Nuclear-Fueled Power Plants - Energy and the Environment - page 122

Most of the fission products are radioactive (see Section 6.3 below). Because more than one
neutron is released in the fission reaction, a chain reaction 6 develops, with an increasing rate of
release of energy. The greater portion (about 80%) of the released energy is contained in the kinetic
energy of the fission products, which manifests itself as sensible heat. A part of the remaining
energy is immediately released in the form of

rays and neutrons from the excited fission
products. The rest of the fission energy is contained in delayed radioactivity of the fission products.
At the same time that 235 U splits into fission products with the release of 2-3 neutrons, a part
of the neutrons can be absorbed by the more abundant 238 U in the fuel, converting it in a series of
reactions to an isotope of plutonium, 239 Pu:

γ and β

238 U 239 U 239 Np 239 Pu

+ n → + γ → + β → + β (6.2)

This reaction series is accompanied by radiation.

239 Pu is a fissile 7 element that can sustain a chain reaction (see breeder reactors in Section 6.4.4).
Furthermore, because plutonium is a different element than uranium, it can be extracted chemically
from the spent fuel and then used as fresh fuel for reloading a reactor. The extracted plutonium can
also be used in atomic bombs. Thus, reprocessing of spent fuel carries the risk of nuclear weapons
proliferation. Reprocessing of spent fuel for the purpose of extracting plutonium is currently banned
in the United States. 8

γ and β

6.3 RADIOACTIVITY

Radioactivity is the spontaneous decay of certain nuclei, usually the less stable isotopes of the
elements, both natural and man-made, which is accompanied by the release of very energetic
radiation. After the emission of radiation, an isotope of the element (or even a new element) is
formed, which is usually more stable than the original element. It is important to note that radiation
emanates directly from the nucleus, not the atom as a whole. This is an important distinction,
because X-ray radiation, although equally damaging as radioactivity, emanates from the inner
electronic shells of the atom, not the nucleus.
In radioactive decay, there are three types of radiation: . Only the latter is a form

of electromagnetic radiation; the two former are emissions of very high energy particles. All three
are called ionizing radiation because they create ions as their energy is absorbed by matter through

α , β , and γ

6 A chain reaction propagates exponentially due to the release of more than one reactant (in this case a neutron)
per step than the one that initiated it.
7 A fissile nucleus is one that can split after absorption of a thermal neutron. Examples are 235 U, 239 Pu, and
233 U. A fertile nucleus is one that can convert into a fissile nucleus after absorption of a fast neutron. Examples
of fertile nuclei are 238 U and 232 Th.
8 It has to be emphasized that while the chain reaction in a nuclear power plant has to be carefully controlled, a
nuclear reactor cannot explode like an atomic bomb. Atomic bombs contain highly enriched 235 U (95 % + )or
239 Pu, in contrast to nuclear reactors that contain a maximum of 3-4 % enriched 235 Uor 239 Pu. An uncontrolled
chain reaction in the reactor can lead to a “meltdown” and possible dispersal of radioactive fission products,
as happened, for example, at Chernobyl. But there is never any danger of an accident at a nuclear power plant
resulting in an uncontrolled chain reaction with the explosive force of an atomic bomb.