E3 Moderation and Neutron Absorption

Topic

Two types of materials play opposing roles within the neutron balance established by the previous model. Moderators—such as ordinary water, heavy water, or graphite—do not absorb neutrons but rather slow them down; a neutron newly released by fission emerges with very high energy—a state in which it is unlikely to trigger further fission in most fuels. By repeatedly colliding with the moderator's light nuclei, the neutron loses energy until it reaches much lower thermal speeds, at which point its probability of inducing fission in uranium-235 increases significantly.

Neutron poisons do the exact opposite: they are materials selected for their high capacity to absorb neutrons without undergoing fission or producing a useful reaction. Each neutron they capture is permanently removed from the population available to sustain the chain reaction.

Control rods combine both roles in a practical way: they are made of material with a high capacity for neutron absorption. The extent to which they are inserted into the core directly determines how many neutrons they capture before those neutrons can trigger further fission, as the rods displace the moderator material that would otherwise occupy that same space.

Thus, this module—which begins by distinguishing between materials that slow down neutrons and those that capture them—concludes by showing that the control rod is not a third type of material, but rather the physical means of deliberately combining the previous two. Inserting the rod reduces moderation while increasing absorption at that specific point in the core, making it the most direct lever for shifting the neutron balance between chain reaction growth, stability, and shutdown.

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