University of Colorado
University of Colorado: Ph Et Interactive Simulations: Nuclear Fission
Start a chain reaction, or introduce non-radioactive isotopes to prevent one. Control energy production in a nuclear reactor.
Other
Nuclear Threat Initiative: Nuclear 101: Nuclear Weapons
Interactive tutorial helps with understanding nuclear weapons, the different types, how they are different from conventional weapons, and how many countries have developed them.
Lawrence Berkeley National Laboratory
Berkeley Lab: Nuclear Fission Energy
Resource explains and illustrates nuclear fission energy.