Last year was the 75th anniversary of the first controlled nuclear chain reaction in Chicago, USA, on 2 December 1942, which opened the door to nuclear warfare.
This year marks fifty years since the signing of the Treaty on the Non-Proliferation of Nuclear Weapons, an international treaty aimed at preventing the spread of nuclear weapons and promoting, instead, the peaceful use of nuclear technology. This article by Gil Jacobs is a stark reminder of the destructive capability of nuclear weaponry and its threat to life on earth.
Atoms consist of a nucleus with electrons orbiting it. The nucleus consists of protons and neutrons. Protons are positively charged, electrons are negatively charged and neutrons have no charge. Like charges repel and unlike charges attract. There is a force, called the strong nuclear force, which is a force of attraction between protons and neutrons, neutrons and neutrons, and protons and protons. It has an extremely short range and it holds the protons and neutrons in the nucleus together. If there were no strong nuclear force the mutual repulsion of protons would prevent nuclei from forming. When the number of electrons in an atom equals the number of protons the atom has no net charge; it is neutral. An element is a substance that cannot be chemically reduced to simpler substances. The number of protons in the nucleus is called the atomic number - this number determines the chemical properties of an element. The number of protons plus the number of neutrons is the mass number of the element.
The hydrogen atom has a nucleus which consists of a single proton. Its atomic number is thus one. Helium, with two protons and two neutrons, has an atomic number of two and a mass number of four.
Elements with atoms having the same number of protons, but different numbers of neutrons, are encountered very often. Their chemical properties are identical but their atomic weights differ. These atoms are called isotopes of the same element.
The uranium nucleus has 92 protons. Most (about 99.3%) of naturally occurring uranium has 146 neutrons and thus a mass number of 238. This isotope is known as U238. About 0.7% of naturally occurring uranium with 92 protons and 143 neutrons has a mass number of 235. This isotope is known as U235. This is the isotope of uranium which can be used in atomic bombs. The separation of U235 from U238 is a very slow, difficult and expensive process.
Plutonium has an atomic number of 94. It does not occur naturally but is made in nuclear reactors. Pu239 is the isotope of plutonium that can be used in atomic bombs.
Energy is stored when positively-charged protons are forced together. It follows that if a uranium or plutonium nucleus could be split into two more or less equal halves a great deal of energy would be released as the fragments move apart because of their mutual repulsion. This release of energy can be likened to the energy released when a compressed spring expands. The splitting of nuclei is called nuclear fission.
Nuclear fission was achieved in 1938 by Otto Hahn and Fritz Strassmann. Lise Meitner and her nephew Otto Frisch explained nuclear fission theoretically in 1938.
When a U235 or Pu239 nucleus is struck by a neutron it can split into two more or less equal halves. When the nucleus splits neutrons are released. These neutrons, in their turn, split other nuclei. When these nuclei are split, neutrons are again emitted. More nuclei are then split. What we get is a chain reaction. If a chain reaction is to occur, the amount of uranium or plutonium must exceed the critical mass. If the mass of the uranium or plutonium is less than the critical mass, there will not be a sustained chain reaction.
There are three ways to achieve 'criticality': Add more fissionable material - neutrons have a smaller chance of escaping without colliding with a nucleus; surround the fissionable material with a neutron reflector - instead of escaping, neutrons are reflected back into the material; or compress the fissionable material - the nuclei are closer together and are more likely to be struck by a neutron.
A controlled chain reaction was first achieved by Enrico Fermi in Chicago on 2 December 1942.
The Development of the Bombs
When the physicists heard that nuclear fission was a reality, they immediately realized that an atomic bomb was a possibility, and not just speculation. Leo Szilard had conducted experiments which showed that neutrons are emitted when the uranium nucleus is split. He was a refugee from Europe who fully understood the threat of atomic weapons. Some of the world's best physicists were in Nazi Germany and if the Nazis developed atomic bombs before the Allies did, they would be in a position to enslave the world. Szilard wanted to persuade the United States government to start an atomic bomb programme. He thought that the US government would not listen to him, so he asked Albert Einstein to support him. Szilard then drafted a letter to President Roosevelt which explained the urgency of the situation. Einstein signed the letter on 2 August 1939. The fact that Einstein, who was a complete pacifist, initiated the building of atomic bombs shows how dire the situation was.
After a slow start, the Americans launched the Manhattan Project. It eventually became a massive undertaking, with industrial plant in Oak Ridge, Tennessee; Hanford, Washington and the famous establishment at Los Alamos in New Mexico. The bombs were designed and built at Los Alamos. The enrichment of uranium and manufacture of plutonium were done at Oak Ridge and Hanford. The size of the undertaking rivalled that of the American car industry! The development of the bombs was spurred on by the British, who, realizing that they did not have the industrial capacity to develop their own bombs, placed their considerable knowledge and many able scientists at the disposal of the American bomb programme.
In September 1942 General Leslie Groves was appointed the military head of the Manhattan Project. He was a brilliant military engineer and an outstanding administrator who had overseen the building of the Pentagon. Groves appointed Robert Oppenheimer as the scientific head of the project. This was in spite of Oppenheimer not having had any administrative experience. One of his colleagues said that he would not have been able to 'run a hamburger stand'. He also was a theoretical physicist with not much practical ability. He was not a Nobel laureate this was considered a disadvantage as a number of the scientists who would be working under him were Nobel prizewinners. His detractors were to be proved wrong.
Two kinds of bomb were developed at Los Alamos. The simplest was the uranium bomb, which fired a sub-critical slug of U235 into a hollow piece of U235 to form a super-critical mass of U235. A chain reaction would take place, resulting in a massive explosion. The two masses of uranium had to be brought together very fast, otherwise the bomb would pre-detonate with a very much smaller explosion. This bomb was also called a 'gun bomb'.
The other bomb was the plutonium bomb. They also wanted it to be a gun bomb, but this proved to be impossible. Pu239 is much more likely to predetonate than U235 and the gun method would have brought the two masses together too slowly. A spherical bomb was designed, with a plutonium sphere at the centre. This was surrounded by a tamper and explosive lenses which consisted of a combination of fast and slow explosives shaped in such a way that the shock waves initiated by 32 detonators would be bent so as to form a symmetrical shock wave converging on the plutonium sphere. This became known as implosion. The plutonium would then be compressed, become critical, and explode.
The uranium bomb was not tested, because the designers were confident that such a relatively simple design would work. They only had enough uranium to make one bomb.
As the plutonium bomb was far more complicated than the uranium bomb, it was decided that a test was necessary. A test bomb was placed on a tower 30 metres high and exploded on 16 July 1945. The test was a complete success. The explosion was more powerful than anticipated, and success could be reported to President Truman, who was attending the Potsdam conference.
The Destruction of Hiroshima and Nagasaki
Many scientists were greatly relieved when Germany surrendered to the Allies in May 1945. They thought that the atomic bomb would not be used. They were wrong; work on bomb development continued. When President Roosevelt died in April 1945, he was succeeded by Harry S Truman. Truman did not know about the atomic bombs under development and his staff had to brief him on the matter.
America planned to invade the Japanese home islands in 1946. Truman was told by his advisers that such an invasion could cost up to a million American casualties, and as many, or more, Japanese. Americans were tired of war and their economy was suffering. So why not use atomic bombs to bring the Second World War to an end? This was how many people started thinking. The Potsdam Declaration warned of 'Prompt and utter destruction'. President Truman made the decision to use the atomic bomb. On 6 August 1945, a 829 Superfortress took off from Tinian Island. This bomber was commanded by Colonel Paul Tibbets and named the Enola Gay after Tibbets' mother. In the enlarged bomb bay was 'Little Boy', a uranium gun bomb. The bomb was dropped over Hiroshima and it exploded about 2 000 feet (Just over 600 metres) above the city. About 70 000 people died instantly. The explosion was the equivalent of 13 000 tons of TNT. Japan did not surrender. On 9 August 1945, a B29 named Bock's Car, under the command of Major Charles Sweeney, took off from Tinian. It carried 'Fat Man' in its bomb bay. This was a plutonium implosion bomb. The bomber flew to Kokura, which was the primary target. There was cloud over Kokura so the B29 went to its secondary target, Nagasaki. A break in the clouds over that city made it possible to bomb visually. The aircraft was short of fuel because of a defective fuel pump. If they could not bomb visually, they would have had to bomb by radar, which was less accurate. If the bomb was not dropped, they would not make it back. The bomb weighed four and a half tons. The bomb was dropped over Nagasaki. It also exploded about 2 000 feet above the city, but fell wide of its target. Nonetheless about 50 000 people died instantly. The explosive yield was that of 21 000 tons of TNT. Photographs of the two cities show almost complete destruction. A few buildings were still standing. These were built of reinforced concrete. Their interiors were completely destroyed. The victims who were close to ground zero (the point on the surface directly under the blast) were completely vaporized. People further away were turned to carbon. Those still further away suffered terrible burns from the intense heat and nuclear radiation. Many who suffered from burns started to recover and there was hope that their recovery would be complete. They then became very ill and their hair started falling out. They were suffering from radiation sickness and nothing could be done for them.
An estimated 120 000 people died instantly, 160 000 were injured and 250 000 were dead by the end of 1945. Japan formally surrendered on 2 September 1945, ending the Second World War. The Soviet Union exploded a plutonium bomb in August 1949. The United States then came under pressure to build the hydrogen bomb, thousands of times more destructive than the Hiroshima bomb. The ensuing nuclear arms race between the US and USSR resulted in life on Earth being threatened with extinction. Albert Einstein deeply regretted the part he had played. In 1945, he said: 'The release of atomic power has changed everything except our way of thinking ... the solution to this problem lies in the heart of mankind. If only I had known, I should have become a watchmaker.' Hiroshima and Nagasaki have been rebuilt. They are thriving and beautiful cities today.
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