Video: Understanding Nuclear Fission Reactors

The control rods in a nuclear fission reactor are rods made of boron that are placed in the reactor core. Which of the following describes the purpose of control rods in a nuclear fission reactor? a) Control rods are used to speed up the fission neutrons such that it is more likely that they will interact with another fissionable nucleus and cause it to split. b) Control rods are used to slow down the fission neutrons such that it is more likely that they will interact with another fissionable nucleus. c) Control rods are used to remove heat from the reactor core, which is used to make steam for power generation. d) Control rods are used to absorb some of the neutrons. This keeps the chain reaction under control.

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Video Transcript

The control rods in a nuclear fission reactor are rods made of boron that are placed in a reactor core. Which of the following describes the purpose of control rods in a nuclear fission reactor? a) Control rods are used to speed up the fission neutrons such that it is more likely that they will interact with another fissionable nucleus and cause it to split. b) Control rods are used to slow down the fission neutrons such that it is more likely that they will interact with another fissionable nucleus. c) Control rods are used to remove heat from the reactor core, which is used to make steam for power generation. d) Control rods are used to absorb some of the neutrons. This keeps the chain reaction under control.

Okay, since all of our answer options can’t fit in the same screen, let’s summarize them briefly. Option a says that the general purpose of control rods in a nuclear fission reactor is to speed up the neutrons in that reactor. Option b said that the purpose is to slow down neutrons in a reactor. Option c says that control rods are there to remove heat from the core. And option d says that their purpose is to absorb some of the neutrons in the core.

When we think about a chain reaction in a reactor core, the overall idea is that a single neutron runs into a fissionable atom. And then that atom itself releases three more neutrons. Then each one of those neutrons can go and run into its own fissionable atom and create yet three more. Then each one of those neutrons can create three more of its own through another fission reaction, and so forth and so on. This is what it means for a chain reaction to speed up.

Not only does the reaction continue, but it grows as it does. Control rods are used in the reactor core to keep this reaction from growing at a higher and higher rate. Ideally, from a power generation perspective, it would continue but at a manageable steady rate. To help that happen, control rods are made of boron, which are likely to absorb these fission neutrons. So, instead of a particular neutron going on to increase the rate of this chain reaction, instead, it’s absorbed by the control rod. That way fewer overall neutrons exist. And therefore, the reaction rate is kept under control.

We see then that the function of control rods is to absorb some of the neutrons in this reaction. And that is answer option d which says that control rods are used to absorb some of the neutrons. This keeps the chain reaction under control. Now, let’s look at part two of this question.

Part two asks: Does removing the control rods from the core of the reactor increase or decrease the rate of the chain reaction?

In part one of this question, we saw that when no control rods are in place in the reactor core, the reactor is free to propagate and grow at an increasing rate. The control rods, which are put in the reactor core in order to soak up some of these neutrons and keep the reaction rate from getting out of control, are unable to do that, of course, when they’re removed from the reactor core.

In that case, so long as there continues to be fissionable atoms within the core, the reaction can continue growing at an ever-increasing rate. Since the purpose of the control rods is to decrease the rate of the chain reaction, removing them increases it. This is the effect of removing control rods from the reactor core.

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