Tag Archives: weapon

Bhangmeter

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A bhangmeter is a radiometer, a device that measures the power of electromagnetic radiation. Most people are already familiar with one type of radiometer, the Crooke’s radiometer, which detects infrared radiation.

crookes-radiometer

A Crooke’s radiometer. The higher the flux of infrared radiation the faster it spins.

Bhangmeters are placed on reconnaissance satellites* in order to detect nuclear weapon detonations and to measure their yield. Bhangmeters are designed to look for the characteristic “double flash” created when nuclear weapons detonate: the first initial bright flash being caused by the actual detonation of the weapon and the second being caused when the ionised gas shock wave cools enough to allow light from the fireball to escape.

The name “bhangmeter” was created by Frederick Reines (who later won the Nobel Prize for Physics for his work on detecting neutrinos), who suggested that one would have to be on bhang (an Indian drink made from marijuana) to believe that the detector would work.

* The US Department of Defense’s GPS satellites also carry bhangmeters.

How are mushroom clouds formed?

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Mushroom clouds (perhaps more properly known as pyrocumulus clouds) are traditionally associated with nuclear explosions, but any sufficiently large explosion (for example, a volcanic eruption) will create a mushroom cloud.

The mushroom cloud resulting from the Priscilla test of Operation Plumbbob.

When a large explosion occurs a cloud of very hot gas is created. This hot gas, being less dense than the surrounding air, rises rapidly upwards. As this cloud of hot gas rises it pushes against the air above it and this air resistance causes the top layer to move sideways whilst the hotter gas below continues rising upwards, creating a swirling doughnut-shaped vortex (in the photograph above a very hot “filament” is visible at the centre of this vortex). As the “cap” rises this swirling vortex pulls in cooler air from ground level, creating the “stalk” on which the cap sits.

The formation of a mushroom cloud during the Tumbler-Snapper series of nuclear tests.

The shape of a mushroom cloud is the result of a Rayleigh-Taylor instability at the interface between the hot less-dense and cold more-dense air. These instabilities occur in a number of different situations, and can be easily demonstrated at home by dropping coloured oil into water, creating tiny upside-down mushroom clouds as shown below in photographs by James Riordon of AIP.

The simulated formation of a Rayleigh-Taylor instability.

Nuclear art forgery

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Nuclear weapons* are triggered by neutron initiators, devices that produce a sudden burst of neutrons on activation. They are most often constructed from a mixture of beryllium-9 and polonium-210. The polonium emits high-energy alpha particles, and when brought into contact with the beryllium it causes the beryllium to transmute into carbon with the release of a neutron. This neutrons causes an atom of uranium-235 to split (to fission) and in the process release a huge amount of energy and more neutrons that go on to cause further fissions.

This uncontrolled chain reaction results in the production of many exotic isotopes, as the uranium atoms split to form “chunks” of other elements. For example, it was in the aftermath of the ‘Ivy Mike’ test of the first thermonuclear bomb that the elements einsteinium and fermium were discovered.

The existence of rare isotopes can be used to demonstrate that a painting or other work of art was not produced before the 1940s or 1950s, when nuclear weapons testing was at its peak. Strontium-90 and caesium-137 are isotopes that did not exist in nature before the age of nuclear weapons and which permeate soils and are taken up by plants and other living things as they are very soluble in water. If these organic materials are used in the production of paints, or binders for paint, or in other ways in a piece of art then the presence of Sr-90 or Cs-137 can be used to prove that the item in question was created after the beginning of the nuclear age.

* This paragraph details the operation of a fission bomb. Fusion (thermonuclear) bombs work differently, but all use a fission stage to initiate the fusion process.

Separative Work Units

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Seperative Work Units (SWUs) are a measurement of the effort required to seperate isotopes of uranium for use in nuclear power stations or nuclear weapons.

The maths behind the calculation of SWUs is quite complicated (Kirk Sorenson has written a great article about calculating SWUs) but what is interesting is to compare the effort required in various situations.

Examples

Little Boy, the sixteen kiloton nuclear weapon that was dropped on Hiroshima during World War II contained fifty kilograms of uranium enriched to 88% and a further fourteen kilograms enriched to 50%. This would require 10800 SWUs (9350 + 1450).

Aside from its work enriching uranium to 5% for use in the Bushehr nuclear power station, Iran has also enriched 98 kg of uranium to 20% [source], requiring 3740 SWUs. To further enrich this fuel, to produce 20 kg of highly enriched uranium – enough for a nuclear weapon – would require a further 370 SWUs.

Data about nuclear-powered submarines is hard to come by, but unclassified sources state that Ohio Class SSBNs of the US Navy are powered by General Electric S8G nuclear reactors using fuel that has been enriched to 97.3%, probably with an initial fuel load of around 400 kg. To produce 400 kg of fuel enriched to 97.3% would require 83700 SWUs.

Sizewell B is the UK’s newest nuclear power station and produces about two gigawatts of electricity (about seventeen billion kWh per year). It uses about thirty tonnes of uranium enriched to about 3.5% per year, which would require 129000 SWUs.

A graph showing the effort required to produce a given amount of enriched uranium to a given level. The area of the bubbles is proportional to the number of SWUs required. Click to enlarge.

It’s worth looking in these cases at the amount of initial uranium required. The greater the desired enrichment level, the greater the initial feed required to yield a given mass of enriched uranium is. In the case of Little Boy, to produce 64 kg of uranium enriched to around 80% would have required more than 12 tonnes (12 096 kg) of initial uranium (and a much larger amount of uranium ore, depending on the grade of ore*). This would result in 12 032 kg of waste depleted uranium, good only for use as ballast, shielding or armor-piercing projectiles. The amount of effort required (the number of SWUs) to enrich this depleted uranium to a usable level would be far too great for proliferation to be a problem.

By far the predominant current method of isotope separation is the use of gas centrifuges, at a cost of around $100 per SWU†; thus the cost of the enrichment required to run Sizewell B for a year would be about $13 million. A newer method, laser enrichment, promises to cut this cost to around $30/SWU, which would bring down the cost of running Sizewell B to only $3.9 million. Unfortunately this would also make enrichment for more nefarious uses cheaper.

SWU calculations depend on the amount of uranium left behind in the “tailings” of the enrichment process. For the purposes of all the figures above this is assumed to be 0.3%. If uranium were to become scarce then this percentage would obviously decrease.

* The highest grade ore in the world comes from the Athabasca Basin in Canda, with a grade of 18%. To yield one kilogram of uranium from Athabasca would require 5.56 kilograms of ore.

† The figures for cost per SWU come from Sharon Weinberger, “Laser plant offers cheap way to make nuclear fuel”, Nature 487: 16-17. DOI: 10.1038/487016a.

The Calutron Girls

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One of the most difficult aspects of the Manhattan Project that built the first nuclear bombs was obtaining enough enriched uranium to make the bomb work. The enrichment of uranium took place at a site near the Oak Ridge National Laboratory and used three different methods: electromagnetic separation, gaseous diffusion and thermal diffusion. The gas centrifuge method of separation that is the modern standard could not be made to work at the time.

The final stage of enrichment was the electromagnetic separation stage that took place in a building known as Y-12; the output from the S-50 thermal diffusion plant and the K-25 gaseous diffusion plant (which at the time was housed in the world’s largest building by floor space) was used as input for Y-12.

Electromagnetic seperation was carried out on calutrons, which used giant electromagnets made of silver* to deflect the paths of ionised uranium-235 by a little more than ionised uranium-238. Initially these calutrons were operated by scientists from the University of California, Berkeley where the calutron was invented by Ernest Lawrence, but when a reasonable rate of return was achieved the operation of the calutrons was turned over to operators from the Tennessee Eastman Company.

These Tennessee Eastman operators were mostly women and all of them were only educated to high school level.

“The Calutron Girls”

Major General Kenneth Nichols, the man in charge of ore procurement and feed materials, pointed out to Ernest Lawrence that Eastman’s “hillbilly girl” operators were achieving better rates of production that his scientists and engineers had and a competition took place, with Eastman’s operators beating out Lawrence’s scientists. Nichols put this down to the fact that the girls were “trained like soldiers not to reason why … [whilst] the scientists could not refrain from time-consuming investigation of the cause of even minor fluctuations of the dials”.

During the operation of the calutrons the staff from Tennessee Eastman had no idea what they were doing: they operated switches and dials and monitored meters, but had no idea what those switches and dials did or were related to.

Gladys Owens, seated in the foreground of the photograph above, only discovered what her job was when taking a public tour of the K-12 facility fifty years later. Owens stated that she was told by a manager during a training session “We can train you how to do what is needed, but cannot tell you what you are doing. I can only tell you that if our enemies beat us to it, God have mercy on us!” and said that “Everywhere you looked it told you to keep your mouth shut!”.

Gladys Owens returns to K-12, fifty-nine years later.

* Normally copper would be used to construct electromagnets but this was in short supply due to the war. Kenneth Nichols met with the Under Secretary of the Treasury and arranged to borrow 13 300 tonnes of silver (worth about £170 million at today’s prices) from the US’s West Point Bullion Depository. After the war ended the silver was melted down and returned, with only a tiny fraction being lost in the process.