Controlling the Chain Reaction
A nuclear reactor sustains a controlled chain reaction: each uranium fission produces 2-3 neutrons, which go on to split other uranium atoms. The key parameter is k-effective — the average number of neutrons from one fission that cause another fission. At exactly k=1 (criticality), the reaction is self-sustaining at constant power. The operator's job is to keep k as close to 1.0000 as possible.
Control Rods: The Throttle
Control rods are the primary mechanism for adjusting reactor power. Made of neutron-absorbing materials like boron carbide or hafnium, they slide into the reactor core between fuel assemblies. Pushing rods in absorbs more neutrons, reducing k below 1 and lowering power. Withdrawing rods allows more neutrons to sustain fission, raising k above 1 and increasing power. Fine positioning controls power output with remarkable precision.
The Gift of Delayed Neutrons
About 0.65% of fission neutrons are delayed — emitted seconds to minutes after fission by unstable fission products. This tiny fraction makes reactor control possible. Without delayed neutrons, the reactor period (time for power to change by factor e) would be milliseconds, far too fast for any control system. Delayed neutrons stretch this to seconds, giving operators and automatic systems time to respond.
Simulating Reactor Dynamics
This simulation models the point kinetics equations that govern reactor behavior. Adjust control rod insertion to change reactivity and watch power level respond. Notice how the reactor takes time to reach a new equilibrium — this is the effect of delayed neutrons. Push k-effective above 1.005 to see exponential power rise. The visualization shows the reactor core, control rod positions, neutron flux, and real-time power trace.