In
order to answer this question, one needs to look at the anatomy of an
H-bomb. Lee Wha Rang, October 25, 2002
The US CIA has known for several years that North Korea has one or two (or even three) plutonium implosion bombs of the type the United States dropped on Nagasaki in1945. The Nagasaki bomb (the Fat Man) had the destructive power of mere 22 kt and killed about 70,000 residents. The 15 kt Hiroshima bomb (the Little Boy), a uranium gun-assembly, killed about 200,000. More people were killed in Hiroshima, although the bomb was smaller, due to the bombardier's error of dropping the Fat Man one and half miles off its target.
Though sounds scary, plutonium bombs are limited in destructive power unless one exploded several hundreds of them at the same time. One or two (or even 50) plutonium bombs would do little damage to a large nation such as Japan and the United Sates: at best, they could take out a few hundred blocks of a city and kill a few hundred thousands people - a drop in the bucket.
A crude plutonium bomb requires 35.2 lbs (16 kg) but this critical mass drops sharply with the fissile density (inverse of the square of the density) achieved with better bomb designs. A critical mass of less than one kg has been achieved by the United States, Russia and China. The Nagasaki bomb had 6.2 kg of plutonium. The US CIA estimates that North Korea has at least 70 lbs (31.5 kg) of plutonium and so it could theoretically have made 1-3 plutonium bombs. It has been reported that China has told the United States that North Korea has 3-5 plutonium bombs and that these bombs are small enough to be carried by North Korean missiles aimed at the US military bases in the Far East. How many nukes North Korea has depends on how advanced their bomb technology is. The 207,000 American soldiers in the Far East are in danger of getting hit by 1-5 plutonium bombs and biochemical warheads. This is an acceptable risk for the United States.
But this is not the end of the story. North Korea told the US that it has more 'powerful' weapons. What would be more powerful than A-bombs? The answer is H-bombs. It is no secret that North Korea has a large stockpile of bio-chemical weapons. Chemical weapons are limited in range and so, they are used mainly for tactical purposes. For example, North Korea would use chemicals to disable US forces dug in along the DMZ or Inchon-style landing. Biological weapons are more effective as psywar weapons. Germ bombs dropped on large population centers would cause panic among civilians and disrupt, momentarily, industrial production and transportation. On the other hand, even a handful of H-bombs, with mega-tons of TNT, could wipe out large cities and would present unacceptable risks to the United Stated.
Fission bombs ('A-bombs') are notoriously inefficient. For example, the two bombs dropped in Japan burned less than 0.1 per cent of the fissile material. Practically the entire fissile matters obtained at a huge expense turned into worthless dust without killing a single Japanese. How would one go about increasing the efficiency and at the same time reduce the cost? The H-bomb is the answer for getting more bang cheaper using less fissile expensive materials. H-bombs use only a small amount of costly plutonium and use cheap uranium-238 and light elements for the main punch. North Korea is known to have two active uranium mines and a huge recoverable deposits of U-238. Since an H-bomb requires only a tiny amount of plutonium to trigger U-238 explosion, North Korea's suspected stock pile of 70 lbs of plutonium and virtually unlimited supply of U-238 may mean scores, if not hundreds, of H-bombs hidden in some of the 11,000 underground facilities in North Korea.
In
order to answer this question, one needs to look at the anatomy of an
H-bomb.
Photo: The US Mk-28 H-bomb, dating back to 1958, is still an active weapon system. It is capable of a ground or air burst and may be carried internally or externally, with a free-fall or parachute retarded drop, depending upon its configuration. MK-28 can be carried on bombers, submarines or ground vehicles.
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.
Photo:
Basic components of an H-bomb: The front piece, colored red in the above
photo, houses fusing and detonating devices such as an altimeter, timers, remote
control fuses, etc. It also houses 'safing' devises for preventing
accidental detonation. Connected to the front piece is the business end of the
bomb - affectionately called the "physics" package. The rest of
the bomb is for navigation or propulsion. How
to make an H-bomb?
The physics package itself is a small part of an H-bomb. Safing, fusing and guidance take up more space and care than the business end of the bomb. A simplified diagram of the physics package of Mk-28 is given below.
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.
Photo:
A simplified diagram of the Mk-28 physics package. How
to make an H-bomb?
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.
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.
Photo:
A simplified diagram of a gun-assembly. It is basically a pipe bomb.
A sub-critical mass of enriched uranium is shot into another block at the nozzle
end of the gun. The tampers are made of U-238 and help built up
temperature and pressure.
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.
Photo:
An implosion bomb. A sub-critical mass of fissile substances is surrounded by
conventional high-explosives, that are exploded uniformly around the fissile
core. The core is squashed into a tiny volume and explodes. Nuclear
Weapon Design
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.
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.
Photo:
The Teller-Ulam design. The
Teller-Ulam Design
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.
The United States tested its first H-bomb in 1952, seven years after its first A-bomb. China, France, Great Britain and the Soviet Union have tested H-bombs much sooner after testing their first A-bomb. India has tested a booster bomb - a precursor of H-bombs. Israel most likely has H-bombs. Pakistan, India's archenemy, most likely has H-bombs and North Korea, Pakistan's close ally, most likely has a number of H-bombs as well. Any nation that can made booster bombs can make H-bombs as well. North Korea's 31.5 kg of plutonium, assuming that is all North Korea has to work with, would mean at least 30 H-bombs. If this is true, one can no longer pooh pooh North Korea's 1-2 'primitive' nukes. North Korea's 30 or so H-bombs can do serious damage to Japan and the United States.
Kang Sang Wuk told Kelly, Bush's special envoy to Pyongyang, that North Korea has not only 'nuclear programs' but also, more "powerful' weapons. Other North Korean spokesmen have in the past alluded to waging thermo-nuclear war with the United States using nuclear-tipped ICBMs. Is it possible that they are not bluffing?