Experiment 3

Half-Life: The Energizer Bunny^reg. Effect

Objective: This experiment will illustrate the principles of a half-life.

Review of Scientific Principles:

The half-life of a radioactive substance is the amount of time it takes for one half of a substance to change into something else. After each half-life only half of the original substance remains. During that change some type of "radiation," either alpha (a helium nucleus), beta (an electron), or gamma (high-energy light), is emitted. A substance which undergoes this type of decay is called radioactive. Radioactivity is all around you--from the food you eat to the bricks in the buildings surrounding you. Radioactive elements that occur naturally are considered part of background radiation. Background radiation comes from anything that is part of the natural world that is around all of the time. Because of this, you can easily conclude that all radioactivity is not deadly. Rather, your body is bombarded with radiation every minute of every day, especially if you get lots of exposure to the sun. Several every day ordinary objects are slightly radioactive, including table salt substitute and bananas!

Nuclear power plants do not emit radioactivity. The radioactive material used in nuclear power facilities is contained in the fuel rods inside the core of the reactor. In some reactors the water coolant also becomes slightly radioactive, but has a short half life and is contained inside the plant.

Fortunately, very little high level waste is made per reactor per year. Unlike a coal plant which produces about 15 tons of carbon dioxide, 200 pounds of sulfur dioxide, and about 1,000 tons of solid ash per minute, the high level waste from one year of nuclear power plant operation produces about 1.5 tons and would occupy a volume of about half a cubic yard, which could easily fit under your coffee table! The amount of high level radioactive waste produced per person from nuclear power for a 70 year life span is about the size of a soda can.

Other things become radioactive in the process of operating a nuclear power plant, however. Objects like water and air filters for trapping radioactive material, rags, gloves, lab equipment, pipes, and mops are considered low-level radioactive waste. They have been used near or in the reactor and were exposed to neutrons. About 25% of all low-level waste comes from hospitals, research labs, and industry. Although the radioactivity in low-level waste is about a million times lower than that in high level waste, it occupies about 1,000 times the volume of high-level waste. Because the radioactivity is so low, low-level waste is buried at about 20 feet underground in controlled areas and allowed to decay.

Practical Applications: How long do we need to store nuclear waste?

Time: 20 minutes

Materials and Supplies:

• 80 M & M's ( or 80 Pennies) per lab group
• one box per lab group (the boxes copier paper comes in work well)
• 1 meter stick
• graph paper
• tape
• red pencil

Procedure:

1. Put 80 M & M's "M's up" in a box.
2. Put the lid on the box and shake.
3. Open the box; leave all the "M's up" M & M's in the box, remove all the blank M & M's.
4. Record the number of removed M & M's in the data table.
5. Repeat steps 2-4 more times
6. Write your results on a large table on the board.
7. Plot on graph paper the number of half-lives on the x-axis and the number of non-decayed atoms on the y-axis in red pencil.
8. Average the class data on the board.
9. Plot the class data in normal pencil color on your graph in the same manner described above, and be sure to label each of the graphed lines clearly .

Video Clip

Data and Calculations

Questions:

1. Treat the M & M's as radioactive units. Using your data, how many half-lives does it take to have an inconsequential amount of radioactive material left? Is it ever really gone (HINT: think about the Energizer Bunny^reg.--it keeps going and going and going...)?

2. Suppose you were given $1,000,000. How long would it take to have less than five dollars left if you spent half of it every day?

3. Uranium-238 has a long half life. If 450,000 people live to be 70, how many "soda can" size pieces of nuclear waste will we have to store? How much room would that take up is stacked together?

4. Suppose a material emits 25 rem/year (which causes radiation sickness if felt in a single dose), and has a half-life of 5 years. How many years before it emits only 200 millirem/year, which is approximately the amount of background radiation you receive every year. HINT: 1 rem = 1,000 millirem.

Experiment 3: Teacher's Notes

Half-Life: The Energizer Bunny^reg. Effect

Objective: This experiment will illustrate the principles of a half-life.

Practical Applications: How long do we need to store nuclear waste?

Prep time: 10 minutes

Time: 30 minutes

Sample Data and Calculations (One example) :

Questions:

1. Treat the M & M's as radioactive units. Using your data, how many half-lives does it take to have an inconsequential amount of radioactive material left? Is it ever really gone? Five half-lives. No; there is always half of the amount left.
2. Suppose you were given $1,000,000. How long would it take to have less than five dollars left if you spent half of it every day? 17 days.
3. Uranium-238 has a long half life. If 450,000 people live to be 70, how many "soda can" size pieces of nuclear waste will we have to store? 450,000 * 1 = 450,000

   450,000*(~18 in²) = 8,100,000 in² = 56,250 ft² = 6250 yd²

4. Suppose a material emits 25 rem/year (which causes radiation sickness if felt in a single dose), and has a half-life of 5 years. How many years before it emits only 200 millirem/year, which is the amount of background radiation you receive every year.
   HINT: 1 rem = 1,000 millirem.
   The easiest way to do this is to divide the amount by 2 for every 5 years.
   25,000 mrem / 2 = 12,500 mrem after 5 years
   10 years: 6,250 mrem 15 years: 3,125 mrem
   20 years: 1,562.5 mrem25 years: 781.25 mrem
   30 years: 390.6 mrem35 years: 195.31 mrem
   The material will decay to background level after 35 years!
   

   Next Topic: Experiment 4

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