Reference: Nelson Physics VCE Units 1&2 Chapter 12 Page 257

In 1933 Frederic Joliot (Nobel prize 1935) and Irene Curie (Pierre & Marie's daughter) succeeded in bombarding ²⁷Al with a -particles to produce radioactive ³⁰P. The first fission reaction.

Following this discovery Enrico Fermi and his Italian colleagues made a study of nuclear reactions that involved bombarding atoms with neutrons and nuclear fission was born.

In 1939 Otto Hahn (Nobel 1944) and Fritz Strassmann identified that atoms are split in fission reactions by isolating the products, but because of the way the atom was viewed at that time they would not formally state that the atom could be split.

It was left to Lise Meitner and Otto Frish to confirm the idea that a nucleus could capture a neutron and then disintegrate producing two nuclei of approximately equal size. This process is called Nuclear Fission. They also correctly predicted that the fragments would have high kinetic energy (i.e. move very quickly).

In the nuclear reactor ²³⁵U is usually used. Two typical reactions that involve ²³⁵U are:

and

Each of the products is unstable and will decay. Thus we have produced new radioactive substances (daughter nuclei) which all have their own half-life and must be disposed of carefully.

In general, the fission reaction is represented as follows:

X and X' are the new nuclei.

The energy liberated can be used in power stations to make steam. The amount of energy produced is about 20 times that of radioactivity and many hundred times the energy produce from burning fossil fuels.

Note: Only certain nuclides can undergo nuclear fission.

In section 3.2 we saw that for one bombarding neutron it is possible to receive more than one neutron in the products. One of four things can happen to these neutrons:

1. Capture by the Uranium ore, but no fission
2. Capture by another material in the ore
3. Escaping altogether.
4. Capture by the nuclei and further fission.

Let us now look at option four in more detail.

If fission results it is possible to get a chain, as follows:

As this reaction continues more and more neutrons are produced increasing the chance of fission occurring. And of course an enormous amount of energy is released.

However if the mass of the Uranium is too small most of the neutrons will escape. For an uncontrolled reaction to occur, the material available for fission must be above a certain mass, called the critical mass. The rates of reaction can be controlled so that just enough neutrons are produced to keep the reaction going. This is what is done in a nuclear reactor.

Problem Set #1: Text Page 277 Questions 1 - 11

Controlled fission reactions are useful because they can satisfy our power needs and produce radioactive isotopes used in medicine.

There are two types of reactor:

1. Fast breeder reactor which use fast moving neutrons.
2. Thermal reactors using slow moving neutrons.

of the two thermal reactors are the safest.

A Typical Reactor is Shown

The fuel is located in the reactor core and is usually Uranium-238 enriched with Uranium-235, which is a better fission material.

A moderator slows down the neutrons to speeds that will easily allow absorption by the Uranium core. The moderator must have low mass and be a poor absorber of neutrons.

Eg. Water, heavy water, graphite, beryllium

These control the speed of the fission reaction by absorbing neutrons. They are raised or lowered to speed up or slow down the reaction. If left in the lowered position the reaction can be stopped.

Because of the huge amounts of energy produced the reactor would soon melt, so a coolant is used to cool it down.

Eg. Carbon dioxide, heavy water, liquid sodium

Outside the reactor (in the heat exchanger) the hot coolant exchanges its heat to water to produce steam, which is then used to generate electricity.

Nuclear reactors are enclosed in rooms with thick concrete walls. This absorbs any radiation and protects workers and communities from radiation.

As the name suggests fast breeder reactors use quickly moving neutrons. The fuel used in these reactors is Uranium-238 and the process produces Plutonium-239 ( Pu) which can be used as a fuel. The process can be written in a series of three equations:

The product plutonium is very radioactive and can be used as a reactor fuel. However because it has a half-life of 24,000 years and the fact that its decay produces very penetrating g -rays it is extremely dangerous. The use and production of plutonium is strictly controlled.

Fusion reactions occur when two light nuclei (usually hydrogen) combine or 'fuse' to form a single, heavier nucleus. Like fission, it results in the release of a large amount of energy without the dangerous radiation products.

The diagram shows the fusion of hydrogen-2 (deuterium) and hydrogen-3 (tritium) to form helium-4 a neutron and energy. The equation for the reaction is

Albert Einstein (Nobel 1921) was the first person to calculate the amount of energy produce in nuclear reactions. His famous formula is:

where E = energy

m = mass lost

c = speed of light (3 x 10⁸ ms^-1)

Example: Consider the equation

Measure mass in atomic mass units. 1 u = mass of a proton =

Mass before fission

Neutron 1.0086649 u

Uranium-235 235.0439231 u

Total 236.052588 u

Mass after fission

Neutrons 3.0259947 u

Barium-141 140.9144064 u

Krypton-92 91.9261528 u

Total 235.8665539 u

There is a loss of

236.052588 - 235.8665539

= 0.1860341 u

=

=

applying E = mc² gives

1 eV =

E = 171,620,612 eV

E » 172 M eV

Nuclear weapons cause destruction in three ways:

1. the shock wave of the explosion
2. the fallout
3. the heat produced

eg. Strontium 90 destroys bone marrow, strong ionising radiation damages living cells, causing mutations.

Peaceful use of nuclear fission produces waste which must be disposed of at sea or deep under ground. But containers could decompose or the waste stolen by terrorists.

Problem Set #2: Text Page 277 Questions 12 - 42