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24.9: Laboratory Activities for Chapter 24

Difficulty Level: At Grade Created by: CK-12

Carbon-14 Dating

If you’re a fan of the television CSI shows or other mystery or crime programming, you are probably aware of different means to estimate the timing of the poor unfortunate’s demise, dealing with factors such as body temperature, etc. For an archaeologist or an anthropologist, the trail of evidence is much colder. In the late 1940’s, Willard Libby of the University of Chicago devised a method to establish the age of even the oldest unearthed fossils based on the remaining amount of radioactive 14C\begin{align*}^{14}C\end{align*}. This isotope is one of three for the element carbon, which is ubiquitous in living systems. Carbon has an atomic number of six, and exists in three nuclear configurations: with six, seven and eight neutrons respectively. Thus the 14C\begin{align*}^{14}C\end{align*} isotope has a nucleus consisting of six protons and eight neutrons. This assembly spontaneously decays into Nitrogen-14 and the release of beta radiation.Radioisotopic carbon has been measured to decay at a constant rate, with half the initial amount remaining, after 5730 years. If it is assumed that the 14C\begin{align*}^{14}C\end{align*} is not replaced, the loss of 14C\begin{align*}^{14}C\end{align*} suggests the time interval since the artifact last exchanged CO2\begin{align*}CO_2\end{align*} with the atmosphere.

When the ratio of the remaining amount of 14C\begin{align*}^{14}C\end{align*} to 12C\begin{align*}^{12}C\end{align*} is compared to the same ratio in a living organism, the amount of time elapsing since the organism’s death can be analyzed.

Thus, over time, in any material that contains the element carbon, the amount of remaining 14C\begin{align*}^{14}C\end{align*} in a sample is an indicator of the age of the artifact.

One of the best-known applications of this technique was in the analysis of Ötzi, the alpine Ice Man. Found in a region straddling the Austrian-Italian border, by hikers in 1991, Ötzi was the name given to the partially mummified remains of a hunter located still frozen into a glacier. Ötzi provided a wealth of information to anthropologists in that he was still dressed with fur boots, and a pack with tools including a copper hatchet and arrows. Researchers even discovered the menu of his last meal, probably deer meat and wheat bran, by analyzing his stomach contents. Analysis of small tissue samples from his corpse suggest that he lived from 5300 to 5100 years ago, before the Bronze Age. The construction and contents of his clothing and possessions provide an invaluable insight into the culture and technological sophistication of his age.

Other applications of this technology include dating of the Dead Sea scrolls, and analysis of the time period of the cave art found in central Europe. Ocean sediment samples, and even a meteorite believed to have originated on Mars!

Facts and Myths About Civilian Nuclear Power Plants

• MYTH: Nuclear reactors may undergo a nuclear explosion killing tens of thousands of people.
• FACT: Civilian nuclear power plants in the U.S. never contain a supercritical mass of fissionable material and therefore, cannot explode even if operators tried to make them explode.
• MYTH: Nuclear power plants are not safe.
• FACT: 1. The radiation levels measured outside of the containment building of nuclear power plants are essentially the same as background radiation. 2. There was a nuclear accident in 1986 at the Chernobyl nuclear installation in the Soviet Union that resulted in the immediate death of 28 people (mostly employees and fire fighters), the subsequent death of 19 people, and 9 deaths from thyroid cancer apparently due to the accident. The number of injuries due to fall-out radiation from the accident in unknown (at that time, news from the Soviet Union was highly censored). Predictions of numbers of injuries by UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) were in excess of 4,000 but the number was disputed by the IAEA (International Atomic Energy Agency).

In either a nuclear explosion or a severe nuclear accident, a large amount of radioactive material rises into the upper atmosphere. At some later time, this radioactive material falls back to the earth, usually downwind from where the explosion or accident occurred. This material that falls back to the earth is known as fall-out. It is known that the fall-out from the Chernobyl accident was 30 times the fall-out from the Hiroshima and Nagasaki bombings and that 300,000 people were evacuated from the Chernobyl area. It is also known that the fall-out reached as far as Sweden because Swedish workers at a Swedish nuclear power plant began testing positive for radiation, and after a thorough check of their own plant, officials determined the radiation was coming from fall-out from the Chernobyl plant. The greatest threat from fall-out is to children because children are growing and developing rapidly and radiation like all toxic materials have the greatest effect on children. It may still be years before all the effects of the Chernobyl accident are known. A small area called the Chernobyl Exclusion Zone is still closed, but the rest of the fall-out area is now considered safe.

The Chernobyl accident occurred in an early model of Soviet reactor that had no reaction vessel and no containment building. The fuel rods and control rods were inserted into graphite blocks. The graphite blocks worked well as a moderator, but graphite is combustible. When a fire started in the reactor, the employees and emergency workers were unable to control it, and the graphite burned away releasing radiation into the environment. There are some military reactors of this type in the United States, such as the one at Hanford, WA and there have been some radiation injuries at that facility. Civilian nuclear power plants in the U.S., however, do not use the graphite block reactor design. All U.S. civilian nuclear generating plants use LWR reactors, which have the reactor core submerged in a vessel of water and surrounded by a containment building. They also have a series of fail-safe shut down safety measures.

• MYTH: Hundreds of uranium miners die every year from radiation sickness.
• FACT: Hundreds of uranium miners died in the early days of uranium mining but that problem was solved long ago. On the other hand, 5,000 coal miners continue to die every year worldwide due to cave-ins, explosions, and black lung disease.
• MYTH: Nuclear reactors produce a large amount of radioactive waste that will be dangerous for thousands of years.
• FACT: When the percentage of U235\begin{align*}U-235\end{align*} in fuel rods gets below a certain level, they will no longer function as fuel and must be replaced. Even though the radioactivity is too low to function as fuel rods, they are still extremely dangerous, and must be isolated for a long period of time. Several suggestions have been made for storage of this used fuel, but even though the method is considered safe by nuclear scientists, the people who live in the area where the waste is to be stored strongly oppose having the material stored near them. At the present time, the used fuel rods are still submerged in the reaction vessels where they were replaced. Now that the US government has removed the ban on recycling used fuel, the amount of radioactive waste will be reduced to approximately one-fourth of the present amount. Not only will recycling help with the waste disposal problem, it will also reduce the cost of fuel. The Department of Energy is considering space disposal (rocket the waste into the sun), geological disposal (burying the waste thousands of feet underground in geologically stable areas), transmutation disposal (building a nuclear reactor that will consume nuclear waste; this idea was banned by President Carter, reinstated by President Reagan, and was being investigated by President Bush). For a complete discussion on the handling of nuclear waste, the internet has several dozen sites.
• MYTH: Nuclear reactors are particularly vulnerable to terrorist attack.
• FACT: In 1988, Sandia National Laboratories conducted a test by slamming a military F-4 Phantom jet fighter into a concrete block built to simulate a nuclear reactor containment building. The airplane hit the block at 481 miles\begin{align*}481 \ miles\end{align*} per hour and while the airplane was demolished, the six-foot thick wall suffered a dent 2.5 inches\begin{align*}2.5 \ inches\end{align*} deep. The Turkey Point Nuclear Generating station (near Miami, Florida) suffered a direct hit by hurricane Andrew in 1992. Turkey Point has two fossil fuel units and two nuclear units. The fossil fuel plants suffered \$90 million of damage while the nuclear containment buildings were undamaged.
• COMPLAINT: Some countries may divert nuclear reactor materials to weapon building.
• FACT: True.
• COMPLAINT: When the cooling water from nuclear power plants is dumped back into the original source (river, lake, bay), the temperature of the water over a period of time may be raised several degrees. The amount of oxygen that water will hold in dissolved form is highly dependent on the temperature. Active fish (so-called sport fish) frequently move away from areas where the temperature has increased a few degrees and less active fish (so-called trash fish) move in. To keep everyone happy, the use of cooling towers needs to be greatly increased so that the water returned to its natural source is at the same temperature as when it was taken.
• FACT: True.

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