Energy Recovery by Benign Hydrothermal Processes
37
to a secondary boiling water loop. The main difference between this reactor and the
light water reactor is the use of heavy water in CANDU.
3.2.5 grAPhiTe-moderATed, direCT CyCle
(Boiling WATer) PreSSure TuBe reACTor
The graphite-moderated, direct cycle (boiling water) pressure tube reactor (RBMK)
was designed in the former Soviet Union [2]. The reactor uses ordinary boiling water
as a coolant and graphite as a moderator. This type of reactor is also capable of
online fueling. Both CANDU and RBMK circulate water through pipes rather than
in a pressure vessel surrounding the entire reactor. In both of these reactors, fuel rods
can be reached while the reactor is in operation, and refueling takes place almost on
a continuous basis. RBMK is, however, not currently used.
3.2.6 SuPerCriTiCAl WATer-Cooled reACTor
Twenty percent of all electricity in the United States uses light water nuclear reactors [3,4]. The next-generation reactors, called supercritical water reactors, promise to
increase reactor energy efficiency by as much as 13% while simplifying plant design.
Water at about 374°C and about 220 atm pressure becomes supercritical where a phase
difference between gas and liquid disappears. Heat produced by fission can also be
converted into electricity in a reactor cooled by supercritical water. The supercritical
state of water offers some distinct advantages of physical, thermal, and chemical properties for an efficient energy transformation operation in a nuclear reactor. The building of such reactors will, however, require materials that withstand high temperature
and pressure. The commercial use of this type of reactor is still in its infancy.
It is clear from the above descriptions that water plays an essential role as an
energy carrier and a reactor safety moderator in the nuclear power industry.
3.3 hydrOthermal PrOCesses FOr reCOVery
OF GeOthermal enerGy
Geothermal energy is thermal energy generated and stored in the Earth [5–20]. This
energy of the Earth’s crust originates from the original formation of the planet (20%) and
from the decay of radioactive minerals (80%). The difference in temperature between the
core of the Earth and its surface drives a continuous conduction of heat from the core
to the surface. The temperature of the Earth increases with an increased depth from the
surface. The core of the Earth is believed to be over 5000°C due to radioactive decay.
The hot water and steam generated by the geothermal heat can be used for power
generation. Approximately 10,715 MW of geothermal power is collected in 24 different countries [19]. The worldwide installed geothermal electric capacity is illustrated
in Table 3.2 [19]. While the United States has more geothermal capacity than any other
nation in the world, it has also been extensively explored in other parts of the world
because geothermal power is renewable, reliable, sustainable, environmentally friendly,
and cost effective [5,6]. For example, Philippines obtain >25% of its electricity from
geothermal energy. The United States produces more than 3000 MW of power from