CHEMystery: Nuclear Reactions: Nuclear Reactors

Nuclear Reactors

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Current nuclear power plants are powered by nuclear fission, with the science of nuclear fusion emerging.
The key parts to a nuclear power plant (fission) reactor are:

Fuel rods - Fuel rods are usually composed of fissionable isotopes such as ²³⁵U, ²³³U and ²³⁹Pu. Any isotope present in critical mass will do. In the U.S. enriched tri-uranium oct-oxide, U₃O₈, pellets are placed in a Zirconium alloyed tube where they are lowered into its core.
Control rods - Control rods, normally composed of ¹⁰B or Cd, absorb neutrons. Neutrons are absorbed so as to prevent the reaction from going at an unsafe rate (i.e. meltdown).
Moderators - It is job the of the moderator to slow down neutrons without absorbing them. Moderators must slow down neutrons without absorbing or reacting with them. D₂O (deuterium oxide--heavy water), H₂O, and graphi te are common moderators. If neutrons were permitted to continue uninhibited a chain-reaction would occur causing a meltdown of the facility.
Shielding/Containment - Several products are produced by nuclear reactions, most of which are lethal to humans. The storage and disposal of these materials is an enormous task. Should the facilities where the materials are being held be com promised, that area must be sealed for hundreds to millions of years, until the level of radioactivity has dropped to suitable levels. Inside the reactor several layers of concrete and steel must be laid to prevent radiation from escaping. The steel mus t be replaced periodically be cause exposure to radiation causes it to warp.
Coolant - The job of the coolant is to carry the heat from the reactor to a steam turbine system where it is converted to electricity, as well as keeping the reactor cool enough to prevent a meltdown.

Nuclear Fission

The greater stability of nuclei with mass number of intermediate values on the periodic table suggest the possibility of spontaneous splitting of less stable nuclei of the heavy elements into fragments of approximately half size and that nuclear fission would be accompanied by the release of large quantities of energy. In 1939 Hahn and Strassman reported that bombardment of a U-235 sample with slow neutrons split the atoms into barium, lanthanum, cerium as well as other elements from the middle of the table, all of which were more stable then the original U-235. This process came to be known as fission. There were to other important by products neutrons and energy.

²³⁵U + ¹₀n ==> (Ba, La, Ce, etc.) + ¹₀n + ENERGY

It was found that 0.2 amu per atom were converted to energy. Pound per pound that amounts to 2.5 million times as much energy as coal. A chain reaction can occur where neutrons released from this reaction cause other fusion reactions to take place. If a chain reaction becomes self-sustaining due to its mass, that is the number of neutrons produced by the reaction is equal to, or exceeds, the number of neutrons lost in the reaction, it has reached its critical mass. Critical mass is dependent on the material as well as its shape.

Enrichment

Enrichment is the process of purifying a specific isotope. In nuclear chemistry enrichment of fissionable materials is usually done in order to yield higher products in reactions. One example of enrichment is the fraction distillation of uranium. Uranium is mixed with fluorine to form UF₆, a gas. At low pressures the compound is allowed to go through a diffusion barrier permitting the lighter ²³⁵U to pass through faster. This is done hundreds of times to finally produce a 3% mixture of ²³⁵U.

Breeder Reactors

Fission reactors are dependent on the supply of uranium, which is expensive and being depleted at a rate which will be exhausted in approximately 50 years. Although the ²³⁸U present in enriched uranium is not fissionable it can still be transmutated into ²³⁹Pu, a fissionable material. Some, but not all ²³⁸U reacts as follows:

²³⁸U + ¹n ==> ²³⁹Pu + 2 ⁰e

The plutonium produced then put through the fission reaction

²³⁹Pu + ¹n ==> ¹⁴⁷Ba + ⁹⁰Sr + 3 ¹n

The neutrons produced are then used to make more ²³⁹Pu from ²³⁸U. Reactors that are specially designed to harvest the Pu are breeder reactors. By definition a breeder reactor is any reactor that creates more fissionable matter then it started out with. This sounds great but plutonium is the most toxic material known to man. It is widely held that one atom of plutonium can kill you if it gets into your lungs.

Nuclear Fusion

The process of converting light nuclei into heavy nuclei is accompanied by an even larger amount of energy. This type of reaction is known as nuclear fusion. When deuterium (hydrogen 2--with one neutron) and tritium (hydrogen 3--with two neutrons) are exposed to very high temperatures helium and neutrons are produced.

²H + ³H ==> ⁴He + ¹n + ENERGY

In this reaction MUCH more energy is released. For this reason, there are numerous research projects going on in order to find a way to control this reaction within so its energy can be harvested. Should this energy source be tapped, the worlds oceans would be an inexhaustible supply of cheap, clean, energy.

Fusion Reactors

Harvesting the energy released by a fusion reaction has attracted the attention of many research operations. Fusion is cheap and clean taking deuterium from ordinary water and producing stable, non-radioactive isotopes. The major draw back to this is the required energy to overcome the charges on the positive hydrogen nuclei. Temperatures of about 10⁸ K are required to overcome the positive charge on the reacting nuclei. NOTHING known to man can withstand such temperature for any extended period of time. Even if we knew of such materials, the nuclei would a have lot of energy when colliding with the container walls. Magnetic confinement and laser implosion methods are ways of implementing fusion power.
Inerital confinement, or laser implosion means inclosing the fuel, tritium and deuterium, with a pellet and having high energy lasers shoot at it with the same force from all sides. This causes the pellet to implode into a dense, hot ball of plasma, where fusion could occur.
Magnetic confinement holds the plasma to prevent it from leaking out, since no known material can withstand the temperatures. This is done by shaping and holding magnetic forces in configurations that prevent the plasma from moving out.