Environmental Engineering Reference
In-Depth Information
The heaviest of these, uranium, has so many protons that it naturally sheds neutrons. In a
controlled environment, this property can be harnessed to generate electricity.
Like crude oil, uranium in its naturally occurring state is not of much use as a fuel.
It occurs as an ore (most commonly uraninite, also known as pitchblende), from which
uranium fuel is extracted. Though generally associated with the modern age, uranium has
been known and used for centuries, mainly as a colourant for stained glass. It was also used
in early photography to produce tints and shades.
Most people are at least vaguely familiar with the periodic table, the colourful graph
representing all known chemical elements (see
Figure 2.6
)
. The table was devised in 1869
by the Russian chemist Dimitri Mendeleev to arrange the different elements based on
certain characteristics, such as mass, size and chemical properties. The elements close to
the bottom of the table are heavy, and some are unstable, as they contain a high number
of protons that repel each other. An unstable atom releases particles and electromagnetic
waves in a process known as radioactive decay, thereby transforming the atom into a
different element.
Figure 2.6.
The periodic table of elements. An element is a chemical substance made up
of just one type of atom. Every atom contains protons, neutrons and electrons. The
number above the symbol denotes number of protons in the atom (this is known as the
atomic number). The number below the symbol denotes the combined weight of the
protons and neutrons. Electrons are so tiny that they have little influence on mass.
Uranium, like all elements, occurs in different isotopes. Isotopes are forms of the same
element containing different numbers of neutrons. By far the most common uranium
isotope found on Earth is uranium-238. The number refers to the fact that the nucleus of
this isotope contains 92 protons and 146 neutrons (92 + 146 = 238). The radioactivity
level of uranium-238 is very low, and it is hazardous because of its toxicity rather than its
radioactivity. Uranium-238 is therefore not suitable as a fuel for nuclear reaction.
A far rarer isotope (representing less than 1 per cent of naturally occurring uranium)
is uranium-235, which is sufficiently radioactive to sustain a chain reaction. Uranium-235
contains 92 protons and 143 neutrons. Scientists found a way to extract uranium-235 from
