Nuclear Reactor

Nuclear Reactor

Nuclear power provides about 20% of the US electricity supply. To understand how it works we need to understand the nucleus of an atom.

We tend to think of atoms as solid balls linked together with chemical bonds forming molecules. The reality is that the “hard” shell that represents an atom is a representation of the electron that is whizzing around the central placed nucleus. The “solid” ball is a poor representation; instead think of a ping-pong ball. Most of the ball is empty; in the case of an atom the shell(s) (mass of electrons) does not contribute much to the mass, as electrons are very low in mass. The bulk of the weight is in the nucleus where the proton(s) and neutron(s) reside. These each have a mass of 1 atomic mass units (also called Daltons in the US). So hydrogen the simplest atom has 1 proton and 1 electron. It is the number of protons that determines the element so 1 proton is hydrogen represented as 1H. If there is also a neutron then the element is still hydrogen but it is a different isomer. We call these deuterium atoms and if we produce water from it we have “heavy water”.

There are many elements that are radioactive. For some, only certain of the isotopes are radionuclide (they are radioactive). However, some will have very long half-lives, others decay very quickly

Nuclear Plants are scattered around the world, but its only the rich nations that can afford them! We are not happy that some “Axis of Evil” countries are joining the nuclear club.

We have a nuclear engineering degree at Penn State and a small nuclear reactor!

The nuclear reactor consists of a reactor with fuel rods, control rods and a moderator. The heat produces steam which is heat exchanged (so the water is not radioactive) before going through the turbine to turn the generator which produces electricity. A cooling tower helps to cool the low temperature steam before it is returned to the reactor. A containment structure prevents radioactivity from escaping.

Basically when the 235 U is induced into decay via capturing a neutron, the atom breaks apart violently producing 2 fragments that are the new atoms and several neutrons. Thus, when at the critical state 1 neutron is allowed to induce another 235 U to decay, while other neutrons are modulated such that they do not have the energy to induce decay. The fragments are moving quickly, they have lots of kinetic energy or in the case of atoms and molecules we can say they have a high temperature. Thus, we can create steam as discussed before to turn a turbine. We have about 104 reactors in the US producing electricity, far more reactors than any other single country! The oldest one running is the same age as I am! They are licensed for 40 years with a 20-year extension possible. As most were built in the 70’s & 80’s soon we are looking at dismantling a number of nuclear reactors.

Source: Public Education and Outreach

Uranium

Uranium is a very heavy metal which can be used as an abundant source of concentrated energy. Uranium occurs in most rocks in concentrations of 2 to 4 parts per million and is as common in the Earth’s crust as tin, tungsten and molybdenum. Uranium occurs in seawater, and can be recovered from the oceans. Uranium was discovered in 1789 by Martin Klaproth, a German chemist, in the mineral called pitchblende. It was named after the planet Uranus, which had been discovered eight years earlier. Uranium was apparently formed in supernova about 6.6 billion years ago. While it is not common in the solar system, today its slow radioactive decay provides the main source of heat inside the Earth, causing convection and continental drift. The high density of uranium means that it also finds uses in the keels of yachts and as counterweights for aircraft control surfaces, as well as for radiation shielding. Uranium has a melting point is 1132°C. The chemical symbol for uranium is U.

The Uranium Atom

On a scale arranged according to the increasing mass of their nuclei, uranium is the heaviest of all the naturally-occurring elements (Hydrogen is the lightest). Uranium is 18.7 times as dense as water. Like other elements, uranium occurs in several slightly differing forms known as ‘isotopes’. These isotopes differ from each other in the number of uncharged particles (neutrons) in the nucleus. Natural uranium as found in the Earth’s crust is a mixture largely of two isotopes: uranium-238 (U-238), accounting for 99.3% and uranium-235 (U-235) about 0.7%. The isotope U-235 is important because under certain conditions it can readily be split, yielding a lot of energy. It is therefore said to be ‘fissile’ and we use the expression ‘nuclear fission’. Meanwhile, like all radioactive isotopes, they decay. U-238 decays very slowly, its half-life being about the same as the age of the Earth (4500 million years). This means that it is barely radioactive, less so than many other isotopes in rocks and sand. Nevertheless it generates 0.1 watts/tonne as decay heat and this is enough to warm the Earth’s core. U-235 decays slightly faster.

Energy from the uranium atom

The nucleus of the U-235 atom comprises 92 protons and 143 neutrons (92 + 143 = 235). When the nucleus of a U-235 atom captures a moving neutron it splits in two (fissions) and releases some energy in the form of heat, also two or three additional neutrons are thrown off. If enough of these expelled neutrons cause the nuclei of other U-235 atoms to split, releasing further neutrons, a fission ‘chain reaction’ can be achieved. When this happens over and over again, many millions of times, a very large amount of heat is produced from a relatively small amount of uranium. It is this process, in effect “burning” uranium, which occurs in a nuclear reactor. The heat is used to make steam to produce electricity. Nuclear power stations and fossil-fuelled power stations of similar capacity have many features in common. Both require heat to produce steam to drive turbines and generators. In a nuclear power station, however, the fissioning of uranium atoms replaces the burning of coal or gas.

Inside the reactor

In a nuclear reactor the uranium fuel is assembled in such a way that a controlled fission chain reaction can be achieved. The heat created by splitting the U-235 atoms is then used to make steam which spins a turbine to drive a generator, producing electricity. The chain reaction that takes place in the core of a nuclear reactor is controlled by rods which absorb neutrons and which can be inserted or withdrawn to set the reactor at the required power level. The fuel elements are surrounded by a substance called a moderator to slow the speed of the emitted neutrons and thus enable the chain reaction to continue. Water, graphite and heavy water are used as moderators in different types of reactors. Because of the kind of fuel used (ie the concentration of U-235, see below), if there is a major uncorrected malfunction in a reactor the fuel may overheat and melt, but it cannot explode like a bomb. A typical 1000 megawatt (MWe) reactor can provide enough electricity for a modern city of up to one million people.

 

Plutonium

Plutonium-238 (Pu-238)
Half-life: 87.7 years

Plutonium-239 (Pu-239)
Half-life: 24,110 years

Plutonium-240 (Pu-240)
Half-life: 6,564 years

Mode of decay: Alpha particles

Chemical properties: Solid under normal conditions, plutonium can form compounds with other elements.

What is it used for?

Plutonium-238 generates significant heat through its radioactive decay process, which makes it useful as a heat source for sensitive electrical components in satellites, as a well as a power source (for example, battery power) for satellites. Plutonium-239 is used to make nuclear weapons. Pu-239 and Pu-240 are byproducts of nuclear reactor operations and nuclear bomb explosions.

Where does it come from?

Plutonium is created from uranium in nuclear reactors. It is a by-product of nuclear weapons production and nuclear power operations.

What form is it in?

Plutonium is a solid material that is fashioned into rods for use in nuclear reactors and into ceramic ?buttons? for use in satellite systems.

What does it look like?

Plutonium is a silvery-gray metal that becomes yellowish when exposed to air. Most plutonium in the environment is in the form of microscopic particles that are the remnants of nuclear weapons testing and nuclear reactor accidents.

How can it hurt me?

Because it emits alpha particles, plutonium is most dangerous when inhaled. When plutonium particles are inhaled, they lodge in the lung tissue. The alpha particles can kill lung cells, which causes scarring of the lungs, leading to further lung disease and cancer. Plutonium can enter the blood stream from the lungs and travel to the kidneys, meaning that the blood and the kidneys will be exposed to alpha particles. Once plutonium circulates through the body, it concentrates in the bones, liver, and spleen, exposing these organs to alpha particles. Plutonium that is ingested from contaminated food or water does not pose a serious threat to humans because the stomach does not absorb plutonium easily and so it passes out of the body in the feces.

For more information about plutonium, see the Public Health Statement by the Agency for Toxic Substances and Disease Registry at http://www.atsdr.cdc.gov/toxpro2.html, or visit the Environmental Protection Agency at http://www.epa.gov/rpdweb00/radionuclides/plutonium.html.

For more information on protecting yourself before or during a radiologic emergency, see CDC’s fact sheet titled ?Frequently Asked Questions (FAQs) About a Radiation Emergency? at emergency.cdc.gov/radiation/emergencyfaq.asp, and ?Sheltering in Place During a Radiation Emergency,? at emergency.cdc.gov/radiation/shelter.asp.

For information about possible countermeasures for internal contamination with plutonium, please see CDC’s fact sheet on DTPA.

Source: The Centers for Disease Control and Prevention (CDC) protects people’s health and safety by preventing and controlling diseases and injuries; enhances health decisions by providing credible information on critical health issues; and promotes healthy living through strong partnerships with local, national, and international organizations.

 How nuclear power plants work

Just as many conventional thermal power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear power plants convert the energy released from the nucleus of an atom, typically via nuclear fission.

When a relatively large fissile atomic nucleus (usually uranium-235 or plutonium-239) absorbs a neutron, a fission of the atom often results. Fission splits the atom into two or more smaller nuclei with kinetic energy (known as fission products) and also releases gamma radiation and free neutrons. A portion of these neutrons may later be absorbed by other fissile atoms and create more fissions, which release more neutrons, and so on.

This nuclear chain reaction can be controlled by using neutron poisons and neutron moderators to change the portion of neutrons that will go on to cause more fissions. Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if unsafe conditions are detected.

A cooling system removes heat from the reactor core and transports it to another area of the plant, where the thermal energy can be harnessed to produce electricity or to do other useful work. Typically the hot coolant will be used as a heat source for a boiler, and the pressurized steam from that boiler will power one or more steam turbine driven electrical generators.

There are many different reactor designs, utilizing different fuels and coolants and incorporating different control schemes. Some of these designs have been engineered to meet a specific need. Reactors for nuclear submarines and large naval ships, for example, commonly use highly enriched uranium as a fuel. This fuel choice increases the reactor’s power density and extends the usable life of the nuclear fuel load, but is more expensive and a greater risk to nuclear proliferation than some of the other nuclear fuels.

A number of new designs for nuclear power generation, collectively known as the Generation IV reactors, are the subject of active research and may be used for practical power generation in the future. Many of these new designs specifically attempt to make fission reactors cleaner, safer and/or less of a risk to the proliferation of nuclear weapons. Passively safe plants (such as the ESBWR) are available to be built and other designs that are believed to be nearly fool-proof are being pursued. Fusion reactors, which may be viable in the future, diminish or eliminate many of the risks associated with nuclear fission.

There are two types of nuclear power in current use:

The Radioisotope thermoelectric generator produces heat through passive radioactive decay. Some radioisotope thermoelectric generators have been created to power space probes (for example, the Cassini probe), some lighthouses in the former Soviet Union, and some pacemakers. The heat output of these generators diminishes with time; the heat is converted to electricity utilising the thermoelectric effect.

Nuclear fission reactors produce heat through a controlled nuclear chain reaction in a critical mass of fissile material. All current nuclear power plants are critical fission reactors, which are the focus of this article. The output of fission reactors is controllable. There are several subtypes of critical fission reactors, which can be classified as Generation I, Generation II and Generation III.