Video Transcript
In a nuclear reactor, it is
important that the reaction does not proceed too quickly or too slowly. What is the name of the process by
which neutrons are slowed down to ensure more successful collisions occur? (A) Neutron transmutation, (B)
neutron absorption, (C) neutron ionization, (D) neutron moderation, or (E) neutron
activation.
In a nuclear reactor, nuclear
fission can occur. Nuclear fission is the splitting of
one heavy atomic nucleus into two or more lighter atomic nuclei. When atomic nuclei break apart, or
are uncombined, nuclear energy is released. We can harness the energy from the
nucleus to generate electrical current at nuclear power plants. Generally, the uranium-235 isotope
is used as the fuel for the reaction. It can be found in the form of
pellets in the sealed metal tubes called fuel rods. The atoms of uranium-235 are
bombarded with neutrons. The neutron enters the nucleus
becoming uranium-236, which is unstable. Thus, the nuclear splits and
releases energy. It splits into two smaller nuclei,
such as barium and krypton. And it also releases three
neutrons.
The three neutrons can then collide
with other uranium-235 atoms. But the neutrons can only be
captured by the uranium nuclei if they are slowed down. The fuel rods are placed inside a
moderator, which is usually water but sometimes graphite, to slow down the
neutrons. This ensures that more successful
collisions occur. So the speed of the neutron is
slowed because of a moderator.
Therefore, the answer to the
question, “What is the name of the process by which neutrons are slowed down to
ensure more successful collisions occur?” is (D), neutron moderation.
What is the name of the process by
which neutrons are blocked to prevent collisions with the fuel rods? (A) Neutron activation, (B) neutron
transmutation, (C) neutron moderation, (D) neutron absorption, or (E) neutron
ionization.
Assuming that the neutrons are
moving slowly enough, when they collide with the uranium-235, they will be absorbed
into the nucleus, forming uranium-236. Since uranium-236 is unstable, the
nuclei will split, releasing energy. Just as before, each uranium-236
nucleus will produce two smaller nuclei and three neutrons. Each of these neutrons can then go
on to collide with another uranium-235 nucleus. And each of those nuclei will
produce another three neutrons, which can then go on to react again.
It is easy for this reaction to
occur too quickly, so the reaction needs to be carefully monitored. If the reaction does occur too
quickly, there is a chance of explosion. The nutrients can be blocked with
control rods. Control rods are often made from
boron and are capable of absorbing neutrons, so will decrease the fission reaction
rate. Inserting the control rods deeper
into the reactor core, or adding more control rods, will decrease the reaction rate
further. Neutrons are blocked by the
absorption from control rods.
So the answer to the question,
“What is the name of the process by which neutrons are blocked to prevent collisions
with the fuel rods?” is (D), neutron absorption.
