Splitting the Atom: Spontaneous vs Induced Fission Quiz

Spontaneous fission occurs in very heavy nuclei where the electrostatic repulsion between the large number of protons nearly overcomes the strong nuclear force. This is a form of radioactive decay where the nucleus splits into two smaller fragments and neutrons without any outside influence.

In induced fission a target nucleus captures a neutron becoming extremely unstable for a brief moment. This added energy causes the nucleus to oscillate and eventually deform until it splits. This is the fundamental subsystem utilized in nuclear power plants.

This model treats the nucleus like a drop of liquid. Surface tension holds it together while internal proton repulsion tries to push it apart. When energy is added the drop stretches into an elongated shape before pinching off into two separate drops.

Fission results in fragments that are usually unequal in mass along with several neutrons and a massive release of energy. The released neutrons are critical because they can go on to induce further fission in a chain reaction.

If the mass is too small too many neutrons escape before they can hit another nucleus. At critical mass enough neutrons are retained to ensure that at least one neutron from each fission event triggers a new one maintaining a stable flow of energy.

While Uranium 238 can undergo spontaneous fission its primary mode of decay is alpha emission. Spontaneous fission is a much rarer event for most isotopes becoming more common only in heavy transuranium elements like Californium 252.

This is due to the cross section or probability of interaction. A slower neutron spends more time near the nucleus allowing the strong nuclear force to pull it in. Modern reactors use moderators like water to slow down fast neutrons to thermal speeds.

Fissile isotopes are unique because they can be split by neutrons of any energy level. Other isotopes like Uranium 238 are fissionable but not fissile because they require very high energy neutrons to trigger the split.

According to mass energy equivalence the total mass of the fission products is slightly less than the original nucleus and neutron. This missing mass is released as energy heating the surrounding material in the reactor subsystem.

A functioning reactor is kept exactly at a critical state where the number of neutrons is constant. If it were subcritical the reaction would die out. If it were supercritical the reaction would grow exponentially potentially leading to a meltdown.

Control rods are made of materials like Boron or Cadmium that are excellent at capturing neutrons without fissioning themselves. By sliding them into the core operators can slow down or shut down the chain reaction.

Increasing purity means fewer non fissile atoms get in the way. A reflector bounces escaping neutrons back into the fuel. Both increase the neutron economy of the system driving it toward a higher reaction rate.

Every nucleus has a certain amount of potential energy it must overcome to split. In spontaneous fission the nucleus tunnels through this barrier. In induced fission the captured neutron provides the energy needed to climb over the barrier.

Because heavy nuclei require more neutrons for stability than lighter nuclei the fragments produced find themselves with too many neutrons for their new size. They typically undergo several beta minus decays to reach a stable state.

Fission involves breaking apart heavy elements to release energy while fusion involves forcing together light elements like Hydrogen. Both processes move the nuclei toward the most stable nuclear organization near Iron.