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(Anzar Abbas) At a nuclear plant,
unstable Uranium nuclei are undergoing radioactive decay.
All the actions take place inside this reactor,
where nuclear fission occurs.
It's where the nuclei of uranium isotopes break apart,
creating energy that heats up water,
turning it into the steam that powers the turbines
and converts that energy into useful electricity.
One of the biggest challenges is storing the used fuel
left over from these fission reactions.
These biproducts are radioactive.
In this segment, we're gonna talk about half-life
and why some radioactive materials can last a long time.
Joining me is Michael McCracken at Plant Vogtle.
Michael, explain what happens to the spent fuel
and why you take so much care to store and contain it.
The used fuel that we, uh, placed into the, uh,
spent fuel pool can stay in there for decades.
We've had fuel in there since the 19-- 1987-1988 time period.
After a number of years, after at
you can take out that used fuel from the wet storage
or the pool storage and place it into canisters,
and that's what we have behind us.
Our spent fuel is called dry cask canisters,
and it's basically the fuel is inside a, uh,
a stainless steel canister that's welded shut,
and then that stainless steel canister is placed inside
a metal-- a-a double wall metal canister
that's then filled with concrete.
So, there's multiple layers of metal and concrete,
and that keeps the-the used fuel safe inside,
so people can walk right by it and-and not receive--
uh, uh, uh, absorb much radiation, at all.
Thanks, Michael.
Half-life is the time it takes for one-half
of the radioactive nuclei in a sample to decay.
After half of the radioactive atoms of the material
have decayed into another element,
half of it still remains, right?
Then, it takes the same length of time
for the next half of the material to decay.
Here's an example.
Phosphorus-32 is an isotope of phosphorus
with a half-life of 14.3 days.
Let's say this rectangle is 100 grams of Phosphorus-32.
In 14.3 days, half of the Phosphorus-32 will decay
into another element, Sulfur-32.
You have 50 grams of Phosphorus-32 left.
In another 14.3 days, half of that, or 25 grams,
will decay into Sulfur-32.
In another 14.3 days,
another half decays,
and so on.
It continues to decay
at this exponential rate,
but it never really goes away.
It gets down to minuscule amounts
but never gets down to zero.
How long the half-life is, depends on the material.
The chart shows you the half-life of different isotopes.
Uranium-238, for example,
has a really long half-life of about
4.5 billion years, and, remember,
that's only how long it takes for just half of this isotope
to decay.
See if you can figure this out.
Let's say you have a radioactive material
with a half-life of eight minutes,
and let's say you start with 200 grams of the substance.
How much of the substance will be left after 32 minutes?
The first thing to do is determine the number
of half-lives that occur in the total time.
You do this by dividing the total time by the half-life.
The total time we know is 32 minutes,
and the half-life time is 8 minutes.
By dividing 32 by 8,
we know that 4 half-life periods have occurred.
So, you divide the initial amount of the sample,
which is 200 grams, by 2, four times.
So, 12.5 grams of the substance remains
after 4 half-life periods.
For more a general equation to determine the amount
of a substance that remains when the number of half-lives
isn't a nice even number,
multiply the initial amount times one-half
raised to the number of half-lives that occur.
The equation would look something like this.
If we plug in the numbers from our example problem,
the remaining math would be equal to the starting material
of 200 grams times one-half to the fourth power,
and you end up with 12.5 grams of the substance remaining.
You can plot it out on a graph.
The rate of decay graph for this substance looks like this.
Another thing to know is that the half-life of a substance
is based on how stable the radioactive isotope is.
Half-lives range from less than a millionth of a second
up to billions of years.
Isotopes that have very short half-lives are very unstable,
and those with very long half-lives are much more stable
and not as ready to decay.
And another valuable thing to know is that the half-life
of a radioactive substance is a constant value.
It's not affected by external factors.
So, no matter the temperature, the pressure,
or the presence of a magnetic or an electric field,
the half-life of a radioactive substance does not change.
It's so consistent that scientists use the amount
of a substance present in a sample, like a fossil,
to determine its age.
For example, isotopes with very long half-lives
have been used to determine the age of fossils.
It's a process called radiometric dating.
It was first used at the turn of the 20th century
by Ernest Rutherford.
He's a key guy since he also discovered the atomic nucleus.
To determine the age of a rock, for example,
you can see that there is a ratio between
an unstable isotope found in the rock
that's called the parent isotope,
written NP, and the stable isotope that's created
during the parent's radioactive decay.
That's the daughter isotope, written in ND.
A common parent isotope for determining
the age of rocks is Uranium-235.
It takes roughly 710 million years for it to decay
into the lighter, more stable Lead-207.
That's the daughter isotope.
So, the higher the radioactive metric ratio,
the more decay has taken place.
The smaller the ratio, the younger the rock.
The larger the ratio, the older the rock.
Carbon-14 is one of the most commonly used
radioactive isotopes in dating artifacts.
Carbon-14 is a natural occurring isotope,
and it's extremely rare compared to the more common Carbon-12
and Carbon-13 isotopes.
The half-life of Carbon-14 is about 5,700 years.
So, scientists can use Carbon-14 dating
to estimate the age of artifacts up to 60,000 years old.
Here's how.
Organic substances,
which all living substances are,
ingest Carbon-14,
and they have the same percentage of Carbon-14 in them
as is found in the atmosphere.
Once that organic substance dies,
it no longer takes in Carbon-14,
and whatever Carbon-14 is present
in the substance decays.
The scientists then use the percentage of Carbon-14
that remains in the substance
to estimate the age of the artifact.
So, hopefully, now you have a better understanding
of radioactive decay, half-lives,
and how to use this information
to get a better understanding of the world around us.
That's it for this segment of "Physics in Motion",
and we'll see you next time.
(female) For more practice problems,
lab activities, and note-taking guides,
check out the "Physics in Motion" toolkit.