Nuclear weapons .. part 1

January 29, 2008 at 9:20 pm | Posted in blogging | Leave a comment
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Fission weapons

Nuclear weapons exploit two principle physical, or more specifically nuclear, properties of certain substances: fission and fusion.

Fission is possible in a number of heavy elements, but in weapons it is principally confined to what is termed slow neutron fission in just two particular isotopes: 235U and 239Pu. These are termed fissile, and are the source of energy in atomic weapons. An explosive chain reaction can be started with relatively slight energy input (so-called slow neutrons) in such material.

Pu239Ga.jpg (6513 bytes)
An actual 239Pu ingot, alloyed with gallium for improved physical properties

Isotopes are ’varieties’ of an element which differ only in their number of neutrons. For example, hydrogen exists as 1H 2H and 3H — different isotopes of the same chemical element, with no, one, and two neutrons respectively. All the chemical properties, and most of the physical properties, are the same between isotopes. Nuclear properties may differ significantly, however.

The fission, or ’splitting’ of an atom, releases a very large amount of energy per unit volume — but a single atom is very small indeed. The key to an uncontrolled or explosive release of this energy in a mass of fissile material large enough to constitute a weapon is the establishment of a chain reaction with a short time period and high growth rate. This is surprisingly easy to do.

Fission of 235U (uranium) or 239Pu (plutonium) starts in most weapons with an incident source of neutrons. These strike atoms of the fissile material, which (in most cases) fissions, and each atom in so doing releases, on average, somewhat more than 2 neutrons. These then strike other atoms in the mass of material, and so on.

If the mass is too small, or has too large a surface area, too many neutrons escape and a chain reaction is not possible; such a mass is termed subcritical. If the neutrons generated exactly equal the number consumed in subsequent fissions, the mass is said to be critical. If the mass is in excess of this, it is termed supercritical.

Fission (atomic) weapons are simply based on assembling a supercritical mass of fissile material quickly enough to counter disassembly forces.

The majority of the energy release is nearly instantaneous, the mean time from neutron release to fission can be of the order of 10 nanoseconds, and the chain reaction builds exponentially. The result is that greater than 99% of the very considerable energy released in an atomic explosion is generated in the last few (typically 4-5) generations of fission — less than a tenth of a microsecond.

This tremendous energy release in a small space over fantastically short periods of time creates some unusual phenomena — physical conditions that have no equal on earth, no matter how much TNT is stacked up.

Plutonium (239Pu) is the principal fissile material used in today’s nuclear weapons. The actual amount of this fissile material required for a nuclear weapon is shockingly small.

Below is a scale model of the amount of 239Pu required in a weapon with the force that destroyed the city of Nagasaki in 1945:

Pu32inch.jpg (4643 bytes)

In the Fat Man (Nagasaki) weapon design an excess of Pu was provided. Most of the remaining bulk of the weapon was comprised of two concentric shells of high explosives. Each of these was carefully fashioned from two types of explosives with differing burn rates. These, when detonated symmetrically on the outermost layer, caused an implosion or inward-moving explosion.

The two explosive types were shaped to create a roughly spherical convergent shockwave which, when it reached the Pu ’pit’ in the center of the device, caused it to collapse.

The Pu pit became denser, underwent a phase change, and became supercritical.

A small neutron source, the initiator, placed in the very center of this Pu pit, provided an initial burst of neutrons — final generations of which, less than a microsecond later, saw the destruction of an entire city and more than 30,000 people..

Nearly all the design information for weapons such as these is now in the public domain; in fact, considering the fact that fission weapons exploit such a simple and fundamental physical (nuclear) property, it is no surprise that this is so. It is more surprising that so much stayed secret for so long, at least from the general public.

A neutron reflector, often made of beryllium, is placed outside the central pit to reflect neutrons back into the pit. A tamper, often made of depleted uranium or 238U helps control premature disassembly. Modern fission devices use a technique called ’boosting’ (referred to in the next section), to control and enhance the yield of the device.

Today’s nuclear threat lies mostly in preventing this fissile special nuclear material (often referred to as SNM) from falling into the wrong hands: once there, it is a very short step to construct a working weapon.

What we do now to keep these devices out of the hands of groups like Al-Qaeda is vital to civilized peoples.

abomb.jpg (13766 bytes)
A schematic of a hypothetical ’boosted’ fission weapon
(showing unnecessary 235U)

trinity.jpg (22048 bytes)
The gadget device used in the Trinity test: the world’s first nuclear weapon test.
Note spherical geometry and the HE detonator arrangement. New Mexico, 21KT, 1945.

grable.jpg (16126 bytes)
Typical fission weapon, shortly after detonation at the Nevada test site, with roughly the same yield as the weapon that destroyed Hiroshima. Reddish vapor surrounding the plasma toroid includes intensely radioactive fission fragments and ionized nitrogen oxides from the atmosphere. (Grable, 15KT,

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