What is heavy water?

Normal water is made of two hydrogen atoms and an oxygen atom covalently bonded together. This gives the famous H20 formula. Heavy water has exactly the same structure, except the hydrogen atoms are isotopes of hydrogen called 'Deuterium'. Standard hydrogen has one proton in its nucleus (Relative Molecular Mass = 1), deuterium has one proton AND one neutron in it nucleus, (RMM = 2) hence it is 'heavier' than normal hydrogen. This leads to the water being heavier. Mass of water = 18, mass of heavy water = 20.

Why is heavy water poisonous?

That’s a very interesting question. There have been no detailed studies carried out that I am aware of, but decades ago a crude experiment was done in which mice were given water which had various percentages of heavy water, which is water in which both hydrogen atoms were replaced by deuterium. Low percentages of heavy water didn’t have noticeable effects, but more than 20% heavy water did have adverse health effects and mice given 80% heavy water died within days. In another experiment, bean plants grown from seed given increasing fractions of heavy water showed stunted growth compared with control plants given normal water.

The reason for these adverse effects is that replacing hydrogen with its heavier isotope deuterium slows down the rate of any chemical reaction in which the chemical bond to the hydrogen atom is broken. This includes a great many chemical reactions occurring in biological systems, and not just those involving water; the hydrogen atoms from water end up in a number of other biomolecules, so any process involving these hydrogen atoms will also be slowed down. Thus the heavy water acts like a brake on a large number of metabolic processes.

The amount by which an isotopic substitution like this slows down a chemical reaction is called a kinetic isotope effect. Such effects are a major tool in the study of chemical reactions, including enzymatic reactions. Deuterium isotope effects can be as large as 6 or 7, which means that the reaction rate is 6 or 7 times slower when deuterium is substituted for hydrogen. In rare cases where a quantum mechanical effect called tunneling occurs in the reaction, deuterium isotope effects of 20 or more have been observed.

The major reason for the difference in the rates of the chemical reactions involving the two isotopes of hydrogen is the difference in their masses. Deuterium atoms have an atomic mass of 2, which is double that of normal hydrogen. Of course other atoms have isotopes also, and your comment that for isotopes of other elements these effects would not be present is perceptive but not quite completely correct. Isotope effects do occur with the heavier elements but they are much smaller. For instance if we replaced the oxygen of water (which is normally oxygen 16) with oxygen 18 we end up with a water molecule having the same mass as in the heavy water discussed above, but in which the isotope effects on its reactions would be very small. This is because changing the oxygen atom’s mass from 16 to 18 is a much smaller fractional change than the doubling of mass of hydrogen when we go from hydrogen to deuterium. Oxygen-18 isotope effects are never more than about 1.07, or 7 % slower with the heavier isotope.

Heavy water and uses

Preparation of heavy water: Heavy water is prepared either by prolonged electrolysis or by fractional distillation of ordinary water.

Electrolysis of ordinary water: Multistage electrolysis of ordinary water containing NaOH gives heavy water. The cell used for electrolysis, contains a cylindrical vessel made of steel as cathode while a perforated cylindrical sheet acts as the anode. The electrolysis is carried out in different stages.

Different stages

First stage: Thirty electrolytic cells are used are used in the first stage. Each cell is filled with about 3% solution of NaOH. Electrolysis is carried out for about 72 hrs using a current of 110 volts till the volume reduces to about l/6th of the original volume taken. Gases like 1H2 and O2 are evolved and discarded. The volume left contains about 2.5% of heavy water.

Second stage: The residue left from the first stage is electrolyzed using six electrolytic cells. The gases evolved are burnt and water formed is returned to the first stage cell. The residual liquid contains about 12% of heavy water.

Third stage: Electrolysis of residue of second stage is carried out in this stage. The gases evolved are burnt to get water that is fed to 2nd stage cells. The content of heavy water is raised to about 60%.

Fourth stage: This stage involves the electrolysis of residue of third stage and here; nearly 99% of heavy water is obtained. The gases evolved are burnt as usual, and sent to third stage cells.

Fifth stage: The 99% heavy water is made free from alkali and other impurities by distillation. The distillate is further electrolysed in the fourth stage. Here, the gases evolved are D2 and O2, which are burnt to get 100% pure heavy water.

Fractional distillation of ordinary water: In the partial separation of heavy water from ordinary water, advantage is taken of the small difference in the boiling points of protium oxide (373.2 K) and deuterium oxide (374.3 K). Since the difference in boiling points is very small, a long fractionating column (about 13m) is used for distillation and the process is repeated several times. The lighter fraction (H2O) is distilled first while heavier fraction (D2O) is left behind. The heavier fraction becomes rich in D2O.

Properties of Heavy Water

Physical properties

Heavy water is colourless, tasteless and odourless liquid.

It has all higher values for physical constants than the corresponding values of ordinary water.

Physical Properties of Water and Heavy Water at 298 K

Chemical Properties of Heavy Water: Although heavy water is chemically similar to ordinary water, chemical

reactions of heavy water are slower than those of ordinary water.

Reaction with metals

Alkali metals and alkaline Earth metals react with heavy water to form heavy hydrogen (D2).

Reaction with metal oxides

D2O reacts slowly with metal oxides to form corresponding deuteroxides

Reaction with non-metallic oxides: Non-metallic oxides react with D2O to form corresponding deutero acid,

Reaction with carbides, nitrides, phosphides, arsenides etc.

Electrolysis

A solution of heavy water containing Na2CO3, when electrolysed evolve heavy hydrogen at cathode

Exchange reactions

Compounds having labile hydrogen react with heavy water when hydrogen is exchanged by deuterium partially or completely.

Deutero-hydrates

Heavy water like ordinary water may be associated with salts as water of crystallization, giving deutero hydrates, e.g., NaSO4.10D2O, CuSO4.5D2O, MgSO4.D2O, etc.

Deuterolysis

Water brings hydrolysis of certain inorganic salts. D2O gives similar reactions, which are termed deuterolysis.

Biological and physiological effects

Heavy water of high concentration retards the growth of plants and animals. For example, tobacco seeds do not grow in heavy water. Pure heavy water kills small fishes, tadpoles and mice when fed. Heavy water has germicide and bactericide properties too. Water containing small quantity of D2O acts as a tonic and stimulates vegetable growth. Certain moulds have been found to develop better in heavy water in comparison to ordinary water.

Uses of Heavy Water

The following are the important uses of heavy water:

As a neutron moderator

Fission in uranium-235 is brought by slow speed neutron. Heavy water is used for this purpose in nuclear reactors as moderators.

For the preparation of deuterium

Heavy water produces deuterium on electrylosis or by its decomposition with metals.

As a tracer compound

Heavy water is used as a tracer compound for studying various reaction mechanisms. It has also been used for studying the structure of some oxyacids of phosphorus such as H3PO2 and H3PO3, to determine the number of ionizable hydrogen atoms.

Why is heavy water used as a moderator in a nuclear reactor?

Moderator is required in a thermal reactor to slow down the neutrons produced in the fission reaction to .025 ev so that the chain reaction can be sustained. Different moderators normally in use are Heavy Water, Graphite, Beryllium and Light water. Heavy Water is an excellent moderator.as it has excellent slowing down power and low absorption cross section for neutrons.

Heavy Water (D2O) is a compound of an isotope of hydrogen called heavy hydrogen or Deuterium (D) and oxygen. Deuterium has an atomic mass of 2, as against 1 for normal hydrogen (H) due to presence of an extra neutron in the nucleus. Deuterium is present in hydrogen and hydrogen bearing compounds like water, hydrocarbons, etc. and has a small natural occurrence (D/D+H) of about 140 to 150 ppm. So it is necessary to process large quantities of the low concentration feed stock to produce the final product which is enriched to the reactor grade i.e. 99.8 mole %. Heavy Water has great similarity in its physical and chemical properties to ordinary water. But its nuclear properties display a significant variation which makes it an extremely efficient material for use as moderator in a nuclear reactor.

A good neutron moderator like heavy water is a material full of atoms with light nuclei which do not easily absorb neutrons. The neutrons strike the nuclei and bounce off. In this process, some energy is transferred between the nucleus and the neutron. More energy is transferred per collision if the nucleus is lighter, see elastic collision. After sufficiently many such impacts, the velocity of the neutron will be comparable to the thermal velocities of the nuclei; this neutron is then called a thermal neutron.

What exactly is heavy water?

Heavy water and its importance to nuclear technology both have to do with isotopes of chemical elements.

Elements - the basic building blocks of chemistry, like hydrogen, oxygen, and uranium - are made up of single atoms. Atoms in turn, are made of a nucleus containing protons and neutrons, and a cloud of electrons loosely orbiting that nucleus.

Each element is characterized by its atomic number - how many protons are contained in the atom's nucleus. All hydrogen atoms contain one proton, all oxygen atoms contain eight protons, and all uranium atoms contain 92 protons.

While the number of protons is the same for all atoms of a particular element, different atoms can have different numbers of neutrons. Different isotopes of elements have different numbers of neutrons.

The most common isotope of hydrogen is called protium, or hydrogen-1. The 1 refers to the total number of protons and neutrons together in a particular isotope. Hydrogen-1 atoms are made up of a just a single proton and no neutrons.

A much rarer isotope of hydrogen is hydrogen-2, or deuterium. Hydrogen-2 atoms are made up of one proton and one neutron.

Water is made up of two hydrogen atoms and one oxygen atom, bonded together. In light water - by far the most abundant type of water in nature - the two hydrogen atoms are both of the hydrogen-1 isotope. In heavy water, the hydrogen atoms are both of the hydrogen-2 isotope.

The reason heavy water is important in some types of nuclear reactors also has to do with different isotopes. Uranium has two main naturally occurring isotopes - uranium-235, with 92 protons and 143 neutrons, and uranium-238, with 92 protons and 146 neutrons.

Those three neutrons make a huge difference. Uranium-238 cannot sustain a nuclear chain reaction, but uranium-235 can.

In naturally occurring uranium, there is vastly more of the uranium-238 isotope than of the uranium-235 isotope. The latter makes up only a fraction of a percent of the overall mass of the ore.

Most nuclear reactors require a much higher percentage of uranium-235 in order to maintain a nuclear reaction. This is where uranium enrichment is important - giant rows of centrifuges slowly increase the amount of uranium-235 in a sample of uranium, allowing it to be used in power plants or weapons.

However, some reactors that use heavy water can use unrefined uranium as a fuel, removing the expensive and time consuming enrichment process. These reactors also tend to produce more plutonium as a waste product that can be used in weapons.

This is why heavy water reactors are a concern, and why we are talking about heavy water this week.

Light Water Reactors

The nuclear fission reactors used in the United States for electric power production are classified as "light water reactors" in contrast to the "heavy water reactors" used in Canada. Light water (ordinary water) is used as the moderator in U.S. reactors as well as the cooling agent and the means by which heat is removed to produce steam for turning the turbines of the electric generators. The use of ordinary water makes it necessary to do a certain amount of enrichment of the uranium fuel before the necessary criticality of the reactor can be maintained.

The fission of a U-235 nucleus in one fuel rod releases an average of 2.4 fast neutrons per fission. These neutrons are slowed down or "moderated" by the water between fuel rods, increasing the cross-section for neutron capture and fission by a U-235 nucleus in a neighboring fuel rod.

The two varieties of the light water reactor are the pressurized water reactor (PWR) and boiling water reactor (BWR).

Uranium Enrichment: Natural uranium is only 0.7% U-235, the fissionable isotope. The other 99.3% is U-238 which is not fissionable. The uranium is usually enriched to 2.5-3.5% U-235 for use in U.S. light water reactors, while the heavy water Canadian reactors typically use natural uranium. Even with the necessity of enrichment, it still takes only about 3 kg of natural uranium to supply the energy needs of one American for a year.

Uranium enrichment has historically been accomplished by making the compound uranium hexaflouride and diffusing it through a long pathway of porous material (like kilometers!) and making use of the slightly higher diffusion rate of the lighter U-235 compound. There have been tests of centrifugal separators, but modern efforts are directed toward laser enrichment procedures.

The uranium fuel for fission reactors will not make a bomb; it takes enrichment to over 90% to obtain the fast chain reaction necessary for weapons applications. Enrichment to 15-30% is typical for breeder reactors.

Uranium Diffusion Enrichment: To produce the highly enriched uranium-235 needed for the development of nuclear weapons, a huge diffusion plant was built during World War II at Oak Ridge, Tennessee. Two other massive

plants for uranium enrichment were built at Paducah, KY and Portsmout, OH after the war. The compound uranium hexafluoride was produced and allowed to diffuse through thousands of stages of porous material, making use of the fact that the slightly lighter U-235 compound would diffuse faster than the U-238 compound. While electric power reactors require only enrichment from the 0.7% of natural uranium ore to about 3% U-235, the weapons applications required enrichment to over 90% U-235. Part of the enriched uranium was used to breed plutonium-239 for the more widely used plutonium devices.

Heavy Water Reactors: Nuclear fission reactors used in Canada use heavy water as the moderator in their reactors. Since the deuterium in heavy water is slightly more effective in slowing down the neutrons from the fission reactions, the uranium fuel needs no enrichment and can be used as mined. The Canadian style reactors are commonly called CANDU reactors.

Heavy water (D2O) is 10% heavier than ordinary water and has a neutron moderating ratio 80 times higher than ordinary water. As of January 2002, 32 of the 438 nuclear reactors in operation around the world were of CANDU type.

The CANDU (CANada Deuterium Uranium) nuclear reactor got its name because this heavy water reactor design was developed in Canada. Deuterium is the primary element in heavy water and uranium is the fuel used in this reactor class.

All the 22 reactors in Canada, some of which are offline for refurbishment, are of the CANDU design. Other nations with CANDU and “CANDU derivatives,” or generic heavy water

moderated reactors, include: India, South Korea, Romania, Pakistan, Argentina, and China. The 47 CANDU and derivatives worldwide comprise 11 percent of the 423 reactors worldwide.

It is estimated that power plants using the CANDU design generate more than 23,000 megawatts, about 21 percent of the electricity produced by nuclear energy. A megawatt is generally enough to power 750 average-sized homes.

CANDU Reactors Differ from Light Water Reactors: Heavy water nuclear reactors and light water nuclear reactors differ in how they create and manage the complex physics of nuclear fission or atom-splitting, which produces the energy and heat to make steam to drive generators. Nuclear reactors in use in the U.S. are all light water designs.

The major differences between light-water reactors and CANDU/heavy water moderated design are:

Core – The core of a CANDU reactor is kept in a horizontal, cylindrical tank called a calandria. Fuel channels run from one end of the calandria to the other.

Each channel within the calandria has two concentric tubes. The outer one is called the calandria tube and the inner one is called the pressure tube. The inner tube holds the fuel and pressurized heavy water coolant. This design allows refueling during operation.

By contrast, the core of a light-water reactor is vertical and contains vertical fuel assemblies, which are bundles of metal tubes filled with fuel pellets. The reactor core is kept in a containment vessel.

Fuel – Unlike other nuclear reactors that are designed to use enriched uranium fuel and light water (H20) as a moderator, CANDU reactors use non-enriched (natural) uranium oxide as fuel and heavy water as a moderator.

Basics of uranium fuel processing for nuclear power

Moderator – Moderator is the material in the reactor core that slows down the neutrons released from fission so they cause more fission and sustain the chain reaction. The moderator in light-water reactors is ordinary water, but in the CANDU reactor, the moderator is heavy water or deuterium oxide, which has a chemical formula of D2O.

Unlike ordinary water, with its familiar chemical composition of H20, heavy water includes two atoms of deuterium. Deuterium (D20) has a neutron at its center, unlike ordinary hydrogen, which in its most common form has no neutron and a proton.

Coolant – Coolant circulates through a nuclear reactor core to transfer the heat from it and prevent a melt-down that would halt energy production. In light-water reactors the water moderator functions also as primary coolant. The CANDU reactor uses either light or heavy water coolant.

How a CANDU Reactor Works to Make Electricity

A CANDU reactor core is cooled by heavy water or deuterium oxide, D2O, which also serves as the moderator.

In a closed loop, the heavy water coolant is pumped through the reactor core’s tubes containing the fuel bundles, picking up heat generated from the nuclear fission taking place in the core.

The heavy water coolant loop passes through steam generators where the heat from the heavy water boils ordinary water into high-pressure steam.

The heavy water, now cooler, is circulated back to the reactor as the closed loop cooling cycle continues.

The high-pressure steam from the steam generator is piped outside the reactor containment building to power conventional turbines, which drive generators to produce electricity which is distributed to the grid. In this way, the nuclear reactor is separate from the equipment used to produce electricity.

The steam coming out of the turbine is condensed back into water and is pumped back into the steam generator.

Introduction to Nuclear Power - Uranium Atoms Are Split to Release Fission Energy

Nuclear power plants contain reactors that create controlled chain-reaction fission, a process that continuously splits the nuclei of uranium atoms. This process produces a lot of energy, radiation, and very high heat.

Nuclear power plants harness the energy released by fission and put it to use to drive generators that produce electricity. Although nuclear power contributes only about 20 percent of

the electricity generated in the United States, the nation’s nuclear capacity is the highest of any other country – 101 gigawatts in 2010.

Nuclear reactors have these components in common:

Fuel – Uranium, a radioactive, heavy metal ore, is the most common fuel for nuclear reactors. Following the enrichment process, uranium becomes a very concentrated fuel.

A commercial nuclear reactor requires thousands of pounds of enriched uranium fuel in order to operate. Civilian nuclear power plants in the U.S. purchase approximately 50 million pounds of uranium (U3O8 equivalent) fuel annually, the majority of which comes from overseas.

Uranium is mined in locations worldwide, primarily in Kazakhstan, Canada, Australia and Africa. The United States is among the top 10 producers of uranium.

Control Rods – Made from neutron-absorbing material such as cadmium, hafnium, or boron, control rods are inserted or withdrawn from the core to control

the rate of reaction or to stop it if necessary.

Moderator – Material in the reactor core which slows down the neutrons released from fission so they cause more fission. The moderator is usually ordinary (light) water, but may be heavy water (D20) or graphite.

Coolant – Liquid or gas that circulates through the core to transfer the heat from it. In light water reactors the water moderator also functions as primary coolant.

Containment – Nuclear reactors are encased in heavily reinforced concrete structures to prevent radioactivity from escaping into the atmosphere.

Basic Process of Nuclear Energy

Nuclear physics is very technical, but the basic process for producing electricity with nuclear power is as follows:

The reactor core produces heat and radioactivity in a process called fission, commonly known as atom-splitting. Inside the reactor core is uranium nuclear fuel. As the nuclei of the uranium split they release neutrons. When the neutrons hit other uranium atoms, those nuclei also split, releasing their neutrons to strike other atoms, causing more fission. This continuous atom-splitting is a chain reaction.

The heat from controlled fission reactions is used to produce steam from water, either directly as in the boiling water reactor (BWR), or indirectly as in the pressurized water reactor (PWR), which contains a steam generator.

The steam drives a turbine that powers a generator.

The generator produces electricity that is distributed to the power grid.

Nuclear Reactor Types: Worldwide, various types of nuclear power reactors are used. However, the most common types are pressurized water reactors (PWR) and boiling water reactors (BWR), which are classified as light water reactors. In the United States, PWR and BWR are the only two types of commercial nuclear power plants in operation.

Boiling water reactor (BWR) – In this type of reactor, fission produces heat that boils water in the reactor core. Steam from the boiling water powers a turbine that drives a generator to produce electricity. The reactors at northeastern Japan’s Fukushima Naiishi plant damaged in the March 2011 earthquake and tsunami are BWRs.

Pressurized water reactor (PWR) – This type of reactor is the most common for producing energy. It uses water as coolant and moderator, an agent that helps control the speed of fission. In the closed primary coolant system the water, heated by thermal energy from fission while passing through the core, is kept under high pressure and therefore it doesn’t boil. Steam is produced in a secondary coolant loop and is used to power a turbine that drives an electric generator.

CANDU and heavy water moderated reactors – These designs use heavy water as moderator. The heavy water – with deuterium replacing the two hydrogen atoms – as moderator slows down neutrons in the fission process and allows use of natural uranium, rather than enriched uranium as a fuel.

Pebble bed modular reactor – A high temperature reactor that uses helium coolant and fuel encased in spheres of graphite and silicon carbide to ensure fission product containment and resistance to meltdown.

Nuclear Power, Pros and Cons (What are the advantages and disadvantagaes of nuclear energy?)

In the wake of the Fukushima nuclear disaster that struck Japan in 2011, nuclear energy has come under renewed scrutiny worldwide. Though there are many advantages to nuclear power, a close analysis of the history of uranium-based energy production reveals a darker side to nuclear energy.

Nuclear Energy: Just the Facts

Nuclear power plants have been around since 1951, when the Experimental Breeder Reactor

I (EBR-I) in Idaho produced enough electricity to illuminate four 200-watt light bulbs. Larger, commercial-scale nuclear plants were soon built throughout the United States, Canada, the Soviet Union and England.

A typical nuclear reactor uses enriched uranium -- usually uranium 235 or plutonium 239 -- to generate power. The radioactive uranium is formed into long rods that are submerged into water; the rods of uranium heat the water, creating steam, which then drives a steam turbine. The movement of the steam turbines is what generates electricity. (The plumes of water vapor seen rising from the large cooling towers of nuclear power plants are just harmless steam.)

Currently, there are over 430 nuclear power plants in service all around the world, and just over 100 in the United States. (Because plants go online or offline regularly, the exact number changes yearly.) Nuclear power provides about 15 percent of the world's electricity, and about 20 of the electricity in the United States.

France, Japan and the United States are the largest users of nuclear power, generating over half of the total nuclear power available worldwide.

Advantages of Nuclear Power

Nuclear energy generates electricity very efficiently when compared to coal-generated power plants. It takes millions of tons of coal or oil, for example, to duplicate the energy production of just one ton of uranium, according to some estimates. And because coal and oil combustion is a major contributor to greenhouse gases, nuclear power plants don't contribute to global warming and climate change as much as coal or oil.

Some analysts have pointed out that another advantage to nuclear power is the distribution of uranium across the Earth. There isn't one global center of uranium mining -- no "Mideast of uranium" exists. Many of the countries that do mine uranium, like Australia, Canada and the United States, are relatively stable, so uranium supplies aren't as vulnerable to political or economic instability like oil can be.

In Case of Nuclear Accident ...

When things work exactly like they're supposed to, nuclear energy is a very safe source of power. Trouble is, things don't always work out that way in the real world. A partial meltdown at Three Mile Island in Pennsylvania in 1979 released radiation into the atmosphere; cleanup costs topped $900 million dollars.

In 1986, a flawed reactor design at the Chernobyl nuclear power plant in the Soviet Union caused an explosion in the plant. Nuclear radiation was released for several days, resulting in a major disaster that killed hundreds of people throughout the region. And in 2011, the Fukushima reactor in Japan was hit by an earthquake and a tsunami, causing another huge environmental disaster.

Despite the assurances of nuclear engineers and proponents of nuclear energy, disasters like this are entirely unpredictable and all too common, and will no doubt continue. The price for these crises is extraordinarily high. After Chernobyl, for example, roughly five million people were exposed to high levels of radiation; the World Health Organization estimates that some 4,000 cases of thyroid cancer resulted, and an untold number of children in the region were born with severe deformities.

If a nuclear accident like Fukushima should strike the United States, the repercussions would be catastrophic. Four nuclear reactors in California are located near active earthquake fault lines. The Indian Point nuclear power plant, for example, is just 35 miles north of New York City, and it's ranked by the Nuclear Regulatory Commission as the riskiest nuclear plant in the country.

A Word About Nuclear Waste

Another undeniable problem is the safe disposal of spent nuclear fuel rods. Nuclear waste remains radioactive for tens of thousands of years -- far beyond the planning capacity of any government agency. And each year, an active nuclear power plant produces about 20 to 30 tons of radioactive waste. Even in an advanced country like the United States, nuclear waste is currently being stored at temporary sites around the country while politicians and scientists debate the best course of action.

And speaking of waste, some critics point out that the enormous government subsidies the nuclear energy industry receives are the only thing that makes nuclear power feasible. Roughly $58 billion in loan guarantees and subsidies from the U.S. federal government shore up the nuclear industry, according to the Union of Concerned Scientists. Without those taxpayer subsidies, they argue, the entire industry could collapse since the subsides are greater than the average market price of the electricity that's produced.

Is Nuclear Energy Renewable?

In a word: No. Like oil, natural gas and other fossil fuels, uranium is not renewable, and there are finite supplies of uranium that can be mined for nuclear energy. And mining uranium has its own risks, including the release of potentially deadly radon gas and the disposal of radioactive mining waste.

The fact that nuclear energy is not renewable is, of course, a significant disadvantage that makes renewable sources of energy like solar, geothermal and wind energy seem much more attractive. Given the complexities and challenges of the world's energy needs, the pros and cons of nuclear power will continue to be a hot topic for many years to come.

What Is a Greenhouse Gas?

Greenhouse gases play an important role in climate change

The phrase "greenhouse gas" has become a political hot potato, especially in conversations about global warming and climate change. But not everyone understands what a greenhouse gas is, how the greenhouse effect works, or why greenhouse gases are so critical to life on Earth. Here's a brief introduction to greenhouse gases and their effect on the Earth's climate.

What Is a Greenhouse Gas?

To put it

simply, a greenhouse gas, or GHG, is any gas that captures and stores heat. The Earth's atmosphere -- like the atmosphere of some other planets -- is filled with dozens of different gases, some of which function as heat-trapping greenhouse gases.

The most prevalent GHG in the Earth's atmosphere is water vapor, which comes from the evaporation of water in the oceans, lakes and soil. Carbon dioxide (CO2) is another important greenhouse gas; CO2 occurs naturally from animal respiration -- you release CO2 each time you exhale -- and from the burning of plant material (wood, paper) and of fossil fuels (gasoline, oil, natural gas, coal, etc.).

Methane, another greenhouse gas, is also created from biological activity and from the production of fossil fuels. Nitrous oxide, ozone and gases like chlorofluorocarbons (CFCs), hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride are created by agricultural and industrial processes -- these gases also contribute to the total amount of greenhouse

gases in our atmosphere.

Not all greenhouse gases are created equal, or pose the same risks: Methane, for example, has about 30 times the heat-trapping ability of CO2. Some researchers therefore believe that releases of methane gas may be a more serious concern than CO2 or other greenhouse gases.

Greenhouse Gases and the Greenhouse Effect

To fully understand how greenhouse gases work, it's important to understand a little bit about how the Earth's atmosphere functions. A simple, everyday example of this occurs in a parked car.

If you park a car outside on a sunny day, sunlight enters through the car's glass windows. That incoming sunlight warms the inside of the car when it shines on the dashboard, the car seats, the steering wheel and other areas. But once the sunlight hits those surfaces, it loses some of its energy, and it can't shine back out of the car. The sunlight's energy -- also called solar radiation -- becomes trapped inside the car, and after an hour or two of sitting in the sun, the inside of the car becomes very warm, even on a cold winter day.

A similar process happens when sunlight shines on the Earth. Some of that solar radiation is absorbed by the atmosphere and by the Earth's surface, so it loses energy. Technically speaking, the powerful incoming shortwave solar radiation loses energy when it shines on the Earth's surface and is converted into longwave radiation, which is weaker and can't escape through the atmosphere. That longwave solar radiation becomes trapped in the atmosphere by greenhouse gases, which get warmer and warmer as the gases absorb more and more solar radiation.

This whole process -- where solar radiation is trapped in the Earth's atmosphere by greenhouse gases -- is often referred to as the "greenhouse effect." A greenhouse, which has glass windows instead of walls, acts like a parked car and absorbs incoming sunlight. That's how tropical plants are able to grow inside a greenhouse, even when the weather outside is cold.

How Do Greenhouse Gases Affect Climate Change?

Ideally, the amount of solar radiation coming into the Earth's atmosphere should be about equal to the amount of solar radiation absorbed, converted into plants, or radiating out into space. That stable situation would result in a stable, steady atmospheric temperature.

However, scientists around the world have noticed two important things in recent decades: The amount of greenhouse gases in the atmosphere has increased dramatically, and the overall temperatures of the atmosphere and the Earth have also increased.

According to the U.S. Energy Information Administration, levels of greenhouse gases have increased 25% since 1850, when fossil fuels like coal and oil started to be widely burned for heating, transportation, manufacturing and other processes. Carbon dioxide is the gas that's largely responsible for this overall increase -- in the United States, roughly 82% of our greenhouse gas emissions are CO2 from burning fossil fuels like gasoline, oil, natural gas and coal.

Scientists worldwide agree that greenhouse gases from human activity -- primarily, the burning of fossil fuels -- are responsible for climate change and global warming. Furthermore, as humans continue to add more greenhouse gases to the atmosphere, the rise in temperatures worldwide is expected to increase dramatically. The combined effects of increased industrial activity (especially in developing countries like China and India), political inertia and ever-increasing global temperatures could have catastrophic impacts to life on Earth.

What You Can Do to Lower GHG Emissions

In the face of an overwhelming challenge like global warming and climate change, it's easy to feel utterly helpless. However, there are a great number of things that people can do on an individual and group level to change things like greenhouse gas emissions. Discover 8 easy things you can do to reduce green house gas levels and help to stave off global climate change.

What is the purpose of using heavy water in nuclear reactor?

Basicaly heavy water is used as a moderator in a nuclear reactor. It is used to slow the neutrons being directed at the fissionable material, by means of the molecules of the moderator physicaly impacting the incoming neutrons and absorbing some of the kenetic energy they posses, thus slowing them down, in the same way that two billiard balls impacting each other would slow down the incoming one (or both if they were both moving). The reason that the neutrons have to be slowed is that most fissionable materials are more likely to absorb thermal neutrons (2.2km/s) than fast neutrons (14,000km/s).Light water (the name usually used for regular H2O when talking about nuclear reactors), is the most common type of moderator, because it is cheap, very available, and is more effecient at slowing the incoming neutrons, due to the fact that the hydorgen atoms in the water posses only one proton and one electron, and thus are almost exactly the same mass as the incoming neutrons (the hydrogen atom weighs only as much as one electron more than the neutrons, and electrons are very light when compard to protons and neutrons, which are equal in mass). The problem with using light water as a moderator, however, is that the hydrogen atoms may absorb some of the neutrons, thus preventing them from getting through to the fissionable material. Thus, once the percentage of U-235 (the fissionable isotope of uranium) is too low (such as in natural uranium, where the percentage of U-235 is about 0.72%), then the amount of neutrons getting through the moderator without being abosorbed is not high enough to maintain criticality (the point at which the amount of neutrons being produced is equal to the amount escaping the system or being absorbed but not resulting in fission), and the chain reaction can no longer continue, and the reactor can no longer produce power.Heavy water, however, is deuterium oxide. Deuterium is an isotope of hydrogen with one proton and one neutron. Thus the hydrogen atom already has one extra neutron, and is much less likely to absorb another. This means that when heavy water is used as a moderator, enough neutrons get through that even with very low levels of U-235 (even the very low levels found in natural uranium), criticality can be maintained, and power is produced. So even though the efficiency of the D2O (heavy water) molecules at slowing the neutrons is slightly less than that of regualr H2O (water, or light water) molecules, the use of heavy water as a moderator allows natural uranium to be used as a fuel with little, if any, enrichment (which is a costly process, and controversial, as enriched uranium can be used to make nuclear weapons).This is why CANDU (Canadian Deuterium-Uranium) reactors can use natural uranium, or even the waste uranium from conventional light water reactors as fuel.