MPN - Nuclear Reactions

Nuclear Reactions

Chemical reactions involve the making and breaking of bonds between different atoms. With chemical reactions you begin and end with the same number and type of atoms, just in a new configuration. Nuclear reactions involve nuclei striking each other or nuclei being struck by simpler particles like neutrons or gamma rays. The result of a nuclear reaction is a new set of atoms.

An example of a nuclear reaction is the collision between a neutron and an oxygen-16 nucleus. In this reaction deuterium (an isotope of hydrogen containing one proton and one neutron,, is produced along with another, unknown nucleus. To help understand this reaction we can write the nuclear reaction equation.

Remembering the law of conservation of nucleon number we can see that the unknown nucleus must have 7 protons and an atomic mass of 15 (oxygen has 16 nucleons and the free neutron counts as the 17^th). This leaves us with only one option: nitrogen-15,.

Sometimes a neutron shot into a nucleus is captured, resulting in an isotope. However, when shot into a large, unstable nucleus the neutron triggers one of the two most energetic nuclear reactions, nuclear fission.

Nuclear Fission

In nuclear fission a large parent nucleus is split to create smaller, daughter nuclei.

A common fission reaction is the fission of uranium-235. When bombarded with a free neutron, this isotope of uranium undergoes nuclear fission, producing multiple fission fragments and plenty of energy. The daughter nuclei produced can vary, but there are always extra neutrons. For example, a typical reaction would involve the production of barium-141 and krypton-92.

Notice, again, that the nucleon numbers are conserved. The reaction begins with 236 total nucleons (92 protons and 144 neutrons). The products of the nuclear reaction also contain a total of 236 nucleons (92 protons and 144 neutrons). Understanding the law of conservation of nucleon number can allow you to determine unknown products of nuclear reactions.

Nuclear Fission Practice

Identify the unknown element X in the fission reaction:

We start with 236 nucleons, so we must end with 236 nucleons. Between the strontium-93 and the 2 free neutrons we have 95 nucleons. This leaves our unknown with an atomic mass of 236 - 95 = 141. Since there is no radioactive decay indicated (this could change the relative numbers of protons and neutrons from beginning to end of the reaction) we can assume that we will also begin and end with the same number of protons. 92 - 38 = 54. Consulting our periodic table we find that atomic number 54 is xenon. Our unknown element is xenon-141,.

The fission of uranium-235 releases a great deal of energy. Since its discovery we have found ways to harness that energy. One way is in nuclear power plants. In order to release this energy we can't have just one fission event. We need a chain reaction that, once started, can be self-sustaining. As we saw above, shooting just one neutron into a uranium-235 atom can produce energy and two or three new free neutrons. These free neutrons can then each cause the fission of other uranium nuclei, continuing the process.

This video shows a classic demonstration of how such a chain reaction operates.

Nuclear Fusion

In nuclear fusion, small nuclei are built into larger atoms. At the core of our Sun, hydrogen nuclei (protons) are being smashed together to create helium. This process, when consuming tons of hydrogen per second, produces an enormous amount of energy. As we saw in the previous unit, if you compare the rest mass of the helium nucleus to the four protons that smashed together to create it, there is a tiny loss in mass. This mass is converted to energy, as calculated using the equation.

The fusion of hydrogen into helium is actually a multistep reaction that is described by what is called the proton-proton chain.In the first reaction, two protons combine to form deuterium, a positron (the antimatter version of an electron), a neutrino, and about 0.42 MeV of energy.

Next, another proton combines with the deuterium to form a helium-3 nucleus, gamma rays, and about 5.49 MeV.

Finally, two helium-3 nuclei combine to form helium-4 (standard helium), two additional protons, and about 12.86 MeV of energy.

Each of the first two reactions must take place twice in order to have the helium isotopes necessary for the final reaction. In total, this process produces 26.7 MeV (the additional energy not shown in the equations comes from annihilation of the positrons with electrons).

For many reasons (including some you should be discussing in the forum for this unit) it might be preferable to use nuclear fusion rather than nuclear fission as an energy source on our planet. Here is an example of fusion reactions using isotopes of hydrogen (deuterium, and tritium).

The problem with nuclear fusion reactors, thus far, has been that the amount of energy needed to start the reaction is greater than the amount of energy produced in the reaction. In early 2014 scientists at the National Ignition Facility in Livermore, California reported the first successful fusion reaction in a lab to produce more energy than it absorbed. As critics have since pointed out, the method used by the NIF researchers is not likely to directly lead to a usable fusion reactor, but it is a step, along with many others, towards achieving something new.

Mechanics of Particle Motion

During nuclear processes subatomic particles can be shot into unstable nuclei and/or particles may be accelerated away from nuclei. As long as the particles are travelling less than about 10% the speed of light (3 x10⁷ m/s) the effects of relativity on their motion can be ignored and standard Newtonian mechanics can be applied to describe their motion. This means you should re-familiarize yourself with equations of motion and energy from your previous physics coursework. This includes, but is not limited to:

• kinematics
• 
• force
• 
• energy and momentum
• 

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