The pressurized water reactor (PWR) is the most widely used design in nuclear reactors for electrical power generation in the world.  The PWR was originally intended for use as a propulsion system for nuclear submarines. It was used in the second commercial power plant in the United States. Since then it has seen widespread use in the United States and other countries such as France for power generation. The PWR reactors currently in use in the United States are referred to as Generation II reactors.

            In the PWR reactor ordinary or light water is used as the coolant. The water that circulates through the reactor core to capture the heat of the fission reaction is kept under high pressure and does not turn into steam despite the high temperature. The water from the core is passed to a steam generator where its heat is transferred to a separate water circulation system where steam is created.

            In a power generation plant, this steam is used to spin a turbine which generates electricity. A third water system passes water through a condenser which cools the steam and turns it back into liquid water. This third system requires large amounts of water which is drawn from and sent back to a lake, river or ocean at a higher temperature.

            In propulsion systems, the steam can be fed through a system of gears to a propulsion shaft which turns propeller blades. The expansion of steam can be used in a piston system to drive a catapult to launch aircraft. In some applications, the heat from the steam can be directly extracted for use in industrial processes.

            The water in PWRs also acts as a moderator for the fission reaction in the core. As the fast neutrons generated by fission collide with water molecules, they are slowed down which increases their ability to trigger more fission events in the core. As the water heats, the molecules move apart and reduce the ability of the water to slow down the fast neutrons. This in turn reduces the activation of fission events which cools the core. This creates a negative feedback control system which keeps the core within a particular temperature range. The position of the control rods determines the center or set point of the temperature range. This system is an important safety feature in PWRs. Light water is an excellent moderator and allows the construction of compact reactor cores.

            The fuel used in PWRs is uranium dioxide (UO₂) ceramic pellets in Zircaloy fuel rods. Between two hundred and three hundred fuel rods will be used in a typical reactor. They are arranged in a square configuration with a hole in the center for a control rod. One hundred and fifty to two hundred and fifty rods are assembled into the reactor core. Usually every eighteen to twenty four months, one third of the rods will be replaced.

            Boron control rods are used in PWRs to start the reactor, shut down the reactor and to control short terms changes in power demand.  The control rods can also compensate for changes in the fuel due to depletion or poisoning by isotopes generated during fission. Boric acid is mixed with the coolant water to change the neutron absorption rate.

            PWRs are very stable and the separate coolant and steam systems prevent radioactive contamination of the steam. If offsite power is lost, the electromagnets holding the control rods shut off and the rods are inserted completely into the core by gravity which results in automatic shutdown. This automatic shut down is an important safety feature, however the radioactive decay in the fuel still proceeds at a low rate and requires up to three years of continuous coolant circulation in order to prevent overheating and the possibility of a meltdown. The high pressure required in a PWR demands very strong piping and a heavy reactor vessel. Neutron flux makes the metal in PWRs brittle over time and limits the lifespan of the reactor.

           

            Most of the hydrogen in the universe consists of a single proton orbited by a single electron and is also known as H-1 or protium. A small percentage of hydrogen atoms contain a neutron in the nucleus as well as the proton. This form of hydrogen is referred to as H-2, heavy hydrogen or deuterium. It is a stable atom like H-1. In the Earth’s crust, for every six thousand four hundred and twenty H-1 atoms, there is a single H-2 atom. There is a third isotope of hydrogen called tritium that has two neutrons in the nucleus. This form of hydrogen is unstable and undergoes radioactive decay into stable helium-4 with a half life of twelve and a third years and emits a beta particle when it decays.

            Tritium atoms can combine with oxygen to form tritiated water also call THO. Tritium is very rare in the natural environment and mostly occurs in the form of a few THO molecules mixed in with ordinary water.

            Most tritium is formed from the collision of a high energy cosmic ray from space with nitrogen in Earth’s atmosphere. The cosmic neutron combines with nitrogen-14 to yield an atom of carbon-12 and one atom of tritium or H-3. Small amounts of tritium are produced in reactors by interaction of deuterium or H-2 with lithium-6 and neutron absorption by deuterium. About one in ten thousand decays of U-233, U-235 and P-239 atoms produces tritium. Tritium is also produced in the explosion of nuclear weapons. Release of tritium from reactors must be below a threshold set by U.S government regulation.

            Tritium was created and identified in 1934 by physicists Rutherford, Oliphant and Harteck shortly after the discovery of deuterium. Deuterium was bombarded with neutrons and, through absorption, tritium was produced.

            Tritium was intentionally produced in a special reactor at Savannah River until it was shut down in 1988. Up to 1996, only about five hundred pounds of tritium were produced in the United States. In late 2006 a Tritium Extraction Facility was started up at Watts Bar Nuclear Generating Plant. The new facility recovers tritium from nuclear control rods containing lithium.

            Tritium emits weak beta particles when it decays and the titrated water in which it usually enters the body is excreted in a short time. This makes tritium one of the least dangerous radioisotopes to human health.

            Tritium is used in research dedicated to fusion reactors because of the large amount of energy released when it is mixed with deuterium in the reactor. This energy production also makes it useful for the triggering mechanisms in thermonuclear fusion weapons. It is also used in luminescent  exit signs in buildings, in dials and gauges, in luminous paints and on the faces of wristwatches. Some tritium that has been detected in ground water has been traced back to landfills where people illegally disposed of old exit signs.

            Most of the hydrogen in the universe consists of a single proton orbited by a single electron and is also known as H-1 or protium. A small percentage of hydrogen atoms contain a neutron in the nucleus as well as the proton. This form of hydrogen is referred to as H-2, heavy hydrogen or deuterium. It is a stable atom like H-1. In the Earth’s crust, for every six thousand four hundred and twenty H-1 atoms, there is a single H-2 atom.

            Most of the water molecules in the universe contain two of these ordinary hydrogen atoms combined with an oxygen atom. A few of the water molecules will have H-2 atoms combined with oxygen. In all the water on Earth, in ten thousand water molecules there will be about 2 with H-2 atoms instead of H-1 atoms. Water with H-2 atoms is referred to as heavy water.

            Just about all the deuterium in the universe is thought to have been created in the big bang. Heat can be used to separate the isotopes of hydrogen. The ratio of H1 to H-2 in gas giants and comets varies because of the effects of internal heat and solar heating. The fact that the ratio of H1 to H2 found in comets is close to that found in the oceans on Earth has been used to argue that the oceans were created by cometary impacts on the young Earth.

            Deuterium was identified in the early 1930 soon after the discovery of the neutron. Harold Urey won a Nobel Prize in 1934 for discovering and naming deuterium. Since the discovery of deuterium, water containing deuterium in its molecules has been extracted from ordinary water through a steam distillation process. Canada used to be the leading world supplier of deuterium until its last heavy water production plant was closed in 1997.

            The chemical and physical properties of compounds containing deuterium are similar to the behavior of the same compound without deuterium. However, there are still differences that are greater that those caused by any other change of particular isotopes in compounds. Heavy water is more viscous than ordinary water and ice created from heavy water will sink in ordinary water in contrast to ordinary ice which floats.

            Heavy water is slightly toxic to multi-cellular creatures and single cell life forms whose cells contain a nucleus. More primitive single cell life that has no nucleus appear to not be harmed by it. A average person could consume five quarts of heavy water without serious injury but it half the water in the body was replace with heavy water, death would result.

            Deuterium is used in experimental fusion reactors. When fusing hydrogen to helium, neutrons must be part of the mix because even though most hydrogen does not include neutrons, all helium nuclei do include neutrons. Heavy water is used as a moderator to slow neutrons in some nuclear reactor designs because it does not absorb neutrons like ordinary water. The Canada reactor design CANDU uses heavy water as a moderator. Deuterium is a useful tracer for chemistry and biochemistry because it is a non-radioactive and easily identified.

 

            About two thirds of the heat generated by nuclear reactors is dumped into the cooling system. Ordinary water is a popular coolant for reactors. The water for cooling is drawn from either a large river or the ocean. While this makes it convenient to locate reactors near a river or the ocean, it also makes them more vulnerable to floods and tidal waves such as the recent disaster at Fukushima.

            About two thirds of the nuclear power reactors in the United States are pressurize-water reactors. A one thousand megawatt pressurized-water reactor with a cooling tower consumes about twenty thousand gallons of water per minute for cooling and returns about five thousand gallons of water back into the environment. A one thousand megawatt pressurized-water reactor without a cooling tower can consume five hundred thousand gallons of water per minute for cooling and returns most of it back into the environment. These figures are for a core running at a temperature differential of thirty degrees Fahrenheit. In order to reduce that to a differential of 20 degrees Fahrenheit, about fifty percent more cooling water is required. For the reactor without a cooling tower, this would require over seven hundred and fifty gallons of water per minute. There are designs for new reactors that would need over one million gallons of water per minute for cooling.

            The cooling water released from nuclear power plants can be more than thirty degrees Fahrenheit warmer than the water that was drawn from the river in the first place. In addition, radioactivity isotopes from the core leaks into the cooling water. While the exact type and amount of these isotopes released from a particular reactor cannot be known we know that the heated water and such isotopes have an adverse impact on the ecology of the plants, fish and other life forms in the river. Additional radioactive gases leak into the air from the reactor and wind up in soil and water near the reactor. Government regulations permit such releases during routine operation and do not require plants to monitor their normal releases. Some of these isotopes have half-lives of thousands to millions of years.

            Full power production in nuclear reactors that use water for a coolant is dependant of a steady powerful flow of water. If the water is coming from a river and the flow of the river decreases substantially, then power production must be cut back or the plant must be shut down entirely. There have been many problems at nuclear power plants where something blocked the intake system and the reactors had to be shut down. Many major rivers are so depleted from drawdown for irrigation, municipal water supply and industrial production that some rivers do not reach the sea anymore. In addition, a terrorist could carry enough explosives in a small boat to destroy the intake for a nuclear power station and force it to shut down.

            Even power plants with cooling towers are not immune to problems. Structural problems leading to collapse, clogging of screens that caused overflow, ice build up on nearby transmission lines from vapor release, as well as radioactive gases and isotopes released with the water vapor have all caused partial or total shutdown of reactors.

            Use of water as a primary coolant for nuclear power plants will continue but there are many design and environmental problems that need will require additional research and development.

So far we have focused on uranium and plutonium in our discussion of nuclear fuel and reactors because they are the fuels for most of the world’s reactors. There are other nuclear fuels used in existing reactors and atomic batteries or suggested for use in new designs. One of these alternative nuclear fuels that holds great promise is thorium.

            Thorium is a naturally occurring radioactive element. It is abbreviated as Th and has ninety protons in its nucleus. Almost all natural thorium is the isotope Th-232. It has a half-life of about fourteen billion years or the estimated current age of the universe. There is about four times as much thorium in the Earth’s crust as uranium.

            Mining and processing of rare earths yields thorium as a by-product. Pure thorium is a silvery metal that is soft and ductile. Powdered thorium is can ignite and burn when exposed to air. A common use of thorium was as a component in alloys such as a magnesium alloy called Mag-Thor that was used in the aerospace industry for engines because of its stability at high temperatures. It has also been used in electronic applications and welding rods.  Thorium dioxide has a very high melting point was used in the mantles of gas lamps and as an additive for high temperature ceramics and laboratory glassware Recent concerns over radioactivity have ultimately made thorium and thorium dioxide unattractive for these application.

            Thorium has a long history in the nuclear industry. Thorium is attractive as a nuclear fuel because it is safer and more abundant than uranium. It was used in an experimental reactor built in 1962 and was the fuel chosen in 1977 for a reactor at the first civilian nuclear plant in built in the U.S. at Shippingport Pennsylvania. In the mid 1990s, the US Department of Energy and the International Atomic Energy Commission funded research into thorium as a nuclear fuel. A consortium of major corporations was formed for thorium fuel research as well. India and China have funded major projects to use thorium for power production.

            One benefit of thorium fuel is the fact that is it more difficult to extract weapons grade uranium-233 from a thorium reactor that from a uranium reactor. A second benefit is that thorium products much less nuclear waste with long half-lives. A third benefit is that thorium can be used for fuel just as it comes out of the ground without the need to expensive and complex enrichment. And, fourth, thorium cannot sustain a chain reaction without an external source of neutron flux and so, if the external neutron source is deactivated, there is no risk of a core meltdown.

            Some of the problems with using thorium for nuclear fuel include the need for research and development in fabricating thorium fuel rods, thorium reactor design, thorium reprocessing and possible production of weapons-grade in special breeder reactors.

            Thorium use for nuclear fuel was not attractive as long as there was plenty of cheap uranium. But rising uranium prices, problems with nuclear waste, fear of nuclear weapons production and other problems have made thorium an attractive alternative nuclear fuel in the twenty first century. There is enough thorium inside the United States to provide for our current level of power consumption for over one thousand years.

            When spent fuel rods are removed from a nuclear reactor, they are giving off heat and emitting radiation, primarily from fission products.  They are stored in special pools of water or boric acid to allow the heat and radiation to diminish. The cooling fluid absorbs the radiation and  is circulated through heat exchangers to get rid of the heat. It can take several years for the heat and radiation to drop to a safer level. The rods may be transferred to dry storage casks after at least a year has elapsed.

            Spent fuel is either considered waste and stored indefinitely or is reprocessed into fuel to be used again in reactors. If it is not going to be reprocessed, the spent fuel may be kept at the reactor site indefinitely or transported to a central repository for storage. If it is going to be reprocessed, then it will ultimately be transported to a reprocessing facility. Special spent nuclear fuel casks are required to transport spent nuclear fuel. They are designed to withstand possible accidents such as crashes without releasing their contents.

            Nuclear waste is classified into three different categories. Low-level waste is created during all the stages of the fuel cycle. Intermediate level is produced during the burning of the fuel in the reactor and during reprocessing. High-level waste contains fission products from fuel reprocessing and includes the spent fuel itself.

            Nuclear waste contains many different radioactive isotopes which have half-lives that vary from a few years to billions of years. It is possible to process nuclear waste in some types of reactors where radioactive isotopes can be converted to other elements which may have lower or no radioactivity.

            There is currently no facility where nuclear waste can be disposed of permanently. The technical issues of such disposal have been studied extensively and permanent disposal sites have been propose. Some of the major issues involve how to package the waste in a safe and stable way, geological stability of the area, possibility of leaching into ground water, and other potential problems. There have even been efforts to create such sites but none have been completed to date.

            The various radioactive isotopes that are present in the spent fuel can be separated with chemical processing and used to make more fuel. The amount of U-235 remaining in spent fuel is higher than the amount found in natural ores. The uranium in spent fuel can be enriched to the point where it can be sent back to the reactor and burned again. Alternatively, the P-238 produced by fission can be used to make mixed oxide fuel where plutonium oxide is combined with uranium oxide. . This fuel is burned in what are referred to as mixed oxide or MOX reactors. The P-238 takes the place of U-235 found in uranium oxide fuel 

            Spent fuel is reprocessed in Europe for European and Japanese reactors. The United States has banned reprocessing because of concerns about nuclear weapons proliferation utilizing P-238. There has been work setting up reprocessing plants under international agreement where plutonium can be processed into a type of fuel that could be burned in a reactor but cannot be used to make nuclear weapons.

            After manufacture, nuclear fuel is transported from the production facility to a nuclear power plant for use in a reactor. Specialized transport companies transport nuclear fuel assemblies which release little radioactivity and do not require special shielding.

            In a typical nuclear reactor, sets of fuel rods called cells surround a control rod which can be inserted or withdrawn to control the neutron flux and thus, the rate of the chain reaction.

            U-235 atoms are bombarded by neutrons and fission which produces heat and more neutrons. Some of the U-235 transmutes into plutonium which also undergoes fission producing about one third of the heat in the reactor core. The heat from the core is used to produce steam which drives the turbines that produce the electricity.

            As the nuclear fuel in the rods fissions, the heat generated causes thermal expansion which can cause cracking. The nuclear fuel reacts with cladding materials such zirconium alloy which forms the shell of the fuel rod. The chemical composition of the fuel near the edge of the pellet changes as does its thermal conductivity. The purer uranium oxide in the center of the pellet will reach higher temperatures than the fuel near the outer edge of the pellet.

            One ton of natural uranium can generate about fourty four million kilowatt hours. It would require over twenty thousand tons of coal or eight million cubic meters of natural gas to generate the same amount of electricity.

            The rate at which the fuel is consumed is measured in gigawatt-days per ton of fuel and it is proportional to the level of concentration of U-235 in the nuclear fuel contained in the rods. The level of heat generation that can be safely handled by the current reactors limits the enrichment to about four percent which will yield a burn up rate of fourty gigawatt-days per ton. With improvements in materials and design, enrichment as high as five percent can be utilized, ultimately producing seventy gigawatt-days per ton.

            Only a third of the heat produced by the core is captured in steam production. The other two thirds of the heat is passed to the water of the cooling system and either released in into a body of water such as a large river or the ocean. Alternatively, the water may be sent into cooling towers for evaporative cooling. Normally, a small amount of radioactivity is released into the cooling water.

            As the fission fragments build up in the nuclear fuel rods, the heat generated by the fuel declines. Typically, nuclear fuel is depleted in eighteen to thirty six months and is no longer useful. The total amount of energy produced by a given amount of nuclear fuel depends on the configuration of the core and the overall design of the reactor.

            Periodically, new sets of fuel rod assemblies must be inserted into the core. About one third of the rods in a core are replaced at one time. Different rods burn up at different rates which is partially dependant on the exact location of a given assembly of rods in the core. The exact placement of the new set of rods is referred to as the optimal fuel reloading problem.

            The progression of nuclear fuel through a series of stages of production, use and disposal is referred to as the nuclear fuel cycle. When the spent fuel is used once and then disposed of, the cycle is called open or once-through. When the spent fuel is reprocessed and used again, the cycle is called closed.

            Uranium is a very common element that is found in most rocks and soils as well as river and sea water. Granite comprises about sixty percent of the Earth’s crust and it contains minute amounts of uranium.

            Naturally occurring uranium contains about ninety nine percent U-238, a stable isotope and one percent U-235, a radioactive isotope. In order to be considered for exploitation, a deposit of uranium ore has to have a high enough percentage of uranium in a recoverable form at a low enough cost to be profitable.

            Uranium ores contain uranium oxide in concentrations of less than one percent. Sources of uranium ore in the US are under three tenths of one percent with greater percentages found in ores in other countries. Open pit and underground techniques are the primary methods of uranium mining. Some uranium is mined by passing oxygenated groundwater through porous ore which dissolves the uranium oxide and allows it to be brought to the surface.

            Uranium ore is ground to produce particles which are then subjected to a chemical process called leaching to recover the uranium. This process results in uranium oxide in the form of a dry powder known as yellowcake which is usually about eighty percent uranium. This processing is usually done near the mine.

            Most yellowcake is converted to uranium hexafluoride in a process that involves nitric acid, ammonium and hydrogen. Uranium hexafluoride is a solid at room temperature but becomes a gas when heated above one hundred fourty degrees. It is highly corrosive and toxic. It reacts violently with water and must be transported in sealed containers.

            In order to be utilized as a nuclear fuel, the uranium must be enriched in order to raise the percentage of U-235. Light water reactors use normal water for cooling and require a concentration of U-235 to about three and one half percent. Uranium is enriched by isotope separation. The uranium hexafluoride is heated to a gaseous state and spun in high speed centrifuges to separate the two isotopes which have slightly different weights. Most of the resulting depleted uranium byproduct is stored as uranium hexafluoride. Some of it is converted to metallic uranium and used in armor, artillery, radiation shielding and ballast.

            Enriched uranium fluoride is processed into uranium dioxide powder which is then mixed with binders and formed into pellets. The pellets are fired in a sintering furnace to produce ceramic pellets. These pellets are then machined into a standard size and stacked into alloy tubes to produce fuel rods. The rods are combined into bundles and bundles are combined to create the reactor core. The size and shape of the rods and bundles depend on which type of reactor they are intended for.

            The common nuclear fuel uranium oxide also known as UOX is usually compressed and cooked into cylindrical ceramic pellets. These pellets manufactured to exacting standards and are machined to precise dimensions. The pellets are then sealed into long metal tubes called fuel rods

            Different reactor designs use different metals for the rods. This layer of metal is called cladding and prevents the nuclear fuel from contaminating the coolant. Originally, stainless steel was used because of its strength and resistance to corrosion. Now a zirconium alloy is used in most reactors because it is even more corrosion resistant than stainless steel and has low neutron absorption. Fuel rods are combined into assemblies which are used to create the core of a reactor.

            In a pressurized water reactor (PWR), the one centimeter diameter tubes are filled with helium to improve heat conductions. They are assembled into square bundles that are from fourteen rods by fourteen rods to seventeen rods by seventeen rods. Many rods are placed end to end to from the bundles with from one hundred eighty to two hundred sixty rods per bundle. One hundred twenty to one hundred ninety bundles are loaded into the reactor core. There is a hollow center in the bundles which allows control rods to be inserted.

            Boiling water reactors have rod and bundle configurations that are similar to the pressurized water reactors but, in addition, each fuel rod bundle is surrounded by a thin tube. This helps prevent local variations in neutron flux and heat exchange with the circulating coolant. There are about ninety fuel rods in each bundles and up to 800 bundles in the core.

            Canadian Deuterium Uranium or CANDU reactors have fuel bundles that are ten centimeters in diameter and one half meter long. They contain ceramic uranium oxide pellets in zirconium alloy tubes that are welded to zirconium alloy plates at each end. Each bundle has about fourty fuel rods arranged in concentric rings. They weight about twenty kilograms and cores have from fourty five hundred to sixty five hundred bundles.

            There are many other configuration of nuclear fuels, rods, bundles and cores but these are used mostly for research and military applications.

            In the high radiation environment of a reactor core, materials can swell, crack, change porosity. As different isotopes are produced by the fission process, there can be pitting, formation of bubbles, buildup of such isotopes, out gassing and other processes that change chemical properties and material properties. One important property is thermal conductivity. As all these changes take place, the thermal conductivity of uranium oxide changes and overheating of the center of the fuel pellets can occur.

            Every configuration of nuclear fuel elements has its benefits and drawbacks. There is ongoing research into what chemical composition and physical configuration is best for the nuclear fuel needed to create a economical, reliable and safe nuclear reactor.