Nuclear Fission and Fusion

During radioactive decay, atoms of one element are changed into atoms of another element through the emission of alpha or beta particles from their unstable nuclei.

With alpha decay the nucleus emits an alpha particle, which is essentially a helium nucleus; a group of two protons and two neutrons. It is a form of nuclear fission where the parent atom splits into two daughter products. The atomic nucleus emits an alpha particle and transforms (or ‘decays’) into an atom with a mass number 4 less and atomic number 2 less. For example:

An alpha particle is the same as a helium-4 nucleus. Unlike beta decay, alpha decay is governed by the strong nuclear force.

In beta decay, a neutron in the nucleus of an atom is converted into a proton and an electron. The electron is released as a beta particle. Below is the beta decay of Thorium to produce Protactinium.

In beta minus decay, the weak interaction converts a neutron into a proton while emitting an electron and an anti-neutrino. During beta-plus decay, a proton in an atom’s nucleus turns into a neutron, emitting a positron and a neutrino. Alpha rays can be blocked by a sheet of paper, shielding against beta rays needs a sheet of metal like aluminium.

Nuclear fission differs from other forms of radioactive decay in that it can be harnessed and controlled via a chain reaction: free neutrons released by each fission event can trigger yet more events. Radioactive decay is spontaneous. Most nuclear fuels undergo spontaneous fission only very slowly.

Nucleosynthesis in stars

Hydrogen and helium are the most abundant elements in the universe. Elements heavier than lithium are all synthesized in stars. This first process of primordial nucleosynthesis may also be called nucleogenesis. In the Sun hydrogen is converted to helium in nuclear fusion reactions:

41H � 4He + subatomic particles

During the late stages of stellar evolution, massive stars burn helium to carbon, oxygen, silicon, sulphur, and iron. The production of small amounts of hydrogen and helium nuclei makes it possible for the star to synthesise most of the elements in the first three periods of the Periodic table. Two routes for the generation of lithium are:

Route 1: 4He + 3H � 7Li

Route 2: 4He + 3He � 7Be 7Be + electron � 7Li

The second route is interesting; this is because it is a form of electron capture. This is where the collision between an atom and an electron causes the proton to convert to a neutron and a neutrino is released. It is sometimes called inverse beta decay, the proton number and the structure of the nucleus is changed.

Fission and Fusion

Nuclear fission is the splitting of the nucleus of an atom into lighter nuclei often producing free neutrons and other smaller nuclei. The emission of these neutrons can cause further fission in other nuclei, thus producing a chain reaction. Fission of heavy elements is an exothermic reaction which can release large amounts of energy both as electromagnetic radiation and as kinetic energy of the fragments. Below is an example of nuclear fission, showing Uranium-235 splitting to create strontium-90 and xenon-143:

Inside the reactor of an atomic power plant, Uranium atoms are split apart in a controlled chain reaction. This chain reaction gives off heat energy. This heat energy is used to boil water in the core of the reactor. So, instead of burning a fuel, nuclear power plants use the chain reaction of atoms splitting to change the energy of atoms into heat energy.

This water from around the nuclear core is sent to another section of the power plant. Here, in the heat exchanger, it heats another set of pipes filled with water to make steam. The steam in this second set of pipes turns a turbine to generate electricity.

Uranium-238 is used as a natural way to control the reaction due to the fact that it does not undergo fission. Two other mechanisms used for controlling the reaction are the graphite moderator and the control rods, which are made of boron coated steel.

The neutrons that are produced when a nucleus splits are very fast moving; the graphite slows them down enough so that they cause fission reactions when they collide with Uranium-235 nuclei. The control rods which are made of Boron which absorb neutrons; they can be moved in and out of the reactor to control the rate of fission reactions. A typical absorption reaction is:

Below is a cross section of the inside of a typical nuclear power plant:

Notable advantages of fission include the fact that relatively little fuel is needed and the fuel is relatively inexpensive and available in trace amounts around the world. Also, it is not believed to contribute to global warming or other pollution effects associated with fossil fuel combustion.

However, its major concerns include the possibility for a nuclear meltdown; an example could be the Chernobyl Disaster. Also, waste products can be used to manufacture weapons; waste from plutonium power stations remains dangerous for thousands of years. There is also high initial cost because the plant requires containment safeguards; even then, the power plants are still vulnerable from sabotage and attacks.

Nuclear fusion is the process by which multiple atomic particles join together to form a heavier nucleus. It is accompanied by the release or absorption of energy. The fusion of two nuclei lighter than iron or nickel generally releases energy. Below is an example of a fusion reaction:

Excess energy is released from the fusion reaction because of the lower binding energy of the helium nuclei compared to those in deuterium and tritium. The combined mass of the products is less than the mass of the reactants; the ‘lost’ mass is converted to energy, according to Einstein’s equation:

E=mc2

For the below reaction to occur the particles need to form a high-density, super hot, ionized gas – plasma.

2H + 3H � 4He + 1n

Tritium is produced by using lithium in the reactor, where neutrons from the deuterium-tritium reaction in the plasma will react with the lithium to produce more tritium:

6Li + 1n � 4He + 3H

A way to control the plasma is to keep it away from the walls, which minimizes heat loss. To do this a tokamak is used. This device contains hot plasma in a doughnut shape within a vacuum vessel. Powerful magnetic fields created by large coils that run around the vessel keep the plasma away from the walls. Powerful electric currents heat the plasma as well as by microwaves that are directed into it and beams of fast neutron particles.

Notable advantages to using nuclear fusion include the fact that there is significantly less chance of a fatal accident occurring than that of a fission reactor, because the fuel contained in the reaction chamber is only enough to sustain the reaction for about a minute. Also, Deuterium and tritium are virtually inexhaustible. Unlike fission reactors, whose waste remains dangerous for thousands of years, most of the radioactive material in a fusion reactor would remain dangerous for about 50-100 years.

At present the disadvantage is merely the fact that scientists have not yet been able to contain a fusion reaction long enough for there to be a net energy gain. This is, in turn, causing many countries to phase out fusion research because of the failure to reach a breakthrough.

Challenges to the future of fusion power stations

The main challenge that scientists face is the growth of hydrocarbon films. Where plasma touches the walls, carbon tiles are eroded by deuterium and tritium ions, producing hydrocarbons. Further reactions result in the formation of reactive radicals, which combine with each other to form hydrocarbon films.

These films cause problems because they trap the tritium and deuterium fuel ions in the walls of the device so that they are not circulating in the reacting plasma to produce any energy. Also if the film gets thicker, it begins to flake off, resulting in dust particles which can be absorbed into the plasma, affecting its purity and performance.

Other issues include the potentially prohibitive costs of building, and the difficulties of repairing and maintaining the reaction vessel. This massive “blanket” of lithium and rare metals will degrade and become radioactive over time, requiring regular dismantling and replacement.

No. Words – 357+368+294+145 = 1164 (excluding equations, titles, annotations and text in diagrams) – sorry!

Sources

http://www.answers.com/topic/nucleosynthesis?cat=technology – Nucleosynthesis

http://helios.gsfc.nasa.gov/nucleo.html – Nucleosynthesis

http://physics.bu.edu/py106/notes/RadioactiveDecay.html – Alpha and beta equations

http://map.gsfc.nasa.gov/universe/bb_tests_ele.html – Nucleosynthesis

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch23/modes.php – Alpha and Beta decay

http://media.nasaexplores.com/lessons/01-060/images/Uran235.jpg – Fission of uranium-235

http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/fusion.html – Nuclear fusion

http://www.iter-india.res.in/images/jet_tokamak.jpg – Tokamak image, Fig 5

http://www.newscientist.com/channel/fundamentals/dn8827-no-future-for-fusion-power-says-top-scientist.html – Problems for fusion power

http://eazyvg.linuxoss.com/2007/08/21/fusion-is-the-future-choice-for-nuclear-power-generation/ – Fission and fusion diagrams, advantages and disadvantages of fission and fusion. Fig 1 and 3

http://www.physlink.com/Education/AskExperts/ae534.cfm – Bond energy per nucleon, Fig