Environmental Engineering Reference
In-Depth Information
The lack of permanent waste disposal systems and the unfavorable economics has deterred
privately and publicly financed electricity producers from building new nuclear power plants in
the United States. No other country has yet solved the permanent disposal problem of radioactive
waste from nuclear power plants.
6.6
FUSION
As noted in Section 6.2, a large amount of energy is evolved when light nuclei fuse together. For
example, the following fusion processes are accompanied by energy evolvement:
2
D
3
T
4
He
+
→
+
n
+
17
.
6 MeV
(6.13)
2
D
2
D
3
He
+
→
+
n
+
4 MeV
(6.14)
2
D
3
He
4
He
+
→
+
p
+
18
.
3 MeV
(6.15)
The sum of the masses of the nuclei that fuse (the left-hand side of the equations) is not exactly the
sum of the masses of the fused nucleus plus the mass of the ejected neutron or proton (the right-hand
side of the equations). The “mass deficit” appears as the evolved energy. The ejected neutrons or
protons collide with surrounding matter so that their kinetic energy is converted into sensible heat.
Such fusion reactions power the sun and other self-luminous stars, as well as thermonuclear bombs,
also called hydrogen bombs.
It would be desirable to perform fusion reactions under controlled conditions, so that the
evolved heat energy can be transferred to a coolant working fluid, which in turn would drive
a turbo-machinery. The advantages of fusion-based power plants are threefold: (a) The “raw”
material or fuel available for fusion reactors is almost unlimited, because deuterium is a natural
isotope of hydrogen to the extent of 1 deuterium atom in 6500 hydrogen atoms.
17
Tritium is not
found in nature, but can be manufactured from an isotope of lithium in the following reaction:
6
Li
3
T
4
He
+
n
→
+
+
4
.
8 MeV
(6.16)
(b) The fusion reactions would produce a minimal amount of radioactivity. Some radioactive
isotopes may be created due to absorption of neutrons in materials surrounding the fusion reactor.
Also, tritium is mildly radioactive, emitting low-energy
rays with a half-life of about 12 years.
(c) There is no spent fuel waste from which ingredients could be extracted for fabricating fission
nuclear weapons.
The difficulty of achieving a controlled fusion is in overcoming the electrical repulsion force
of the positively charged nuclei. To overcome the repulsive force, the colliding nuclei must have a
kinetic energy comparable to a temperature of tens of million degrees. At such temperatures, atoms
are completely dissociated into positively charged nuclei and free electrons, the so-called
plasma
state. For the release of significant amounts of energy, many nuclei must collide. Hence the plasma
needs to be confined to a small volume at a high pressure.
β
17
Of course, separating deuterium from hydrogen requires considerable amounts of energy.
