Control Room Simulator for Cook Nuclear Power Plant, Michigan
One aspect of nuclear power that I find interesting is that there are people who love nuclear because they believe that this source is environmentally friendly and practical, and there are those who are vehemently opposed to nuclear because they feel very strongly that nuclear is dangerous to local communities and the waste is excessively harmful to the environment. To be fair, I should share my personal bias; I like nuclear power for a few reasons; its safer than people know, it does not produce any air pollution, its relatively in-expensive to operate, the fuel is plentiful, and most importantly its proven technology that we are taking advantage of now. Fusion may be the holy grail of electrical generation, but it won't be viable for decades, if ever, and we have to address our energy needs now while balancing national security, public health and environmental concerns. As I have said before, nuclear has benefits and drawbacks, and the huge drawback with nuclear is the management of very dangerous waste products. Since I am not terribly concerned with a reactor going critical and exploding, or otherwise causing a catastrophe, the real question is does the prospect of having to manage highly radioactive waste for a very long period of time out weight the drawbacks of fossil fuel burning plants? Depending on how a community, country, or the world answers that question will determine how the future of nuclear power.
How Nuclear Reactor Works
As strange as this may sound, a nuclear power plant is fundamentally a cousin of the coal, or natural gas plant in that the water is heated to create steam, which turns a turbine that spins a generator that produces electricity. A radioactive substance, in commercial reactors that fuel is uranium, creates heat as the atoms break down, and vast amounts of heat is generated. So the operation of a nuclear reactor is basic in concept since all that is required is to gather a sufficient quantity of nuclear fuel, submerge it in water, and use the steam to produce power. Of course, in practice it is not that simple for when nuclear fuel is amassed, the radioactivity feeds off itself and starts a chain reaction that has the potential to perpetuate it self uncontrollably and that is when the reactor has the potential to explode or melt down. However, a reactor works more efficiently if the reaction does not have to be stimulated
Let me explain, in a little more detail how the reactor actually works. Uranium is radioactive, which means that it is inherently unstable. Eventually, given its instability, a uranium will decay or break apart. In nuclear reactors, the fuel is enriched with an isotope of uranium that will decay when struck with a neutron, and in turn throws off two or three more neutrons. Now what makes a nuclear reactor work is that those two or three neutrons will likely trike two or three uranium atoms, which will in turn each decay releasing heat and two or three neutrons each potentially releasing up to 9 neutrons. If those 9 neutrons hit 9 uranium atoms, then 27 neutrons could be released, and so on and so on. Or, in other words, this is the start of a chain reaction. If this reaction continues without abatement, then the fuel will start to get very hot, melt, and the the reactor could melt down.
Chain Reaction Video:
Uranium fuel is formed into pellets that are about the size of the end of a grown mans pinky finger. These pellets are arranged in long stacks within a metal tube called fuel rods. A fuel rod may be as long as 12 feet, and they batched together to form an assembly. A typical fuel assembly is arranged into 8x8, 14x14, 17x17 blocks of fuel rod. These assemblies are placed into the reactor core in a manner that balances safety and efficiency, and intermixed with the fuel are control rods that can slow or even stop the reaction. Control rods are made up of a material, such as cadmium, that is very good at capturing neutrons, and if you remember the sequence of atoms decaying, releasing neutrons that strike other uranium atoms causing them to decay and release more neutrons. If those free neutrons are stopped, then the chain reaction is slowed or stopped.
Reactor design does vary from plant to plant or country to country, but the basic design for capturing the heat energy released as a uranium atom decays is for the whole reactor core to be contained within a large stainless steel vessel filled with water similar in many respects to a large boiler. This water is able to circulate through the fuel assemblies where is gets heated. The heated water either its self becomes steam (see Boiling Water Reactor, abbreviated BWR) or is used to make steam (see Pressurized Water Reactor, abbreviated PWR). Like a coal or natural gas plant, the steam then turns a turbine that turns a generator.
Picture of the reactor vessel from EIA (Energy Information Administration). This is from a PWR (Pressurized Water Reactor)
Water is circulated through the reactor vessel and used to generate steam. One safety feature of a nuclear reactor is the water that is passed through the reactor and nuclear fuel is isolated from the water that becomes steam and passes through the turbine. Often, a power plant will have three separate water loops, a primary that cools the reactor, a secondary that is heated by the primary loop to become steam, and a tertiary loop that is used to convert the steam back into water. Typically the tertiary loop is drawn from an environmental source such as a river, lake, or ocean. The diagram below from the Tennessee Valley Authority (TVA) illustrates the multiple loops, the primary loop is pinkish, the secondary is blue and goes to the turbine, and the tertiary loop (in this diagram) goes to the cooling tower.
http://www.ocrwm.doe.gov/factsheets/doeymp0010.shtml
