Since humans learned to split the atom in the 20th century, the process has been put to great use. The most well-known application was the

use of nuclear fission in the “atomic” bomb. A runaway chain reaction happens very fast, releasing a lot of energy in a burst which can be used

to devastating effect.

On the positive side of the ledger, nuclear power has been used as an energy source. A slowly progressing chain reaction produces a steady

flow of heat that can be used to boil water, which then drives steam turbines to generate electricity. The poster-child for nuclear power is

France, which supplies about three-quarters of its electrical needs with nuclear reactors.

However, nuclear power is not without its risks or costs. Some of the byproducts of the fission process are highly radioactive and remain

dangerous for tens of thousands of years. A typical way to dispose of these wastes is to bury them deep in the Earth. If they leak, they

contaminate water sources.

38.14 - Fusion

Fusion: Two light nuclei fuse into a heavier one,

releasing energy.

Fusion is a process in which nuclei join together to become a single, larger nucleus.

This process also releases energy. Because positively charged nuclei repel one

another, fusion does not occur spontaneously under normal conditions on Earth.

However, fusion is commonplace in the Sun and other stars where hydrogen atoms

fuse into helium atoms. Fusion provides the energy to keep the star going, which Earth

ultimately experiences as light and heat. Fusion occurs in the Sun because of the high

temperature within the star; its interior is at about 100 million Kelvin.

At this high temperature, the atoms are in an ionized state of matter called plasma.

While normal matter consists of distinct neutral atoms, plasma is a “soup” of positive

nuclei and negative electrons. In the interior of the Sun, nuclei are hot enough and

moving quickly enough to overcome the electrostatic repulsion of their positive nuclei.

They move close enough to be bound by the attractive strong force between them, then

fuse into a single nucleus.

Why is energy released in the process? For light elements (atoms to the left of the peak on the binding energy curve), the binding energy per

nucleon increases with atomic number. As smaller nuclei are fused into larger ones, the result is a more efficient arrangement of nucleons; one

that is harder to break apart. Since the new nucleus is more efficient í more tightly bound í than the previous ones, there is energy to spare.

It may be tempting to think that fusion and fission are opposite processes, since one combines nuclei and the other splits them. It may seem

confusing as to how two “opposite” processes can release energy. The key is that fission with energy release occurs only when very heavy

nuclei break apart into medium-size nuclei, and fusion with energy release occurs when very light nuclei fuse into heavier ones. The answer

again resides on the curve of binding energy; “mid-sized” elements have the highest binding energy per nucleon. As predicted by the curve, it

requires energy to combine two medium-sized nuclei into a single large nucleus, just as it requires energy to split a medium-size nucleus into

smaller nuclei.

Fusion is critical to the universe we observe. Without it, there would be no stars, and in fact no elements heavier than Lithium(Z = 3).

However, there is also a practical reason for scientific interest in fusion. If we could create and control fusion, we could use it as an energy

source. Using fusion as an energy source has two huge appeals: the fuel (hydrogen isotopes) is present in the oceans in essentially unlimited

quantities (compared to the relative scarcity of, say, uranium) and fusion creates no radioactive byproducts.

However, before fusion reactors become commonplace, some daunting engineering challenges will have to be solved. Recreating the

conditions inside the Sun, with its enormous temperatures, is no easy feat. Furthermore, simply getting the atoms hot is not sufficient; there is a

minimum plasma density that must also be achieved so that collisions between nuclei will occur frequently enough to release energy, which

keeps the plasma hot and keeps the fusion reaction going. The combination of high temperature and high density requirements necessarily

means that the pressure must also be very high, to hold all of the reactants together.

As in the case of fission reactions, early work on fusion was directed toward nuclear weapons. Scientists solved the problem of achieving the

ultrahigh temperature and pressure conditions necessary for fusion by using a fission (“atomic”) bomb as a trigger, to both heat up and

compress the fusion fuel. Some trigger! Using atomic bombs to power a nearby fusion reactor is not a very popular proposal.

There are two less-explosive schemes that are currently being pursued to keep the superhot plasma together for fusion to occur.Magnetic

confinement uses electromagnetic fields to hold the charged particles together. Inertial confinement fusion uses a solid pellet of deuterium and

tritium that is crushed by the light pressure of perfectly timed, short-duration, high-powered laser beams from all directions. The term comes

from the fact that the particles’ own inertia keeps them in place during the laser pulse.

Fusion

Light nuclei fuse into larger, more stable

pieces

Releases energy

(^710) Copyright 2007 Kinetic Books Co. Chapter 38