radionuclides are usually neutron deficient and therefore decay by b+-
emission or electron capture.
Reactor-Produced Radionuclides
A variety of radionuclides is produced in nuclear reactors. A nuclear reactor
is constructed with fuel rods made of fissile materials such as enriched^235 U
and^239 Pu. These fuel nuclei undergo spontaneous fission with extremely low
probability.Fissionis defined as the breakup of a heavy nucleus into two
fragments of approximately equal mass, accompanied by the emission of
two to three neutrons with mean energies of about 1.5 MeV. In each fission,
there is a concomitant energy release of ~200 MeV that appears as heat and
is usually removed by heat exchangers to produce electricity in the nuclear
power plant.
Neutrons emitted in each fission can cause further fission of other fis-
sionable nuclei in the fuel rod, provided the right conditions exist. This
obviously will initiate a chain reaction, ultimately leading to a possible melt-
down of the reactor core. This chain reaction must be controlled, which is
in part accomplished by the proper size, shape, and mass of the fuel mate-
rial and other complicated and ingenious engineering techniques. To main-
tain a selfsustained chain reaction, only one fission neutron is needed and
excess neutrons (more than one) are removed by positioning cadmium rods,
called control rods, in the reactor core (cadmium has a high probability of
absorbing a thermal neutron).
The fuel rods of fissile materials are interspersed in the reactor core with
spaces in between. Neutrons emitted with a mean energy of 1.5 MeV from
the surface of the fuel rod have a low probability of interacting with other
nuclei and therefore do not serve any useful purpose. It has been found,
however, that neutrons with thermal energy (0.025 eV) interact with many
other stable nuclei efficiently, producing various radionuclides. To make the
high-energy neutrons, or so-called fast neutrons, more useful, they are ther-
malized or slowed down by interaction with low molecular weight materi-
als, such as water, heavy water (D 2 O), beryllium, and graphite (C), which
are distributed in the spaces between the fuel rods. These materials are
called moderators. The flux, or intensity, of the thermal neutrons so obtained
ranges from 10^11 to 10^14 neutrons/cm^2 · sec, and they are useful in the pro-
duction of many radionuclides. When a target element is inserted in the
reactor core, a thermal neutron will interact with the target nucleus, with a
definite probability of producing another nuclide. The probability of
formation of a radionuclide by thermal neutrons varies from element to
element.
In the reactor, two types of interaction with thermal neutrons occur to
produce various radionuclides: fission of heavy elements and neutron
capture or (n,g) reaction. These two nuclear reactions are described next.
46 5. Production of Radionuclides