ELI5- Why does heat decrease reactivity in a nuclear reactor?

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ELI5- Why does heat decrease reactivity in a nuclear reactor?

In: Physics

It doesn’t always; depends on the design. For most water-based reactors (so, most reactors), it’s because the hotter the water is, the less dense it is. There’s literally fewer hydrogen atoms per unit volume. That means it’s worse at moderating (slowing down) the fast neutrons that come from fission. Since most reactors rely on moderated (slow) neutrons to sustain a chain reaction, less moderation means a decrease in reactivity.

In a reactor, a nuclear reaction is created by splitting atoms of some fissile material. When these atoms split, they release very, very fast neutrons (particles that make up a portion of the nuclei of an atom).

These neutrons then smash into other atoms of the fissile fuel, causing those atoms to then split. This is called a “nuclear chain reaction”.

However, the fissile atoms are very small and so are the neutrons. These fast neutrons are moving *so fast* that the chances of them hitting another atom is *astronomically* low. In order to increase the chances of them continuing the reaction, we use what is called a ‘neutron moderator’ to slow some of these neutrons down.

A perfect neutron moderator is meant to slow neutrons without absorbing them. In most reactors, we just use water because Hydrogen performs his role very well (Hydrogen has approximately the same mass as a neutron). Water is also an effective coolant, so it is also used to accumulate the heat energy from the reactor (which is then eventually used to drive turbines).

However, as you heat up water it becomes less dense (i.e. it wants to turn into steam) – It’s atoms are further apart from each other. Therefore, it becomes less efficient as a moderator since the fast neutrons are more likely to just pass through.

More heat = hotter water. Hot water is less dense, so fewer neutrons are moderated by it. Fewer moderated neutrons means fewer collisions with the fissile fuel. Fewer collisions means fewer atoms being split. Fewer atoms being split means less reactivity in the reactor and thus less heat.

Heat where?

In the fuel? The Doppler effect occurs and makes it harder for a good fission reaction to occur or can affect resonance absorption.

In the moderator? If you have an under moderated reactor, then rising temperature causes the moderator density to decrease, which lowers reactivity.