Engineering Chemistry S Datta

NUCLEAR CHEMISTRY 51
• Types of fuel:
(a) Natural uranium ( 238 U = 99.28%; 235 U = 0.714%)
(b) Enriched uranium (Higher % of 235 U)
(c) Uranium 233 (obtained from 232 Th)
(d) 234 Pu (obtained from 238 U)
• Types of fuel moderator assembly:
(a) Homogeneous–in which fuel is dissolved in the moderator.
(b) Heterogeneous–where moderator remains separated from the fuel.
• Moderators:
(a) Heavy water (D 2
O) (b) Ordinary water (H 2
O)
(c) Graphite
(d) Beryllium
(e) Beryllium oxide.
• Types of Coolants:
(a) Gaseous (air, CO 2
)
(b) Water or organic liquids
(c) Liquid metals.
• Purposes:
(a) For research
(b) For production of 239 Pu
(c) For power generation.
Depending on the above factors, the nuclear reactors are:
(i) Thermal, natural uranium, graphite moderated, Heterogeneous, air cooled, research
reactor.
(ii) Fast, enriched uranium, heterogeneous, liquid metal cooled, power breeder reactor.
A nuclear reactor is constructed of the following components:
1. Reactor Core: Reactor cores are generally a right circular cylinder with dia. 0.5 m to
15 m. The cylinder is enclosed in a pressure vessel. The cylinder with the pressure vessel is
considered as reactor core. The reactor core consists of an assembly of fuel elements, controlled
rods, coolant and moderator. In the reactor core, the fission chain reaction takes place and the
energy is liberated in the form of heat.
The fuel elements are either in the form of plates or rods of uranium metals. They are
usually clad in a thin sheath of Zr or Al stainless steel. This provides retention of radioactivity
and corrosion resistance. For the passage of the coolants, enough space is provided between
the individual rods (Fig. 3.5).
Reactor
Control rods
Steam to
turbine
Moderator
Coolant
Water
Core
uranium fuel
Shielding
Pressure
vessel
Fig. 3.5 Nuclear reactor.