Title: Nuclear Reactors
Course: 2016FA-MCO-115-401 > Section: Module 4: Nuclear

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Nuclear Energy (Reactors)

Basics

To understand the basic science of nuclear energy it is important to remember that all matter is made up of atoms. Atoms are the smallest quantity of an element that can take part in a chemical reaction.  There is a lot of energy in the bonds that hold atoms together [8]. The word nuclear means having to do with the nucleus (center of an atom). A nuclear reaction is simply a reaction that makes a change to the nucleus.

Similarities and Differences with Fossil Fuel

With nuclear energy, heat comes from a (non-carbon dioxide forming) chain reaction that happens in the nucleus of uranium atoms. Fossil fuel power plants burn the fossil fuels for heat to create steam and power a turbine. With fossil fuels there are by-products like carbon dioxide that pollute and cause harm to our environment. There is less waste produced with nuclear power, but the waste that is produced (like spent fuel rods) is dangerous and must be sealed and placed underground for many years. The areas in which this waste are stored must be well-thought out to avoid flood zones, areas of likely seismic activity, and the area must be very secure.

Types of Nuclear Reactions

Nuclear reactions can either happen by an atom splitting apart (into two or more different atoms), called nuclear fission, or by two or more atoms fusing themselves together in a reaction called a fusion reaction. During fission a great deal of energy and radiation is expelled.

The most common way to harness the power of a nuclear reaction is to cause nuclear fission and use the heat energy created from that reaction to create steam. The steam then is used to turn a turbine which is able to produce electricity. The process is similar to that in a fossil fuel burning power plant.

Reactor

To obtain electricity from a nuclear chain reaction a nuclear reactor is used. A reactor is a system that controls and contains a nuclear chain reaction. Nuclear reactors can vary in size and are often used to power submarines and aircraft carriers. Nuclear reactors can be used to produce radioactive isotopes, for medical imaging, and for cancer treatments. In the United States there are some nuclear reactors that are dedicated to research, training, and testing. The reactors dedicated to these fields are not producing electricity for the power grid.

Three Main Components of a Reactor

1. Fuel
2. Moderator
3. Control Rods

Fuel

Nuclear reactors use uranium mixed into ceramic pellets that are inserted into heat resistant zirconium alloy tubes. These tubes are often referred to as rods or fuel rods. Many rods are put together to cause the nuclear chain reaction (fission). This creates energy and radiation and more neutrons (causing even more fission – hence the chain reaction).

Moderator

The fuel rods are usually emerged in water. Water acts as a moderator. It can slow down or moderate the production of energy neutrons formed during fission. This slows the motion of the neutron down enough to allow it to trigger multiple chain reactions.

Control Rods

To control the rate of fission, and to stop it in case of an emergency (or to perform maintenance), control rods are used. Control rods are made up of materials that absorb many neutrons without fissioning.  Control rods can be either partially or completely added amongst the fuel rods; they can either halt or slow down the fission reaction. Typically these can be made of boron or cadmium.

Types of Reactors

There are two main nuclear reactor designs that are used. One is called the Boiling Water Reactor (BWR) and the other is called the Pressurized Water Reactor (PWR) [9].

In a BWR the heat from the split atoms causes water to boil, create steam, and turn a turbine. The turbine powers a generator used to create electricity. Later the steam is collected and converted back into water and returned to the reactor core [10].

In a PWR the water in the reactor tank is under pressure to prevent it from boiling. This water reaches extremely high temperatures and is sent through a steam generator. In the steam generator the water is forced through thousands of small pipes and becomes steam. This steam then powers a turbine. The turbine powers a generator used to create electricity. Steam is then collected in a condenser for reuse as water. The water is now pumped back into the reactor tank and heated again. Water in the primary circulation system never leaves the containment building since it is radioactive from coming into contact with the fuel [10].

Every nuclear power plant has its own unique attributes and can vary slightly in their setup.

Research into other types of reactors and into variations of these reactor types is ongoing. Many improvements are made to reactors and other aspects of nuclear energy as we learn from research and experience.