The above image is of the Limerick Power Plant. It utilises two GE Boiling Water Reactors and generates a nameplate capacity of 2268MW, delivering 19 Million KiloWattHours of electrical power to consumers, at a power delivery efficiency rate of 95%. Click on the image to open it in a new window, and then click on the image again for even larger detail.
POWER GENERATION PROCESS In the previous post in this series, I showed how the low enriched pellets were manufactured, then placed into the thin rods (fuel rods) which were then placed into assemblies with differing numbers of those rods in the assembly, depending upon which type of plant they are to be used in.
These assemblies are then placed into the reactor with what are called control rods, which are manufactured from a different material. With the control rods in place, no reaction can take place. As the control rods are slowly removed, more of the fuel pellets in those fuel rods are exposed and the nuclear reaction takes place. What is generated from this reaction by exposing the fuel rods is heat. The more the control rod is removed, the more pellets in the fuel rods now become exposed, and more heat is generated by the greater reaction.
This controlled reaction makes these plants ideal to generate large volumes of pressurised boiling steam to drive a turbine which then drives the generator. For large amounts of power to be generated, a large generator is required, and these large generators are around 35 feet in length and up to 15 feet across. A large three stage turbine is required to drive that huge weight. The added to the generator makes for a rotating length around 100 to 120 feet long, and can weigh in the vicinity of 200 to 350 tons. To drive that weight, huge amounts of steam are required. All this rotates at around 3600RPM or 60 times a second. That is the scale, and the reason huge amounts of steam are required.
Typically, Nuclear power plants have two reactors, one reactor for each large turbine/generator complex.
The generated heat from the reaction of the exposed low enriched fuel pellets in the fuel rods is used to heat water, and the water is either converted to steam, or can be used to heat another tank of water to steam, depending upon the type of plant.
The two most common forms of generating power are Pressurised Water Reactors (PWR), and Boiling Water Reactors, (BWR) and I will give a basic method of operation for both methods to show you the difference.
PRESSURISED WATER REACTOR.
All the ‘action’ takes place inside the Containment structure. You can see the control rods poking out the top of the reactor vessel on the left there. The water in the reactor is heated but does not boil. It is then pressurised and this then boils the water in the second tank on the right. The steam produced in this second tank is then used to drive the turbine/generator. The steam drives the three stages of the turbine as it slightly cools, and is then recycled through the cooling condensor under the cooling towers, and then the cycle continues. Go to the above image of the Limerick power plant and when it opens click on it again for greater detail, and look at the base of the tower. You’ll see the water there with vapor coming off it, but most of the vapor goes up and out of the stack, harmless water vapor.
The water that drives the turbine is never mixed with the water that cycles through the reactor vessel.
Keep the control rods in mind because I’ll come back to them later.
BOILING WATER REACTOR.
Again it all happens inside the containment structure, only this time there is only the one vessel. The reaction boils the water directly, and the steam drives the turbine/generator, the steam cools and is recycled. You may notice with this method, those control rods come up from underneath.
BWR’s have advanced over the years and have gone through three or four different types. The basic principle is still the same. These are the Advanced BWR, the Simplified BWR, and the Economic Simplified BWR.
Both methods, PWR, and BWR have advantages and disadvantages and these two types are the most prevalent of the U.S. reactors. The BWR’s run at a lower water pressure, and also a lower nuclear fuel temperature.
These are what controls the reaction process. All nuclear power plants have many inbuilt safety measures and these are usually triple redundant, so that if one shut down procedure fails, then a second, or even a third can take over the shut down procedure. This shut down procedure is the complete stopping of the nuclear reaction, by driving those control rods back into the reactor so none of the pellets in any of the rods can still contribute to the reaction process. This is called Scramming the reactor. Typically, the process takes 4 seconds, and once the control rods are back in the pile, there is no further nuclear reaction.
Even the Scram process is triple redundant. The rods are removed from the pile by electric motors, which either remove the rods or drive them back in. If the current fails to the motors, powerful springs drive the rods back into the pile. The third form is possibly where the term Scram originated, but there are a few explanations that it could actually mean. At one of the early reactors, a man was told that if the motors, and the springs failed, then the rods were also held up by a thick rope. His job was to take an axe and cut the rope, hence the term, Safety Control Rod Axe Man (SCRAM) Needless to say the axe was never used. Another explanation says it as Super Critical Reactor Axe Man, and a third has it as Safety Cut Rope Axe Man. Whichever may or may not be true, an axe has never been used, not that it matters, because the term has stuck, and now the shut down procedure is given the name Scramming the reactor.
There are other claims about the term, some saying that the control rods just scrammed back into the reactor, and the U.S. Navy has given it an official title of Super Critical Reaction Abatement Mechanism.
Either way, it is the overarching safety mechanism for all Nuclear reactors now.
There are other types of reactors that are being introduced, and with each new process, that process becomes more inherently safe.
In the next post in this series, I will show you just how many of these reactors there are in the U.S. and in other parts of the World, and how other Countries are moving ahead with new plants while in the U.S. the process seems to have stalled, as people falsely believe that renewable power plants can fill the electrical power generating void, if there is to be one.
Previous posts in this series.