Advertisement
Some isotopes of uranium and plutonium have nuclei that are so close to being unstable that they fragment and release energy when bombarded with neutrons. A fission chain reaction builds up because each fragmenting nucleus produces several neutrons that can initiate further reactions. An explosion occurs if the piece of uranium or plutonium exceeds a certain critical mass— thought to be a few kilograms (i.e., smaller than a grapefruit). In order to bring about the explosion, this critical mass has to be assembled very quickly, either by firing together two subcritical pieces or by compressing a subcritical sphere using conventional explosives. The US developed the first atom bombs in great secrecy during World War II at Los Alamos, New Mexico. The first test weapon, exploded in New Mexico in July 1945, had a force equivalent to 21 kilotons of high explosive. A few days later, bombs of similar size devastated the Japanese cities of Hiroshima and Nagasaki.
Producing the fissile materials for such weapons was difficult and expensive and required an enormous industrial complex. Less than 1% of natural uranium is the “explosive” isotope 235U, and separating it from the more abundant 238U is very difficult. Plutonium does not occur naturally at all and must be manufactured in a fission reactor and then extracted from the intensely radioactive
waste. Moreover, the size of a pure fission bomb was limited by the requirement that the component parts be below the critical mass. Fusion does not suffer from this size limitation and might allow bigger bombs to be built. The fusion fuel, deuterium, is much more abundant than 235U and is easier to separate. Even as early as 1941, before he had built the very first nuclear (fission) reactor in Chicago, the physicist Enrico Fermi speculated to Edward Teller that a fission bomb might be able to ignite the fusion reaction in deuterium in order to produce an even more powerful weapon—this became known as the hydrogen bomb, or H-bomb. These ideas were not pursued seriously until after
the war ended. Many of the scientists working at Los Alamos then left to go back to their academic pursuits. Robert Oppenheimer, who had led the development of the fission bomb, resigned as director of the weapons laboratory at Los Alamos to become director of the Princeton Institute of Advanced Study and was replaced by Norris Bradbury. Edward Teller (Figure 6.1), after a brief period in academic life, returned to Los Alamos and became the main driving force behind the development of the H-bomb, with a concept that was called the Classical Super.
FIGURE 6.1 Edward Teller (1908–2003). Born in Budapest, he immigrated to the US in 1935.
As well as proposing the hydrogen bomb, he was a powerful advocate of the “Star Wars” military
program.
There was, however, much soul-searching in the US as to whether it was justified to try and build a fusion bomb at all. In 1949, Enrico Fermi and Isidor Rabi, both distinguished physicists and Nobel Prize winners, wrote a report for the Atomic Energy Commission in which they said:
Necessarily such a weapon goes far beyond any military objective and enters the range
of very great natural catastrophes…. It is clear that the use of such a weapon cannot
be justified on any ethical ground which gives a human being a certain individuality
and dignity even if he happens to be a resident of an enemy country…. The fact that no
limits exist to the destructiveness of this weapon makes its existence and the knowledge
of its construction a danger to humanity as a whole. It is an evil thing considered in
any light.
The debate was cut short in early 1950 by the unexpected detonation of the first Soviet fission bomb. Prompted by the suspicion that East German spy Klaus Fuchs had supplied information about US hydrogen-bomb research to the Soviet Union, President Truman ordered that the Super be developed as quickly as possible. However, no one really knew how to do this, so new fears were raised that Truman’s statement might simply encourage the Soviet Union to speed up its own efforts to build a fusion bomb and, more seriously, that the
No comments:
Post a Comment