Nuclear fission and uranium fuel

Uranium is the element most used to produce energy by fission. That occurs naturally in the form of two isotopes, uranium 238 and uranium 235.  The isotope used to produce energy from nuclear fission is Uranium 235.

Uranium mines are huge. Before the uranium can be extracted the rock has to be crushed. There is only a small percentage of uranium in the ore, generally about 0.1 to 0.25%.

Uranium mine

Tiny amount of uranium in a tonne of ore

Centrifuges

To be useful in a nuclear power station the fuel must have a high percentage of uranium 235 but because the isotopes of uranium have absolutely identical chemical properties the only way of (partly) separating the two is by using the vey slight difference in density (only about 1% different). To partly separate the two isotopes they are turned into a liquid or gas that will flow, usually the gas, uranium hexafluoride. The two isotopes are then separated in a huge centrifuge, however, the difference in density is so small that the separation is far from complete. The concentration of uranium 235 may be around 50%.

What is fission

Nuclear Fission is the splitting of the nucleus of an atom into two or more parts by hitting it with a small particle, almost always a neutron (a proton would be repelled from the positive nucleus and an electron would have too little energy).

The isotope most commonly used to produce energy from nuclear fission is Uranium 235. If a neutron strikes the uranium nucleus with the “right” amount of kinetic energy the neutron enters the nucleus and destabilises it. The nucleus then splits into two large parts and releases a large amount of energy.

The right amount of kinetic energy to give the best possible split for the most energy in uranium fission is about the same as the neutron would have if it were at room temperature. It would be travelling at about 2.2Km per second, these neutrons are called “thermal neutrons”.

 

The process of fission

Fission equation

 

Where does nuclear fission energy come from?

The graph shows how the binding energy, that is the amount of energy that holds the nucleus of atoms together, varies with the size of the nucleus.

Smaller atoms, on the line with the blue shading, release energy if they are fused together. Larger atoms on the line with the red shading, can release energy if they are split - that especially applies to very big atoms like uranium (with the deep red shading).  The most stable atom is iron 56, at the peak of the curve.

If a uranium atom were to be split into two fairly equal parts there would be energy released due to the change in binding energy, as shown by the arrow on the graph. This doesn’t look much but remember the arrow shows the change of energy for each nucleon in the nucleus and there are 235 of them.

The energy change is so large that the mass of the parts after splitting is slightly less than the mass of the atom and extra neutron in the first place.

That loss of mass is converted to energy a calculation you can do using the famous equation E = mc2

Before you do that you have to convert the mass to kilograms and the energy produced by one atom being split is about 3.24 × 10−11 J

But a kilogram of uranium contains a very large number of atoms, if one kilogram of uranium 235 underwent fission the total energy produced would be about 83.14 TJ

Now a one kilogram cube is about 37 mm (just under 1.5 inches) on each edge.   To produce an equivalent amount of energy we would have to burn around 10,000 tonnes of coal

A nuclear fission reactor

The fuel (coloured yellow) is usually uranium 235 or plutonium 239. The neutrons split the atoms producing new elements and a large amount of energy.

The control rods (coloured green) contain boron which will absorb neutrons, to slow down the reaction and reduce the amount of heat produced the rods are lowered further, if more energy is needed then the rods are raised.

The graphite (coloured black) is a moderator. It slows the neutrons down so they react more effectively

A coolant, usually gas or water, is pumped through the reactor core to absorb the heat.  Water in the heat exchanger removes the heat energy from the coolant. The water is very hot and under very high pressure and is used to drive a steam turbine, which drives a generator to produce electricity.

A nuclear fission reactor

Advantages and disadvantages of nuclear fission reactors

Nuclear power

  • Causes no atmospheric emissions
  • A flexible and reliable source of energy
  • Fuel can be recycled to some extent
  • Low cost power production once a power station is set up
  • The fuel is easy to transport

BUT

  • There is a chance of high risk disaster due to accident, earthquake or tidal wave
  • The waste produced is highly radioactive and will remain so for a long time
  • Leaks can cause contamination of the environment
  • The power station is expensive to build
  • The lifetime of a nuclear power plant is limited and it is expensive to demolish at the end of its life.

Notes on nuclear fission can be downloaded and copied here:Nuclear fission notes (5 pages)