Untitled - Kelly Walsh High School

Nuclear Chemistry 299
Don’t Forget!
If you want to calculate the energy in joules, the mass must be in units of kilograms.
A joule is a kgm 2 /s 2 .
20-5 Fission and Fusion
Nuclear fission is the breakdown of a nucleus into two or more smaller nuclei
with the release of energy. The most useful fission process involves the decay of
U-235 when hit by a neutron:
1 235
0n 92U l
Notice that the reaction consumes one neutron, but the reaction releases three
neutrons. Those three neutrons are then free to initiate additional fission reactions.
This type of situation in which there is a multiplier effect is a chain reaction.
We can use isotopes that undergo chain reaction in both the production of
bombs and in nuclear power plants. U-235 is fissionable, but U-238 is not. There
is a certain minimum quantity of fissionable matter needed to support a chain
reaction, the critical mass.
Nuclear reactors are useful in the production of electricity, but they are not
without their problems. These problems include disposal of nuclear wastes,
accidents, and sabotage. The eventual answer may lie in nuclear fusion.
Fusion is the combining of lighter nuclei into a heavier one. Such reactions can
release a great deal of energy. Isotopes of hydrogen fuse into helium and power
the sun. For the past few decades, scientists have been investigating the fusion
process as a way of providing the world with energy. One of the more promising
fusion reactions is:
2
1H 3 1H 142 91
56Ba 36Kr 3 1 0n l
4
2He 1 0n In this reaction, two isotopes of hydrogen fuse into helium and a neutron. Three
major problems in this reaction have arisen—time, temperature, and containment.
The nuclei must be held together long enough (~1 s) at high enough temperatures
to provide the activation energy for the reaction (~40,000,000 K). At
this temperature, every substance is a gas or plasma so containment is proving
to be the biggest obstacle. Scientists are conducting major investigations into the