BBC News Online – 2005-05-14 11:39:59
The Nuclear Fuel Cycle: How to Make Nuclear Weapons
BBC News Online
Uranium is the basic raw material of both civilian and military nuclear programmes. It is extracted from either open-cast pits or by underground mining. Although uranium occurs naturally all over the world, only a small fraction is found in concentrated ores. When certain atoms of uranium are split in a chain reaction, energy is released. This process is called nuclear fission.
In a nuclear power station this fission occurs slowly, while in a nuclear weapon, very rapidly. In both instances, fission must be very carefully controlled. Nuclear fission works best if isotopes – atoms with the same atomic number, but different numbers of neutrons – of uranium 235 (or plutonium 239) are used. Uranium-235 is known as a “fissile isotope” because of its propensity to split in a chain reaction, releasing energy in the form of heat.
When a u-235 atom splits, it emits two or three neutrons. When other u-235 atoms are present, these neutrons collide with them causing the other atoms to split, producing more neutrons.
A nuclear reaction will only take place if there are enough u-235 atoms present to allow this process to continue as a self-sustaining chain reaction. This requirement is known as “critical mass”.
However, every 1,000 atoms of naturally-occurring uranium contain only seven atoms of u-235, with the remaining 993 being denser u-238.
Once extracted, uranium ore is taken to a mill to be crushed and ground into a fine powder. This is then purified in a chemical process and reconstituted in a solid form known as “yellow cake”, due to its yellow colouring. Yellow cake consists of 60-70% uranium, and is radioactive.
The basic aim of nuclear scientists is to increase the amount of u-235 atoms, a process known as enrichment. To do this, uranium must first be converted into a gas, uranium hexafluoride by heating it to about 64 degrees centigrade.
Uranium hexafluoride is corrosive and reactive and must be handled very carefully. Pipes and pumps at conversion plants must be specially constructed from aluminium and nickel alloys. The gas is also kept away from oil and grease lubricants to avoid any inadvertent chemical reactions.
Nuclear reactors work on the principle that nuclear fission releases heat, which can be harnessed and used to heat water into steam to drive turbines.
A typical nuclear reactor uses enriched uranium in the form of fuel ‘pellets’, each roughly the size of a coin and about an inch long. The pellets are formed into long rods known as bundles, and housed inside a heavily insulated, pressurised chamber.
In many power stations, the bundles are submerged in water to keep them cool. Other types use carbon dioxide or liquid metal to cool the reactor core.
To function in a reactor – ie produce heat through a fissile reaction – the uranium core must be supercritical. This means that the uranium must be in sufficiently enriched form to allow a self-sustaining chain reaction to occur.
To regulate this process, and allow the nuclear plant to function, control rods are inserted into the reactor chamber. The rods are made of a substance, typically cadmium, which absorbs neutrons inside the reactor.
Fewer neutrons means fewer chain reactions are started, slowing down the fission process. There are more than 400 nuclear power stations across the globe, producing about 17% of the world’s electricity. Nuclear reactors are also used to power submarines and naval vessels.
The aim of enrichment is to increase the proportion of fissile uranium-235 atoms within uranium. For uranium to work in a nuclear reactor it must be enriched to contain 2-3% uranium-235. Weapons-grade uranium must contain 90% or more u-235.
A common enrichment method is a gas centrifuge, where uranium hexafluoride gas is spun in a cylindrical chamber at high speeds. This causes the slightly denser isotope u-238 to separate from the lighter u-235.
The dense u-238 is drawn towards the bottom of the chamber and extracted; the lighter u-235 clusters near the centre and is collected. The enriched u-235 is then fed into another centrifuge. The process is repeated many times through a chain of centrifuges known as a cascade.
The remaining uranium – essentially u-238 with all the u-235 removed – is known as depleted uranium. Depleted uranium, a heavy and slightly radioactive metal, is used as a component in armour-piercing shells and other munitions.
Another method of enrichment is known as diffusion. This works on the principle that of the two isotopes present in uranium, hexafluoride gas, u-235 will diffuse more rapidly through a porous barrier than its heavier cousin, u-238. As with the centrifuge method, this process must be repeated many times.
The aim of all nuclear bomb designers is to create a supercritical mass which will sustain a chain reaction and violently release vast amounts of heat.
One of the simplest is a so-called ‘gun’ design. Here, a smaller subcritical mass is fired at a larger one, causing the combined mass to go supercritical triggering a nuclear explosion. The process occurs in less than a second.
To make fuel for a uranium bomb, highly-enriched uranium hexafluoride is first converted into uranium oxide, and then uranium metal ingots. This can be done using relatively simple chemical and engineering processes.
The most powerful basic fission weapon – an atom bomb – will detonate with an explosion the force of 50 kilotons. This force can be increased by a technique called boosting, which harnesses the properties of nuclear fusion.
Fusion consists of the joining together of the nuclei of atoms of hydrogen isotopes to produce nuclei of helium. This process occurs when hydrogen nuclei are subjected to intense heat and pressure, both of which are produced by a nuclear bomb.
Nuclear fusion has the effect of producing more neutrons and feeding the fission reaction, resulting in a bigger explosion. Such boosted devices are known as hydrogen bombs, or thermonuclear weapons.
Reprocessing is the chemical operation which separates useful fuel for recycling from nuclear waste.
Used fuel rods have their metallic outer casing stripped away before being dissolved in hot nitric acid. This produces uranium (96%), which is reused in reactors, highly radioactive waste (3%) and plutonium (1%).
All nuclear reactors produce plutonium, but military types produce it more efficiently than others. A reprocessing plant and a reactor to produce sufficient plutonium could be housed inconspicously in an ordinary-looking building.
This makes extracting plutonium by reprocessing an attractive option to any country wishing to pursue a clandestine weapons programme.
Plutonium offers several advantages over uranium as a component in a nuclear weapon. Only about 4kg of plutonium is needed to make a bomb. Such a device would explode with the power of 20 kilotons. To produce 12kg of plutonium per year, only a relatively small reprocessing facility would be needed.
A warhead consists of a sphere of plutonium surrounded by a shell of material such as beryllium, which reflects neutrons back into the fission process. This means that less plutonium is needed to achieve critical mass, and produce a self sustaining fission reaction.
A terrorist group or country may find it easier to acquire plutonium from civil nuclear reactors, rather than enriched uranium, to produce a nuclear explosive.
Experts believe a crude plutonium bomb could be designed and assembled by terrorists possessing no greater level of skill than needed by the AUM cult to attack the Tokyo underground with nerve gas in 1995.
A nuclear explosive of this nature could explode with the power of 100 tonnes of TNT – 20 times more powerful than the largest terrorist bomb attack to date.