20.3.1 Nuclear Fission

Question 1

Nuclear Fission

Answer 1

• The reaction of 235 U with a neutron produces more neutrons, which can then be used to initiate further reactions.
• Critical mass is the amount of fissionable material required for a self-sustaining nuclear fission reaction to occur.
• Nuclear fission reactors harness the energy given off by nuclear fission to boil water, which turns a turbine to produce electricity.

Question 2

note

Answer 2

• A chain reaction is a series of reactions in which the products of one reaction initiate the next reaction. Nuclear fission, the exothermic splitting of a heavy nucleus to form lighter nuclei, is such a reaction.
• The neutrons produced in such reactions can be used to initiate further 235 U fission reactions.
• Uranium is enriched in 235 U by forming uranium hexafluoride gases. 235 UF 6 (g) and 238 UF 6 (g) can be separated based on Graham’s law of effusion.
• The problem with nuclear fission is that many different products can be formed from the fission of 235 U.
• Nuclear fission has been used in atomic bombs. If uranium becomes too compressed, a spontaneous nuclear fission reaction can occur.
• Fission can also be used to produce electricity. However, fission reactions produce hazardous radioactive waste. Since some of the products of fission reactions have half-lives on the order 10,000 years or longer, radioactive waste is a major environmental concern.
• Nuclear fission reactions are controlled by adding a substance that absorbs neutrons without reacting.
• Typically, 12 C is used in the form of graphite control rods. The reaction rate is controlled by moving these rods into and out of the reacting uranium.
• The energy given off by these reactions is harnessed by boiling water to create steam to turn a turbine, which drives a generator.
• Breeder reactors use these reactions to produce energy. The advantage of breeder reactors is that the fissionable materials are more completely consumed, greatly reducing radioactive waste.

Question 3

Which statement about nuclear fission reactors is not correct?

Answer 3

The United States generates more than half of its electricity from fission reactors.

Question 4

Let’s look at a nuclear fission event
235 92 U + 1 0 n -> 236 92 U -> 141 56 Ba + 92 36 Kr + 3 1 0 n
Which statement about this event is not correct?

Answer 4

235 92 U comes from 2387 92 U. 235 92 U grabs neutrons and pulls them out of the reaction

Question 5

Nuclear fission reactors produce a lot of energy. Which statement about this process of producing energy is not correct?

Answer 5

The rate of the reaction is controlled by graphite because carbon-12 is a non-fissionable element

Question 6

Which statement about nuclear fission is not correct?

Answer 6

Currently, the only way to dispose of nuclear waste is to store it in the ground. The only problem with that process is that the contaminated rods have to be stored separately so that a critical mass does not occur.

Question 7

A breeder reactor is a nuclear reactor _________________

Answer 7

that produces a useful, isotopic fuel.

Question 8

Let’s take a closer look at what happens in a nuclear fission reactor. Which statement about this process is not correct?

Answer 8

Boiling of the water produces steam. The original source of this steam heat is the water.

Question 9

Look at the following nuclear fission chain reaction
235 92 U + 1 0 n -> 236 92 U -> 141 56 Ba + 92 36 Kr + 3 1 0 n
Which statement about this event is not correct?

Answer 9

238 92 U and 235 92 U are combined for this reaction. The 238 92 U is used to capture excess neutrons

Question 10

Which statement about a chain reaction is not correct?

Answer 10

As long as one neutron is generated by a chain reaction, the “domino” effect will occur, causing the rate of the subsequent chain reactions to increase more and more.

Question 11

Which statement about a nuclear fission reactor is not correct?

Answer 11

Nuclear fission reactors are very “clean” because they do not produce CO2 (which contributes to the greenhouse effect).

Question 12

Which statement about breeder reactors and nuclear fission reactions is not correct?

Answer 12

One problem with nuclear fission reactors is that the fuel rods become contaminated. As a result, the neutrons are no longer captured by the rods.

Question 13

Look at the following nuclear fission chain reaction
235 92 U + 1 0 n -> 236 92 U -> 141 56 Ba + 92 36 Kr + 3 1 0 n
Which statement about this event is not correct?

Answer 13

Ba and Kr are more stable isotopes in this endothermic reaction.