Nuclear Fusion

Terrestrial Fusion Reactions

The reactions that appear most promising for a fusion power reactor involve deuterium and tritium:

Potential energy as a function of separation distance between two deuterons. R is on the order of 1 fm. If we neglect tunneling, the two deuterons require an energy E greater than the height of the barrier to undergo fusion.

Nuclear Fusion

The temperature at which the power generation rate in any fusion reaction exceeds the loss rate is called the critical ignition temperature Tignit.

Power generated versus temperature for deuterium–deuterium (D–D) and deuterium–tritium (D–T) fusion. When the generation rate exceeds the loss rate, ignition takes place.

Nuclear Fusion

In addition to the high-temperature requirements, two other critical parameters determine whether or not a thermonuclear reactor is successful: the ion density n and confinement time τ, which is the time interval during which energy injected into the plasma remains within the plasma.

The product nτ is referred to as the

Lawson number of a reaction.

In particular, Lawson’s criterion states that a net energy output is possible for values of nτ that meet the following conditions:

Nuclear Fusion

Magnetic Confinement

Many fusion-related plasma experiments use magnetic confinement to contain the plasma. A toroidal device called a tokamak.

A combination of two magnetic fields is used to confine and stabilize the plasma: (1) a strong toroidal field produced by the current in the toroidal windings surrounding a doughnut-shaped vacuum chamber and (2) a weaker “poloidal” field produced by the toroidal current. In addition to confining the plasma, the toroidal current is used to raise its temperature. The resultant helical magnetic field lines spiral around the plasma and keep it from touching the walls of the vacuum chamber.

Nuclear Fusion

Magnetic Confinement

Nuclear Fusion

Inertial Confinement

The second technique for confining a plasma, called inertial confinement, makes use of a D–T target that has a very high particle density.

In inertial confinement, a D–T fuel pellet fuses when struck by several high-intensity laser beams simultaneously.

Nuclear Fusion

Fusion Reactor Design

Diagram of a fusion reactor.

Nuclear Fusion

Advantages and Problems of Fusion

If fusion power can ever be harnessed, it will offer several advantages over fissiongenerated power: (1) low cost and abundance of fuel (deuterium), (2) impossibility of runaway accidents, and (3) decreased radiation hazard. Some of the anticipated problems and disadvantages include (1) scarcity of lithium, (2) limited supply of helium, which is needed for cooling the superconducting magnets used to produce strong confining fields, and (3) structural damage and induced radioactivity caused by neutron bombardment. If such problems and the engineering design factors can be resolved, nuclear fusion may become a feasible source of energy by the middle of the twenty-first century.

Nuclear Fusion

Advantages and Problems of Fusion

Several units have been used historically to quantify the amount, or dose, of any radiation that interacts with a substance.

The roentgen (R) is that amount of ionizing radiation that produces an electric charge of 3.33·10-10 C in 1 cm3 of air under standard conditions.

For most applications, the roentgen has been replaced by the rad (an acronym for radiation absorbed dose):

One rad is that amount of radiation that increases the energy of 1 kg of absorbing material by 1·10-2 J.