Nuclear Fission Reactor

About this simulation

Nuclear Fission Reactor is a cutaway 3D simulation of a thermal nuclear reactor core. Nine Uranium-235 fuel assemblies stand inside a Cherenkov-glowing water moderator while a four-finger Boron-10 control rod spider lowers from above. Fast yellow neutrons released by fission bounce through the water moderator, slow into cyan thermal neutrons, and strike U-235 fuel rods to split nuclei into Barium-141 and Krypton-92 daughter fragments. Students drag the control rod spider or adjust U-235 enrichment and moderator density to steer the reactor between Subcritical (k_eff below 1), Critical (k_eff = 1.00), and Supercritical (k_eff above 1) operation.

What students can explore

Drag the orange Boron control rod assembly slightly upward to reduce insertion below 45% and watch k_eff rise above 1.00 as the neutron population and thermal power climb.
Press the SCRAM button to drop the Boron control rods to 100% insertion and watch neutron absorption drive the reactor into a safe subcritical shutdown.
Turn on Hold k_eff = 1.00, change U-235 enrichment or Water moderator density, and see how the control rods automatically reposition to maintain steady critical power.
Click any green U-235 fuel rod to fire a starter burst of neutrons and watch the green Ba-141 and pink Kr-92 daughter nuclei fly apart.
Lower Water moderator density to 40% and notice how k_eff drops even with the control rods raised, illustrating why pressurized water reactors cannot sustain a chain reaction if coolant boils away.

Learning objectives

1Describe the nuclear fission of Uranium-235 induced by a thermal neutron and identify its products (daughter nuclei, fast neutrons, and thermal energy).
2Define the effective neutron multiplication factor k_eff and distinguish subcritical, critical, and supercritical reactor states.
3Explain the distinct roles of the water moderator (slowing fast neutrons) and Boron control rods (absorbing excess neutrons).
4Explain why losing water moderator density provides an inherent negative-reactivity safety feature in light-water reactors.

Questions teachers ask

What does k_eff = 1.00 (critical) mean in a nuclear reactor?

The multiplication factor k_eff is the average number of neutrons from one fission that go on to cause another fission. When k_eff = 1.00, each fission triggers exactly one new fission on average, keeping the neutron population and reactor thermal power steady.

Why does a nuclear reactor need a moderator like water?

Newly emitted fission neutrons travel very fast (~20,000 km/s), where Uranium-235 has a small probability of capturing them. Collisions with light hydrogen nuclei in water slow them down to thermal speeds (~2 km/s), increasing the U-235 fission probability by hundreds of times.

How do Boron-10 or Cadmium control rods regulate reactor power?

Boron-10 and Cadmium nuclei have huge neutron-absorption cross-sections without undergoing fission. Lowering the control rods deeper into the core soaks up free neutrons and lowers k_eff; withdrawing them slightly lets power rise.

What does SCRAM mean in a nuclear power plant?

SCRAM is an emergency rapid shutdown in which all control rod assemblies are driven or dropped completely (100%) into the reactor core within seconds, immediately making k_eff much less than 1 and stopping the fission chain reaction.

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