Fission chain reaction and nuclear reactors
PhysicsQuantum & Nuclear PhysicsAges 17–18
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Sign in to playSlow neutrons split uranium-235 nuclei, and each fission releases 2–3 neutrons. Choose the U-235 enrichment, moderator, control-rod depth and core size to make the chain reaction subcritical, critical or supercritical, with the neutron multiplication factor k, a bar chart of neutrons per generation and a breakdown of what happens to the neutrons. The Reactor screen runs a pressurised water reactor with coolant, power output, shutdown and decay heat; Energy calculates E = Δmc² for one fission and compares it with fusion; Spent fuel shows how the waste decays.
Lesson: Nuclear fission, chain reactions, critical mass, nuclear reactors and fusion
What it shows
In nuclear fission a slow neutron is absorbed by a uranium-235 nucleus, which splits into two smaller nuclei and releases 2 or 3 neutrons and about 200 MeV of energy. If on average exactly one of these neutrons causes another fission, the multiplication factor k = 1 and the chain reaction is critical; with k < 1 it dies out and with k > 1 it grows. A reactor keeps k = 1 using enriched fuel, a moderator that slows the neutrons, control rods that absorb them and a core large enough that few escape. The energy comes from a loss of mass, E = Δmc².
How to use
On Chain reaction, choose the U-235 enrichment and Moderator, set Control rods in and Core size, then press Fire 1 neutron or Fire 100 neutrons and watch the bars for each generation. On Reactor, move the control rods, reduce Coolant flow or press Emergency shutdown (SCRAM). Energy compares fission with fusion; Spent fuel shows how the waste decays over time.
Parameters you can change
- Screen Chain reaction, Reactor, Energy, Spent fuel
- U-235 enrichment 0.7 % (natural), 3 %, 5 %, 20 %, 90 %
- Moderator None, Water, Heavy water, Graphite
- Control rods in 0–100 %
- Core size (relative) 1–10
Questions to explore
- Why can natural uranium sustain a chain reaction with graphite or heavy water but not with ordinary water?
- What happens to the number of neutrons in each generation when k is slightly greater than 1?
- Why must a reactor still be cooled after it has been shut down?
