Why does the US prefer fusion warheads to fission warheads?

[Involute]

I understand that the vast majority of our atomic warheads are fusion devices. I realize that fusion yields are potentially much larger than fission yields, but our most modern fusion warhead, the W88, has a maximum yield of "only" 475 kilotons, while the maximum theoretical yield of a fission warhead is around 500 kilotons.

I believe the limiting factor in fission is how much fissile material can be made to undergo fission before the device destroys itself. Fusion isn't subject to a similar limitation since it can go on fusing as much fuel as is provided.

Given the relative simplicity of fission devices, and our apparent desire for roughly half-megaton yields, why don't we just use pure fission warheads? I assume they'd be significantly larger and heavier than the W88 and its predecessors (which I gather achieve much of their yield from fusion-boosted fission), but is that true? Are there other reasons?

 

[scottdave]

It may have something to do with cost. Fission is more than just mining uranium. It must be enriched (get the proper amount of fisionable isotopes)

 

[jim mcnamara]

Fusion (thermonuclear fusion) bombs are "ignited" by a small fission bomb, but are orders of magnitude more powerful than a pure fission bomb of the same mass. So, more bang for a buck. A lot harder to build.

 

[jim hardy]

You might enjoy John McPhee's "The Curve of Binding Energy".

It is a very basic introduction to nuclear terminology and concepts written to explain enrichment and proliferation risks.

But John McPhee can't resist doing character studies and this one is about Ted Taylor the guy who miniaturized our weapons.
Of course talking to someone like Taylor he gets quite a little ways into the subject.

old jim

 

[mheslep]

I believe the limiting factor in fission is how much fissile material can be made to undergo fission before the device destroys itself. Fusion isn't subject to a similar limitation since it can go on fusing as much fuel as is provided.

Yes.

The fissionable material for a fission weapon, either HEU or Pu, is difficult to produce.

 

[scottdave]

The fusionable materials (deuterium) is not that easy to acquire either. So maybe it is not cost, but something like payload weight. A lighter weight warhead will need less fuel to go the same distance.

 

[mheslep]

The fusionable materials (deuterium) is not that easy to acquire either. So maybe it is not cost, but something like payload weight. A lighter weight warhead will need less fuel to go the same distance.

http://www.praxair.com/gases/buy-deuterium-gas

Separating isotopes that differ 2:1 in mass is not that challenging. Few hundred $ for small tank.

Anyway, the major nuclear powers use other isotopes in their thermonuclear weapons per reports. Tritium, lithium something.

 

[scottdave]

http://www.praxair.com/gases/buy-deuterium-gas

Separating isotopes that differ 2:1 in mass is not that challenging. Few hundred $ for small tank.

Anyway, the major nuclear powers use other isotopes in their thermonuclear weapons per reports. Tritium, lithium something.

Hmm... Compressed Deuterium gas. So I guess if you burn that in oxygen, you produce heavy water?

 

[Astronuc]

Hmm... Compressed Deuterium gas. So I guess if you burn that in oxygen, you produce heavy water?

Heavy water is D₂O.

There is a lot of speculation on the type of device. I expect it was a boosted device rather than what one would consider a thermonuclear device.
NY Times have several articles on the matter: https://www.nytimes.com/2017/09/03/world/asia/north-korea-tremor-possible-6th-nuclear-test.html

 

[jim hardy]

D₂O isn't at all hard to get . I bought 100ml back when "cold fusion" was the rage. It's still in my sock drawer.

old jim

 

[smartalek86]

"I understand that the vast majority of our atomic warheads are fusion devices. I realize that fusion yields are potentially much larger than fission yields, but our most modern fusion warhead, the W88, has a maximum yield of "only" 475 kilotons, while the maximum theoretical yield of a fission warhead is around 500 kilotons. "
You are generally on the right track. Yield and efficiency is substantially increased, not from fusion, but from fusion induced fission...of the tamper and Pu239. Uranium238 is used in the tamper of many nukes, and is considered non fissile(and cheap)...except from fusion neutrons, this is where a lot of money is saved and power gained...
From memory, the original tsar bomb design had a U238 tamper, which would have doubled its yield.

 

[Rive]

There is a lot of speculation on the type of device. I expect it was a boosted device rather than what one would consider a thermonuclear device.

I too thought that for a time, but the shape (if that thing on the published vid is any 'real' one) does not really fit.
Right now I think it is some 'real' thermonuclear device, but the secondary fizzled at the test.
Not as if it would matter.

 

[Delta Force]

The United Kingdom pursued a fission approach in the 1950s due to difficulties developing thermonuclear bomb technology. They found a lot of problems with pure fission devices and moved to thermonuclear designs as soon as possible. Those are weight, inefficient materials usage, safety and readiness concerns due to the large number of critical masses in a single core and the safeguards that must be employed to prevent a criticality accident or even a nuclear fizzle partial detonation.

The Orange Herald boosted fission device achieved worse performance than the unboosted Mark 18 Super Oralloy Bomb. The Orange Herald design used 117 kilograms of highly enriched uranium for 740 kilotons yield (about 6 kilotons per kilogram HEU) while the Mark 18 used 60 kilograms for 500 kilotons (8 kilotons per kilogram HEU). This is a significant amount of material, the Orange Herald test consumed almost all of the United Kingdom's annual production of 120 kilograms HEU. To put that in perspective, a critical mass of uranium 233 is only 15 kilograms.

There were safety issues with large fission designs because the large amount of material involved increased the risk of accidentally assembling a critical mass. The British designed a smaller fission device, Green Grass, for deployment, but it was inefficient and unsafe:

Green Grass yield was originally stated to the Royal Air Force (RAF) as 500 kilotons of TNT equivalent (2.1 PJ), but the designers estimate was later revised downwards to 400 kt of TNT. The Green Grass warhead was never tested. It used a dangerously large quantity of fissile material – thought to be in excess of 70 kilograms (150 lb), and considerably more than an uncompressed critical mass. It was kept subcritical by being fashioned into a thin-walled spherical shell. To guard against accidental crushing of the core into a critical condition, the shell was filled with 133,000 steel ball-bearings, weighing 450 kilograms (990 lb). In a conflict, these would have had to have be removed before flight. The RAF thought it unsafe.

The United States had similar concerns about the Mark 18, and they were in fact rebuilt to the Mark 6 configuration once thermonuclear bombs started entering service:

The Mark 18 bomb design used an advanced 92-point implosion system, derived from the Mark 13 nuclear bomb and its ancestors the Mark 6 nuclear bomb, Mark 4 nuclear bomb, and Fat Man Mark 3 nuclear bomb of World War II. Its normal mixed uranium/plutonium fissile core ("pit") was replaced with over 60 kg of pure highly enriched uranium or HEU. With a natural uranium tamper layer, the bomb had over four critical masses of fissile material in the core, and was unsafe: the accidental detonation of even one of the detonator triggers would likely cause a significant (many kilotons of energy yield) explosion. An aluminum/boron chain designed to absorb neutrons was placed in the fissile pit to reduce the risk of accidental high yield detonation, and removed during the last steps of the arming sequence.

It's also thought that the pressure to produce materials for Orange Herald and other boosted fission designs led to the use of improper fuel elements that led to the Windscale Fire:

Winston Churchill publicly committed the UK to building a hydrogen bomb, and gave the scientists a tight schedule in which to do so. This was then hastened after the US and USSR began working on a test ban and possible disarmament agreements which would begin to take effect in 1958. To meet this deadline there was no chance of building a new reactor to produce the required tritium, so the Windscale Pile 1 fuel loads were modified by adding enriched uranium and lithium-magnesium, the latter of which would produce tritium during neutron bombardment. All of these materials were highly flammable, and a number of the Windscale staff raised the issue of the inherent dangers of the new fuel loads. These concerns were brushed aside.

When their first H-bomb test failed, the decision was made to build a large fusion-boosted-fission weapon instead. This required huge quantities of tritium, five times as much, and it had to be produced as rapidly as possible as the test deadlines approached. To boost the production rates, they used a trick that had been successful in increasing plutonium production in the past; by reducing the size of the cooling fins on the fuel cartridges, the temperature of the fuel loads increased, which caused a small but useful increase in neutron enrichment rates. This time they also took advantage of the smaller fins by building larger interiors in the cartridges, allowing more fuel in each one. These changes triggered further warnings from the technical staff, which were again brushed aside. Christopher Hinton, Windscale's director, left in frustration.

After a first successful production run of tritium in Pile 1, the heat problem was presumed to be negligible and full-scale production began. But by raising the temperature of the reactor beyond the design specifications, the scientists had altered the normal distribution of heat in the core, causing hot spots to develop in Pile 1. These were not detected because the thermocouples used to measure the core temperatures were positioned based on the original heat distribution design, and were not measuring the parts of the reactor which became hottest.

 

[mheslep]

critical mass of uranium 233

The UK built 233 based weapons, not 235?