Nuclear Physics Year 13 Option 2006 Part 2 – Nuclear Fusion

Fusion

Energy release Nuclear energy can be released by fusion of two light elements The power that fuels the sun and the stars is nuclear fusion. In a hydrogen bomb, two isotopes of hydrogen, deuterium and tritium are fused to form a nucleus of helium and a neutron. This fusion releases 17.6 MeV of energy.

Overcoming the Coulomb Barrier

Energy needed for fusion Protons have positive charge. Like charges repel -- the electromagnetic force. We need to overcome this repulsion to have the nuclei fuse. Calculate the energy needed for 2 protons to fuse At what temperature will this required energy be reached?

Fusion in the stars: The proton-proton cycle

The proton-proton “cycle”

The carbon cycle 1% of sun’s mass due to elements heavier than H and He These heavier elements allow fusion of H into He In massive stars, the He can be fused into C and so on upto Fe-56

The carbon (CNO) cycle

Fusion as an energy source Fusion has the potential of providing an abundant supply of energy. The fuel needed for fusion is readily available. Deuterium must be extracted from water. (About 0.015% of the hydrogen in water is exists as deuterium.) Tritium must be made, since it does not occur naturally in sufficient quantities. Tritium is radioactive (a beta emitter), with a half- life of 12.3 years. It is also toxic.

A possible reaction for harnessing energy Can you show that this amount of energy would be released in this D-T reaction? Larger than D-D reactions and coulomb barrier is less

Plasma Sustaining a fusion reaction may be possible by containing the reactants in a high temperature form of matter called plasma Plasma particles can be contained within a magnetic field. This principle is referred to as magnetic confinement. The purpose of magnetic confinement is to avoid heat loss, not to prevent the walls of the confinement vessel from vaporizing, as often believed. Another possible technique for sustaining a fusion reaction is inertial confinement, in which a fuel pellet containing the fusion reactants is bombarded by a high energy source such as a laser or an electron beam.

What is a Plasma? Plasmas consist of freely moving charged particles, i.e., electrons and ions. Formed at high temperatures when electrons are stripped from neutral atoms, plasmas are common in nature. For instance, stars are predominantly plasma. Plasmas are the "Fourth State of Matter" because of their unique physical properties, distinct from solids, liquids and gases. Plasma densities and temperatures vary widely.

Gravitational Confinement Compression (gravity) Fusion Reactions (such as the p-p chain) Only seen in massive bodies eg stars

Inertial Confinement Compression (implosion driven by laser or ion beams, or by X-rays from laser or ion beams) Fusion Reactions (primarily D+T)

Magnetic confinement

Magnetic Confinement Only solution for large scale power generation Plasma held in toroidal vacuum vessel as a single turn of a secondary transformer coil Huge currents delivered Plasma current produces circular field (remember field around a wire…) Toroidal field coils produce another field in same shape as vacuum chamber Resulting field is helical – charged particles therefore describe helical paths superimposed on helical paths.

Magnetic Confinement RF Electromagnetic Waves can be used to provide additional heating D ions can be injected into the plasma after acceleration outside the vacuum chamber

Confinement cont… Because of the electric charges carried by electrons and ions, a plasma can be confined by a magnetic field In the absence of a magnetic field, the charged particles in a plasma move in straight lines and random directions. Since nothing restricts their motion the charged particles can strike the walls of a containing vessel, thereby cooling the plasma and inhibiting fusion reactions. But in a magnetic field, the particles are forced to follow spiral paths about the field lines Consequently, the charged particles in the high- temperature plasma are confined by the magnetic field and prevented from striking the vessel walls.

Confinement designs

A fusion power station

Fusion power stations cont.. In the most likely scenario for a fusion power plant, a deuterium-tritium (D-T) mixture is admitted to the evacuated reactor chamber and there ionized and heated to thermonuclear temperatures. The fuel is held away from the chamber walls by magnetic forces long enough for a useful number of reactions to take place. The charged helium nuclei which are formed give up energy of motion by colliding with newly injected cold fuel atoms which are then ionized and heated, thus sustaining the fusion reaction. The neutrons, having no charge, move in straight lines through the thin walls of the vacuum chamber with little loss of energy.

Fusion power stations cont.. The neutrons and their 14 MeV of energy (80% of that available) are absorbed in a "blanket" containing lithium which surrounds the fusion chamber The neutrons' energy of motion is given up through many collisions with lithium nuclei, thus creating heat that is removed by a heat exchanger which conveys it to a conventional steam electric plant The neutrons themselves ultimately enter into nuclear reactions with lithium to generate tritium which is separated and fed back into the reactor as a fuel Write equations for the capture of neutrons by Lithium-6 and Lithium-7 The other 20% is carried by alpha particles to sustain the temperature in the plasma – plasma is therefore self-heating This is called IGNITION

A bit of light relief Listen to a Tokamak fusion test reactor How much energy can you extract? html html

Advantages of fusion No greenhouse gases Plentiful raw materials No long-lived reaction products No long-lived radioisotopes following decomission Very quick shut-down <1min BUT

Technological difficulties Containing plasma long enough without contamination due to evaporation of container walls Removing impurities and by-products (mainly helium) Continuous operation rather than pulsed