Radiocarbon Dating Assignment Chemistry

Name: ___________________________ Date: ______________ Hour: ________

Radiocarbon Dating (http://www.c14dating.com/) The radiocarbon method was developed by a team of scientists led by the late Professor Willard F. Libby of the University of Chicago after the end of World War II. Libby later received the Nobel Prize in Chemistry in 1960 for the radiocarbon discovery. Today, there are over 130 radiocarbon dating laboratories around the world producing radiocarbon dates for the scientific community. The 14C method has been and continues to be applied and used in many, many different fields including hydrology, atmospheric science, oceanography, geology, palaeoclimatology, archaeology and biomedicine. Who developed radiocarbon dating? _______________________________________________________ ___________________________________________________________________________________ __ When was this method developed? ________________________________________________________ ______________________________________________ _______________________________________ All plants and animals on Earth are made principally of carbon. During the period of a plant's life, the plant is taking in carbon dioxide through photosynthesis, which is how the plant makes energy and grows. Animals eat plants, and some eat other animals in the food chain. Carbon follows this pathway through the food chain on Earth so that all living things are using carbon, building their bodies until they die. How do animals get carbon into their bodies? _______________________________________________ ______________________________________________________________ _______________________ A tiny part of the carbon on the Earth is called carbon-14 (14C), or radiocarbon. Radiocarbon is formed when 14N in the atmosphere is changed into 14C by cosmic rays. It is then called 'radio'-carbon, because it is 'radioactive'. This means that its atomic structure is not stable and there is an uneasy relationship between the particles in the nucleus of the atom itself. Eventually, a particle is emitted from the carbon-14 atom, and carbon-14 changes into something else. Most of the carbon on Earth exists in a slightly different atomic form, carbon-12, although in terms of chemically properties, all carbon is identical. (There is approximately one radioactive 14C atom for every billion stable 12C atoms.) How is carbon-14 formed? _______________________________________________________________ _____________________________________________________________________________________ What does it mean to be radioactive? _______________________________________________________ ________________________________________________________ _____________________________ If I had two billion stable carbon-12 atoms inside me, how many carbon-14 atoms would also be present? __________________________________________________ ___________________________________ While alive, organisms have a fixed 12C to 14C ratio. After dying, organisms don’t absorb any more C from the atmosphere; the 14C they have is radioactive and decays away. The organism’s carbon ratio is then compared to the “living” ratio to estimate how long the organism has been dead. 14

In the 1940s, scientists succeeded in finding out how long it takes for radiocarbon to decay from a sample of carbon from a dead plant or animal. Willard Libby, the principal scientist, had worked in the team making the nuclear bomb during World War II, so he was an expert in nuclear and atomic chemistry. After the war he became very interested in peaceful applications of atomic science. He and two students first measured the “half-life” of radiocarbon. The half-life refers to the amount of time it takes for half the radiocarbon in a sample of bone or shell or any carbon sample to disappear. Libby found that it took 5568 years for half the radiocarbon to decay. After twice that time (about 11000 years), another half of that remaining amount will have disappeared. After another 5568 years, again another half will have disappeared. You can work out that after about 50 000 years of time, all the radiocarbon will have gone. Therefore, radiocarbon dating is not able to date anything older than 60 000 or 70 000 years old. The job of a radiocarbon laboratory is to measure the remaining amounts of radiocarbon in a carbon sample. This is very difficult and requires a lot of careful work to produce reliable dates. What is meant by the term half-life? ___________________________________________ ____________ __________________________________________________________ ___________________________ __________________________________________________________________________________ ___ What is the half-life of radiocarbon? _______________________________________________________ How many years does it take for all the radiocarbon in a sample to decay? _________________________ What is measured in a radiocarbon lab? _____________________________________________________ _____________________________________________________________________________________ Libby tested the new radiocarbon method on carbon samples from prehistoric Egypt whose age was known. A sample of acacia wood from the tomb of the pharoah Zoser was dated for example. Zoser lived during the 3rd Dynasty in Egypt (2700-2600 BC). Libby figured that since the half-life of 14C was 5568 years, they should obtain a radiocarbon amount of about 50% of that which was found in living wood because Zoser's death was about 5000 years ago. The results they obtained indicated this was the case. Many other radiocarbon dates were conducted on samples of wood of known age. Again, the results were good. In 1949, Libby and his team published their results. By the early 1950s there were 8 new radiocarbon laboratories, and by the end of the decade more than 20. How was Libby sure the radiocarbon method worked? _________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ We can date rice, pollen, seeds, and tiny pieces of charcoal. We can now date a variety of very, very small samples. Many more archaeological and geological samples can be dated than before. It is possible to test radiocarbon dates in different ways. One way is to date things that you already know the age of. Libby did this when he first developed the method, by dating artefacts of Egyptian sites, which were already dated historically. Another way is to use tree rings. Every year a tree leaves a ring, the rings increase in number over time until a pattern of rings is formed. Sometimes the tree has many hundreds of rings. Scientists can date the age of the tree by counting and measuring the rings. Radiocarbon daters can then date the tree rings and compare the dates with the real age of the tree. How can trees be used to verify radiocarbon dates? ___________________________________________ _______________________________________________________ ______________________________ _______________________________________________________________________________ ______

Look at the following diagram about how carbon-14 forms and decays:

(a)

Is radioactive carbon-14 naturally produced or caused by humans? _________________________

(b)

How does radioactive carbon-14 get into humans? ______________________________________ ________________________________________________________ _______________________ _______________________________________________________________________________

(c)

After a plant or animal is dead, will its amount of radioactive carbon-14 increase or decrease? _____________________________________________________ __________________________ _______________________________________________________________________________

(d)

Using information from the article and the diagram, how can scientists use carbon-14 dating to tell how old a bone is?

Look at the following graphs and answer the questions: Graph 1:

What is the half-life of this radioactive substance? _____________________________________ Graph 2:

What is the half-life of this radioactive substance? _____________________________________ Graph 3:

What is the half-life of this radioactive substance? _____________________________________

Read the following articles and answer the questions: How Japan's Nuclear Crisis Works (http://science.howstuffworks.com/japan-nuclear-crisis1.htm) by Marshall Brain Different people have different opinions of the nuclear power industry. Some see nuclear power as an important green technology that emits no carbon dioxide while producing huge amounts of reliable electricity. They point to an admirable safety record that spans more than two decades. Others see nuclear power as an inherently dangerous technology that poses a threat to any community located near a nuclear power plant. They point to accidents like the Three Mile Island incident and the Chernobyl explosion as proof of how badly things can go wrong. What are two positives of nuclear power? ___________________________________________________ _____________________________________________________________________________ ________ What are two negatives of nuclear power? ___________________________________________________ __________________________________________________________________________________ ___ In either case, commercial nuclear reactors are a fact of life in many parts of the developed world. Because they do make use of a radioactive fuel source, these reactors are designed and built to the highest standards of the engineering profession, with the perceived ability to handle nearly anything that nature or mankind can dish out. Strength is built in, and layers of redundancy are meant to handle any operational abnormality. Shortly after an earthquake hit Japan on March 11, 2011, however, those perceptions of safety began rapidly changing. Explosions rocked several different reactors in Japan, even though initial reports indicated that there were no problems from the quake itself. Fires broke out at the Onagawa plant, and there were explosions at the Fukushima Daiichi plant. So what went wrong? Let's take a look. What is redundancy? ____________________________________________________________________ ______________________________________________________________ _______________________ At a high level, these plants are quite simple. Nuclear fuel, which in modern commercial nuclear power plants comes in the form of enriched uranium, naturally produces heat as uranium atoms split. The heat is used to boil water and produce steam. The steam drives a steam turbine, which spins a generator to create electricity. These plants are large and generally able to produce something on the order of a gigawatt of electricity at full power. How is nuclear power produced? __________________________________________________________ ____________________________________________________________________________________ ______________________________________________________________________________________ In order for the output of a nuclear power plant to be adjustable, the uranium fuel is formed into pellets approximately the size of a Tootsie Roll. These pellets are stacked end-on-end in long metal tubes called fuel rods. The rods are arranged into bundles, and bundles are arranged in the core of the reactor. Control rods fit between the fuel rods and are able to absorb neutrons. If the control rods are fully inserted into the core, the reactor is said to be shut down. The uranium will produce the lowest amount of heat possible (but will still produce heat). If the control rods are pulled out of the core as far as possible, the core produces its maximum heat. Think about the heat produced by a 100-watt incandescent light bulb. These bulbs get quite hot–hot enough to bake a cupcake in an Easy Bake oven. Now imagine a 1,000,000,000-watt light bulb. That is the kind of heat coming out of a reactor core at full power. How is a nuclear reactor shut down? _______________________________________________________ _____________________________________________________________________________________

The reactors that failed in Japan are Mark 1 boiling water reactors designed by General Electric in the 1960s. This is one of the earlier reactor designs, in which the uranium fuel boils water that directly drives the steam turbine. This design was later replaced by pressurized water reactors because of safety concerns surrounding the Mark 1 design. As we have seen, those safety concerns turned into safety failures in Japan. A boiling water reactor has a fatal flaw–the cooling system. As the water boils, it creates a huge amount of pressure to spin the steam turbine. The boiling water also keeps the reactor core at a safe temperature. When it exits the steam turbine, the steam is cooled and condensed to be reused over and over again in a closed loop. The water is recirculated through the system with electric pumps. The design’s vulnerability comes into play if the electric pumps lose power. Without a fresh supply of water in the boiler, the water continues boiling off, and the water level starts falling. If enough water boils off, the fuel rods are exposed and overheat. At some point, even with the control rods fully inserted, there is enough heat to melt the nuclear fuel. This is where the term meltdown comes from. Tons of melting uranium flows to the bottom of the pressure vessel. At that point, it’s catastrophic. In the worst case, the molten fuel penetrates the pressure vessel gets released into the environment. What are the two roles of water in a boiling water reactor? ______________________________________ _______________________________________________________ _____________________________ ______________________________________________________________________________________ Because of this known vulnerability, there is huge redundancy around the pumps and their supply of electricity. There are several sets of redundant pumps, and there are redundant power supplies. Power can come from the power grid. If that fails, there are several layers of backup diesel generators. If they fail, there is a backup battery system. With all of this redundancy, it seems like the vulnerability is completely covered. Unfortunately, shortly after the earthquake, the worst-case scenario unfolded. The nuclear power plants in Japan weathered the earthquake itself without difficulty. The four plants nearest the quake’s epicenter shut down automatically, meaning that the control rods were fully inserted into their reactor cores and the plants stopped producing power. This is normal operating procedure for these plants, but it meant that the first source of electricity for the cooling pumps was gone. That isn’t a problem because the plant could get power from the power grid to run the pumps. However, the power grid became unstable, and it shut down as well. The second source of electricity for the cooling pumps was gone. That brought the backup diesel generators into play. Diesel generators are a robust and time-tested way to generate electricity, so there were no worries. But then the tsunami hit. And unfortunately, the tsunami was far larger than anyone had planned for. If the backup diesel generators had been higher off the ground, designed to run while submerged in water or protected from deep water in some way, the crisis could have been averted. Unfortunately, the unexpected water levels from the tsunami caused the generators to fail. This left the last layer of redundancy–batteries– to operate the pumps. The batteries performed as expected, but they were sized to last for only a few hours. The assumption, apparently, was that electricity would become available from another source fairly quickly. What is a tsunami? _____________________________________________________________________ What went wrong at the power plant when the tsunami hit? _____________________________________ _____________________________________________________________________________________ Although operators did truck in new generators, they could not be hooked up in time, and the coolant pumps ran out of electricity. With the batteries dead, the coolant pumps failed. With no fresh coolant flowing into the reactor core, the water that kept it cool began boiling off. As the water boiled away, the tops of the fuel rods were exposed, and the metal tubes holding the uranium fuel pellets overheated and cracked. The cracks allowed water to enter the tubes and get to the fuel pellets, where it began generating hydrogen gas. The process is called thermolysis–if you get water hot enough, it breaks

down into its constituent hydrogen and oxygen atoms. Hydrogen is a highly explosive gas–recall the Hindenburg explosion, in which the Hindenburg was full of hydrogen gas. In Japan’s nuclear plants, pressure from the hydrogen built up, and the gas had to be vented. Unfortunately, so much hydrogen vented so quickly that it exploded inside the reactor building. This same chain of events unfolded in several different reactors. What was the Hindenburg explosion? ______________________________________________________ What gas caused the explosion at the power plant in Japan? _____________________________________ The explosions did not rupture the pressure vessels holding the nuclear cores, nor did they release any significant amounts of radiation. These were simple hydrogen explosions, not nuclear explosions. The explosions damaged the concrete and steel buildings surrounding the pressure vessels. The explosions also indicated that things had gotten out of control. If water were to continue boiling off, a meltdown would be almost assured. So operators decided to flood the reactors with seawater. This is a last-ditch effort to control the situation, since seawater completely ruins a reactor, but it’s better than a meltdown. In addition, the seawater was mixed with boron to act something like a liquid version of the control rods. Boron absorbs neutrons and is one of the main constituents in the control rods. What did the plant operators do to avoid a meltdown? _________________________________________ ________________________________________________________________ _____________________ The nuclear incidents in Japan are described as Level 6 INES events (International Nuclear and Radiological Event Scale). Chernobyl was a Level 7 event, and that is the top of the event scale. Obviously, it's a serious situation. Japan has lost a significant portion of its electrical generating capacity. Approximately a third of Japan's electricity comes from nuclear power plants, and about half of that capacity has been lost (approximately 20 percent of total generating capacity). That capacity will need to be replaced in some way. At 40 years old, these reactors are nearing the end of their design lifespans anyway. One alternative is to simply rebuild the plants. The two problems with this approach are that it will be a very lengthy process–possibly taking a decade or more–and the general public in Japan may have no appetite for new nuclear reactors. It is still too early to tell. The Benefits of Nuclear Energy (http://www.fi.edu/guide/wester/benefits.html) Nuclear energy is the world's largest source of emission-free energy. Nuclear power plants produce no controlled air pollutants or greenhouse gases. The use of nuclear energy in place of other energy sources helps to keep the air clean, preserve the Earth’s climate, avoid ground-level ozone formation, and prevent acid rain. Of all energy sources, nuclear energy has perhaps the lowest impact on the environment, including water, land, habitat, species, and air resources. Nuclear energy is the most eco-efficient of all energy sources because it produces the most electricity relative to its environmental impact. What is one positive of nuclear energy? _____________________________________________________ Of all energy sources, nuclear energy has perhaps the lowest impact on the environment, especially in relation to kilowatts produced because nuclear plants do not emit harmful gases, require a relatively small area, and effectively minimize or negate other impacts. Nuclear energy is an emission-free energy source because it does not burn anything to produce electricity. Nuclear power plants produce no gases such as nitrogen oxide or sulfur dioxide that could threaten our atmosphere by causing ground-level ozone formation, smog, and acid rain. Nor does nuclear energy produce carbon dioxide or other greenhouse gases suspected to cause global warming. Throughout the nuclear fuel cycle, the small volume of waste byproducts actually created is carefully contained, packaged and safely stored. As a result, the nuclear energy industry is the only industry established since the industrial revolution that has managed and accounted for all of its waste, preventing adverse impacts to the environment.

How is nuclear energy emission-free? ______________________________________________________ Nuclear power also provides water quality and aquatic life conservation. Water discharged from a nuclear power plant contains no harmful pollutants and meets regulatory standards for temperature designed to protect aquatic life. This water, used for cooling, never comes in contact with radioactive materials. If the water from the plant is so warm that it may harm marine life, it is cooled before it is discharged to its source river, lake, or bay as it is either mixed with water in a cooling pond or pumped through a cooling tower. Because the areas around nuclear power plants and their cooling ponds are so clean, they are often developed as wetlands that provide nesting areas for waterfowl and other birds, new habitats for fish, and the preservation of other wildlife as well as trees, flowers, and grasses. Is the water used in a nuclear power plant radioactive? Why or why not? __________________________ _____________________________________________________________________________________ Because nuclear power plants produce a large amount of electricity in a relatively small space, they require significantly less land for operation than all other energy sources. For instance, solar and wind farms must occupy substantially more land, and must be sited in geographically unpopulated areas far from energy demand. To build the equivalent of a 1,000-megawatt nuclear plant, a solar park would have to be larger than 35,000 acres, and a wind farm would have to be 150,000 acres or larger. Also, uranium is a concentrated, low-volume fuel source requiring few incursions into the land for extraction or transport. Nuclear plants also contribute to national energy security and ensure stable nationwide electricity supply. Unlike some other energy sources, nuclear energy is not subject to unreliable weather or climate conditions, unpredictable cost fluctuations, or dependence on foreign suppliers. What are two advantages of nuclear power over wind power? ___________________________________ __________________________________________________________________________ ___________ Nuclear power plants have long periods of operation. Nuclear power plants are designed to operate continuously for long periods of time. They can run about 540 days before they are shut down for refueling. US nuclear power plants use an enriched form of uranium for fuel. Uranium is a relatively abundant element that occurs naturally in the earth’s crust, about as common as tin. Compared to natural gas, uranium is already relatively low in cost and less sensitive to fuel price increases. And a little goes a long way: one uranium fuel pellet-the size of the tip of your little finger-is the equivalent of 17,000 cubic feet of natural gas, 1,780 pounds of coal, or 149 gallons of oil. Today, nuclear power plants, the second largest source of electricity in the United States, supply about 20 percent of the nation's electricity each year. The average electricity production cost in 1999 for nuclear energy was 1.83 cents per kilowatt-hour, for coal-fired plants 2.07 cents, for oil 3.24 cents, and for gas 3.52 cents. What is one advantage of nuclear power over natural gas? ______________________________________ _____________________________________________________________________________ ________ Despite popular belief, nuclear plants are relatively safe. For years, America’s commercial nuclear energy industry has ranked among the safest places to work in the United States. Even if you lived right next door to a nuclear power plant, you would still receive less radiation each year than you would receive in just one round-trip flight from New York to Los Angeles. You would have to live near a nuclear power plant for over 2,000 years to get the same amount of radiation exposure that you get from a single diagnostic medical x-ray. Now that you have read these two articles, do you think we should use nuclear power? Why or why not? ______________________________________________________________ _______________________ ______________________________________________________ _______________________________ ______________________________________________ _______________________________________ _____________________________________________________________________________________