Environmental Engineering Reference
In-Depth Information
However, from the efficiency point of view, it has to be considered that the
biomass formation always requires the conversion of solar energy to some type of
biomaterial, and this can be realized only by the photosynthesis process, whose
energy efficiency is lower than 1% (see
Sect. 1.3
).
The well-known limits of new renewable energies reside in their low energy
density (all of them), intermittency (solar, wind) and not always reliable predict-
ability (wind). For this reason in recent years the debate on nuclear energy has seen
a renewed interest, especially in countries where its utilization has not yet accepted
or widely diffused. In 2007 about 6% of the total world energy usage is produced
from nuclear (Fig.
1.1
), in over 400 current operating nuclear plants. Several
recent publications analyze the main issues and last advances associated with
nuclear power [
24
-
28
], here limits and potentialities of this technology are briefly
described, in relation to the usage of electricity and hydrogen as energy carriers on
transportation means [
29
-
33
].
The commercial nuclear technology is still based on fission of isotope 235 of
uranium (
235
U). This is defined as fissile material since it is capable to absorb
neutrons of any kinetic energy (also low energy ''thermal'' neutrons). Uranium is
mainly present in natural deposits as U
3
O
8
, where 0.7% of uranium is constituted by
235
U, the remainder being
238
U. In order to have an amount of fissile nuclei
sufficient for reactor operation all fission reactors are fueled with uranium which
has been enriched to about 4% in the fissile
235
U. High energy neutrons produced by
the fission of
235
U can be absorbed by other
235
U nuclei, so continuing the reaction.
The fission of one nucleus of
235
U produce about 200 MeV of usable energy, to be
compared with 4 eV produced by the oxidation of one C atom. During the overall
process, a huge amount of heat is produced through a controlled nuclear chain
reaction in a critical mass of fissile material. Potential future developments (fission
in fast neutron reactors (breeders), also known as fourth generation nuclear, which
could use
238
U, much more abundant than
235
U, and nuclear fusion) are decades
away from their practical utilization for producing energy.
The risks associated with highly radioactive spent fuel wastes, with the
uncertainties regarding the stability of the geological repositories for the
required length of time (about 200,000 years), and the not eliminable possi-
bility of accidents in plants, constitute fundamental causes of intense opposition
to the current nuclear technology. Another critical aspect is connected to the
ascertained reserves of uranium, which could not be sufficient to fuel over 400
already existing nuclear reactors (65,000 tons/year consumed) plus possible
future plants. The world reserves of uranium, as estimated by IAEA (Interna-
tional Atomic Energy Agency) as function of market price and with reference
to the current uranium production, should be sufficient for about 100 years at
100USD/kgU
3
O
8
(approximately equal to the average price at the beginning
of 2010). However, this evaluation considers not only the reasonably assured
resources, but also the estimated additional ones, deduced from exploration
data relative to known deposits [
34
]. This problem could make impracticable a
substantial expansion of nuclear energy in next years, in particular the
requirements of uranium coming from strategically influent countries would
