NUCLEAR POWER PLANTS[TRANSCRIPT]
Hello and welcome to the Minnesota Homeland Security and Emergency Management’s online training program: The Basics of Nuclear Power.
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Introduction
Nuclear power is and has been an important source of energy for the United States since the first U.S. nuclear generating plant went online in Shippingport Pennsylvania on December second nineteen fifty seven.
Despite a more than thirty year halt in construction of nuclear power plants the U.S. reliance on nuclear power continues to grow.
For example in nineteen-eighty nuclear power plants accounted for only eleven percent of the country’s electricity generation.
By two-thousand nine that output had risen to nearly twenty percent.
Much of that came from reactors approved for construction that came online in the late seventies and throughout the eighties - which more than doubled US nuclear generating capacity.
Today, the U.S.A. is the world largest producer of nuclear power.
It contains one hundred four nuclear power reactors in thirty one states.
Nuclear Power Plants
Nuclear power plants function in a very similar manner to that of traditional power plants - except that the source of energy comes from the splitting of uranium235 atoms instead of the burning of fossil fuels.
There are three main areas in a nuclear power plant that we are going to discuss in this training.
The reactor, the turbine and generator, and the cooling mechanism.
The reactor vessel is housed in the containment structure.
This where the fission process takes place.
During operation, reactor coolant water flows all around and through the fuel assemblies removing the heat produced by the fission process which creates steam.
Steam from the nuclear reactor then travels through pipes and turns the turbines.
The turbines then turn the generator to make electricity.
The steam that turns the turbines is eventually condensed back into water and is used again.
This is done by drawing in water from a nearby lake or river to cool the steam in a condenser.
The condensed steam or feed water is pumped back - where it is reheated into steam and is again sent to the turbines to make electricity which completes the loop.
The condenser water is then returned to the lake or river from where it was drawn.
Although warmed by this process, the discharged water is not radioactive and is completely safe for the environment.
SAFETY OF NUCLEAR POWER
Even from the earliest years of nuclear power generation, there has been a strong awareness of the potential hazards posed by a release of radioactive materials from a nuclear power plant.
Of course the main safety concern has always been the possibility of an uncontrolled release of radioactive material leading to contamination and consequent radiation exposure offsite.
These concerns are often capitalized on and perpetuated by Hollywood and the media leading to misconceptions about the realities of nuclear power.
The realities are this -there have been three major reactor accidents in the history of civil nuclear power.
Three-mile Island, Chernobyl and Fukushima.
These are the only major accidents that have occurred in over fourteen thousand four hundred cumulative reactor-years of commercial operation spread over thirty-two countries.
One of the common misconceptions is that a release at a nuclear power plant might look something like this
The truth is that a release like this is impossible.
The reason is based on the ratio of the two forms- or isotopes - of uranium – Uranium two-thirty-eight and Uranium two-thirty-five.
It is Uranium two-thirty-five that is used as a fuel in a nuclear power plant. But Uranium two-thirty-eight is far more common.
Natural uranium is made up of only about point seven two percent Uraniumtwo-thirty-fivewhereas the fuel pellets used in a nuclear reactor are enriched to about three to four percent U two-thirty-five.
However to achieve a nuclear detonation that could produce a mushroom cloud would require about ninety percent enriched uranium.
So there just isn’t enough fuel in a nuclear power plant to achieve this form of nuclear chain reaction.
In fact, if there ever were a release at a nuclear power plant,it would be completely invisible and the only way to track it would be through the use of special equipment that is sensitive to radioactivity.
At this point you may be remembering back to the news reports from the disaster at the Fukushima nuclear power plant in Japan where through a series of events following the tsunami an excess of hydrogen in the reactor building resulted in explosions on site.
These were not the result of a nuclear detonation of the uranium fuel inside the reactor.
These were chemical explosions caused by a buildup of hydrogen in the reactor building.
This type of situation can occur if there is damage to the reactor core coupled with an inability to adequately vent excess hydrogen from the building.
The Monticello nuclear generating plant is the same style as the Fukushima plant in Japan.
So back in nineteen-ninety-two,foreseeing such a possibility, Xcel energy added a robust vent to the containment building in Monticello.
This upgrade would allow for the ability to vent excess hydrogen to prevent it from exploding as it did in Japan.
Because the primary concern at a nuclear generating plant is the radiation and radioactive materials involved in nuclear fission, nuclear power plants are carefully designed with multiple overlapping physical barriers to contain these materials.
In addition, there are different styles of nuclear power plants.
For example the Monticello nuclear generating plant is a boiling water reactor while the Prairie Island nuclear generating plant is a pressurized water reactor.
As a result, there are various differences between these types of power plants that we should be aware of including the ways they are engineered to generate electricity and the barriers used to contain the radioactive materials.
Click on the Monticello nuclear generating plant to learn more about boiling water reactors or click on the Prairie Island nuclear generating plant to learn more about pressurized water reactors.
A boiling water reactor such as that found at Monticello, is a two loop system.
Water is boiled directly in the reactor vessel and converted to steam.
As discussed earlier, the steam drives the turbine which then turns the generator.
Because the steam from the reactor has picked up some radioactivity and is directly used to power the turbines, some additional shielding is required in the turbine building.
The other loop in this picture can be seen bringing in cooling water from the Mississippi river to the condenser.
In the boiling water reactor there are 3 primary barriers to radiation release.
The first is the fuel assemblies.
If we look closely, we will see that the nuclear fuel pellets are surrounded and protected by a zircalloy fuel cladding.
The second physical barrier is the reactor pressure vessel which surrounds the core and also prevents the release of radioactive material.
If necessary, control rods can be inserted to stop the fission reaction.
The third barrier is a steel containment vessel.
This whole structure is then housed inside a secondary concrete shield wall as shown in this cut-out picture of a mark one style boiling water reactor.
Click Prairie Island to continue on and learn about pressurized water reactors or click Monticello to go back and review boiling water reactors.
A pressurized water reactor such as that found at Prairie Island is a three loop system.
In the first closed loop, water is heated to a very high temperature in the reactor vessel, but because it is under pressure it doesn’t boil and remains liquid.
This heated water is pumped to the steam generator where it heats water in the second closed loop converting it steam which is used to turn the turbines and generate electricity.
This picture also shows cooling water that is being brought into the condenser from the Mississippi River.
A pressurized water reactor also has three barriers designed to prevent the release of radioactive materials to the environment.
And just like a boiling water reactor the first barrier in a pressurized water reactor is the zircalloyfuel cladding that surrounds the fuel pellets.
The second barrier consists of the reactor pressure vessel that surrounds the core as well as the pressurizer and the steam generator.
If necessary, control rods can be inserted to stop the fission reaction.
Surrounding all of this is a very substantial containment structure which constitutes the third barrier and is made from highly reinforced steel and concrete.
The entire structures of both boiling water reactors and pressurized water reactors are designed to withstand a multitude of natural disasters including earthquakes, fires, floods and the high winds and missiles that would occur with tornadoes.
For an environmental release to occur, each of these formidable barriers would need to be compromised.
Click on either of the power plants for a review or click on NEXT to continue.
Test
By clicking next you will begin the assessment portion of the training module. Please complete each question and click the submit button when you have selected your answer. At the end of the assessment you will receive your score.
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