P2bL15 Nuclear Fusion

Key Words

Nebula - cloud of dust coming together to form a star

Nuclear fusion - small nuclei stick together to form a larger nucleus, releasing energy.

Protostar - a ball of hot gas which will become a star.

Grade E

Space is not empty; there are huge amounts of gas (mostly hydrogen) and particles of dust.  These form large clouds called nebulae.

Although gravity is a tiny force, it attracts the dust and gases together over millions of years.  The dust and gas might come together more quickly if there is the shockwave of an exploding star (a supernova)

The dust gets squashed together and gets hot.  It glows and gives out heat.  It's a protostar.

If the temperature gets hot enough, about 15 million Kelvin, hydrogen nuclei start fuse together in a process called nuclear fusion. 

Hydrogen ® Helium + energy

A lot of energy is given out in the process, and, if the star is big enough, the reaction becomes self sustaining.

Grade C

The reason that the temperature is so high is that nuclei, being positively charged, will tend to repel each other.  The nuclei must be slammed into each other with enough kinetic energy to overcome the repulsive force and fuse

The process has three stages:

1. Proton + Proton ® Deuterium + positron + electron neutrino

2. Deuterium + proton ® Helium 3 + photon

3. Helium 3 + Helium 3 ® Helium 4 + proton + proton

Some new words:

• Deuterium is an isotope of hydrogen with a proton and a neutron, also called hydrogen-2. 
• A positron is a positively charged particle the same size as an electron.
• An electron neutrino is a very tiny particle.
• A photon is a particle of light.

Since two protons are left over, the reaction is self sustaining.

Grade A

The most obvious use of fusion is the Hydrogen bomb. The amount of hydrogen fused is enough to fill a small balloon, from which a phenomenal amount of energy is released in a titanic explosion.  Anything within 20 km is fried to a crisp.

However it would be a wonderful idea to harness such huge amounts of energy to generate cheap electricity.  Hydrogen is plentiful.  A cubic metre of hydrogen could keep a power station going for several days.  And scientists at the Culham Laboratory near Oxford have been trying to do just that.  There are some problems:

• The hydrogen has to be cooked to 15 million Kelvin (which is quite hot). 
• The plasma (atoms with all electrons removed) has to be contained in a magnetic field, as it could easily burn a hole in the container.

The huge machine, called a torus due its doughnut shape, has produced fusion energy for a couple of seconds at a time but far more electricity has had to be put in to keep the machine going.  The biggest problem is keeping the plasma stable; it has a habit of going out.