by Warren » Oct 25, 2002 @ 4:00am
Uranium is the 92nd element on the periodic table of the elements. It is named after the planet Uranus, which had been discovered eight years earlier. Uranus comes from the Greek god Ouranos, and the element was named in honor of the planet. The next two elements on the table were named after planets also, neptunium and plutonium. Uranium was discovered by German chemist Martin H. Klaproth in 1789 in Berlin. He located the new atom in a mineral called pitchblende. Pitchblende is a mixture of mostly uranium dioxide (UO2), some uranium trioxide (UO3), and is a source for radium, lead, thorium, and other rare-earth metals. It is brownish, greenish, or blackish color, with a sub-metallic luster. The uranium yield in pitchblende is 50% to 80%. Another mineral high in uranium is uraninite, which has the most uranium in a natural state. Uraninite is almost entirely uranium dioxide (UO2), yielding 80%, but also is a good source for radium and polonium. Martin Klaproth attempted to isolate uranium from pitchblende, but was not successful. It was not until 1841, when Eugène M. Péligot, a French physicist from Paris, isolated the element from uraninite. Uranium is also found in most rocks in proportions of 2 to 4 ppm (parts per million), and in scarce quantities in seawater.
When uranium was discovered, it became the hot “new thing”. In the 1830s, a German glassmaker, Josef Riedel, experimented with uranium dioxide, and created uranium glass. Uranium glass either has a light yellow or light green fluorescent glow. It was called Annagelb for yellow glass, and Annagrün for green glass; today it is called Vaseline glass. One may have a concern about the radiation of the uranium glass, but the concentration of uranium dioxide is very low, usually about 0.1% to 2%, so the whole glasswork will emit only about 2400 Bq/g (berquerels per gram, or disintegrations per second per gram), which is about the same as the human body. It was not until the early 1930s, when scientists started studying the nuclear and radioactive properties of uranium.
Uranium is an atomic number of 92, meaning that it has 92 protons and 92 electrons. Its atomic mass is 238.02891 amu, so the average number of neutrons is 146. Uranium, chemical symbol U, is located in the actinide group, which is the bottom row of the table; its period number is 7. Most actinides are synthetic (man-made), while uranium is the last and heaviest element to be found in nature. The only other actinides that are natural are actinium, thorium, and protactinium. Neptunium, the next element after uranium, is the first and lightest synthetic element.
At room temperature of 20°C, uranium is a solid silvery metal. Its density is very high at 18.95 g/cm3, that’s almost 19 times denser than water! Uranium’s melting point is 1132°C, and its boiling point is 3818°C, so its natural state is solid. Uranium has many isotopes, 14 known ones, and they are all radioactive. An isotope is when an atom of the same element has a different number of neutrons. The following are the 10 most common isotopes with their half-lives: U-230 – 20.8 days, U-231 – 4.2 days, U-232 – 70.0 years, U-233 – 1.59X105 years, U-234 – 2.47X105 years and is 0.0055% of the total on Earth, U-235 – 7.0X108 years and is 0.720% of the total on Earth, U-237 – 6.75 days, U-238 – 4.47X109 years (about the age of the Earth) and is 99.27% of the total on Earth, U-239 – 23.5 minutes, and U-240 – 14.1 hours. Half-life is how long it takes for half the amount of material to decay. The radiations for the main 3 isotopes are, 2.31X108 Bq/g (disintegrations per second per gram) for U-234, U-235 is 8.00X104 Bq/g, and U-238 is 1.244X104 Bq/g. 80,000 disintegrations per second per gram may seem like a lot, but there are about 2.56X1021 U-235 atoms in 1 gram, so it would take a little over 1 billion years for the entire gram to decay to Th-231. All 3 of these isotopes’ radiation emit alpha particles (a). An alpha particle is the equivalent to the nucleus of a helium atom, 2 protons and 2 neutrons; it is ejected from the nucleus of the uranium atom to form a lighter atom. In the case of U-238, after disintegration (decay, when an alpha particle is released), the atom becomes thorium-234. For U-235, it becomes Th-231. Radioactive decay will continue until the atom becomes lead-106, lead-107, or lead-108, which are stable.
The amount of energy needed to take away an electron is the ionization energy. The first ionization energy for uranium is 597.6 kJ/mol (2511 J/g). To remove the second electron after the first (most loosely held one), the amount of energy needed is 1420 kJ/mol (5966 J/g). Electronegativity is the attraction an atom has for an electron in a covalent bond. Water is the most common example: the oxygen has more electronegativity and attracts the shared electrons more than the hydrogen does; this causes a polar covalent bond. Because the oxygen is pulling the electrons closer to it, it has a slight negative charge, making the hydrogen have slightly positive charge, and this accounts for water’s high surface tension, because hydrogen bonds form in between the water molecules. Hydrogen bonds are very weak attractions between the hydrogen and oxygen atoms of adjacent molecules. This explains why water has some peculiar characteristics, like expanding when it freezes. Water expands when it freezes because as the molecules slow down and start to crystallize, the hydrogen bonds have a chance to stick, and because of the polarity of the electronegativity, the molecules crystallize in a less dense form than in liquid. Electronegativity is measured in the Pauling scale, where the most electronegative element, fluorine, is rated as 4.0 (today, it is rated as 3.98), and all other elements below that. Uranium has an electronegativity of 1.38 on the Pauling scale, which is about average, so it’s nothing special in the characteristics of the element.
Uranium can make many bonds, but the usual natural ones include uranium dioxide (UO2), uranium trioxide (UO3), triuranium octaoxide (U3O8), and uranium hexafluoride (UF6). Uranium dioxide and trioxide are found in pitchblende and uraninite, triuranium octaoxide is made from uranium ore and pitchblende, and uranium hexafluoride is used for U-235 enrichment. The electron configuration of uranium is [Rn]5f36d17s2, so its electron orbit shells are 2, 8, 18, 32, 21, 19, 2. When uranium is a solid (as is in nature), the crystalline structure is orthorhombic, which means that, all atoms are at 90° angles in comparison to each other (like a cubic structure), but the distances can be unequal. The heat of fusion of uranium is not actually that much, even though the melting point is 1132°C, as it is only 35.8 J/g. The heat of fusion of ice is 334 J/g; ten times more energy is required for ice than uranium. The heat of vaporization is a lot more though, 2004 J/g, but still less than water at 2261 J/g. The specific heat capacity for uranium though, is very low, at 0.12 J/g°C, while water is high at 4.183 J/g°C. Aluminum is 0.903 J/g°C. Even though uranium is quite rare, it produces so much energy from radioactive decay, that it is the foremost material in warming the interior of the Earth.
Uranium is extremely important in the world industry; it is used in atomic bombs, nuclear power plants, and is used to make synthetic elements. Uranium is mined from uranium ore underground. There are two methods used to extract the uranium, the first is to chisel it off the rock, grind it to a powder, add acid to dissolve the uranium, and then recover it from the solution. The other method is called in situ leaching (ISL), where the mine is filled with a dissolvent, and the uranium is pumped to the surface. The product of both of the techniques results in triuranium octaoxide (U3O8); this is how uranium is sold. The country that is most abundant is Australia, supplying 28% of the world’s uranium, then Kazakhstan at 15%, and Canada at 14%. The cost of uranium fluctuates with demand and supply. The average for 2001 was about $12/kg for U3O8, but remember, this is 99.27% U-238, U-235 is the isotope which is needed for power.
Next, the triuranium octaoxide must go through the process of refinement. In refinement, U3O8 is converted into a gaseous form of uranium hexafluoride (UF6). After it has been amended into uranium hexafluoride, it goes through the procedure called enrichment. Because natural uranium-235 is only 0.72%, it must be filtered out of the uranium mixture and compiled to make an “enriched” uranium fuel. For nuclear power plants, uranium mixtures are usually around 3.5% U-235. To extract the U-235, the uranium hexafluoride gas is blown through thousands of filters, because U-235 is lighter than U-238, it will travel farther and can be separated at the end. There is no chemical procedure known to separate U-235 and U-238. After enrichment, the uranium hexafluoride with 3.5% U-235 is converted into uranium dioxide (UO2) powder; the uranium dioxide is then made into small fuel pellets used by nuclear reactors.
Over 16% of the world gets their power from nuclear power plants. Nuclear power plants are very similar to power plants that burn coal or oil, except that they are actually much cleaner, more efficient, and safer. The UO2 pellets are put into long thin metal tubes, usually made out of either zirconium alloy or stainless steel. The long tubes are then arranged in clusters of usually 25-50. These clusters are then inserted into the reactor core. The core is like one of those trays that hold all the vertical chemical vials, where there is a grid of holes, and the cluster tubes are lowered into the holes. The uranium pellets create energy to heat the water in the reactor core. This happens because of nuclear fission. Nuclear fission is the destruction of the nucleus of an atom. All the uranium in the reactor core emit alpha particles (He+2). Uranium-235 is known as an extremely fissionable atom, while uranium-238 is not (called fertile). Being fissionable means that when a neutron is accepted by the nucleus of the atom, the nucleus divides into two parts very easily. When an alpha particle hits a U-235, the nucleus of the U-235 splits and, depending on the angle and speed on the impact, will release 2 or 3 high-speed neutrons. Those neutrons will then hit other uranium molecules, which would start a chain reaction. Whether those neutrons are going to hit other U-235s depends on the mass and shape of the pellets. The shape is spherical, so there is an equal chance of a neutron hitting another atom anywhere in the pellet. The mass is divided into 3 categories: subcritical mass, critical mass, and supercritical mass. Subcritical mass is when the mass is too small to sustain a chain reaction, meaning that on average, less than 1 neutron emitted from a destroyed U-235 will hit another U-235, so eventually, the chain reaction will cease. In a critical mass, 1 neutron from each atom will hit 1 other atom, making that atom explode, and only 1 of those neutrons released will hit 1 other atom, so the chain reaction is constant and self-sustained. Supercritical mass is when the mass is high enough that on average, more than 1 neutron from each disintegrated atom will hit another atom, so the chain reaction will increase and will not stop until all the fissionable atoms have been demolished. The pellets in a nuclear reactor core are slightly supercritical, so the chain reaction is self-sustained, but not out of control. When an atom is disintegrated, it releases a great deal of energy in the form of gamma radiation. This reaction is controlled by control rods that are lowered and raised from the core. These control rods are made out of graphite, and absorb neutrons to slow the chain reaction. When they are raised, the reaction speeds up. The radiation heats the pressurized water in the core, and the water is pumped to a heat exchanger, which is filled with water. The hot pressurized water boils the water in the exchanger, making steam. The steam rises and spins turbines, which make the electricity. The steam is cooled and recycled back to the heat exchanger. So really, the radioactive material is only used to heat the water, just like how other power plants use coal and oil to heat water too. The top three countries that use nuclear power are Lithuania, France, and Belgium.
Nuclear power plants are much safer too; the reactor core is enclosed in a very thick steel vessel with a thick concrete liner, all to retain any radioactive gases or alpha rays in the reactor core. The housing for the reactor core has three layers. The first is usually graphite or another neutron absorbing metal. The second is a steel layer, and the outer shell is a very thick layer of concrete. Concrete is one of the best radiation deflectors, not even strong gamma rays can penetrate it. The U-238 in the fuel pellets actually absorb neutrons without blowing up, keeping the reactions much more stable. But even with all this, and many safety procedures, accidents can still happen. The most infamous incident was the meltdown at Chernobyl, Ukraine. The Chernobyl power plant was a very large plant, containing four reactor cores. On the night of April 25th, 1986, the inexperienced night crew was underway with a test of reactor core number four. During the experiment, the temperature of the core began to rise. It continued to rise, and the crew did nothing, until it was too late. As the temperature and the nuclear fission chain reactions grew faster and higher, the team lowered the control rods, but it was not enough to stop it. The reactor exploded, blowing off the concrete building, and spewing radioactive gases into the air. 30 people died immediately, and 135,000 people had to evacuated in a 20-mile radius. Analysts say that if the usual day crew was performing the procedure, they would have seen the danger much sooner, and would have prevented the meltdown.
The main concern about nuclear energy is where do you put the used uranium? The chain reaction in the core is constant, so when the U-235 is used up, they need to replace it. Technology was improved the recycling of used (depleted) uranium back to usable fuel, but not all uranium can be recycled. When the uranium pellet tubes are used up, they are removed and put into a special spent fuel storage building, usually at the power plant site. This building is like a very large reactor core, except that the water is cooled and not pressurized. The tubes clusters are lowered into the grid, which is submerged in the large pool. Water is a natural deflector of radiation, so it uses up the last of the radiation and heat from the depleted uranium. Depleted uranium pellets are still 96% usable U-238; the 3.5% of U-235 is now less than 1%, about 3% radioactive waste, and the pellets contain about 1% plutonium-240 from U-238 absorbing alpha particles (92U238 + 1 a = 94Pu240 + 2n). Next, the pellets are ready for reprocessing. In reprocessing, the pellets are dissolved in acid to separate the uranium, plutonium, and waste products. The uranium is extracted and changed into uranium hexafluoride for enrichment, and is recycled to UO2 and made into new fuel pellets. The plutonium is sent to fuel fabrication plants for processing. The waste though, is an extremely radioactive sludge, called radwaste. The next process is called vitrification (vitrification means to convert to glass by means of heat and fusion). The liquid radwaste is left to solidify, then is calcined (super heated) to make a dry powder. This powder is then incorporated into borosilicate Pyrex glass, to immobilize the radwaste. The glass is then melted and poured into stainless steel canisters, each able to hold 400 kg of glass. A 1000-MWe power plant (a typical size) produces about 5 metric tons of radwaste borosilicate Pyrex glass, or about 12 canisters a year. Then, the canisters are buried deep underground in stable rock formations. Most countries are planning to bury all their radwaste canisters by 2010.
Another use for uranium, especially U-235, is the atomic bomb. In the 1930s, during World War II, American and British scientists were experimenting with radioactive and unstable materials to compete with Germany and Japan. The result was the Manhattan Project. Two bombs, one an atomic uranium bomb called “Little Man”, and the other, a plutonium bomb called “Fat Man”. The uranium bomb was an extremely simple design, it used uranium-235’s own fissionability for self-detonation, while the plutonium-239 had to be crushed under pressure to explode, and also would need the help of uranium-238. The uranium bomb was built like this: there were two uranium chunks. It had been calculated that for the perfect supercritical mass, there would need to be a total of 50 kg of pure uranium-235. Robert Oppenheimer said that for pure U-235 was impossible to make (for the technology of the time, it was impossible), so they would have to use 100 kg. At the front of the bomb, was a mass of 95% U-235, in the shape of a hollow cylinder with no ends (like a bottomless cup). In the stern was another mass of 95% U-235 in the shape of a rod. In the middle was a lead barrier to separate the two masses. In the far back of the bomb, was a small explosive with an altimeter. When the bomb would be dropped, the altimeter would determine when the bomb should detonate, when at the appropriate elevation, the small explosive would ignite, launching the rod of uranium-235 through the lead barrier, into the uranium cylinder at the other end for a perfect fit. When the two portions unite, supercritical mass would be accomplished. At this mass, an alpha particle would collide with another atom, disintegrating it. The 2 or 3 neutrons released would hit 2 or 3 other atoms (supercritical mass means that every moving neutron would hit another U-235 atom), this would cause a chain reaction, until every single atom in the quantity was disintegrated. This would all take place in approximately 1ms (microsecond, 1X10-6 seconds)! The mass had to perfect, because if there was not enough mass, the explosion could be too slow, and if there was too much mass, the explosion could get ahead of the chain reaction. The uranium-235 atom generates a large amount of heat and gamma radiation when undergoing nuclear fission. For one uranium atom, the energy released is about 215 MeV (million electron-volts), which is 3.44X10-11 Joules; this is very little energy, but it’s for only 1 atom. “Little Man” had 100 kg (although, because of the technology, it is estimated that only about 1 kg of U-235 actually underwent fission within the 1 ms), or 2.56X1024 atoms (in 1 kg), which would be 8.81X1013 Joules (1 J = 1 kg ∙ m2/s2), or 5.50X1028 MeV. That’s the
equivalent of about 20,500 tons (about 41 million pounds, 19 million kilograms) of dynamite! (1 ton of dynamite = 4.3X109 Joules) That’s more power than all the bombs dropped in World War II combined (excluding the atomic bombs).
The aftermath of “Little Man” on Hiroshima was more destructive than what anyone could have imagined. On August 6th, 1945, at 8:16:02AM local time, the “Enola Gay”, an American B-29 bomber, released the “Little Man” atomic uranium bomb over Hiroshima, Japan. An estimated 100,000 people died instantly, and tens of thousands of others would die within a few months. 129,558 people were injured, and 176,987 people were left homeless. The bomb had completely annihilated 68% of the city to the complete nothingness of oblivion. 50% of the people that died were killed by the blast itself. The blast simply vaporized nearly 100% of everyone within a 1.5-mile radius. 35% of the total people killed died from the heat, fires, or wind. The wind generated by the explosion moved at several hundred miles per hour, tossing buildings, cars, and people like toys. The heat measured 3000°C to 4000°C; many of the people in the 1.5 to 2.5-mile radius melted from the heat. Glass bottles melted in people’s hands. Most of the people that died in the 1.5 to 2.5-mile radius died from suffocation, because everything was on fire, and oxygen was burnt up extremely fast. 15% of the total people that expired died from radiation poisoning. This was a long-term effect, causing extreme nausea, hair loss, severe illness (because the radiation would separate molecules in the cells), and in most cases, death. People today are still fighting the long-term effects of radiation poisoning from “Little Man”. Hopefully this will not be the future of the technological use of uranium. After dropping the atomic bomb on Hiroshima, co-pilot Robert Lewis wrote in his journal, “My God what have we done?” (CNN).
In 1970, the Nuclear Non-Proliferation Treaty was inaugurated. A treaty of the United Nations, the NPT was a landmark international treaty designed to prevent the spread of nuclear weapons. A total of 187 parties signed the treaty, the most significant number in any international agreement. This accord was made in the midst of the Cold War, where a nuclear holocaust was only a button push away. On March 5th, 1995, the UN met to again negotiate the NPT. The final product was an accordance to make the ruling of the NPT last for an infinite amount of time. The NPT also stated that countries must set rules about which countries should be allowed to purchase uranium.
Another use for uranium is nuclear fusion. Nuclear fusion is the joining of two nuclei to form a new element. Uranium, being the largest natural element, is the most commonly used element to make new synthetic manmade (transuranic) elements, such as plutonium and americium. Fusion is induced by extreme pressure to force two atoms into each other. Squishing a uranium-238 with a helium-4 will make the synthetic element plutonium-242. Nuclear fusion makes a lot of energy, but it is much harder to control than nuclear fission. Combining bismuth-209 and nickel-64 makes unununium-272 (Uuu).
Uranium has been one of the most important elemental discoveries for modern science. It has created an entirely new spectrum of energy production. Nuclear power plants are among the cleanest and most environmentally sound methods of energy making. Technology for radwaste disposable has been increasing, and in the future, will not be a concern for environmentalists. Yet, uranium has also been one of man’s darkest discoveries. The atomic bomb and modern day nuclear warheads show mankind’s animalistic nature. Modern nuclear missiles are said to be up to 200 times as power as the atomic bombs dropped on Hiroshima and Nagasaki. Overall, the unearthing of uranium unearthed a whole field of knowledge for the world, but it is up to the world to decide how to use it.
Ok, that's it, with bibliography, it's 15 pages, without, it's 13.
U R A N I U M !
