Tag Archives: nuclear

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.

 

Technetium-99m generators

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Technetium-99m is a radioactive tracer that is used in twenty million medical diagnostic procedures per year. At least 31 radiopharmaceuticals based on Tc-99m are used for imaging and studying organs such as the brain, heart muscle, thyroid, lungs, liver, gallbladder and kidneys, as well as the skeleton and blood and for the investigation of tumours.

The ‘m’ in the name of technetium-99m indicates that it is metastable. Tc-99m is radioactive because one or more of the protons and neutrons in its nucleus is in an excited state. Tc-99m decays into Tc-99 with a half-life of six hours and this makes it particularly well suited to use in the body: after one day (four half-lives) only 6.3% of the initial Tc-99m remains. (It’s worth noting that the non-metastable technetium-99 is also radioactive, but with a half-life of 211000 years, it presents a very low risk.)

This short half-life also creates a problem: obtaining Tc-99m when required. Hospitals cannot run their own nuclear reactors and so they rely on technetium generators – machines that produce Tc-99m from the decay of its parent isotope molybdenum-99. Molybdenum-99 has a longer half-life (66 hours) and can therefore be transported to hospitals and still remain useful for up to a week.

Molybdenum-99 is produced in nuclear reactors by bombarding a highly enriched uranium target with neutrons, causing it to fission, forming Mo-99 (and many other isotopes) as it does. The vast majority of Mo-99 is produced by five nuclear reactors around the world that are specifically devoted to the production of nuclear isotopes for medicine: NRU in Canada, BR2 in Belgium, SAFARI-1 in South Africa, HFR Petten in the Netherlands and OSIRIS-1 in France.* Temporary shutdowns of NRU and HFR Petten in the 2000s led to a long-term shortage of Mo-99.

Once Mo-99 has been produced it is placed into a technetium generator and these generators are transported to hospitals. The technetium generators make use of the fact that molybdenum likes to bond with aluminium oxide (alumina) but technetium does not. The generators are “milked” by drawing a saline solution across an inner molybdenum/alumina capsule; during this elution process any technetium that has formed will be drawn away with the saline and can then be used in tests.

A cutaway model of a technetium generator.

The molybdenum/alumina sample is placed in the centre of the device, surrounded by shielding (painted red in this case). Saline is injected through one of the tubes at the top of the device and flows into a shielded container through the other tube, after having passed over the sample and “picked up” radioactive technetium-99m.

* Mo-99 is also produced in much smaller amounts from low-enriched uranium at the OPAL reactor in Australia and at other sites.

Curiosity’s nuclear battery

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The Curiosity rover that is the main part of the Mars Science Laboratory mission is very different from its predecessors Sojourner and the twin rovers Spirit & Opportunity.

L-R: Spirit/Opportunity, Sojourner and Curiosity.

L-R: The wheels of Sojourner, Spirit/Opportunity and Curiosity.

Curiosity is nearly twice as long as Spirit/Opportunity and has more than five times the mass; at 2.1 metres in height it is taller than most of the people that built it.

For me, the most interesting difference between Curiosity and the other Mars rovers is its power source. Both Sojourner and Spirit/Opportunity were powered by solar cells but Curiosity is powered by a radioisotope thermoelectric generator (RTG), in particular the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) built by Pratt & Whitney’s Rocketdyne division.

Curiosity‘s RTG is the large unit attached to the rover’s rear.

The main problem with using solar cells for power is that the cells only work during daylight hours and don’t function well at high latitudes where there is less sunlight; Spirit/Opportunity‘s cells only worked at full strength for about four hours per day, producing about 900 watt hours (about 3.2 megajoules) per day at best. Mars is covered in fine dust and dust covering solar panels was a problem for the Spirit and Opportunity rovers, though this dust was occasionally blown away by high winds.

Spirit‘s solar panels before and after a “cleaning event”.

RTGs work via the Seebeck effect, where a difference in temperature between between the two junctions of a thermocouple cause an electric current to be produced. The heat source in an RTG is the decay of a radioactive isotope; in the case of most RTGs this isotope is plutonium-238 in the form of plutonium dioxide. Pu-238 is a nearly pure alpha emitter and therefore requires only minimal shielding.

A pellet of 238PuO2 glows red hot from internal radioactive decay.

The MMRTG uses 32 marshmallow-sized plutonium pellets and will initially produce about 125 watts of electrical power (from 2000 watts of thermal power), but this will drop off over time as the plutonium decays. The MMRTG will consistently produce about 2500 watt hours of electricity per day compared with Spirit/Opportunity‘s average of 600 Wh and this will enable Curiosity to operate in all seasons and at all times of day.

Curiosity’s MMRTG before installation.

SNIFing out rogue nuclear reactors

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This was my losing entry for the Wellcome Trust Science Writing Prize.

The search for hidden nuclear reactors has traditionally been an intelligence operation run by organisations like the CIA and the SIS (formerly MI6), but in future it might the initials of France’s Atomic Energy Commission (CEA) that become ubiquitous in the fight against nuclear proliferation. In a paper1 accepted for publication in the prestigious journal of nuclear physics Physical Review C, Thierry Lasserre and colleagues from the CEA outline a radical new method for detecting clandestine or rogue nuclear reactors – the IAEA diplomatically calls them “undeclared reactors” – using mobile neutrino detectors transported by supertankers. Lasserre’s group calls the project SNIF: the Secret Neutrino Interactions Finder.

When uranium and plutonium nuclei split (when they “fission”) they produce smaller lighter nuclei called fission fragments. It is these fission fragments that make up radioactive waste and have names like strontium-90. These fission fragments are always heavy in neutrons: iodine-131 and caesium-137, two isotopes that have been in the news recently because of the accident at the Fukushima Daiichi nuclear plant both contain four more neutrons than their stable forms. These unstable neutron-heavy isotopes always become stable by undergoing beta decay, one of the three main types of radioactive decay.

During beta decay a neutron inside the nucleus turns into a proton and in the process releases an electron and an electron antineutrino. For every watt of thermal energy produced in the reactor about a thousand billion electron antineutrinos are produced. As a typical clandestine reactor will have a power of between ten million and two billion watts this equates to a very large release of neutrinos.

Neutrinos are tiny neutrally-charged particles with almost no mass. Because they are so small, and because they have no charge, neutrinos have almost no interactions with matter at all. Right now, no matter where in the world you are, there are millions of neutrinos emitted by the Sun travelling through your body at close to the speed of light.

The fact that neutrinos don’t interact means that it is impossible to prevent them from leaving the reactor. Burying your reactor a mile underground or encasing it in steel and concrete won’t work – these are effectively transparent to neutrinos. The fact that neutrinos don’t interact with matter also makes them very difficult to detect: neutrino detectors have to be very large to have a reasonable chance of capturing a neutrino “event” in a reasonable amount of time. The Super-Kamiokande neutrino detector in Japan contains fifty million kilograms of ultra-pure water in a cylinder 39 m across and 41 m tall and only detects about fifteen events per day.

The French group’s work expands on previous work2 by Eugene Guillian at the University of Hawaii which proposed an array of stationary one megaton detectors by suggesting a flotilla of supertanker-borne mobile detectors. Their paper suggests a cylindrical detector 46 m across and 97 m long, submerged one and a half kilometres underwater to reduce interference from “background” neutrinos present due to solar activity and natural radioactive decay.

The SNIF detector would be filled with a chemical called linear alkylbenzene – normally used in the preparation of detergents – and be “doped” with the element gadolinium to increase the detection rate. The inside of the cylinder containing the alkylbenzene would be covered by thousands of photomultiplier tubes (PMTs), ultra-sensitive light sensors designed to pick up the flashes of ultraviolet light created on the very rare occasions when a proton in the detector “absorbs” the electron antineutrino. The Super-Kamiokande detector uses a little over 13000 of these PMTs, 6600 of which were shattered in a chain reaction in late 2001 and each of which had to be replaced by hand at a cost of three thousand dollars each.

By combining readings from their detector with the location and power of known nuclear reactors and a map of naturally-occurring “geoneutrinos”3; and by using a bit of common sense – reactors are usually located near oceans or rivers for cooling, for example – Lasserre and his colleagues suggest that they could locate a three hundred megawatt research reactor producing fuel for a nuclear weapon to within “a few tens of kilometres” after only sixth months’ observation from three hundred kilometres away. If the number of detectors or the observing time is increased then even tiny research reactors could be accurately located.

Tyrannical despots need not start worrying immediately. The SNIF detector would be three times the size of the largest detectors existing today and would present significant logistical and operational difficulties. Nevertheless, as Lasserre points out, the possibility of non-civilian use of nuclear reactors is a growing one, and may eventually justify the creation of a real-life SNIF project.

1 Thierry Lasserre et al. 2010. “SNIF: A Futuristic Neutrino Probe for Undeclared Nuclear Fission Reactors”. arXiv:nucl-ex/1011.3850v1 available at http://arxiv.org/abs/1011.3850 accessed 17 May 2011.
2 Eugene Guillian. 2008. “Far Field Monitoring of Rogue Nuclear Activity with an Array of Large antineutrino Detectors”. Earth Moon and Planets 99: 309-330. doi: 10.1007/s11038-006-9110-x
3 Fabio Mantovani, Luigi Carmignani, Gianni Fiorentini and Marcello Lissia. 2004. “Antineutrinos from Earth: A reference model and its uncertainties”, Physical Review D 69: 013001-013013. doi: 10.1103/PhysRevD.69.013001.