The Trusty Servant

U²³⁵ is the only naturally occurring material which can be used for fission weapons. Natural uranium is a mixture of 99.3% U²³⁸ and 0.7% U²³⁵ (plus minute quantities of U²³⁴). You need U²³⁵ for fission weapons. So you have to “enrich” the natural element to increase the proportion of U²³⁵. Material with more than 20% U²³⁵ is known as “highly enriched uranium” (HEU).

Just to recap, an atom consists of a nucleus made of protons and neutrons, and electrons which are arranged in shells surrounding the nucleus. Protons and electrons have opposite charges, whilst neutrons have no electrical charge. The chemistry of an atom is determined by its electrons, and hence by the number of protons in its nucleus.

“Element” or “chemical element” is the term used to mean ordinary everyday quantities of material which is made up (largely or almost entirely) of the same kind of atom. For example, “carbon” is a more user-friendly term for material made up of atoms each with six protons. In addition to a name (like “carbon”) chemical elements are also known by one or two letter abbreviations. For example, C denotes carbon, and U denotes uranium which is made up of atoms each having 92 protons.

It makes no difference to the chemical properties how many neutrons the nucleus has. But for fission, the number of neutrons is important. An isotope is material with both the number of protons and the number of neutrons specified. So U²³⁵ is the isotope of uranium with 143 neutrons, giving a total of 235 particles in the nucleus, whereas U²³⁸ has 146 neutrons in the nucleus.

An atom “fissions” when its nucleus splits into two parts, which are then driven apart by electrostatic repulsion. Because when the two parts are a tiny distance apart, each has a positive charge from the protons, and like charges repel.

Many isotopes can fission. But for an explosion you need a “chain reaction”. When a U²³⁵ atom fissions it also sends out a few “slow prompt neutrons”. Prompt just means they are emitted at the same time as the nucleus breaks into two parts. Slow means they have relatively low energy. But a slow neutron will cause a U²³⁵ nucleus to fission if it hits it. On average a fissioning nucleus sends out 2.5 slow prompt neutrons. So if a single nucleus fissions, then a moment later 2-3 more will fission. A moment after that, each of those will cause 2-3 more. So within a split second most of the U²³⁵ nuclei in the material will have fissioned and a huge amount of electrostatic energy will have been released, giving a big explosion.

The detail is somewhat complicated. A particular atom may fission without releasing a slow neutron capable of causing another atom to fission. Or the slow neutron may leak out of the edges or get stopped by the U²³⁸ before it has a chance to cause another atom to fission. So there are elaborate probability calculations. The bottom line is that there is a “critical mass” of material, although this depends on its shape, its density, the proportion of U²³⁵, the nature of any non-U²³⁵ material mixed in, and so on. But if the mass is above the critical figure, then the material will explode spontaneously. If it is below (by a reasonable margin) then it won’t.

The usual technique for making a bomb is to get a lump of material which is slightly below the critical limit and then to increase its density by “imploding it”. Basically you pack ordinary high-explosive around it and set that off, so that the uranium is driven inwards. That increases the density and thus reduces the critical mass below its actual mass, so that it explodes.

The first bomb, which was exploded over Hiroshima was 80% HEU. Subsequent HEU bombs have been rather more highly enriched, in the range 80-94%. However, theoretical calculations suggest that 20% HEU is enough for a carefully designed bomb – or maybe even less than 20%. After the test ban treaty, the computer code for simulating nuclear explosions has become extremely sophisticated, and the weapons labs now have high confidence that a real bomb will explode in the same way as its virtual counterpart.

Designs below 80% are typically more sophisticated than those using 80%+ material. There is a famous quote by US weapons designer, Teddy Taylor, to the effect that given 80%+ material, designing a fission weapon is so easy that “everyone who has tried it has succeeded first try”. That may not be true for material which is much less enriched.

The graph above is taken from a 1998 Los Alamos publication (pdf). Ignore the lower U²³³ line. The upper line shows how the critical mass increases as the enrichment goes down. So for 20% HEU (= 80% U²³⁸ on the horizontal scale) you need about a tonne of enriched material (=1000 kg on the vertical scale). This drops dramatically to around 10kg for 80% HEU. 60% HEU is around 1/5 tonne.

Note that the critical mass is roughly the amount you need for a bomb. So the snag about low enrichment material is that you need far more of it.

The International Atomic Energy Agency (IAEA) believes that Iran is planning to make fission warheads which can be delivered by rocket. It is clearly sensible to enrich the material as much as possible, because a given quantity of natural uranium will yield far more warheads if you enrich it above 80% than if you leave it at 20%. Also your warheads are far heavier and more difficult to deliver with less enriched material. Finally, there must be some doubt about whether they will really explode at the lower enrichments. But I will turn to the IAEA evidence for its beliefs in a later post.