Heavier elements can divide and
release energy
38.13 - Fission
Fission: A heavy nucleus breaks up into two
smaller ones, releasing energy.
You may have heard the term “splitting the atom” as something that humans first
accomplished in the 20th century. In this section, you will learn what it means to “split”
an atom. Fission is the process used both in nuclear reactors to produce electrical
power and also in the first atomic bombs.
When a nucleus breaks up into smaller, more stable pieces, this is known as fission.
Some unstable nuclei, such as uranium-236, do this spontaneously. When an atom
undergoes fission, it changes identity, as the new nuclei it breaks into have different
numbers of protons. There are many ways that the nucleus can break up. For example,
the uranium-236 nucleus can break into Xe-140 and Sr-94, in the process releasing two
neutrons. To see this fission process, press the refresh button in your browser and look
at Concept 1.
However, being the impatient race that we are, humans learned to induce the process
to happen at a greater rate. Induced fission, also known informally as “splitting the
atom,” was first performed by Otto Hahn and Fritz Strassmann. They bombarded
uranium with neutrons and found that lighter elements (such as barium) were produced.
Why are neutrons effective at inducing fission? Was it not stated earlier that the strong
force they supply is crucial to holding a nucleus together? Yes, but it is possible to have
too much of a good thing. For a given number of protons in a nucleus í for a certain
element í the band of stability is quite narrow. Too few neutrons or too many neutrons
make the nucleus unstable.
In fact, neutrons are a natural choice to induce fission. Since they are electrically
neutral, they can easily approach and hit the nucleus without being repelled by
electrostatic forces. If the incoming neutron has the right speed, the nucleus captures it
and becomes even more unstable. The nucleus has one neutron too many for the
number of protons, and that immediately causes the new compound nucleus to fission
into various elements.
Energy is released during fission; in the above process, roughly 200 MeV. You can
understand why this happens by returning to the concept of binding energy per nucleon,
which is lower for very heavy nuclei than it is for intermediate-size nuclei.
The binding energy is the amount of energy that is required to separate a nucleus into its separate nucleons, or it is the energyreleased when
separate nucleons are brought together to form a nucleus. The more tightly bound a nucleus is, the higher the binding energy, that is, the more
energy is released. Since the intermediate-size nuclei have the highest binding energy per nucleon, this means that heavier nuclei will release
energy as they split apart and become medium-sized.
We have discussed fissioning a single atom with the release of energy. This is a crucial scientific achievement, but in order for this to be useful
(for example, in a nuclear reactor), the process needs to be self-sustaining.
What makes fission a practical process is that in a fission reaction, one or more neutrons may be released, which can then induce more fission
reactions in nearby atoms, which produce more neutrons, and so on. Why are there “extra” neutrons? Recall that for heavier nuclei, the
number of neutrons exceeds the number of protons, while for lighter nuclei, the number of neutrons and protons tends to be nearly equal. This
means there are usually some neutrons left over after the nuclear re-arrangement.
When there are enough uranium atoms so that at least one neutron, on average, is captured by another uranium atom, the critical mass has
been reached. The process is self-sustaining, and it is called a chain reaction. Press the refresh button to see the fission process occur in
Concept 2.
Fission
Nucleus breaks up into smaller
elements
·And releases neutrons
·And releases energyFission and chain reactions
Neutrons can be used to cause fission
Fission can be self-sustaining
Releases energy