History



In 1896, Henri Becquerel was fascinated by the radiation of certain materials and looked for X-rays in Uranium salts. However, he discovered something he was not searching for, spontaneously emitted rays, which differed from the X-rays and light. He coined the term "Uranic rays" which created a new field of physics entitled Nuclear Physics.
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Henri Becquerel


Marie Curie and her husband, Pierre Curie followed Becquerel's research and discovered the rays were not only found from Uranium, which led to the uncovering of new elements as well as the term, Radioactivity. Many other radioactive substances were found due to the experiments of the Curies.

Later, Rutherford experimented with alpha particles by shooting them at a piece of gold foil. He found the particles would not all pass through the foil, but most would bounce off or back. Since the positively charged alpha particles would bounce off, he concluded the atom was mainly made of positively charged particles inside of a nucleus. The discovery of the nucleus contributed to the new model of the atom.

Bohr then entered the picture with his atomic model with its stationary states and stable orbits. His atomic model explained how electrons would emit light when jumping from state to state. Beta-rays, a type of radiation, were found to be electrons, but traveled close to the speed of light. Therefore, to account for the energies of the beta-rays, they had to be of nuclear origin. The same is true for gamma-rays as well.

Yet, some energy was not accounted for as the maximum energy corresponded to the difference between the initial and final nucleus. Wolfgang Pauli proposed a new particle was taking energy from the emitted electron away, which was called the neutrino.

Nuclear Decay



Nuclear decay occurs whenever a nucleus is in an energy state which is not the lowest possible for its "nucleon number." (The nucleons are the protons and the neutrons making up the nucleus.) The state may occur either naturally or artificially by neutron or photon irradation. Also, the energy level depends on the angles and spins of the nucleons within the nucleus.

There are 4 modes of decay:

  • alpha
  • beta (electron or positron emission)
  • gamma (high energy photon emission
  • electron or positron capture (opposite of beta decay)


Alpha decay occurs when the nucleus emits an alpha particle which causes its mass number to decrease by 4 and its atomic number to decrease by 2. In alpha decay, the parent atom releases a daughter collection of nucleons, leaving a product behind. Alpha particles have a large mass and a low velocity which contribute to its great loss of energy and the reason why it can only last in a few centimeters of air.

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Beta plus decay


Beta decay is the radioactive decay of a beta particle; an electron or a positron. With an electron emission, it is referred to as beta minus and with positron emission, it's referred to as beta plus. In beta minus decay, the weak interaction converts a neutron into a proton, while emitting an electron for charge conservation and electron antineutrino for energy and momentum conservation. However in beta plus decay, energy converts a proton into a neutron, positron and neutrino. Beta plus decay must have energy so it cannot occur by itself unlike beta minus decay.

Gamma rays differ from both alpha and beta decay. When a nucleus emits an alpha or beta particle, the daughter nucleus is left in an excited state. It will then jump down to a lower energy state and emit a gamma ray.

Finally, electron or positron capture is the opposite of the emission processes.

Nuclear Fission



Nuclear fission is a change in the nucleus of an atom in which a heavy nucleus (atoms in which mass number is greater than 58) captures a neutron and splits into two or more lighter nuclei along with two or three emitted neutrons. The sum of the two or more nuclei, also known as fission products, is slightly less than the original mass. The "missing" mass (approximately 0.1% of the initial mass) has been converted into energy, in the form of photons or kinetic energy of the fission products, based on Einstein's equation (E=mc²).
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When neutrons released in fission are captured by other nuclei, those nuclei become unstable and additional fissions may occur. A series of additional fission events, or a chain reaction, can be triggered by a few stray neutrons. A chain reaction that is controlled and self-sustaining results in nuclear power. In a sustained fission reaction, for every 2 or 3 neutrons released, one must be allowed to strike another nucleus. If the ratio is less than 1, the reaction will stop; if the ratio is greater than 1, the reaction will grow uncontrollably. If the chain reaction is not controlled, a tremendous amount of energy may be released and a violent explosion could result. An uncontrolled chain reaction is main principle behind a fission bomb.

A nuclear reactor uses uranium-235 and plutonium-239 as fuel because of their high fission probability. However, both uranium-235 (0.7% of natural uranium) and plutonium-239 (not naturally occurring) are scarce. To control the number of free neutrons in a chain reaction, boron and cadmium, both highly neutron-absorbent elements, are used.


Nuclear Fusion


What is Nuclear Fusion?
Nuclear fusion is a process that constantly occurs in the sun and the stars. Nuclear fusion is the process where energy is released when atoms collide and fuse together to create one. In the sun two Hydrogen atoms fuse together to form one Helium atom. Fusion occurs when the conditions are right; the atoms are able to overcome the repel of the electrostatic forces between the negative and positive particles of the atoms. When the temperature increases the ions move faster and eventually reach speeds high enough to bring the ions close enough together. The nuclei can then fuse, causing a release of energy. This is what Einstein's formula E=mc² describes: the tiny bit of lost mass (m), multiplied by the square of the speed of light (c²), results in a very large figure (E) which is the amount of energy created by a fusion reaction. Every second the sun turns 600 million tons of hydrogen into helium through nuclear fusion creating large amounts of energy.
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Types of Nuclear Fusion
tokamak.gifMagnetic
Magnetic fields are ideal for containing plasma because the electrical charges on the separated ions and electrons mean that they follow the magnetic field lines. The goal of the magnetic fields is to prevent the particles from coming into contact with the reactor walls as this will dissipate their heat and slow them down (E=mc²). The tokamak chamber was designed in 1951 by Soviet physicists Andrei Sakharov and Igor Tamm. In a tokamak reactor, the toroidal field is created by a series of coils evenly spaced around the reactor, and the poloidal field is created by a system of horizontal coils outside the toroidal magnet structure. Ohmic heating, neutral beam injection and high-frequency waves will work together in the ITER tokamak to buring the plasma to 150,000,000 degrees where fusion can occur. Ohmic heating uses magnetic fields to create a high-intensity electrical current through induction. As the current travels through the plasma, the electrons in the plasma become energized and collide. The collisions create “resistance” which creates heat. Neutral beam injection consists in shooting high energy particles into the plasma. Outside of the tokamak, charged Deuterium particles are accelerated to the required energy level. These accelerated ions, only the neutral, are then injected into the plasma where they collide with the plasma, creating heat. High-frequency waves heat the same way that microwaves transfer heat to food. The energy carried by high-frequency waves introduced into the plasma is transferred to the charged particles, increasing the velocity of their chaotic motion.

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The inside of a European JET reactor.

Inertial
In inertial confinement fusion, which is a newer line of research, laser beams are focused very precisely onto the surface of a target, which is a pellet of Deuterium-Tritium fuel, a few millimetres in diameter. This heats the outer layer of the material, which explodes outwards creating an implosion that compresses and heats the inner layers of material. The core of the fuel may be compressed to one thousand times its liquid density, resulting in conditions where fusion can occur. The energy released then heats the surrounding fuel, which may also undergo fusion leading to a chain reaction (known as ignition) as the reaction spreads outwards through the fuel. The time required for these reactions to occur is limited by the inertia of the fuel (hence the name), but is less than a microsecond. So far, most inertial confinement work has involved lasers.
"These lasers are pulsed, and for a very short amount of time"-one ten-billionth of a second-"the power they produce is more than all the power generated by the entire electrical grid of the United States at an"-Siegfried Glenzer, physicist at Lawrence Livermore National Laboratory in California
.

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Laser from the National Ignition Facility's reactor. This heats up the pellets inertial nuclear fusion.


Types of Nuclear Fusion Fuels
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These three viles all contain Hydrogen: Left-Hydrogen, Middle-Deuterium, Right-Tritium.
Unlike nuclear fission, nuclear fusion is still in the experimenting phase. When doing experiments, scientists do not use Hydrogen atoms, but rather heavier isotopes (E=mc² ) of Hydrogen: deuterium and tritium. Both of these types of fuel are abundant in nature- deuterium can be extracted from seawater and tritium is either made from lithium or is a radioelement from decayed Hydrogen. Only tiny amounts of Deuterium and Tritium are necessary to fuel the fusion reaction. At extreme temperatures, electrons are separated from nuclei and a gas becomes a plasma. The fusion between Deuterium and Tritium will produce one Helium nuclei, one neutron and energy. The Helium nucleus carries an electric charge which will respond to the magnetic fields of the reactor and remain confined within the plasma. However about 80% of the energy produced is carried away from the plasma by the neutron which has no electrical charge and is therefore unaffected by magnetic fields. The neutrons will be absorbed by the surrounding walls of the reactor, transferring their energy to the walls as heat. In the future fusion installations, the heat will be used to produce steam, which will power the turbines that produce the electricity.


Assessing Fusion Power

1) A fusion reaction is about four million times more energetic than a chemical reaction such as the burning of coal, oil or gas. While a 1 000 MW coal-fired power plant requires 2.7 million tons of coal per year, a fusion plant of the kind envisioned for the second half of this century will only require 250 kilos of fuel per year, half of it Deuterium, half of it Tritium.

2) In addition, fusion reactions do not emit greenhouse gases or pollution. Also there is no possibility of a reaction getting out of control or becoming dangerous because the conditions for fusion are precise - any alteration in these conditions and the plasma cools within seconds and the reaction stops.















References



ITER. (2010). Retrieved March 20, 2010, from ITER Organization: http://www.iter.org/sci/Pages/FusionFuels.aspx

Koehler, K. A. (1996). Nuclear decay and radioactive series. Retrieved from http://www.rwc.uc.edu/koehler/biophys/7c.html

Moszkowski, S. A., & Wong, C. W. (n.d.). Early nuclear physics. Retrieved from http://www.physics.ucla.edu/~moszkows/earlynp/earlynpv.htm

Nuclear fission basics. (2008). Retrieved from http://www.atomicarchive.com/Fission/Fission1.shtml

Serway, Raymond A., & Faughn, Jerry S. (2009). Holt physics. New York, NY: Holt, Rinehart and Winston.

Than, K. (2010). Fusion Power a Step Closer After Giant Laser Blast. National Geographic
.