ENERGY SOURCES—ALTERNATIVES 317
Magnetohydrodynamics
MHD generators can utilize either a fossil or nuclear
thermal energy source. In a fossilfueled system, hot combus-
tion gases, seeded with potassium or cesium to make the gas
conductive, are expanded at high velocity through a mag-
netic field. The dc current produced in the moving conduc-
tive gas is picked up at electrodes embedded in the walls of
the gas channel in various geometries depending upon the
particular type of generator. The advantage of this system is
its high thermal efficiency of 50 to 60% when operated at a
gas temperature in the 4000° to 5000°F range. A panel^49 has
conducted a study of MHD for the OST and concluded that
additional research and development should be performed
before an MHD plant is constructed. Much of this develop-
ment effort must be directed toward materials research to
permit extended operation at high temperatures.
The environmental effects of MHD generation would be
similar to those in any fossil-fueled combustion system but
would be mitigated because of the improved efficiency of con-
version. The seed material in the gases could not be released
in any case, for economic as well as environmental reasons,
and must be recovered. It is anticipated that recovery of the
other pollutants can be accomplished in the same operation.Fission Reactors
Current light-water cooled nuclear reactor systems used
for electric power production operate with fuel slightly
enriched in the uranium-235 isotope and have a conversion
ratio of about 0.6, i.e., they generate about 60% as much new
fuel as they consume by converting fertile uranium-238 into
fissionable plutonium-239. These plants have a thermal effi-
ciency of about 33% because of limitations on the tempera-
tures at which they can operate. Alternate reactor systems
that are under study, including the molten salt reactor (MSR)
and the high temperature gas-cooled reactor (HTGCR), can
generate steam at conditions comparable to those of fossil-
fueled plants and the molten salt system, when fueled with
uranium-233 fuel that is produced from thorium by neutron
capture in a reactor, can breed about 5% more fuel than it
consumes. All of these systems operate with a thermal (or
slow) neutron spectrum in the reactor. By operating with a
higher energy neutron spectrum in a fast reactor, better neu-
tron economy can be achieved and more neutrons are avail-
able for the conversion of fertile material. Such reactors may
be cooled with helium, steam, or liquid sodium. These fast
breeder reactors, when developed and integrated into the
centralized power system, will greatly extend the nuclear
energy resources by fully utilizing the more abundant ura-
nium-238 isotope to produce additional fissionable fuel. An
analysis of the various reactor concepts and their potential
roles in a nuclear power economy has been performed by
the AEC^50 and led to the selection of the liquid metal cooled
fast breeder reactor (LMFBR) as the high priority develop-
ment task.
The various topping and MHD systems discussed previ-
ously are also applicable to nuclear power plants and may
be useful additions if higher reactor operating temperatures
can be reached.All reactor systems generate similar quantities of radioac-
tive materials for a given thermal energy output and the han-
dling of these materials is closely controlled during all fuel
cycle and reactor operations. The amount of such materials
produced, and waste heat discharged, for a given electrical
output depends, of course, on the plant efficiency. It is impor-
tant to recognize that in all reactor systems the cooling water
that carries away the waste heat is physically separated from
the reactor primary coolant whether it be gas, water, or liquid
sodium. Because of the large quantities of radioactive materi-
als present in the fuel region of a reactor during operation,
there is concern over incidents that might accidentally dis-
charge this material to the environment and no nuclear plants
have been constructed in regions of high population density.
Protection against such discharge is provided by safety sys-
tems that minimize the possibility of an accident and by incor-
porating several physical barriers within the plant to contain
the radioactive products in the unlikely event that they are
released from the fuel region.Fusion Reactors
The successful development of a controlled and eco-
nomic fusion device would make available the tremendously
large energy reserves of deuterium in sea water. Current
research is directed toward devices in which the combina-
tion of plasma temperature, density, and confinement time
required for a sustained reaction may be attained. The
status of this research and the outlook for fusion power
has been summarized by Rose.^51 The deuterium-tritium
(DT) reaction, wherein these materials are confined as a
plasma at a temperature of about 40 10 6 °C in a high strength
magnetic field while they react, releases most of its energy
in the form of neutrons that must be captured in a lithium
blanket to generate new tritium fuel. The blanket fluid may
then be circulated to generate steam for a conventional steam-
cycle system. Topping cycles may also be used depending on
the operating temperatures and the over-all efficiencies will
be competitive with fossil-fired power plants.
The deuterium-deuterium (DD) reaction appears to be
more difficult to attain, requiring confinement at a tempera-
ture of some 350 10 6 °C, but releases much of its energy in
the form of charged particles which might be used to gener-
ate electricity directly without using a power cycle. Higher
operating efficiencies may be achieved in such a system.
There are still other fusion reactions that are of interest
because of their special characteristics.
It now seems that fusion power will be more economi-
cal in very large plants of greater than 5000 MW capacity
and such plants will pose substantial waste heat disposal
problems unless very high efficiencies are attained. In the
DT concept, tritium must be recovered from the blanket
and handled during the fuel cycle. Another source of radio-
active products the structural material that is activated by
the neutrons produced in either reaction. In any case, the
problems of disposing of radioactive materials appear to be
less severe than those in fission reactors and, because of the
small amounts of fuel material present in the device at any
given time, the accident hazard is apparently minimal.C005_006_r03.indd 317C005_006_r03.indd 317 11/18/2005 12:05:49 PM11/18/2005 12:05:49 PM