Homi Jehangir Bhabha, the scientist largely responsible for India’s atom bomb, was born in Bombay on October 30, 1909. He came from a Parsi family closely associated with the Tatas, one of the richest families in India. His father was a British-educated lawyer for the Tatas and his paternal aunt had married Sir Dorab Tata, the oldest son of the founder of the Tata industrial empire, Jamsetji N. Tata.
Bhabha grew up in an atmosphere of high culture and great privilege. In his aunt’s house could be found people like Nehru and Gandhi. He was educated in the best of the most expensive schools in Bombay. He showed great natural abilities in mathematics and he persuaded his uncle to finance studies in Cambridge. He was supposed to become an engineer, but at Cambridge he attended the lectures of Paul Dirac, one of the founders of the quantum theory, and decided to take his degree in physics. He did his Ph.D. with Ralph Fowler, who had been Dirac’s mentor. As a postdoctoral scholar he studied with the best scientists in Europe. During this time he did a calculation of the collisions of electrons with positrons—a process that bears his name and can be found in any textbook. When he was on vacation in India in 1939 the war broke out and Bhabha decided that he could not go back to England. This was a momentous choice, which led eventually to the creation of the Indian atomic bomb.
Zulfikar Ali Bhutto, the politician who organized Pakistan’s nuclear weapons program, was born on January 5, 1928, in Larkana, then part of British India but now in Pakistan. His father, Sir Shah Nawaz Bhutto, was a landowner in the Sindh. While not in the same financial league as the Tatas they were still very well-off. The family was Muslim and his father was an important political leader. Like Bhabha, Bhutto received his primary education in private schools in Bombay. Also like Bhabha, he left the Indian subcontinent to study abroad. He spent two years at the University of Southern California before transferring to Berkeley. He then went to Oxford to study law. He practiced law in Pakistan briefly before entering government. In 1957 he became the youngest member of the Pakistani delegation to the United Nations.
Six years later he became the foreign minister and angered the Americans by developing a close relationship with China. One of his friends was a nuclear engineer named Munir Ahmad Khan, who had worked extensively in the United States but took a position as a technical associate with the International Atomic Energy Agency in Vienna. This gave Khan access to classified reports and in 1965 he informed Bhutto that India was starting a serious program to build an atomic bomb. Bhutto reacted by saying, “Pakistan will fight for a thousand years. If India builds a bomb Pakistan will eat grass or leaves or even go hungry, but we will get one of our own. We have no other choice.” Munir Khan came back to Pakistan to lead the project that has by now constructed about a hundred such weapons, matching India’s supply. The two countries are like scorpions in a bottle. If one stings the other they will both die. A full-scale attack would reduce both countries to rubble.
While much has been written about the nuclear bombs of India and Pakistan, there is nothing like the collection of essays entitled Confronting the Bomb, by seven Indian and Pakistani scientists with an introduction by John Polanyi, a Nobel Laureate in chemistry. The collection has been edited by the Pakistani physicist Pervez Hoodbhoy. Born in Karachi in 1950, he went to school there and later became a junior colleague of the physicist Abdus Salam, who worked on the Pakistani bomb and is the only Pakistani to have won a Nobel Prize in any subject. Hoodbhoy took four science degrees from MIT and went on to join the faculty at the Quaid-e-Azam University in Islamabad. Apart from physics he is widely known for his courageous and outspoken struggle against Islamic extremism in Pakistan as well as his opposition to nuclear weapons.
The first essay in the book—“Scientists and India’s Nuclear Bomb”—is by the Indian physicist M.V. Ramana, who is currently at Princeton University. Among other things it gives a historical account of the Indian nuclear program.
Fission—the splitting of heavy nuclei like uranium by neutrons—was discovered in Germany at the end of 1938. The disintegrating atoms release more free neutrons, and these can in turn fission other nuclei of fissile isotopes, such as uranium-235 and plutonium-239. This can lead to a chain reaction. (Isotopes are variants of an element with different numbers of neutrons but the same number of protons. They have the same chemical properties but somewhat different masses.) Scientists throughout the world quickly saw that such a chain reaction would release a tremendous amount of energy and might be used not only to generate electricity, but also to create weapons far more destructive than any that had been built before.
In 1948 the newly independent India passed an Atomic Energy Act and created a commission to supervise atomic energy. Bhabha was appointed chairman. In fact, Bhabha had already written to his uncle Dorab Tata’s trust in March 1944 to ask for money to begin a nuclear energy program and fund the Tata Institute of Fundamental Research in Bombay. This date is surprising, since it was over a year before the US set off its first nuclear explosions. What inspired Bhabha to make this request at that time? He envisioned a nuclear power program and believed that India should have a homegrown group of experts to run it. Did he know the details of the US program to make nuclear weapons, perhaps through his British contacts? Once the bombs were exploded in Japan, Bhabha’s intentions were clear.
I think of Bhabha as in some ways the Edward Teller of India. Teller—the Hungarian-American physicist whose work was essential to the development of the hydrogen bomb—was obsessed with the Soviet Union. Bhabha was obsessed with China. He thought that the only way to deter China was to have a nuclear arsenal and he was prepared to do anything to get one for India. He had support from Nehru, who tried to disguise the bomb program as a means of using nuclear power—including atomic explosions—for peaceful purposes. Teller also tried to persuade people that nuclear explosions were important for projects like dredging harbors.
On the average in fission something like two neutrons are produced. These neutrons can have different possible futures. The most likely is to bounce off the uranium nuclei. After a few such bounces they will either escape through the surface of the fuel element—for example, a bomb—or alternatively, cause another fission. (There is, as I will discuss later, a third possibility that leads to the manufacture of plutonium.) When the rate of escape exactly equals the rate of fission the fuel element is said to be “critical.” For a spherical fuel element such as a bomb there is then a critical radius and hence a critical volume. From the density one can find the critical mass of uranium or plutonium that makes up the fuel element. At precisely this mass there is no self-sustaining chain reaction. For this to happen one must create a supercritical mass in which the rate of fission exceeds the rate of escape.
There is a very important distinction to be made between isotopes like uranium-235, which are “fissile,” and isotopes like uranium-238, which are “fissionable.” The latter isotopes require a neutron energy above some threshold to produce fission, while the former can be fissioned by neutrons of any energy. The neutrons produced in a fission have a spectrum of energies and some are not energetic enough to fission uranium-238. Hence to make a chain reaction one needs to increase the amount of uranium-235 above the amount found in natural uranium. This enrichment is frequently done by using centrifuges. These are relatively easy to conceal compared, say, to a reactor. The first Chinese bomb used uranium but we did not know this until after the test. Most reactors require fuel that is at least 3 percent U-235, and weapons-grade uranium must be enriched to about 90 percent U-235.
In December 1953, President Eisenhower announced the Atoms for Peace program, which was intended to share peaceful nuclear research and technology with America’s allies and provided a large number of developing countries, including Iran and Pakistan, with their first reactors. One oddity of these reactors is that they were fueled by weapons-grade uranium, which was available at the time because a surplus of it was created by the US weapons program. No one thought that these countries would ever have the ability to make nuclear weapons—and the United States provided weapons-grade uranium to its Atoms for Peace clients until 1978. Many years earlier, President Truman had announced that the Russians would never have the ability to make a nuclear weapon. There is still some of this weapons-grade uranium in various countries and it needs to be watched carefully.
In August 1956, Indian scientists began operating a very small reactor at a new research facility in Trombay, known as the Atomic Energy Establishment. (It was renamed for Bhabha after his death in 1966.) This reactor burned 80 percent enriched uranium supplied by Britain. But a different path to a weapon—using plutonium instead of uranium as the fuel—was opened by a Canadian-designed reactor that first went critical in 1960.
In a chain reaction, the neutrons emitted by fissioning nuclei must be slowed down by a moderator, such as water or graphite, in order to increase the probability of their fissioning the fissile isotope U-235. In the example of fission I gave earlier the moderator was light water. In the case of the Canadian reactor, the moderator was heavy water, in which ordinary hydrogen, a single proton, is replaced by a relatively rare isotope that also contains a neutron. Light water can capture neutrons, thus taking them out of the fission chain. Heavy water is less capable of doing this and is used in some reactors. What is crucial is that with a heavy water moderator one can use fuel with the low U-235 concentration found in natural uranium, which has a high concentration of U-238.
During the chain reaction of U-235, the U-238 that constitutes a significant portion of any reactor fuel is constantly being converted into U-239 by the absorption of neutrons. The nuclei of this new isotope are unstable; and through a process known as beta decay they become neptunium-239, yet another unstable isotope that in turn also undergoes beta decay, yielding plutonium-239, a fissile isotope that can power both reactors and weapons. Thus, by choosing a heavy water reactor that used natural uranium—which is 99.3 percent U-238—Bhabha and his colleagues maximized the amount of fissile plutonium that could be extracted from the byproducts of their supposedly peaceful energy program.