Nuclear Chemistry -

Fission and Fusion

The atom as a source of energy

In 1904, Rutherford predicted that atoms might be used as a source of energy:

“If it were ever possible to control at will the rate of disintegration of the radio elements, an enormous amount of energy could be obtained from a small amount of matter”. :

Einstein’s Theory

One year later, Einstein developed the theory of relativity that predicted the amount of energy that should be produced when the mass of an atom is converted into energy. Most of us have seen the famous equation that expresses this relationship:

E = m . c 2

The discovery of fission

In 1934, Enrico Fermi was surprised to discover that bombarding heavy atoms such as Uranium with neutrons resulted in the formation of smaller atoms among the products.

In the earlier nuclear transmutations involving alpha and beta decay, the atoms produced in the nuclear reactions were only slightly different than the atoms that they started from.

More evidence

In 1938, German scientists Otto Hahn and Fritz Strassman were able to isolate Barium (atomic number=56) among the products produced when Uranium was bombarded with neutrons.

They concluded that the Uranium atoms were splitting roughly in half to form two smaller atoms.

This process of splitting a large atom into two smaller atoms is called fission.

Meitner confirms mass can be converted into energy

Lisa Meitner, a German scientist who had fled Nazi Germany to Sweden, was able to show that the mass of the products was less than the mass of the original Uranium atoms.

She was able to show that the energy given off in the reaction was equal to the amount of energy that Einstein’s equation (E=mc2) predicted should have been given off.

The Chain Reaction

In 1939, Niels Bohr came to America and discussed with Einstein and Fermi the possibility of creating a sustained chain reaction in which the fission products of one fission are used to cause additional fission reactions.

Einstein writes to the PresidentAt the request of several concerned colleagues, Albert Einstein wrote

a letter to President Franklin Roosevelt in 1939:

“In the course of the last four months it has been made probable - through the work of Joliot in France as well as Fermi and Szilard in America - that it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future”.

Einstein predicted the atom bomb

He continued in the letter:

“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 thus 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”.

The first nuclear reactorOn December 2, 1942, Enrico Fermi and his colleagues achieved

the first controlled nuclear reaction under the stadium at the University of Chicago.

Fuel for the bombs

Once Fermi had shown that the concept of a sustained chain reaction was correct, it was necessary to amass enough of the fissionable isotopes to sustain the chain reaction. The mass required to sustain a fission reaction is called critical mass.

Two isotopes were identified as capable of undergoing fission. They were Uranium-235 and Plutonium-239.

The production of U-235 was carried out at Oak Ridge Tennessee while Pu-239 was produced at Hanford, Washington.

The First Atom BombThe first bomb utilizing the fission reaction was exploded at Los

Alamos, New Mexico on July 16, 1945.

Atom bombs dropped on JapanThe United States dropped its

first atomic weapon on Hiroshima on Aug. 6, 1945

A second bomb (pictured at left) was dropped on the city of Nagasaki three days later on Aug. 9, 1945

On Aug. 15, 1945 Japan surrendered to the Allied Powers.

The Soviets develop an atom bomb

In 1949 the Soviet Union tested an atom bomb, igniting a arms race that would last for the next thirty years.

This arms race led to the development of another type of bomb. This bomb was called the Hydrogen bomb and utilized a different type of nuclear reaction called fusion.

Fission ReactorsNuclear power plants (such as the three plants located at Nine

Mile Point on Lake Ontario) use a controlled form of the fission reaction to produce electricity.

The energy from the fission of Uranium-235 atoms is used to heat water into steam. The steam is then used to drive a turbine to generate electricity.

Nuclear Power Plant

Let’s review• Fission is a type of nuclear reaction in which a large atom

splits into two smaller atoms and several neutrons when it is hit by a neutron.

• In a fission reaction, the mass of the products is less than the mass of the reactants. The missing mass is converted into energy.

• Atoms which undergo fission are U-235 and Pu-239.

• If a large amount of fissionable atoms are located together, the neutrons from one fission may cause additional atoms to split, producing more neutrons and causing more fissions. This process is called a chain reaction.

• The minimum amount of fissionable material required to produce a sustained fission reaction is called critical mass.

The Cold War and the Arms Race

The fact that the Soviet Union tested their own atom bomb in 1949 led to an arms race between the Soviet Union and the United States.

In 1951 Dr. Edward Teller and other scientists at Los Alamos began to work on a bomb that would utilize the nuclear reaction that provides the energy that stars give off – the fusion reaction.

The energy of the stars

In a fusion reaction, small atoms (such as Hydrogen) are pushed together at very high temperatures and pressures. Under these conditions, the nuclei merge into a larger atom and energy is produced.

Mass is converted into energy

• In the fusion reaction, as in the fission reaction, the mass of the products is less than the mass of the products.

• However, this mass defect is even greater in fusion than in fission, so a much greater amount of energy can be produced.

• The mass defect can be used to predict the amount of energy that can be produced in a fusion reaction by using Einstein’s equation E=mc2.

The Requirements for Fusion

The very high temperatures and great pressures required for the fusion reaction are readily available in stars. But how could they be duplicated here on earth?

Dr. Teller and his colleagues suggested that an atom bomb could be used as the trigger to provide the high temperatures and pressures required for a fusion reaction.

The first hydrogen bomb (also known as a thermonuclear device) was detonated by the U.S on an island in the South Pacific in 1952.

The Soviet Union followed this with a test of their own thermonuclear device in 1953.

The Hydrogen Bomb

Scientists are researching ways to control the fusion reaction to generate electricity.

Advantages of such a reactor would be:

• cheap and readily available fuel (hydrogen from water)

• Products are generally not highly radioactive

Disadvantages include:

• Maintaining the high temperatures and pressures required for the H atoms to combine together.

Let’s review• Fusion is a type of nuclear reaction in which a small atoms

are forced together to form larger atoms.

• In a fusion reaction, as in fission, the mass of the products is less than the mass of the reactants. The missing mass is converted into energy.

• The conditions which allow fusion to occur are extremely high temperatures and pressures. Fusion is the source of the energy given off by the sun and stars.

• Man has utilized the fusion reaction to make hydrogen bombs (thermonuclear devices).

• Scientists are trying to develop ways to control the fusion reaction and utilize it to generate electricity.

Comparing fission & fusion