Half-Life
Half-Life
When Uranium-238 (U-238) decays, its final decay product is Lead-206
The figure below is an outline that you can use to show the amount of U-238 and its decay product. The first column represents 100 grams of U-238.
1. In the column labeled “After 1 half-life”, shade in the amount of the column to show the amount of U-238 that would remain after one half-life. Label the column to indicate how much U-238 AND how much decay product would be present.
2. In the column labeled “After 2 half-lives”, shade in the amount of the column to show the amount of U-238 that would remain after two half-lives. Again, label the column to indicate how much U-238 and decay product would be present.
3. In the column labeled “After 3 half-lives”, repeat the procedure you used in steps 1 and 2 to show how much of each substance would be present after 3 half-lives.
After 1 half-life After 2 half-lives After 3 half-lives
100
90
80
70
60
50
40
30
20
10
0
0 4.5 9 13.5
Time (billions of years)
Application:
1. Suppose a rock originally contained 10 grams of U-238
a. After 1 half-life, the rock would probably contain ________ grams of U-238 and ________ grams of decay product.
b. After 2 half-lives, the rock would probably contain ________ grams of U-238 and ________ grams of decay product.
2. Suppose a rock sample contains 2 grams of U-238 today. How much U-238 would it probably have contained 1 half-life ago (4.5 billion years ago)? ________________
Explain: ____________________________________________________________
3. Uranium-235 (the fuel commonly used in the production of nuclear power) has a half-life of 710 million years. Assume that the waste will be safe only when its radioactivity has dropped to .10% of its original level.
a. How many half-lives are required for this degradation? ____________________
b. How long will the fuel need to be stored in order to be safe? ________________
4. Assume a sample of radioactive waste has a half-life of 20 years and an activity level of 4 curies. The waste will only be safe when the activity level has dropped to .25 curies.
a. How many half-lives are required for this degradation? ____________________
b. How long will the fuel need to be stored in order to be safe? ________________
Half-Life
When Uranium-238 (U-238) decays, its final decay product is Lead-206
The figure below is an outline that you can use to show the amount of U-238 and its decay product. The first column represents 100 grams of U-238.
1. In the column labeled “After 1 half-life”, shade in the amount of the column to show the amount of U-238 that would remain after one half-life. Label the column to indicate how much U-238 AND how much decay product would be present.
2. In the column labeled “After 2 half-lives”, shade in the amount of the column to show the amount of U-238 that would remain after two half-lives. Again, label the column to indicate how much U-238 and decay product would be present.
3. In the column labeled “After 3 half-lives”, repeat the procedure you used in steps 1 and 2 to show how much of each substance would be present after 3 half-lives.
After 1 half-life After 2 half-lives After 3 half-lives
100
90
80
70
60
50
40
30
20
10
0
0 4.5 9 13.5
Time (billions of years)
Application:
1. Suppose a rock originally contained 10 grams of U-238
a. After 1 half-life, the rock would probably contain ________ grams of U-238 and ________ grams of decay product.
b. After 2 half-lives, the rock would probably contain ________ grams of U-238 and ________ grams of decay product.
2. Suppose a rock sample contains 2 grams of U-238 today. How much U-238 would it probably
have contained 1 half-life ago (4.5 billion years ago)? ________________
Explain: ____________________________________________________________
3. Uranium-235 (the fuel commonly used in the production of nuclear power) has a half-life of 710
million years. Assume that the waste will be safe only when its radioactivity has dropped to .10%
of its original level.
a. How many half-lives are required for this degradation? ____________________
b. How long will the fuel need to be stored in order to be safe? ________________
4. Assume a sample of radioactive waste has a half-life of 20 years and an activity level of 4 curies.
The waste will only be safe when the activity level has dropped to .25 curies.
a. How many half-lives are required for this degradation? ____________________
b. How long will the fuel need to be stored in order to be safe? ________________
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