May 21, 2005

sCIENTISTS have known about the atom for a long time, possibly since thousands of years, but it was not fully understood until the beginning of the 20th century. Thus, it was after a long gestation period that in the early thirties the idea of an atomic bomb — involving the catastrophic release of energy produced by a chain reaction of splitting atoms — took shape when a physicist in Hungary named Leo Szilard figured it all out.

The idea might have taken a long time coming, but it took only another 12 years before the ultimate weapon was used in a war to kill enemy troops on a large scale. After the atom was first split for energy in 1938, Albert Einstein wrote a letter to Franklin D. Roosevelt, suggesting that atoms could be a major source of energy. He added: “This new phenomenon would also lead to the construction of bombs, and it is conceivable — though much less certain — that extremely powerful bombs of a new type may be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory.”

There are two main types of bombs that are designed to release energy from the nuclei of atoms. The simplest kind is called the atomic bomb. Like a nuclear power plant, it releases great quantities of energy through a process called nuclear fission involving a large unstable (radioactive) element like uranium or plutonium.

Nuclear bombs play with two fundamental forces — strong and weak forces — that hold the nucleus of an atom together, especially in the case of unstable material.

There are two basic ways in which energy can be produced from an atom. The more common one is nuclear fission, which involves breaking up of the nucleus of an atom into two smaller fragments using a neutron. This method usually involves isotopes of uranium (uranium-235 and uranium-233) or plutonium-239.

This material is very unstable. Every atom’s nucleus is ready to fall apart (“decay”) at a nudge, releasing extra energy and extra neutrons. Plutonium is particularly unstable, or radioactive. Its atoms are constantly “falling apart” — breaking up into smaller elements that are more stable.

Every time one nucleus does this, it releases the extra energy it no longer needs to hold it together, in addition to a few leftover neutrons. The release of energy, and the escaping neutrons, is what’s described as the emission of radiation from radioactive plutonium. This phenomenon can easily damage human cells. That’s why radioactivity is seen as dangerous for people.

The fission reaction is confined within a controlling device called a tamper, which is usually made up of uranium-238. The tamper is heated and expanded by the fission core. The expansion of the tamper exerts pressure back on the fission core and slows down the core’s expansion.

The tamper also reflects neutrons back into the fission core, increasing the efficiency of the fission reaction. The simplest way to bring the sub-critical masses together is to make a gun that fires one mass into the other. A sphere of U-235 is made around the neutron generator and a piece of the unstable U-235, which is like a small bullet of U-235, is removed. The bullet is placed at one end of a long tube with explosives behind it, while the sphere is placed at the other end.

A barometric-pressure sensor determines the appropriate value for detonation and triggers a sequence of events. First, the explosives, which have been set for this purpose, fire and propel the bullet down the barrel. The bullet strikes the sphere and generator, initiating the fission reaction.

The fission reaction begins resulting in the explosion of the bomb. A blow from a single neutron is enough to split the less-stable U-235 atom, creating atoms of smaller elements (often barium and krypton) and releasing heat and gamma radiation (the most powerful and lethal form of radioactivity). A chain reaction occurs when “spare” neutrons from this atom fly out with sufficient force to split other U-235 atoms they come in contact with.

The probability of a U-235 atom capturing a neutron as it passes by is high. In a bomb that is working properly, more than one neutron ejected from each fission causes another fission to occur. When they hit another atom, they cause its nucleus to break down too, whether it was ready to do so or not.

The second nucleus releases more energy, and more neutrons, which in turn go on to hit and break up further nuclei. The decaying nuclei give rise to more decaying nuclei, and so on, in a rapidly escalating chain reaction. This is all because the U-235 has been squeezed into such a dense state that the escaping neutrons that normally would fly out of the material now cannot without hitting the other nuclei.

This condition is known as super-criticality. The process of capturing the neutron and splitting the nucleus happens very quickly, in a matter of picoseconds (10 to the power -12) seconds.

Within this very tiny fraction of a second, all the nuclei in the chunk of plutonium get hit by escaping neutrons, and break down. The extra energy in trillions of atomic nuclei is all released at once.

This energy is considerable. The atomic bomb dropped on Hiroshima in the Second World War is an example. The energy released by a single fission is high, because the fission products and neutrons, put together, weigh less than the original U-235 atom. The difference in weight is converted to energy at a rate governed by the equation e = mc2.

A pound of highly enriched uranium, as used in a nuclear bomb, is equal to something of a million gallons of gasoline. When you consider that a pound of uranium is smaller than a baseball and a million gallons of gasoline would fill a cube that is 50 feet per side, you can get an idea of the amount of energy available in just a little bit of U-235.

Nuclear bombs have had devastating effects. At the centre, everything is immediately blasted into oblivion because of high temperatures — up to 300 million degrees Celsius. Outwards from the centre, most casualties are caused by burns, injuries from flying debris of buildings and acute exposure to high radiation.

Beyond the immediate blast area, casualties are caused by heat, radiation and fires spawned by the resultant heat wave. In the long-term, radioactive fallout occurs over a wider area according to the prevailing winds.

The radioactive fallout particles enter the water supply and are inhaled and ingested by people at a considerable distance from the blast. Einstein, speaking about the destructiveness of nuclear bombs, said he did not know which weapons would be used in the WW3, but the only ones that would be used in WW4 would be sticks and stones.

Nuclear bombs are the man’s best weapons against himself and we can only wish that such devices of mass destruction are never again used to kill people. The technology should be used only for peaceful purposes.

The writer subtainnaqvi@hotmail.com is a student of the Institute of Business Administration, Karachi

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