Nuclear+Reactors

Nuclear Reactors Edited by Frank Ur     

__**How Nuclear Power Plants Work: **__ Nuclear Power plants work by a process called Nuclear Fission. Nuclear Fission is a nuclear reaction in which a heavy nucleus splits spontaneously or on impact with another particle, with the release of energy. This also causes another fissionable nuclear to be released, which causes the chain reaction to continue. One type of element used in a nuclear power plant is Uranium, which undergoes the process of nuclear fission. __**Main Components of a Nuclear Reactor **__ **Fuel:** The fuel is generally made up of small enriched uranium oxide rods that are stacked so that they form cylinders.The rods are wrapped in metal sheathes that are made up of steel or zirconium alloy, these allow heat to pass through while blocking the radiation produced by fission.

Here is a Picture of how fuel is placed:

**Moderator:** A moderator is a material placed in the reactor to slow down the neutrons that are produced by fission,this is in the reactor in order to reach the suitable speed allowing the chain reaction to continue and produce more energy. Depending on the type of reactor, the moderator may be made up of graphite, water or heavy water. Heavy water is where the hydrogen is present in a heavier isotope.

 **Heat-transfer fluid (or coolant):** The coolant can be a liquid or gas, and cools the core and disperses the heat that is formed there. There are many types of coolants such as water, heavy water, molten sodium, carbon dioxide, helium and other fluids, but the main coolant is water and it also depends on what kind of reactor is being used.

 **Control Rods:** Control rods are rods that are made up of specific materials (silver, cadmium, boron carbide) which help control fission inside the core. Because control rods absorb neutrons, the rods have the capability of controlling the fission process. Depending on how deep the rods are placed within the core, the chain reaction can be speed-ed up, slowed down, and even stopped. These are control rods placed in a reactor. Cooling Systems:The function of a power reactor is to extract the heat of nuclear fission and convert it to useful power, generally electric power. The coolant system plays a role in performing this function. A coolant fluid enters the core at low temperature and exits at a higher temperature after collecting the energy produced by fission.The higher-temperature fluid is then directed to conventional thermodynamic components where the heat is converted into electric power. In most light-water, heavy-water, and gas-cooled power reactors, the coolant is maintained at high pressure __** The Types of Power Plants Operational in the U.S: **__ **Pressurized Water Reactor: (PWR): **

This is the most common type of nuclear reactor in use, with over 230 in use for the generation of power. This type of reactor uses water as both a coolant and a moderator. The design is distinguished by having a primary cooling circuit which flows through the core of the reactor under very high pressure, and a secondary circuit in which steam is generated to drive the turbine. The turbine generator is what produces the electrical energy. The preasurized water reactor has around 200 to 300 rods each. Water in the reactors core can reach up to 325degrees celcius ( 617f). Since it gets to extreme temperatures the water must be kept at a very high atmospheric pressure to help the water not to boil. In the primary cooling circuit the water is also the moderator, and if any of it turned to steam the fission reaction would slow down. The secondary shutdown system involves adding boron to the primary circuit.

** Boiling Water Reactor: (BWR) : ** This design has many similarities to the PWR, except that there is only a single circuit in which the water is at lower pressure, so that it boils in the core at about 285°C (554F). BWR units can operate in load-following mode more readily then PWRs. The steam passes through drier plates (steam separators) above the core and then directly to the turbines, which are part of the reactor circuit. Since the water around the core of a reactor is always contaminated with traces of radionuclides, it means that the turbine must be shielded and have radiological protection provided during maintenance. The cost of this tends to balance the savings due to the simpler design. Most of the radioactivity in the water is very short-lived* (
 * mostly N-16, with a 7 second half-life), so the turbine hall can be entered soon after the reactor is shut down

** Preasurized Heavy Water Reactor: **

PHWRs generally use natural uranium (0.7% U-235) oxide as fuel, hence needs a more efficient moderator, in this case heavy water (D2O). The PHWR produces more energy per kg of mined uranium than other designs. The moderator is in a large tank called a calandria, penetrated by several hundred horizontal pressure tubes which form channels for the fuel, cooled by a flow of heavy water under high pressure in the primary cooling circuit, reaching 290°C. As in the PWR, the primary coolant generates steam in a secondary circuit to drive the turbines. The pressure tube design means that the reactor can be refuelled progressively without shutting down, by isolating individual pressure tubes from the cooling circuit. The difference between these type of reactors is that one uses heavy water as a moderator because in the case of a pressurized water reactor, it needs a more efficient moderator. Also they all differentiate in the degrees that they all reach and how the energy is produced and used to form energy.

Heavy Water and Why it is Important: <span style="color: #af09a9; font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif; font-size: 130%;">Heavy Water is water in which the hydrogen in the molecules is partly or wholly replaced by the isotope deuterium. Heavy Water is used especially as a moderator in nuclear reactors. Heavy water is important because it helps to reduce the speed of neutrons that are produced. This is important because it helps to increase the probability that the neutrons will react with the type of uranium used and helps leads to more fission reactions instead of being absorbed by the uranium used. Since heavy water has the isotope deuterium instead of hydrogen it does not absorb neutrons like normal water would.

<span style="font-family: 'Arial Black',Gadget,sans-serif; font-size: 170%;">Problems and Environmental Concerns with Nuclear Reactors: <span style="font-family: 'Comic Sans MS',cursive; font-size: 130%;">One major environmental concern with nuclear reactors is where to put the nuclear waste produced by the reactors. The nuclear waste that is produced is radioactive waste and cannot be disposed of my any type of conventional means. The forms of radioactive waste include things such as used up nuclear fuel rods, and a very dangerous type of waste called low-level waste which include the general radiation-contaminated material. Storing radioactive material and disposing it is hard to do because there are no safe ways to dispose of it, but there are ways to keep it contained and somewhat disposed. Current techniques that power plants use include cooling waste underwater for a couple years, sealing them in glass, and storing the waste in concrete bunkers that are overground.

<span style="font-family: 'Comic Sans MS',cursive; font-size: 130%;">Another major environmental concern with the use of Nuclear reactors is the effect on the local ecology. Since the coolant for a reactor is most of the time water, the water is sometimes if not all times released back into a stream. Since the water is extremely hot it can/will kill off any wild life in the stream that cannot withstand the heat.

<span style="font-family: 'Comic Sans MS',cursive; font-size: 130%;">Other concerns are things such as who can get their hands on nuclear waste. Radioactive waste, in the wrong hands. can be produced into bombs. These bombs are very destructive because when set off they spread radioactivity which can kill things around where the bomb went off.

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