Module 8

Module 8—Nuclear Decay, Energy, and the Standard Model of the Atom

 

Explore

 

The Energy of Nuclear Reactions

 

Nuclear reactions involve vast amounts of energy, either creating massive fireballs in a chain reaction or slowly releasing significant amounts of energy over many years in a nuclear reactor. Recall from Module 8: Lesson 1 that particles (nucleons) make up a nucleus that is held together by a strong nuclear force. Both nuclear fission and fusion reactions change the number of nucleon particles, so work must be done against the strong nuclear force during any nuclear reaction.

 

The amount of work required to separate all the nucleons in a given atom is referred to as the binding energy. It is equal to the difference between the energy of all the nucleons when they are free compared to when they are contained in the nucleus.

 

Ebinding = Enucleons – Enucleus

 

Dividing the binding energy of the nucleus by the number of nucleons making it up gives a value for the binding energy of each nucleon.

 

 

Stable nuclei have greater binding energy per nucleon than unstable nuclei. Nuclei with atomic masses in the range of 58–62 (iron-nickel) are the most stable, with the highest binding energy per nucleon. Smaller atoms, such as hydrogen-2, have a very small amount of binding energy per nucleon, making them less stable. At the same time much larger atoms, such as uranium-238 also have a reduced binding energy per nucleon making it unstable. In order to become more stable, very small nuclei can become larger by combining in the process of fusion, and very large nuclei can become smaller by breaking down into smaller nuclei in the process of fission. The result in either process is to move toward a medium-sized stable nucleus with the greatest amount of binding energy per nucleon.

 

For both fission and fusion reactions, the energy released is equal to the difference between the total binding energy of the original nucleus or nuclei and the final binding energy of the nucleus or nuclei.

 

 

For both fission and fusion, the binding energy of the final (resulting) atom(s) is much larger than the binding energy of the initial atom(s), leading to both a more stable nucleus and the release of a large amount of energy.

 

The change in binding energy also corresponds exactly to the change in mass between the original and new nuclei, according to Einstein’s mass-energy equivalency (E = mc2). In this respect, the energy released in a nuclear reaction is based on the change in mass before and after the reaction.

 

 

According to this equation, even a very small change in mass multiplied by the square of the speed of light (9 × 1016) will result in a large release of energy.

 

Nuclear Fission

 

 

The Canadian CANDU nuclear reactor uses the fission of uranium-235 as an energy source. In this reactor a free neutron is absorbed by a uranium nucleus causing it to become unstable and break apart into two smaller nuclei accompanied by the release of several more neutrons.

 

If the free neutrons encounter more uranium atoms they will be absorbed again, causing further fission and the production of more neutrons capable of continuing the process. If enough uranium is present in a small enough area (critical mass), the probability of a neutron causing another uranium atom to split is very high and a chain reaction will occur. This would cause the release of a massive amount of energy in a very short period of time, producing a nuclear explosion. In a nuclear reactor, by contrast, the uranium atoms are spread out in fuel rods and some of the released neutrons are blocked by control rods in order to slow down the chain reaction. When the reaction rate is slow, a smaller amount of energy is released over a long period. In essence, a nuclear reactor is a nuclear bomb going off in a controlled manner over a prolonged period. When all or most of the uranium-235 atoms in the fuel have been spent, the reactor cools, at which point new fuel would have to be inserted to ensure continued energy production.

 

Example Problem 1. How much energy is released by the fission of one U-235 atom?

 

 

Given

 

The atomic masses come from “Table 7.5” and “7.6” on page 881 of your physics textbook.

 

 

Required

 

the amount of energy released

 

Analysis and Solution

 

Determine the mass defect.

 

 

Change the mass defect into kilograms.

 

 

Determine the amount of energy released.

 

 

Paraphrase

The energy released by the fission of one uranium-235 atom is 2.78×10–11 J.

 

This value may seem small by comparison, but in a single kilogram of uranium there are enough fissionable uranium atoms to produce 7.09 × 1013 J of nuclear energy, which is approximately 1.6 million times greater than the chemical energy within one kilogram (≈1.4 litres) of gasoline.

 

Read

Read “Comparing Chemical Energy with Nuclear Energy” on page 820 of the textbook.

 

Watch and Listen

 

When dealing with nuclear reactors the rate of the reaction must be controlled to prevent a chain reaction. In the fission of uranium, each uranium nucleus decays spontaneously, which is a very slow reaction. This decay also occurs when a uranium nucleus absorbs a neutron in a nuclear reactor, which can be slow or fast depending on the position of the control rods. It is slowed down when the ejected neutrons are absorbed by inserting control rods, or it is sped up by removing the control rods, which lets the ejected neutrons strike other uranium nuclei, thus continuing the chain reaction. In the following animation you will see how a chain reaction spreads exponentially.

 

Three animationss are available to explain nuclear reactions. Do an Internet search using the term “atomic archive.” This should take you to a website that explores the histrory, science, and more of the invention of the atomic bomb. When you get to the website, use the website’s search function to locate “Nuclear Chain Reaction Animation.” This animation will show how, in a nuclear bomb, the chain reaction is engineered to occur as rapidly as possible to produce the largest explosion possible.

 

Next, find “Nuclear Fission Animation.” This animation shows the reactants and products of a uranium-based nuclear fission reaction, like the one that powers nuclear power reactors.

 

Finally, find “Nuclear Fusion Animation.” Fusion reactions release much more energy than fission reactions; however, this is also what makes them hard to control. This animation shows the reactants and products of a deuterium (H-2) and tritium (H-3) fusion, which produces a helium nucleus and a neutron. This is the main form of fusion that powers Earth’s Sun.

 

If you're having trouble finding any of the animations, click on “Media” in the top navigation bar on the website, and then on “Animations” on the page that appears.

 

Try This

 

TR 1. Complete “Practice Problems” 1 to 3 on page 819 of the textbook.

 

Nuclear Fusion

 

Nuclear fusion is similar to nuclear fission in that the binding energy of the products is much higher than the starting nuclei. The process to achieve this, however, is based on combining nuclei rather than breaking them down. At temperatures greater than 100 million Kelvin, the small nuclei of tritium and deuterium will combine to form larger, more stable helium atoms while releasing an amount of energy proportional to the change in mass (mass defect) or equal to the difference in binding energies before and after the reaction.

 

 

Nuclear fusion powers the Sun, which has enough hydrogen to maintain its present rate of energy production for another 6 billion years. On Earth, hydrogen is abundant in the water, which, in theory, could provide a seemingly infinite supply of clean, safe nuclear energy.

 

Watch and Listen

 

Watch the video “Sun Flares,” which is about solar flares. The video demonstrates the energy released by the nuclear fusion on the surface of the Sun. When the Sun’s magnetic field fluctuates it allows massive amounts of plasma to arc off into space. The arcs are often larger in diameter than Earth.

 

Commercial fusion reactors could provide phenomenal amounts of non-polluting electrical energy. The EFDA (European Fusion Development Agreement) sponsors JET (Joint European Torus), a commercial fusion reactor development site. EFDA scientists are researching how to safely recreate the reaction that powers the Sun to produce electricity on Earth. They used the information gathered from the JET project to design a large reactor called ITER, which is scheduled to begin operation in 2016. It will be the biggest fusion furnace ever built, twice as large as any previously built, and will produce plasma at temperatures of hundreds of millions of degrees Celsius. The video “Inside a Reactor” shows the inside of the JET reactor during a plasma experiment.

 

Read

 

Read “Fusion” on pages 821 to 823 of your physics textbook. Page 821 provides more extensive detail on the reactions occurring on the Sun and includes references to the products released (neutrinos, positrons or antielectrons, and gamma rays).

 

Try This

 

TR 2. Read “Example 16.16” and complete “Practice Problem" 1 on page 822 of the textbook.

 

How does a nuclear fission reactor work? Why are they so dangerous if they get out of control? What caused the Chernobyl nuclear accident? Open this nuclear reactor simulation, take your operator test and see if you can run the reactor efficiently. You can also discover how you could accidentally cause an explosion. (Be sure to open the “Information” tab in the simulation.)

 

After completing the nuclear reactor simulation, post your response to the following Lab questions in the discussion area of your class. You may need to return to the simulation.

 

Module 8: Lesson 3 Assignment

 

Remember to submit your answer to LAB 1, LAB 2, LAB 3, LAB 4, and LAB 5 to your teacher part of your Module 8: Lesson 1 Assignment. 

 

LAB 1. Which variables control the reactor temperature and how does an operator adjust them?

 

LAB 2. Describe the combination of variables that leads to the quickest meltdown.

 

LAB 3. Describe the combination of variables that prevent the reactor core from generating heat.

 

LAB 4. Which type of control rod is most effective and why?

 

LAB 5. Which type of coolant works best for safely generating electricity?

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