Nuclear-Fueled Power Plants - Energy and the Environment - page 127

Basically, these assumptions constitute the linear-no-threshold (LNT) hypothesis. The assump-
tions mean that even the slightest radiation could lead to delayed somatic or genetic effects, and
the occurrence of delayed effects does not depend on the stochastic nature of radiation, or whether
the given dose is received over a short or extended time.
Based on the LNT hypothesis, the Nuclear Regulatory Commission in the United States set
a standard of exposure for workers in nuclear power plants to 50 millisievert per year (50 mSv
y − 1

5 rem y − 1 ). For the general population—that is, any person in the region outside the plant
boundary—the standard is 1 mSv y − 1

=

100 mrem y − 1 . =

6.4 NUCLEAR REACTORS

A nuclear reactor in a nuclear fueled power plant is a pressure vessel enclosing the nuclear fuel
that undergoes a chain reaction, generating heat which is transferred to a fluid, usually water, that
is pumped through the vessel. The heated fluid can be steam, which then flows through a turbine
generating electric power; or it can be hot water, a gas, or liquid metal that generates steam in a
heat exchanger, the steam then flowing through a turbine.
The first controlled nuclear reactor was built and demonstrated by Enrico Fermi in 1942. It
was constructed under the bleachers of the stadium at the University of Chicago. The reactor had
dimensions 9 m wide, 9.5 m long, and 6 m high. It contained about 52 tons of natural uranium and
about 1350 tons of graphite as a moderator, and cadmium rods were used as a control device. The
reactor produced an output of only 200 W and lasted only a few minutes. Fermi's “pile” ushered
in the nuclear age.
The first commercial scale nuclear power plant of 180-MW capacity went into operation in
1956 at Calder Hall, England. In the United States a 60-MW station started operating in 1957 at
Shippingport, Pennsylvania. Before that, an experimental breeder reactor that produced electricity
was demonstrated in 1951 near Detroit, Michigan. The first nuclear-powered submarine, the Nau-
tilus, was launched in 1954. Submarine reactors produce steam that drives a turbine, which in turn
propels the submarine.
We have discussed before that the energy of a nuclear reactor is derived from splitting a fissile
heavy nucleus, such as 235 Uor 239 Pu. In a nuclear reactor of a power plant, the splitting of the
nucleus and sustaining of the ensuing chain reaction has to proceed in a controlled fashion.
The basic ingredients of a nuclear reactor are fuel rods, a moderator, control rods, and a coolant.
A schematic of a reactor is depicted in Figure 6.1.
The fuel rods contain the fissile isotopes 235 U and/or 239 Pu. Natural uranium contains about
99.3% 238 U and 0.7% 235 U. The concentration of the fissile isotope 235 U in natural uranium is not
enough to sustain a chain reaction in most power plant reactors; therefore, this isotope needs to be
“enriched” to 3-4% (for enrichment processes, see Section 6.5). 14 The fuel rods contain metallic
uranium, solid uranium dioxide (UO 2 ), or a mix of uranium dioxide and plutonium oxide, called
MOX, fabricated into ceramic pellets. The pellets are loaded into zircalloy or stainless steel tubes,
about 1-cm diameter and up to 4 m long.

14 Some power plant reactors use natural uranium containing 0.7 % 235 U as the fuel with different moderator
and coolant combinations.