Unit 8.3 Nuclear Chemistry Nuclear Reactors
State College Area School District
Teacher: Van Der Sluys
http://www.atomicarchive.com
Nuclear Fission: Basics
•
•
When a nucleus fissions, it splits into several smaller fragments. These
fragments, or fission products, are about equal to half the original mass.
Two or three neutrons are also emitted.
The sum of the masses of these fragments is less than the original mass.
This 'missing' mass (about 0.1 percent of the original mass) has been
converted into energy according to Einstein's equation, E = mc2 .
The Fission Reaction
•
A common pair of fragments from uranium-235
fission is xenon and strontium:
•
Highly radioactive, the xenon decays with a halflife of 14 seconds and finally produces the stable
isotope cerium-140. Strontium-94 decays with a
half-life of 75 seconds, finally producing the stable
isotope zirconium-94. These fragments are not so
dangerous as intermediate half-life fragments such
as cesium-137.
The fission products shown above are just one of
the many possible fission product combinations
(see figure).
235
•
92 U
+
1
0n
140
54Xe
+ 9438Sr + 2 10n
1
Nuclear Binding Energy
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html
Nuclear Chain Reactions
• A chain reaction refers to a process in which neutrons
released in fission produce an additional fission in at
least one further nucleus. This nucleus in turn produces
neutrons, and the process repeats. The process may be
controlled (nuclear power) or uncontrolled (nuclear
weapons).
• If each neutron releases two more neutrons, then the
number of fissions doubles each generation. In that
case, in 10 generations there are 1,024 fissions and in
80 generations about 6 x 1023 (a mole) fissions.
Nuclear Chain Reactions
2
Critical Mass
•
•
•
•
•
In order for a nuclear reaction to be self-sustaining, there must be a
“critical mass” of fissile material.
This is the mass necessary so that, on average, at least one neutron
produced by every fission goes on to trigger another fission.
Having less than critical mass will cause the bomb to “fizzle” and have
a very low efficiency.
Actual mass required for criticality depends on the density of the mass,
the shape of its configuration, and the presence/effectiveness of the
neutron reflector.
Due to natural emission of neutrons, any critical mass has a chance of
beginning a nuclear chain reaction.
Controlled Nuclear Fission
•
To maintain a sustained controlled nuclear reaction, for every 2 or 3
neutrons released, only one must be allowed to strike another
uranium nucleus. If this ratio is less than one then the reaction will
die out; if it is greater than one it will grow uncontrolled (an atomic
explosion). A neutron absorbing element must be present to control
the amount of free neutrons in the reaction space. Most reactors are
controlled by means of control rods that are made of a strongly
neutron-absorbing materials such as boron or cadmium.
•
In addition to the need to capture neutrons, the neutrons often have
too much kinetic energy. These fast neutrons are slowed through the
use of a moderator such as heavy water and ordinary water. Some
reactors use graphite as a moderator, but this design as several
problems. Once the fast neutrons have been slowed, they are more
likely to produce further nuclear fissions or be absorbed by the
control rod.
Controlled Nuclear Fission
3
Why Uranium and Plutonium?
•
•
•
•
Scientists knew that the most common isotope, uranium 238, was
not suitable for a nuclear weapon. There is a fairly high probability
that an incident neutron would be captured to form uranium 239
instead of causing a fission. However, uranium 235 has a high fission
probability.
Of natural uranium, only 0.7% is uranium 235. This meant that a
large amount of uranium was needed to obtain the necessary
quantities of uranium 235. Also, uranium 235 can not be separated
chemically from uranium 238, since the isotopes are chemically
similar.
Alternative methods had to be developed to separate the isotopes.
This was another problem for the Manhattan Project scientists to
solve before a bomb could be built.
Research had also predicted that plutonium 239 would have a high
fission probability. However, plutonium 239 is not a naturally
occurring element and would have to be made. The reactors at
Hanford, Washington were built to produce plutonium.
Isotope Separation
•
•
•
Natural uranium is 99.28% 238U and 0.72% 235U.
Only the 235U atoms undergo this kind of fission.
UF6 is the chemical form that is used to separate the
isotopes of uranium.
The most common method is by gaseous diffusion
through a membrane. The 235UF6 diffuses faster.
Fuel Rods, Control Rods and
Moderators
•
Inside the core of a nuclear reactor are
the
– Fuel rods containing enriched
uranium metal or oxide
– Control rods which when inserted
between the fuel rods, absorb
neutrons and slow or stop the chain
reaction
– Moderators are materials that slow
down fast neutrons so that they are
more easily captured by the uranium235
http://library.thinkquest.org
4
Power Plant Design
Control rods
Fuel
rods
Summary
5