Under the Microscope There is much highly enriched uranium (HEU) located around the world in research nuclear reactors writes Prof William Reville
It could be used to build a simple nuclear bomb and so there are understandable concerns that such material might fall into the hands of terrorists. Urgent efforts are under way to immediately improve security at civilian installations holding HEU, pending its replacement by low enriched uranium (LEU) that is unsuitable for making bombs. The story is told by A Glaser and F von Hippel in Scientific American, February 2006.
Uranium is naturally present in igneous rocks and its atoms occur in two forms - U238 and U235. U238 comprises 99.3 per cent of natural uranium and U235 makes up the remaining 0.7 per cent. U235 is the atom of interest for generating nuclear power, and also for making bombs, because it is the only naturally occurring isotope, found in any economic quantity, capable of nuclear fission.
An atom is made of two parts - a central nucleus in which almost all of the mass of the atom resides, and surrounding electrons that fly about and occupy almost all the volume of the atom. The central nucleus is made of two types of subatomic particles called protons and neutrons. The number of protons determines the kind of atom and is called the atomic number. All uranium atoms have 92 protons. However, atoms can have different number of neutrons, giving rise to isotopes of the atom. U238 has 92 protons and 146 neutrons. U235 has 92 protons and 143 neutrons.
The ratio of protons to neutrons determines the stability of the nucleus. The U235 nucleus is unstable in a special way that allows it, when hit by a wandering neutron, to break into two approximately equal fragments - nuclear fission. When fission occurs, some of the mass present in the parent nucleus is converted into a large quantity of energy according to the equation E=MC2, where E is energy, M is mass and C is the speed of light.
When fission occurs, in addition to the two fission fragments, neutrons are also released. One or more of these neutrons can trigger other fissions in nearby U235 atoms and this is the basis of the nuclear chain reaction. If on average each fission causes more than one other fission then you have an accelerating chain reaction which quickly gets out of control. In nuclear power, conditions are manipulated so that you get a steady chain reaction. In an atomic bomb the chain reaction runs out of control and explodes. When nuclear fission reaches a self-sustaining rate the nuclear material is said to be critical. When it accelerates beyond this it is super critical. At a lower than self-sustaining rate it is sub critical.
Some designs of nuclear power plant can use natural uranium as fuel, but almost all designs now require enriched uranium where the U235 content is enriched from its natural concentration of 0.7 per cent to somewhere between three per cent and 5 per cent. In order to build a bomb, the uranium 235 content must be enriched beyond 20 per cent (HEU) and usually enrichment would be brought up to more than 90 per cent. Enrichment of uranium is a slow, sophisticated and expensive technological exercise. It is not something that terrorists would undertake. However, it is relatively simple to build a uranium atomic bomb, once HEU is available.
The first atomic bomb used at Hiroshima in Japan in 1945, called Little Boy, was such a weapon. The design is simple. You need about 60 kg of HEU. This is divided into two subcritical portions, and each portion is placed at either end of a tube. A propellant is placed behind one portion (called the bullet). When the propellant is activated it fires the uranium bullet into the other portion of the uranium giving it sufficient mass to go super critical and explode.
During the 1950s and 1960s the US and Soviet governments built many research reactors and also gifted such reactors to about 50 other countries to win political favour and to establish their own reactor technologies abroad. These research reactors were fuelled with HEU (enriched to about 90 per cent U235), surplus from nuclear weapons programmes. At the end of 2005, about 10 metric tons of bomb grade uranium still remained in countries that do not have nuclear weapons. This is enough to make up to 200 Hiroshima-like bombs.
The worry is that much of this HEU world wide is held in civilian facilities that are not very secure. Some HEU has been stolen in Russia. Efforts are underway to repatriate HEU back to America, where it is diluted down to 20 per cent U235 (LEU) with U238. Many research reactors around the world have also been retrofitted to run on LEU fuel. The International Atomic Energy Agency has ruled that any enriched uranium with U235 content above 20 per cent must be considered direct use material, ie usable to make a nuclear bomb. Below 20 per cent, the critical mass becomes too big to fit into any device of reasonable size. To make a critical mass using 95 per cent enriched uranium requires only 22 kg, whereas 400 kg are required to reach critical mass using 20 per cent enriched uranium.
Glaser and von Hippel are worried about the rate at which HEU clearout programmes are proceeding. They claim that if the US and its allies were to seriously tackle the situation, all civilian HEU could be eliminated from the world in five years, and say: "Continued delay in completing this task only extends the window of opportunity for would-be nuclear terrorists." William Reville is associate professor of biochemistry and public awareness of science officer at UCC - understandingscience.ucc.ie