Nuclear Reactor Components

In a highly simplified view of nuclear fission reactors, we can reduce the key elements to just two components that control the chain reaction. Control rods are used to reduce the number of free neutrons and the moderator is used to slow down the neutrons so that they can be captured by other nuclei to prompt further fission events.

Without control rods, the number of fission events would increase exponentially and the chain reaction would run out of control, with catastrophic consequences. Note that we want a chain reaction to occur so that there is a sustained energy release but we don’t want a chain reaction where the number of fission events accelerates and the amount of energy (heat) increases out of control.

Without the moderator, the neutrons released from fission events would be moving too fast to be captured by other nuclei. The moderator reduces the neutron’s energy to thermal levels so that absorption is possible.

You may spot that the absence of a moderator would stop the reactor from working, even if there were no control rods, because the neutrons would have too much energy to be captured by other nuclei. This fact opens the door to “fail-safe” reactor designs, such as the pressurised water reactor (PWR) where any leak inside the pressure vessel will cause water to be lost (either as liquid or as steam) and, since the water acts as a moderator, the reactor will start to shut down.

Beyond your GCSE Physics exam, things are not really this simple and PWRs can undergo serious failure, as occurred in the Three Mile Island incident, in Pennsylvania USA in 1979, when a partial meltdown happened because heat could no longer be removed from the core.

To check your understanding of the basic components, there is an online test that you can complete at https://www.liveworksheets.com/cj1264423vv. Remember to choose Email My Answers to My Teacher and enter my usual school email address. You will also notice that the test ends with a short writing task that you are expected to email to me separately.