Is there a Nuke in your future?
By James C.
Warf
HOW NUCLEAR REACTORS WORK. –
In the U.S. today, about 22 percent of our electricity derives from nuclear
reactors, which consume uranium as fuel. To understand how nuclear reactors
work, we need a brief introduction to uranium and how energy is derived from
it.
The periodic table identifies families of
similar chemical elements. Each atom consists of a nucleus (positive charge) and
planetary electrons (negative charge). The nuclei consist of protons and
neutrons (zero charge). Currently, 110 elements are known. The number of protons
in each nucleus is the atomic number, and the total number of protons and
neutrons is the mass number. The heavier nuclei are extremely
energy-rich.
Examples of important
elements with light atoms and their atomic numbers are hydrogen (1), carbon (6),
and oxygen (8). Some in the middle elements of the Periodic Table are iron (26)
and copper (29) and toward the end of the table heavy elements such as uranium
(92) and plutonium (94) are found. All elements have several isotopes; this
means that, using iron as an example, all iron atoms have 29 protons in their
nuclei. The number of neutrons is variable, depending on the isotope; its mass
numbers range from 46 to 68 for this
element.
Thus a given element consists
of a mixture of atoms which have a range of masses. This occurs because some
atoms have more neutrons in their nuclei than others. Uranium-238 is the common
variety, and uranium-235 is the more scarce variety (0.7%). This is the one
which supplies us with nuclear
energy.
Lise Meitner was a brilliant
Austrian physicist who worked with Otto Hahn and Fritz Strassmann in Berlin. In
1938-1939, they discovered that the nuclei of uranium-235 undergo the process of
fission, in which bombardment with neutrons causes their nuclei to split into
two fragments with the liberation of extra neutrons and an astonishing amount of
energy. This makes a self-sustaining chain reaction possible.
In 1940, the Italian Enrico Fermi and his
colleagues built the first nuclear reactor at the University of Chicago. It
consisted of fission of U-235 (that is, chain reaction), which liberated huge
amounts of energy and generated more neutrons. The new-born neutrons were too
energetic to cause fission of additional uranium, but after being slowed down
(moderated) by collision with carbon atoms of graphite could cause continued
fission. These new neutrons kept the process going via chain reaction until the
supply of U-235 ran too low.
To date,
at least 100 kinds of nuclear reactors have been constructed. Commercial
American reactors employ fuel which is uranium in the dioxide form and which is
enriched to 4 or 5% in the lighter isotope, U-235. The fuel elements are clad in
zirconium, a high-melting point metal with suitable chemical and nuclear
properties. Commercial reactors generating electric power are cooled by water
under pressure.
Reactors are
controlled by inserting rods of materials which strongly absorb neutrons, such
as boron. A giant American reactor produces more than 1.35 billion watts of
electricity. Some Canadian reactors are cooled with heavy water (deuterium
oxide). Special reactors make isotopes for medical purposes.
Experimental reactors using a molten
salt coolant have been made. Some reactors are cooled using molten sodium metal.
Breeder reactors make fuel from abundant uranium-238. The worst reactor accident
so far was that at Chernobyl, Ukraine, in
1986.
NUCLEAR WASTE – The
principal shortcoming of nuclear reactors is that their waste is extremely
radioactive and poses a challenging disposal problem. The most promising
technique so far to cope with this is to vitrify the solid waste. The glass
immobilizes the waste and makes it much safer for disposal in geological
repositories. Vitrification is, however, rather expensive. Various techniques to
simplify the problem by separation of the most radioactive components have been
studied. One of these is called pyroprocessing. It is hazardous and costly. Both
France and Japan recycle spent fuel and have plans to vitrify the remaining
waste.
SAFETY – Two serious
accidents with nuclear reactors were those at Three Mile Island (U.S.) and at
Sellafield (U.K.) but the worst ever was at Chernobyl in Ukraine in 1986. The
greater the level of safety, the higher the construction and operational costs
are.
Los Angeles area residents are
generally unaware that the first nuclear meltdown in the world occurred in Simi
Valley at the Rocketdyne Santa Susana Field Laboratory in 1959. At that time 14
fuel rod bundles melted out of a total of 43, ionizing radiation being released
into the atmosphere. Only now is the effect of that accident being investigated.
NATURAL NUCLEAR REACTORS – As
strange as it seems, uranium ore occurring in Gabon, in West Africa, has been
found to contain only about half of the normal 0.71% uranium-235. Research
indicates that at 15 sites where uranium ore is found, the uranium-235 is
deficient. At these sites, there are residues containing the fission products
identical to known fission products of uranium-235. The evidence is convincingly
strong that at each site was a natural nuclear rector. There was plenty of
uranium fuel, and water served as the moderator. This was roughly two billion
years ago.
CONCLUSIONS – The
issue of whether or not new nuclear reactors should be built today is
controversial. While some people agree with government and nuclear industry
views that nuclear power should proliferate based on acceptable handling of
waste and safety, other people believe that the severe waste disposal and safety
issues must preclude the building of any new nuclear power reactors. In fact,
the latter group believes that those plants already in operation should either
have stricter oversight or be shut down completely.
The author is a veteran of the
Manhattan Project and is currently Professor of Chemistry at USC. He is a member
of the SCFS Executive Board and is author of ALL THINGS
NUCLEAR.
This is the third in a
Beachhead series from the Southern California Federation of Scientists
(SCFS).
Posted: Sat
- July 1, 2006 at 02:17 AM