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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 1940s and early 1950s, 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 → He4 (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 U.S. 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 lightweight 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 α increases).

The fusion neutrons are extremely energetic (7 times more energetic than an average fission neutron) which causes them to boost the overall alpha far out of proportion to their numbers. This is 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 Pu239).

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 α value for Plutonium (density 19.8) is some 8 times higher than for an average fission neutron (2.5x109 vs 3x109).

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 α 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 U.S .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).

U.S. 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 He3 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 U.S. 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 9 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/cm3) 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 5-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 2-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 107 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 Helium4 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 (modeled 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 α and the fission spectrum neutron population in the core. The fusion contribution is determined by the fusion reaction rate and the fusion neutron α 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 at 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.