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 [F] New York Times on the Web Forums  / Science  /

    Missile Defense

Technology has always found its greatest consumer in a nation's war and defense efforts. Since the last attempts at a "Star Wars" defense system, has technology changed considerably enough to make the latest Missile Defense initiatives more successful? Can such an application of science be successful? Is a militarized space inevitable, necessary or impossible?

Read Debates, a new Web-only feature culled from Readers' Opinions, published every Thursday.

Earliest Messages Previous Messages Recent Messages Outline (5464 previous messages)

manjumicha - 01:38am Nov 5, 2002 EST (# 5465 of 5470)

Is North Korea smart enough to build H-bombs? In order to answer this question, one needs to look at the anatomy of an H-bomb.

The basic physical theory of an H-bomb is that the higher energy neutrons are more likely to split plutonium and U-238 and so the basic design principle is to produce high energy neutrons using nuclear fusion and creating high temperature. Since the probability of explosion increases with particle density, extremely high pressure is created inside the bomb. In brief, a small plutonium bomb is exploded, which triggers nuclear fusion that creates a large flux of high energy photons and neutrons, which creates high pressure and temperature, and nuclear fission of U-238.

Plutonium bombs require a small neutron flux to get started and a neutron generation is activated an instant after the high-explosive is ignited that surround plutonium.

The primary trigger requires a tiny amount of plutonium and contains fusile elements in its core. Wehn the trigger fires, an intense radiation and a flux of particles are created. This high intensity radiation creates high pressure and temperature. The U-238 casing of the package holds in the radiation and pressure long enough for the secondary core to explode, which in turn cause the U-238 casing to explode.

In the process a huge amount of destructive power is generated and much of the fusible and fissile materials are consumed. Since only a tiny amount of plutonium is used, H-bombs are much cheaper than A-bombs in terms of fissionable materials.

The plutonium trigger is an implosion-type bomb. What is an implosion bomb? Before we answer this question, let us look at the other type of nukes - the gun-assembly.

manjumicha - 01:39am Nov 5, 2002 EST (# 5466 of 5470)

The Hiroshima bomb, a gun-assembly type, used 64.1 kg of uranium enriched to 80% of U-235 and 20% U-238. South Africa made six gun-assembly bombs, each using 50 kg of uranium enriched to 80-93%. Since the gun assembly bombs rely on smashing two blocks of sub-critical fissionable material into each other in order to achieve critical mass for explosion, plutonium, which requires fast neutrons at high pressure and temperature, cannot be used as the main fissile source.

The Hiroshima bomb was of this type and burned less than 0.1% of the 64.1 kg of uranium enriched to 80% at an enormous cost. Nuclear Weapon Design

Any nation that makes artillery pieces and shells can in principle make gun-assembly nukes, provided it has 50kg or more of uranium enriched to 80% or more.

As mentioned earlier, the Nagasaki bomb was an implosion type that used only 6.2kg of plutonium (viz. 64.1 kg of enriched uranium in the Hiroshima bomb.) The idea is based on supernova explosion. When certain stars reach an old age, a thin shell of fissile and fusion elements surrounds an inner core of burning hydrogen. When the gravity becomes larger than the inner radiation pressure, the outer shell collapses and the whole star explodes in a giant fireball - a supernova.

In an implosion bomb, the collapse is caused by high-explosives. A thin shell of plutonium or enriched uranium collapses on a core of fusile elements.

North Korea has conducted a series of high-explosive tests. It is believed that North Korea's noted chemist, Prof. Lee Yong Ki, has invented extremely potent chemical explosives not only for nuclear triggers but also for North Korea's conventional artillery shells. The US CIA believes that North Korea has 1-3 (or maybe 2-5) implosion bombs.

manjumicha - 01:41am Nov 5, 2002 EST (# 5467 of 5470)

How does one go from implosion to 'hydrogen' bombs?

A small amount of LiD (Lithium-6 deutride) placed in the inner core of an implosion bomb can significantly increases the bomb yield. LiD powder turns into Li, D and tritium gases that undergo fusion releasing fast neutrons, which in turn enhance nuclear fission of plutonium and U-238. The US CIA believes that the Indian H-bomb tested and advertised as such was in fact an implosion bomb with a 'fusion' core - a 'booster' bomb but not a true H-bomb. Any nation that can make implosion bombs can make fusion booster bombs.

Referring to the Mk-28 physics package diagram given earlier, one sees that an implosion bomb is used as a trigger, which ignites fusion on a larger scale than in a booster bomb. Fusile substances surround the implosion bomb trigger and another much larger implosion bomb, and the whole thing is placed inside an explosion bomb made of an uranium-238 casing. U-238 fission if bombarded by high-energy neutrons, photons and alpha particles.

The secondary stage is made of a hollow lithium-6 deuteride cylinder or ellipsoid case in by a layer of U-238. At the core of the cylinder is a Pu-239 or U-235 rod about one inch in diameter. The casing is wrapped in a layer of plastic foam and a plug of U-238 separates the secondary from the trigger.

The Teller-Ulam bomb is often called a "2-stage bomb" because the fission trigger ignites the fusion stage. Since the shock wave dies out in a few micro-second, a 2-stage bomb has a limit on the bomb yields and additional stages are required for super bombs. The rule of thumb is each stage can be 10-100 times the previous stage in explosive power. The largest bomb tested so far is the 59 Mt super bomb tested by the Soviets.

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