If you follow UK news at all you’ll know that recently there was a bit of a brouhaha over the Royal Navy’s failed Trident II missile test just off the coast of Florida. Some of the more hysterical accounts of the incident have the missile veering towards the US mainland before self-destructing; these sound a little dubious, but there’s at least a sense of irony to the idea as Trident is a US-developed weapons system. The UK abandoned its own nuclear weapons development program back in 1958 in favour of simply buying the technology from the Americans, and there are some very good reasons why this is so. One of them is Violet Club.

The UK’s first serious attempts to develop a nuclear weapon actually predate the Manhattan Project, but after it became apparent just how much industrial capacity the US was willing to devote to producing a working fission bomb the British sensibly dropped their parallel development and instead swung their resources behind the American program. The British contribution consisted of both materials and scientists and significantly accelerated the completion of the bomb, but unfortunately one of the scientists in question was the spy Klaus Fuchs, who handed over a vast quantity of atomic data to the Soviet Union. Once this came out it really turned the Americans off of the idea of post-war collaboration on nukes, and after Roosevelt died and the only American copy of the agreement detailing post-war cooperation between the UK and US was lost (the British copy was sent to them, but certain suspicious figures in the US government weren’t convinced it was authentic) the US unilaterally broke off all collaboration, forcing the British to resume independent development of nuclear weapons.

British efforts culminated in the shipborne Operation Hurricane test detonation in 1952 and the first operational nuclear bomb — Blue Danube, which was essentially a copy of the Manhattan Project Fat Man — being tested in 1953. Unfortunately by this point both the US and the Soviets had tested their first thermonuclear weapons – if you want an explanation of the difference between nuclear and thermonuclear weapons you can read either the Wikipedia article or my own blog post on the subject, but a quick primer would be that a thermonuclear weapon uses a regular fission bomb as a first stage to kick-start nuclear fusion in hydrogen isotopes, which releases several times more energy than nuclear fission and creates a commensurately bigger bang.  Thermonuclear weapons effectively rendered pure fission bombs obsolete, as there were hard limits on the amount of fissile fuel that could be converted to energy by the chain reaction in the tiny fraction of a second before that energy was released and the bomb destroyed itself – much of the nuclear material inside a fission bomb therefore ends up being wasted, and cramming more uranium or plutonium into your weapon results in diminishing returns in terms of yield because it ends up being scattered into the shockwave as fallout material rather than being released as explosive power.

This was something of a problem for the British military establishment, as it was additionally becoming very clear that the delivery system of choice for a nuclear weapon was going to be a missile rather than a bomber. A missile has a far smaller payload capability than a bomber, which made the small, efficient thermonuclear devices ideal in comparison to large wasteful fission bombs. Unfortunately for them they’d expected fission bombs to have a longer lifespan and were stockpiling large amounts of highly-enriched uranium to build them. Not only would there be a gap in capability while they developed their own thermonuclear weapons, but said weapons would only require small amounts of this fissile material to trigger the second stage. As they were unwilling to see this expensive nuclear material go to waste, and using a smaller (yet saner) amount of plutonium would have been even more expensive, it was eventually proposed that an “Interim Megaton Device” be constructed using uranium to fill the perceived need for a megaton-grade weapon until the UK could build its own hydrogen bombs. Hence Violet Club.

Even on paper Violet Club sounds like a tremendously stupid idea. It’s a classic implosion design: a sphere of highly-enriched uranium surrounded by a set of high explosive lenses. When triggered the lenses explode inwards, compressing the uranium core so that it achieves critical mass; the uncompressed uranium core on its own is sub-critical and can’t go nuclear unless the explosive lenses trigger, and the explosive lenses are very difficult to trigger accidentally, making this a relatively safe design for fission weapons up until this point. The problem with Violet Club was that the target yield of one megaton was so much larger than a typical fission yield (which at this point was on the order of a hundred kilotons or so) that they had to cram in a truly ridiculous amount of uranium to achieve it. So much uranium, in fact, that the uncompressed mass of the uranium sphere was actually greater than one critical mass. The reason it wasn’t instantly going nuclear was because the Violet Club core was spread out into a thin, hollow uranium shell – critical mass is a bit of a misleading term and should be thought of as more like critical density as it relies on having a lot of fissile atoms in very close proximity so that they can bounce neutrons off one another — and the big gap in the middle was enough to stop the chain reaction from running out of control.

Now, a key feature of nuclear weapons, even back then, was that they should be fail-safe. If the weapon is not armed then it should be impossible for it to go off, even if e.g. the bomber it’s being carried on gets shot down and crashes, or if there’s a fire on-base and the building that it’s being stored in collapses. Violet Club was very emphatically not fail-safe, however; if the uranium core was crushed or damaged in any way that led to that hollow gap in the middle being squeezed out of the sphere, the chain reaction would start and the bomb would detonate. It wouldn’t be as destructive as if it were triggered intentionally as without the explosive lenses to compress the core the reaction would be even more inefficient than it already was, but it would certainly have been enough to ruin the day of anyone caught within a mile or two of the epicentre. This is why the bomb designers included a rather dubious safety feature: a small hole was bored through the uranium shell through to the hollow gap inside through which 133,000 steel ball-bearings were inserted. The ball-bearings were then sealed inside with a plastic bung. The theory went that as long as the ball-bearings were present inside the weapon, that hollow gap that prevented it from going nuclear couldn’t be crushed out of the core and the bomb would remain safe.

That was the theory, anyway. In practice the ball-bearings were even dumber than the bomb itself:

• Having to remove 133,000 pieces of steel from the interior of your weapon before it can be considered armed turned out to take some time – a minimum of half an hour, in fact (and up to 90 minutes during bad weather), which was far too slow considering the increasingly truncated warning time that was expected to precede any hostile nuclear action.

• The ball-bearings increased the weight of the Violet Club bomb assembly by half a tonne. The V-bomber force of the time (so-called as the UK’s nuclear deterrent consisted of Valiant, Vulcan and Victor bombers) could not carry a bomb that heavy, so the ball-bearings had to be removed from the bomb before takeoff. This ensured that Violet Club would always be armed while it was in the air, and this made it far too dangerous to be flown on exercises or even sent to a dispersal base to mitigate the effects of an expected nuclear strike.

• The bombs had to be stored inverted when not in use, as otherwise there was a risk that the plastic bung would fall out and the ball-bearings would exit the core, arming the bomb.

• By their nature nuclear bombers spent a lot of time sitting around on exposed airstrips waiting for an alert, and so did the bombs they carried. If it was a particularly cold day the ball-bearings would freeze solid inside the bomb, rendering it useless.

And so on. The Violet Club bombs were quite possibly the most dangerous and impractical nuclear weapons ever made, and the RAF knew it: 12 were ordered, but only 5 were delivered, and the ground crew who had to handle them were absolutely terrified of them. To add insult to injury, it was estimated (we’ll never know for sure as Violet Club was deemed too dangerous to even test) that thanks to the inherent inefficiency of pure fission bombs Violet Club would have a yield of around 400 kilotons – far short of the one megaton target yield.  In short, it was not an ideal outcome for the UK’s homegrown nuclear weapons development program, and god knows what else we might have come up with if we’d been allowed to continue bodging together the safety features that kept our nuclear deterrent from prematurely exploding.

Fortunately for us the Operation Grapple tests of 1958 proved to the US that we had the capability to build thermonuclear warheads of our own, and at that point the Americans figured that if we were going to get them anyway, they might as well make a few bucks by giving us their warhead blueprints and then selling us the delivery systems to go with them. The 1958 Mutual Defense Agreement swiftly followed and ensured Violet Club was the last purely British nuclear weapon ever deployed; Red Beard (the more prevalent — and conventional — successor to Blue Danube) remained in service until the switch to a primarily submarine-based deterrent force and the first Polaris patrol in 1968. And whatever else you can say about the American designs, at least they aren’t armed by physically yanking out a plastic bung from the warhead casing.

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I said I’d talk about thermonuclear weapons at some point. We have now reached it.

So you’ve got your fission weapon designs which are coming up on seventy years old now. They make a pretty loud bang, can seriously mess up a city if detonated over one, and refinement of the designs over the years have allowed us to squeeze ever larger booms out of the same amount of fissile material. Eventually, though, you run up against hard physical limits enforced by the fission reaction itself. Past a certain point you’re converting the vast majority of fissile material inside the bomb into explosive energy and further attempts to increase the bomb’s efficiency have drastically diminishing returns. However, modern nuclear weapon delivery systems – ICBMs with MIRV warheads – mean warhead weight is at a premium. It’s all very well to physically lug your fission bomb over to the target city in a WW2 bomber, but if you want to strike your enemies with genocidal death weapons launched from the other side of the planet you need something even lighter than even the most efficient fission weapon designs.

This is where thermonuclear weapons come in. Thermonuclear weapons are also known as fusion bombs or hydrogen bombs, and that’s because they rely on fusing together isotopes of hydrogen to produce most of their energy instead of a simple fission reaction to split atoms in a fissile material. You may be familiar with the nuclear fusion process as the thing that goes on inside the heart of a star to give us the heat and light we need to survive on Earth, as well as something that alternative energy researchers are very interested in as the possible future of large-scale power generation on this planet. Fusion can liberate a tremendous amount of energy from a very small quantity of fuel. Unfortunately for the energy researchers the pressures and temperatures required to naturally induce fusion are basically those found inside the core of a star, and are understandably difficult to reproduce in a stable form inside a fusion reactor. This is why fusion power was just around the corner twenty years ago, is just around the corner today, and will be just around the corner in twenty years time; achieving steady-state fusion requires materials and technologies that haven’t been invented/perfected yet like room-temperature superconductors, and while these technologies always seem tantalisingly within reach they never quite seem to materialise.

I’m digressing here, though. We’re interested in how fusion bombs work, not fusion reactors, and the cunning thing about the fusion bomb is that while you cannot yet reliably reproduce a star-like environment over a period of time appreciable to a human being (say five minutes or so) you can use a fission bomb as a first-stage to recreate the pressures and temperatures required for nuclear fusion for a very, very brief moment of time. As we saw in the first nuclear weapons post, given the runaway nature of nuclear reactions that one moment is all you need to make a very big bang indeed.

A simple thermonuclear weapon design looks like this (thanks, Wikipedia):

You have a fission bomb primary which works exactly as described in the original nuclear weapons post, so there’s no mystery there. What we’re interested in is the fusion secondary, although the interstage separating the two deserves a brief mention; that has to correctly direct the energy and radiation released by the explosion of the fission primary towards the fusion secondary in such a way that it successfully triggers fusion. If you don’t build your interstage correctly you get a “fissile fizzle”, which is just a small fission explosion and nothing else since the secondary stage didn’t trigger. Sadly I can’t tell you exactly how an interstage works because – as you’d expect – the details are highly classified.

The secondary itself is a column of fusion fuel contained within a tamper. The tamper works just like the tamper in a fission bomb: it is a very dense material such as uranium which serves to contain the expanding energies of the fusion explosion for just a few microseconds longer than they otherwise would, but which nevertheless makes all the difference in amplifying the power of the nuclear explosion. Inside the fusion fuel is a uranium rod which acts as a fission sparkplug; because it’s rod-shaped it does not constitute a critical mass until compressed by the detonation of the fission primary, at which point it itself will go nuclear and further heat and compress the fusion fuel to the point where it starts to fuse.

The fusion fuel is a heady mix of deuterium and tritum formed by lithium deuteride.  The flood of neutrons created by the chain reaction in the fission primary bombards this lithium deuteride, splitting the lithium component into helium-3 and tritum. Tritum is one of the isotopes of hydrogen which can undergo nuclear fusion, the other being the deuterium component of the lithium deuteride. The explosion of the primary bookended with the now-nuclear fission sparkplug compresses the tritum and deuterium together, forcing them to fuse together into helium-4. Incidental byproducts of this fusion reaction happen to include a neutron, which can then go off and split another lithium atom, and 17.59 MeV of energy. The heat provided by the fissionable bomb, tamper and sparkplug is key to the nuclear fusion reaction, hence the term “thermonuclear bomb”.

One of the interesting things about the fusion reaction inside a hydrogen bomb is that it creates very few harmful radioactive byproducts, with the main result of the reaction being a crapload of ionising radiation that is reflected back into the bomb core to increase the power of the explosion. If this radiation was released it would certainly kill anything unlucky enough to be in the blast radius but it wouldn’t persist, making a fusion bomb far cleaner in theory than an equivalent fission bomb. Unfortunately the two-stage design of fusion weapons means that they include fission bombs by default, and furthermore the fissionable material providing the heat and pressure for the fusion reaction means that it’s still going to scatter radioactive fission fragments over a wide area, making the typical hydrogen bomb just as bad as any fission weapon in terms of irradiating the blast zone. Designs which replace the fissionable uranium tamper with another dense, non-fissionable material such as lead do exist, and while they reduce the yield of the bomb by about half they’re also far “cleaner”. These are the fabled neutron bombs, which are supposed to kill people but leave infrastructure intact¹, and they do this via the spray of neutron radiation released by the fusion reaction described above.

How much more efficient are thermonuclear weapons compared to common-or-garden fission bombs? As a rough guide, the explosive power of a fission bomb ranges from a few kilotons for the most basic, primitive types (Fat Man and Little Boy) to about half a megaton for a modern fusion-boosted design – that is, a fission bomb with a small amount of fusion material mixed in to boost the yield. By contrast thermonuclear weapons start at about half a megaton and work their way up from there. The largest nuclear weapon ever detonated was the fifty-megaton Tsar Bomba, which actually added a third fusion stage to the fission primary and fusion secondary described here to achieve its obscene yield, but Tsar Bomba was a product of Cold War willy-waving and was subsequently deemed overkill for the practical use of nuclear weapons: destroying cities. Most modern thermonuclear warheads have a yield range anywhere up to about 1.5 megatons, this being more than sufficient to do the job as long as your ICBM targeting is good enough. Nuclear warheads are designed to explode above cities, not on them; this allows the heat and the shockwave to flatten a wide areas below them rather than having their explosive energy soaked up by having to travel through buildings, and if you do it like this it turns out you simply don’t need a very big bomb to devastate a city. Also many small explosions are more efficient in terms of distributing energy than one big one², so scattering a series of smaller MIRV warheads across a large area is a much better way of doing the job.

Well, that was educational. I don’t know about you, but I’d managed to go through 28 years on this planet without knowing just why these nightmare inventions were called hydrogen bombs. And now I do. Isn’t learning fun? The actual effects of the explosions on a civilian population are somewhat beyond my purview, but if you really want to be depressed for the rest of the day you could do worse than watch the War Game, a sixties mockumentary that was banned from broadcast for twenty years because it was “too horrifying for the medium of broadcasting”, and also Threads, the 80s equivalent.

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1. Although they’d still do a fair bit of damage thanks to having a fission bomb jammed in the top.

2. The power of the explosion diminishes exponentially with distance in accordance with the inverse square law, which is another reason why Tsar Bomba was a stupid sabre-rattling exercise.

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