The science of nuclear energy
The science of nuclear energy

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

The science of nuclear energy

2.3.1 Half-life

When quantifying the risk posed by a particular isotope, it’s important to consider the amount of time that it will remain radioactive and its activity during this time. Both of these quantities relate to the half-life of the isotope.

Last week, we defined activity as a measure of the number of particles (alpha, beta or gamma) that are emitted in a given time. As the particles are emitted, the isotope is said to decay and changes into an isotope of another element.

Let’s consider the alpha emission that you looked at last week:

cap u 92238 emits an α particle and decays to times times Th 90234 postfix times.

If there are a certain number of uranium-238 atoms in a particular sample, the half-life is the time taken for half of these radioactive atoms to decay. After another half-life, half of the remaining atoms will decay and so on.

Imagine you were given 1200 atoms of uranium (in reality it would be a much larger number). After one half-life, half the uranium atoms will have decayed into thorium, so you will only have 600 uranium atoms left. After another half-life, another half will have decayed so you will have 300 uranium atoms, after another half-life you will have 150 uranium atoms. After four half-lives you would be left with 75 atoms of uranium and 1125 atoms of thorium.

Now, in fact, the half-life of uranium is 4.5 billion years so you would have to watch your atoms for a long time to see them decay! Half-lives can vary from billions of years to nanoseconds. Some half-lives are shown in Figure 14.

Described image
Figure 14 The main decay chain for uranium-238; other radioisotopes similarly have their own characteristic decay chains

If an isotope has a short half-life, it will decay quickly, and emit more particles in a given time than an isotope with a longer half-life. So isotopes with shorter half-lives have higher activity than those with longer half-lives. The activity of all isotopes will diminish over time as the number of atoms that are present decay away. It is worth noting that, unlike chemical reactions, the rate at which radioactive isotopes decay at any particular moment cannot be changed; heating them, subjecting them to high or low pressures, or to any other physical process, does not alter the half-life.

Both the high rates of decay, or activities, of radioisotopes with short half-lives and the longer life span of those with long half-lives have an impact on the disposal of radioactive waste. Some products of fission have half-lives of the order of hours or days, while others have half-lives of thousands of years or more. This requires both short- and very long-term planning when considering what to do with the waste.

Next, you’ll look in more detail at the different sorts of radioactive waste produced.


Take your learning further371

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses372.

If you are new to university level study, we offer two introductory routes to our qualifications. Find out Where to take your learning next?373 You could either choose to start with an Access courses374or an open box module, which allows you to count your previous learning towards an Open University qualification.

Not ready for University study then browse over 1000 free courses on OpenLearn375 and sign up to our newsletter376 to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371