The Fusion Reaction

Energy is released by nuclear reactions between light atoms at temperatures of millions of degrees. This process is called thermonuclear fusion because high temperatures cause light nuclei to fuse together to produce heavier nuclei. For example, in the center of stars four hydrogen atoms (atomic number 1) react to produce one atom of helium (atomic number 2). In the nuclear fusion process, the weight of the hydrogen consumed is more than the weight of the helium atoms produced. The mass difference is converted to energy. 127

Thermonuclear fusion is the source of energy of most stars including the sun. At the center of the sun, the high temperature and pressure drive a number of thermonuclear reactions. The majority of the energy is produced by reactions of four hydrogen nuclei to form one helium nuclei. In this reaction, 0.711% of the mass of the four hydrogen atoms is converted into energy. This does not seem like a large percentage change, but the energy equivalent of matter given by Einstein's E = mc2 is extremely large. The conversion of one gram of hydrogen (a United States nickel weighs about 5 grams) to helium produces as much energy as the combustion of 15 million grams of oil (112 barrels).

Based on the energy produced per liter, the conversion of hydrogen to helium at the center of the sun proceeds at a relatively slow rate. The large energy production of the sun is the result of the colossal volume of the core where hydrogen is converted to helium. In earth-based laboratories, using current technology, it is impossible to produce the combination of temperature and pressure existing at the core of the sun. Even if it were possible to produce solar conditions in an earth based reactor, the energy output for a reasonable sized reactor would be small. If the solar conversion of hydrogen to helium were the only fusion reaction known its use would be beyond current technology. Fortunately, there are several other thermonuclear reactions with the potential for production of fusion energy on

127 Bromberg, J. L., "Fusion: Science, Politics and the Invention of a New Energy Source", MIT Press, Cambridge, Massachusetts, 1982

earth. These reactions use a variety of hydrogen (deuterium) that was consumed in the core of the sun in the first million years of its life. Today it is only present as a trace constituent of the sun's outer layers.

Hydrogen is unique among the elements because its isotopes (atoms of identical chemical properties with different atomic weights) all have separate common names. The name hydrogen is applied to the mixture of isotopes found in nature. Hydrogen with an atomic weight of one is accurately called protium. Hydrogen with an atomic weight of two is called deuterium (or in the vernacular "heavy hydrogen"). Hydrogen with an atomic weight of three is called tritium. The hydrogen on earth is composed of 99.985% protium and 0.015% deuterium. Tritium is unstable with a half-life of 12.5 years and exists only in nearly undetectable quantities as a product of cosmic interactions with atoms of other materials. The reactions of the heavy isotopes of hydrogen useful in producing energy are defined in Table 3.1. 128

Most current research is probing the use and control of the D + T reaction because it requires the lowest ignition energy (temperature) and produces the highest ratio of energy return. The disadvantage to this reaction lies in its production of neutrons that cause induced radioactivity in the structure of the reactor. Other, more challenging, reactions may be used if the proposed development shows that they are feasible. The following discussion will concentrate on the D + T reaction because it can be used in the near term.

Water is about 11% hydrogen. A metric ton of water (one cubic meter) contains 111 kilograms of hydrogen. The hydrogen in water is 0.0157% deuterium, or 0.0174 kilograms deuterium per metric ton of water. At first glance, this looks like a small number, but remember a huge amount of energy is produced by each gram of deuterium. When reacted with tritium the 0.0174 kilograms of deuterium will produce 1.4 xlO13 Joules of energy. This is equivalent to about 2500 barrels of oil.

When considered with regard to the amount of water available on earth it is sufficient to provide energy for many thousands of years. Moreover, deuterium can be separated from the water by electrolysis. Electrolysis is a simple, efficient process. The other fuel, tritium, is a slightly radioactive, electron emitting, isotope of hydrogen with a 12.5 year half-life. Tritium is produced from lithium by reaction with neutrons.

The requirement for a successful fusion reactor is the production of the proper conditions for the D + T reaction to occur. The reaction rate must be fast enough to make up for the energy required to start and maintain the reaction and provide net excess for use as the power output. To achieve a fusion reaction, the nuclei of the atoms must be forced to collide with enough energy to react. This requires a temperature of millions of degrees. There must be sufficient reactions in a volume to make up for the heat loss and provide net excess energy for use. The reaction must continue long enough to be self-sustaining. 129,130

128 Fowler, T. K. and Post, Richard, "Progress Toward Fusion Power", Scientific American, Vol. 215, No. 6, December 1966, Page 23

129 Chen, Francis F., "The Leakage Problem in Fusion Reactors", Scientific American, Vol. 217, No. 1, July 1967, Page 76

130 Gough, William C. and Eastlund, Bernard J., "The Prospect of Fusion Power", Scientific American, Vol. 224, No. 2, February 1971, Page 50

Nuclear fusion reaction — Symbol definition

D

= Deuterium (hydrogen isotope weight 2)

T

= Tritium (hydrogen isotope weight 3)

He3

= Helium isotope, weight 3;

He4

= Helium isotope, weight 4;

P

= proton

n

= neutron

B

= Boron isotope, weight 11

keV

= energy in units of 1000 electron volts per particle

RATIO

= energy produced/particle divided by the energy required to start the reaction

YIELD

TEMP.

RATIO (Y/T)

Reaction

Products

KeV

KeV

d2 + t3

He4 + n1

17,600

10

1760

d2 + d2

He3 + n1

3,300

50

66

d2 + d2

T3 + p'

4,000

50

80

D + He3

He4 + p1

18,300

100

183

p' + B"

3He4

8,700

300

29

Table 3.1 Nuclear Fusion Reactions

At ordinary temperatures, the electrons surrounding the nucleus of the atom serve as a buffer and prevent the nuclei from colliding and reacting. As the temperature increases collisions become more violent and ultimately the electrons are stripped from the atoms leaving bare nuclei. This mixture of bare positively charged nuclei and negatively charged electrons is called plasma.

In plasma, the positive charged nuclei are strongly repelled from each other by electrostatic forces. At low temperatures, the repulsion prevents any nuclear reactions. At higher temperatures, the nuclei move faster and begin to overcome the repulsive forces. When the temperature approaches 100 million Kelvin (at this temperature the average kinetic energy of the particles is about 10 KeV) the velocity of the nuclei are high enough to overcome the inter-nuclear electrostatic repulsion. At this, and higher temperatures, they can react when they collide. Sufficient reactions occur, as outlined in Table 3.1, (above) to keep the mixture hot and provide enough excess energy for external power generation.

The number of positive and negative charges in the plasma is equal. The overall plasma has no charge and is termed neutral plasma. The plasma exerts a pressure in the same manner as any confined gas. Unlike most gas, the large number of electrically charged particles present in the plasma makes it an exceedingly good conductor of electricity. The high conductivity allows intense electric currents to flow through the plasma. The electric currents generate strong magnetic fields. The currents and magnetic fields provide a handle by which the plasma can be manipulated and confined.

To achieve a thermonuclear reaction it is necessary to heat the plasma to the 100 million Kelvin ignition temperature and contain it for sufficient time that more energy is produced than was used in the heating process. Heating, controlling, and maintaining this plasma is the central challenge in the production of fusion energy.

To produce a fusion reaction the critical factors are the plasma confinement time, the ion density and the temperature. When a reactor achieves a product of these three variables greater than 1016, the reactor is a net producer of energy. 131 These factors can be traded against each other in the design of a reactor. If the confinement time is long, the temperature and density can be low. If the confinement is short, the temperature and ion density must be high. There are some practical limitations in this relationship. If the ion density is low, even with a long confinement time at high temperature, the reaction may produce net power, but at such a low rate per reactor volume that the reactor would be too large for use. The various reactor schemes strive to produce a combination of temperature, density and confinement time that will result in a practical reactor design.

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