheme is used however.

One approach is to opt for a "once-through" design. In this scheme the escaping fission neutrons breed tritium, the tritium fuses, and the fusion neutrons fission the fusion tamper, thus completing the process. Since each fission in the trigger releases about one excess neutron (it produces two and a fraction, but consumes one), which can breed one tritium atom, which fuses and release one fusion neutron, which causes one fast fission, the overall gain is to approximately double the trigger yield (perhaps a bit more).

The gain can be considerably enhanced though (presumably through a thicker lithium deuteride blanket, and a thicker fusion tamper). In this design enough of the secondary neutrons produced by fast fission in the fusion tamper get scattered back into the fusion blanket to breed a second generation of tritium. A coupled fission-fusion-fission chain reaction thus becomes established (or more precisely a fast fission -> tritium breeding -> fusion -> fast fission chain reaction). In a sense, the fusion part of the process acts as a neutron accelerator to permit a fast fission chain reaction to be sustained in the uranium tamper. The process terminates when the fusion tamper has expanded sufficiently to permit too many neutrons to escape.

The advantage of the once-through approach is that a much lighter bomb can be constructed. The disadvantage is that a much larger amount of expensive fissile material is required for a given yield. Yields exceeding a megaton are possible, if a correspondingly large fission trigger is used. This design was developed by the British. The Orange Herald device employed this concept and was tested in Grapple 2 (31 May 1957). A U-235 fission trigger with a yield in the 300 kt range was used, for a total yield of 720 kt - a boost in the order of 2.5-fold. A variant design was apparently deployed for a while in the fifties under the name Violet Club.

The second approach was adopted by the Soviets and proven in the test known as Joe-4 to the West (actually the fifth Soviet test) on 12 August 1953 at Semipalatinsk in Kazakhstan. This resulted in a very massive, but much cheaper bomb since only a small amount of fissile material is required.

Since there is an actual multiplication effect between the fusion reaction and the tamper fast fission, an improved yield can be obtained at reasonable cost by spiking the fusion layer with tritium prior to detonation.

The Joe-4 device used a 40 kt U-235 fission bomb acted as the trigger and produced a total yield of 400 kt for a 10-fold enhancement, although tritium spiking was partly responsible. 15-20% of the energy was released by fusion (60-80 kt), and the balance (280-300 kt) was from U-238 fast fission. A later test without tritium spiking produced only 215 kt.

This design has a maximum achievable yield of perhaps 1 Mt (if that) before becoming prohibitively heavy. The USSR may never have actually deployed any weapons using this design.