9.4 Nuclear fission and fusion — Practise with help

1. 92 protons and $235 - 92 = 143$ neutrons.
2. A neutron has no electric charge.

If question 1 went wrong, look again at the atomic number and mass number in Section 9.1. If question 2 went wrong, look at the table of particles in Section 9.1.

The name tells you that something is being split.

Say what kind of nucleus splits, and what it splits into.

Nuclear fission is the splitting of a large and unstable nucleus, such as a nucleus of uranium or plutonium, into two smaller nuclei that are roughly equal in size. Energy is released.

After the neutron is absorbed, the nucleus becomes unstable and splits.

Say what the two products are, then say what else is released.

The nucleus absorbs the neutron and becomes unstable. It splits into two smaller nuclei that are roughly equal in size. Two or three neutrons and gamma rays are released. Energy is released, and the products carry it away as kinetic energy.

One fission releases more than one neutron. What can each neutron do?

Trace one neutron from a fission to the next fission, then say what happens to the number of fissions at each step.

Each fission releases two or three neutrons. Each of those neutrons can be absorbed by another uranium-235 nucleus, which then fissions and releases more neutrons. The number of fissions grows with each generation, so the reaction spreads through the sample. This is a chain reaction.

Think about how fast the energy would be released if every neutron caused another fission.

Say how many neutrons are allowed to cause the next fission in a reactor. Then describe the result if all of them did.

In a reactor the chain reaction is controlled so that, on average, only one neutron from each fission causes the next fission. The energy is then released at a steady rate that can make electricity. If every neutron caused another fission, the number of fissions would grow very quickly and release a huge amount of energy in a very short time, as in a nuclear weapon.

The name tells you that two nuclei are joining.

Say what kind of nuclei join and what they form. Then say what the missing mass becomes.

Nuclear fusion is the joining of two light nuclei to form a heavier nucleus. Some of the mass of the nuclei is converted into the energy of radiation.

One process splits a nucleus and the other joins two nuclei.

Write one line for fission and one for fusion, then name a place for each.

Fission splits a large nucleus into smaller nuclei, and it happens in a nuclear reactor in a power station. Fusion joins two light nuclei into a heavier nucleus, and it happens in the core of a star such as the Sun. Both release a large amount of energy.

The nuclei repel each other, so they need a very high temperature to join.

Say what is difficult to reach and what is difficult to hold.

Nuclei are positively charged, so they repel each other. They must move very fast to get close enough to join, which needs a very high temperature. It is difficult to reach such a high temperature and to hold it under control, so fusion is not yet used to generate electricity.