A nuclear chain reaction occurs when neutrons released by one fission event cause further nuclei to undergo fission. In a nuclear reactor, this process is carefully controlled so that, on average, one neutron from each fission produces another fission, maintaining a steady power output.
How the chain reaction develops
This content is part of IB Physics E.4 Fission and applies to both SL and HL.
- A slow neutron is absorbed by a fissile nucleus, commonly uranium-235.
- The resulting uranium-236 nucleus is unstable and splits into two smaller nuclei called fission fragments.
- The fission event releases energy and usually two or three additional neutrons:
- Some released neutrons are absorbed by other uranium-235 nuclei, causing further fission events. This repetition creates the chain reaction.
The energy is released because the products have slightly less mass than the original nucleus and neutron. The mass defect is converted into energy according to .
| Reactor component | Role in controlling or using the reaction |
|---|---|
| Moderator | Slows fast neutrons, increasing the probability that they cause further uranium-235 fission. |
| Control rods | Absorb neutrons. Inserting them further reduces the reaction rate; withdrawing them increases it. |
| Coolant | Transfers thermal energy away from the reactor core. |
| Heat exchanger | Uses the transferred energy to produce steam without mixing reactor coolant with turbine water. |
| Shielding | Absorbs ionizing radiation and protects workers and the environment. |
A common misconception is that the moderator absorbs most neutrons. It primarily slows them; control rods are designed to absorb neutrons. If too few neutrons continue the reaction, it becomes subcritical; if the number remains steady, it is critical; if it increases, it is supercritical.
Exam technique
For an IB Explain question, link neutron absorption, nuclear fission, neutron release, and further fission as a causal sequence. Clearly distinguish the roles of the moderator and control rods.