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Analysis Friday, 5 June, 1998, 03:13 GMT 04:13 UK
India - the search for nuclear capability
A nuclear device provides the largest and most rapid release of energy of any weapon yet developed. Our Science Correspondent David Whitehouse explains what makes a nuclear explosion awesome and investigates India's long search for nuclear capability.

India first decided to build its own nuclear weapons after China began nuclear tests in the mid-1960s.

A key factor in India's desire to be a nuclear power has been China's presence on its northern border as well as Pakistan's nuclear capability.

Its nuclear programme is based at the Bhabha Atomic Research Centre near Bombay.

By 1971 it has a design for a nuclear device and the then prime minister Indira Gandhi gave the order to put it into production.

information box on India's recent tests
The Indian prime minister said there was no release of radioactivity into the atmosphere
It took two years to build and was beset with problems, especially with the electronic detonators that force the nuclear material together.

India's first test was the so-called 'Smiling Buddha' detonations in the Rajasthan Desert in 1974.

Like the most recent tests they were also carried out in an underground chamber 107 metres below the surface.

India has many nuclear reactors that can provide the basic material required for a nuclear device.

However it is not thought that it has a stockpile of weapons. In 1993 the United States Central Intelligence Agency said that it believed that India did not have assembled nuclear weapons.

Indian Government officials have said that they have the ability to put a nuclear device together if the need arose.

A resumption of India's nuclear tests was expected.

In 1995 leaked information from US spy satellites suggested a renewed flurry of activity at the Rajasthan testing site.

What is a nuclear device?

A nuclear device works by a very different principle than conventional chemical explosives.

Even so, a nuclear weapon's power is often expressed in how much TNT, a conventional explosive, would be needed to produce an explosion of the same size.

Basically, a so-called critical mass of Plutonium or enriched Uranium has to be forced together to produce a nuclear reaction. Then within a few thousandths of a second vast amounts of energy are released.

The main component of the device is a shell of Plutonium or Uranium. The shell is built to contain a little less mass than that required for an explosion.

Chemical explosives, detonated by electronics, are wrapped around the shell. These compress the nuclear material until it becomes critical and explodes.

To help the explosion begin a material called a neutron reflector is placed outside the nuclear material to reflect neutrons back into the explosion.

Without the reflector the explosion may not occur. Also a source of neutrons called an initiator is placed at the core of the bomb.

The bombs that exploded over Hiroshima and Nagasaki had yields of 12,500 and 20,000 tonnes of TNT respectively. The recent tests carried out by India were of sub-kiloton bombs.

Advances in the power of computers has rendered much testing of nuclear weapons obsolete. Nuclear detonations can now be simulated with remarkable precision in supercomputers.

However sometimes scientists will argue that a test is needed to provide data for future simulations and India has said that this is the case with its latest detonations.

Links to more Analysis stories are at the foot of the page.

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