Greeting...
Increasingly, our country has become a nation of electricity users. We depend on an abundant affordable
supply of energy to power the many machines we use in our complex society. Can you imagine what it would
be like not having electricity in your home? About one-third of our energy resources are used to produce
electricity.
Electricity can be produced in many ways — most of which you already know about. Today, we're
going to talk about one of those ways — nuclear fission. In America, nuclear energy plants are the
second largest source of electricity after coal — producing approximately 20 percent of our
electricity. There is something I want all of you to be aware of: The purpose of a nuclear power plant
is to produce electricity.
While nuclear power plants have many similarities to other types of electricity generating plants, there
are some significant differences.
With the exception of solar, wind, and hydroelectric plants, all others including nuclear convert water
to steam that spins the propeller-like blades of a turbine that spins the shaft of a generator. Inside
the generator coils of wire and magnetic fields interact to create electricity. [Turbine/Generator]
The energy needed to boil water into steam is produced in one of two ways: by burning coal, oil, or gas
(fossil fuels) in a furnace or by splitting certain atoms of uranium in a nuclear energy plant. Nothing
is burned or exploded in a nuclear energy plant. Rather, the uranium fuel generates heat through a
process called fission.
What Is Fission? Where Does It Take Place?
Uranium is an element that can be found in the crust of the Earth. This element, quite abundant in many
areas of the world, is naturally radioactive. Certain isotopes of uranium can be used as fuel in a
nuclear power plant. The uranium is formed into ceramic pellets about the size of the end of your
finger. [Reactor Fuel Assembly] These pellets are
inserted into long, vertical tubes (fuel rods) within the reactor. The reactor is the heart of the
nuclear power plant. Basically, it is a machine that heats water.
A reactor has four main parts: the uranium fuel assemblies, the control rods, the coolant/moderator, and
the pressure vessel. The fuel assemblies, control rods, and coolant/moderator make up what is known as
the reactor core. The core is surrounded by the pressure vessel. [Franklin's Core]
We also have to understand that uranium cannot just be thrown into a reactor the way we shovel coal into
a furnace. The fuel rods, containing the uranium, are carefully bound together into fuel assemblies,
each of which contains about 240 rods. The assemblies hold the rods apart so that when they are
submerged into the reactor core, water can flow between them.
When the uranium atom splits, it releases energy and two or more neutrons from its nucleus. These
neutrons can then hit the nuclei of other uranium atoms causing them to fission. The sequence of one
fission triggering others, and those triggering still more, is called a chain reaction. When the atoms split, they release energy in the form of heat. The
heat is transferred from the reactor core to the water flowing past. That water becomes high-pressure
steam which turns the turbine in the electric generator. [Turbine/Generator]
This next slide shows the basic design of the nuclear fuel steam plant and its various components. [Nuclear Power Plant]
In the past few minutes I've introduced several new terms. Let me explain what each means in the
energy generating process. First, control rods. What are they? How are they used?
The control rods slide up and down in between the fuel assemblies in the reactor core. They control or
regulate the speed of the nuclear reaction by absorbing neutrons. Here's how it works: When the
control rods absorb neutrons, fewer neutrons hit the uranium atoms thus slowing down the chain reaction.
On the other hand, when the core temperature goes down, the control rods are slowly lifted out of the
core, and fewer neutrons are absorbed. Therefore, more neutrons are available to cause fission. This
releases more heat energy.
Just as there are different types of houses and cars, there are different types of nuclear power plants
that generate electricity. The two basic types being used in the United States are the boiling water reactor (BWR) and the pressurized water reactor (PWR). These power plants are
often referred to as light water reactors.
[Pass out Activities 4 – Boiling Water Reactor (BWR) and 5 – Pressurized Water Reactor (PWR). Students can label
components of each type reactor during the discussion.]
Boiling Water Reactor (BWR)
The boiling water reactor operates in essentially the same way as a fossil fuel generating plant. Neither
of these types of power plants have a steam generator. Instead, water in the BWR boils inside the
pressure vessel and the steam water mixture is produced when very pure water (reactor coolant) moves
upward through the core absorbing heat. The water boils and produces steam. When the steam rises to the
top of the pressure vessel, water droplets are removed, the steam is sent to the turbine generator to
turn the turbine. [BWR schematic]
Pressurized Water Reactor (PWR)
The pressurized water reactor differs from the BWR in that the steam to run the turbine is produced in a
steam generator. Water boils at 212°F or 100°C. If a lid is tightly placed over a pot of boiling
water (a pressure cooker), the pressure inside the pot will increase because the steam cannot escape. As
the pressure increases, so does the temperature of the water in the pot. In the PWR plant, a pressurizer
unit keeps the water that is flowing through the reactor vessel under very high pressure to prevent it
from boiling. The hot water then flows into the steam generator where it is converted to steam. The
steam passes through the turbine which produces electricity. About two-thirds of the reactor power
plants in the U.S. are of the PWR type. [PWR schematic]
Answers to Questions from "Nuclear Reactors/Energy Generation" Unit Outline:
| 1. |
Q: |
Is there a nuclear power plant near where you live? What type is
it? |
| A: |
Example: 40 miles south of Annapolis, MD — Calvert Cliffs 1 & 2 (PWR).
|
| 2. |
Q: |
Why don't boiling water reactors have steam generators?
|
| A: |
Because the water is boiled inside the pressure vessel and the steam is used
directly to turn the turbine. |
| 3. |
Q: |
What is the purpose of a "cooling tower"? |
| A: |
To remove excess heat from the reactor circulating water system. |
| 4. |
Q: |
What percentage of our electricity in the U.S. is produced in nuclear
power plants? |
| A: |
Approximately 20 percent. |
| 5. |
Q: |
Name the two types of reactor power plants in operation the U.S. What
are the basic differences? |
| A: |
a. |
Boiling water reactor (BWR) — water is boiled in the pressure vessel and the steam is
used directly to turn the turbine. |
| |
b. |
Pressurized water reactor (PWR) — water flows through a steam generator where it is
heated to produce steam that then flows to the turbine to generate electricity. |
Turbine and Generator
Reactor Fuel Assembly
Where Does Fission Take Place? – Franklin's
Core
Chain Reaction
Nuclear Power Plant
Boiling Water Reactor
Pressurized Water Reactor