Wednesday, April 13, 2011

Nuclear Technology Basics: Part 4 Reactor Components


Part 1

Part 2

Part 3

The process of nuclear fission can be carried out by several different fissile isotopes, depending on the design of the nuclear reactor. Most reactors that have been built in America for energy generation are what are called light water reactors or simply "LWRs" as an abbreviation. Light water reactors are thermally-based reactors that use regular water for both coolant and as a neutron moderator as opposed to heavy water which has a high ratio of deuterium which is an isotope of hydrogen that has a greater atomic mass than normal hydrogen.

In future posts I will go over the different types of light water reactors, but in this case I will use the layout of a typical pressurized water reactor or "PWR". PWRs constitute the most common type of nuclear reactor in the world, including the US. Although many different types of reactor designs exist, the PWR remains the most common design because of both politics and technical familiarity with engineers in the nuclear field.

While I normally frown on Wikipedia as a source of accurate technical information, I did find a nice animated diagram of a PWR reactor layout there.

So, here are the basic components of our nuclear reactor:

1. The Reactor Vessel
The reactor vessel is the component within a nuclear reactor that contains the reactor core and where coolant circulates to prevent the core from overheating. Some designs lack a reactor vessel but the BWR and the PWR types of light water reactors both have this component in their similar designs. The basic layout is that of a cylindrical tube made out of a steel alloy containing manganese and molybdenum because of its durability, while the interior of the reactor itself lined with a layer of stainless steel to prevent corrosion from rust since it comes into contact with the coolant fluid, which is water in LWR designs. The top of the reactor vessel is designed to be removable to facilitate the replacement and insertion of fuel assemblies. Coolant is pumped in through the inlet nozzles where it flows around the fuel assembly; removing heat in the process. The heated coolant is then pumped out of the reactor vessel and into the steam generator.

2. The Pressurizer

The coolant circulation system and the steam that is produced within the steam generator is under constant pressure. Maintaining the degree of pressure within the coolant circulation system and the steam generator is an important task and this is carried out by the pressurizer. An increase in the temperature of the circulating coolant causes the density of the fluid to drop, and the volume of the liquid to expand in volume. This pushes the fluid into the pressurizer, causing the steam within the top of the component to become compressed, and pressure levels to increase. A drop in the temperature of the coolant increases the density of the water, causing it to contract. Fluid drops out of the pressurizer, reducing the degree of steam compression within the top of the pressurizer. Should the pressure increase too much or decrease below safe levels, the pressurizer will bring fluid pressure levels back to a safe equilibrium by either spraying cold water through the top of the pressurizer which would cause the compressed steam to cool down and turn back into water. Lower than normal pressure levels will cause the pressurizer to activate a series of electric heaters embedded within the walls of the component to raise the ambient temperature of the water within the pressurizer. Should pressure levels continue to fall, safety systems will cause the reactor to shut down automatically.

3. The Control Rods

The control rods regulate the speed and ratio of fission within a nuclear reactor. Each control rod is composed out of materials that can capture and absorb neutrons without undergoing fission themselves, such as indium, cadmium, and boron to name a few. When nuclear fission occurs within the nuclear fuel rods, control rods serve to prevent some of the neutrons from striking fissile atoms within the fuel assembly which slows down the nuclear reaction. The speed of fission can be increased by lifting the control rods further out of the reactor vessel, leaving more neutrons available to initiate fission. Lowering the control rods into the reactor vessel can decrease the rate of fission as they absorb more the neutrons that were being emitted by the fuel rods. In the event of an emergency, the control rods will be pushed into the reactor vessel at their maximum depth in order to slow down the rate of fission as much as possible.

4. The Neutron Moderator

A neutron moderator is a material that serves to slow down the speed of fast neutrons so that they will cause a fission reaction when they strike the nucleus of a fissionable atom. As reactor designs are grouped by their moderator type, light water reactors use ordinary water as their neutron moderator but moderators can be made out of many substances. In water moderated reactors, a bluish glow can be seen around the control rods as charged particles are moving faster than the speed of light within the medium. Since the electric field of the particles is unable to keep up with them as they travel, photons are produced in an optical equivalent of a "sonic boom", producing light towards the blue wavelengths of the color spectrum.

5. The Containment Structure

All nuclear reactors in the world are now constructed with a containment building over the reactor vessel to protect the reactor from damage and to physically prevent the release of radiation in the event of a core meltdown. The containment building is a solid concrete or steel shell that is several feet thick that is extremely durable and tests have shown their ability to withstand earthquakes or impacts with aircraft with minimal damage. The containment building is what prevented the release of any significant degree of radiation during the Three Mile Island incident in 1979 and what prevented the damaged reactors at Fukushima Daiichi in Japan from irradiating the populace after being subjected to a massive earthquake. If the infamous Chernobyl reactor had been built with a containment building over the reactor vessel, the effects of the meltdown on the surrounding area would have been negligible.

6. The Steam Generator

The PWR light water reactor design has a steam generator, the BWR design does not. Hot coolant flows from the reactor vessel into pipes that are surrounded by secondary coolant within the steam generator. The pipes containing the hot coolant cause the fluid surrounding them to heat up and begin to boil and generate steam. Heat energy is transferred from the hot pipes into the secondary coolant causing the primary coolant to cool down as it is pumped back into the reactor vessel. The steam generated by the boiling secondary coolant rises and is forced through a moisture separator into the turbine chamber.

7. The Reactor Turbine

Within the reactor turbine, steam is pushed through the center of the turbine which then turns the blades of the turbine assembly as it expands outward. Most turbine designs have a moisture separator after the first high-pressure turbine which separates the condensed water from the steam and forces the steam through a series of low pressure turbines. This is how pressurized steam is converted into mechanical energy.

8. The Generator

The generator coverts mechanical energy into electricity. A rod runs from the generator to the turbine that spins along with the motion of the turbine. Within the generator, the rod is wrapped with a piece of wire that is surrounded by magnets. Electrical current is generated within wires surrounding the magnets on the side of the generator shaft as it spins, which is then sent out of the nuclear power plant.

9. The Condenser

Steam passes into the condenser chamber after passing through the turbine where cool water circulated from a water box causes the steam to condense into water. The water is then circulated within the secondary coolant loop to the steam generator. Then it is used to generate steam from the heated pipes of the primary coolant loop.

This was just a brief summary on the major components of a "typical" nuclear reactor such as a pressurized water reactor, or PWR. On my next entry, I will be taking a look at how reactor designs differ, starting with the different types of light water reactors. The PWR is by no means the final word in nuclear reactor technology.

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