What is Nuclear Fission: All details are here

Nuclear fission is one of the most powerful processes known, forming the basis of nuclear power generation and certain types of weaponry. It's a fascinating phenomenon that involves fundamental forces within the atom.

What is Nuclear Fission?

In simple terms, nuclear fission is the process where the nucleus of a heavy atom splits into two or more smaller, lighter nuclei. This splitting releases a tremendous amount of energy, along with several neutrons and other particles (like gamma rays).

Think of it like this: Imagine a very large, overstuffed water balloon. If you poke it just right, it doesn't just leak; it bursts violently into smaller pieces, sending water (energy) in all directions. In nuclear fission, the "water balloon" is the heavy atomic nucleus, and the "poke" is usually a neutron hitting it.

This process can occur spontaneously in some highly unstable isotopes, but more commonly, it's induced fission, where a neutron strikes a suitable nucleus, causing it to become unstable and split. The neutrons released from one fission can then go on to hit other nuclei, causing them to split as well, leading to a chain reaction. This chain reaction is controlled in nuclear power plants to generate electricity and uncontrolled in atomic bombs.

Elements That Support Fission (Fissile Materials)

Not all heavy elements can undergo fission in a way that is useful for energy generation or chain reactions. The key term here is fissile. A fissile material is one that can be fissioned by slow-moving (thermal) neutrons and produce enough additional neutrons to sustain a chain reaction.

Video Guide:

The most important fissile elements are:

1. Uranium-235 (²³⁵U):

• This is the only naturally occurring isotope that is fissile with thermal neutrons. Natural uranium contains only about 0.7% ²³⁵U, with the vast majority being Uranium-238 (²³⁸U). This is why uranium used in most nuclear reactors needs to be "enriched" to increase the concentration of ²³⁵U.
• Why it supports fission: Its nucleus is on the verge of instability. When a slow-moving neutron strikes a ²³⁵U nucleus, it's easily absorbed. This absorption adds enough energy to the nucleus (forming ²³⁶U, which is highly unstable) to distort it into an elongated shape, causing the strong nuclear force (which binds the nucleus together) to be overcome by the electrostatic repulsion between the positively charged protons. The nucleus then splits, typically into two smaller nuclei (fission products like Krypton and Barium) and releases an average of 2 to 3 new neutrons. These new neutrons can then go on to cause more fissions.
• Example: Imagine a delicately balanced stack of building blocks. A gentle tap (slow neutron) can make it topple over (fission) and knock over other stacks (release more neutrons).

2. Plutonium-239 (²³⁹Pu):

• This is a synthetic fissile isotope, meaning it's not found naturally in significant quantities but is produced in nuclear reactors when ²³⁸U absorbs a neutron.
• Why it supports fission: Similar to ²³⁵U, its nuclear structure allows it to absorb slow neutrons readily and undergo fission, releasing more neutrons and energy. It's an excellent fuel for reactors and weapons.

3. Uranium-233 (²³³U):

• Another synthetic fissile isotope, produced by bombarding Thorium-232 (²³²Th) with neutrons. This is part of the "thorium fuel cycle," which is being explored as an alternative to the uranium cycle.
• Why it supports fission: Its nuclear properties make it suitable for fission by thermal neutrons, similar to ²³⁵U and ²³⁹Pu.

Why Other Heavy Elements Don't Support Fission

While many heavy elements are "fissionable" (meaning they can be split), most are not "fissile" in a practical sense for chain reactions.

The primary reasons other heavy elements (like most isotopes of Uranium-238, Thorium-232, and even heavier transuranic elements) do not readily support a chain reaction with slow neutrons are due to their nuclear structure and binding energy properties:

1. Neutron Requirement (Energy Threshold):

• For most heavy nuclei, absorbing a slow (thermal) neutron does not provide enough extra energy to overcome the strong nuclear force and cause fission. They require a much more energetic "kick" – a fast neutron (with higher kinetic energy) – to split.
• The "Even-Odd" Rule: Nuclei with an odd number of neutrons (like ²³⁵U and ²³⁹Pu) gain extra binding energy when they absorb an additional neutron (making their neutron count even). This "bonus" binding energy is sufficient to excite the nucleus enough to fission, even with a slow-moving neutron.
• Nuclei with an even number of neutrons (like ²³⁸U) don't get this "bonus" energy from absorbing a slow neutron. They need the kinetic energy of a fast neutron to push them over the fission threshold.
• Example: Imagine trying to open a stubborn jar. For fissile elements (odd neutron count), a gentle twist (slow neutron) is enough because the lid is already loose (extra binding energy from becoming even). For non-fissile elements (even neutron count), you need to hit it hard with a hammer (fast neutron) to get it open, as a gentle twist won't do anything.

2. Neutron Capture vs. Fission:

• Many heavy nuclei, like the abundant Uranium-238 (²³⁸U), are more likely to simply absorb a neutron without fissioning, especially slow neutrons. When ²³⁸U absorbs a slow neutron, it often becomes ²³⁹U, which then undergoes a series of radioactive decays to eventually form Plutonium-239 (²³⁹Pu) – this process is called "neutron capture" and is how ²³⁹Pu fuel is bred in reactors. This absorption "removes" neutrons from the system, preventing a chain reaction from sustaining itself.
• Chain Reaction Sustainability: For a chain reaction to sustain itself, at least one of the neutrons released from a fission event must cause another fission. If too many neutrons are simply absorbed without causing fission, or if they escape the material, the chain reaction dies out.
• Example: You're playing dominoes. With fissile elements, knocking one domino over (fission) reliably knocks over two or three more, keeping the chain going. With non-fissile heavy elements, knocking one domino might just make it wobble (neutron capture) or fall without touching another domino (neutron escape), stopping the chain reaction.

The Role of Binding Energy

The ultimate reason for fission and why some elements are fissile lies in the nuclear binding energy. This is the energy required to disassemble an atomic nucleus into its constituent protons and neutrons.

• The binding energy per nucleon (the average binding energy for each proton or neutron in the nucleus) is highest for elements around iron (Fe-56).
• For elements heavier than iron, the binding energy per nucleon decreases as the atomic mass increases. This means very heavy nuclei are less stable per nucleon than medium-sized nuclei.
• When a very heavy nucleus (like Uranium or Plutonium) splits into two smaller, medium-sized nuclei (like Krypton and Barium), the total binding energy of the products is greater than the binding energy of the original heavy nucleus. This difference in binding energy is released as the tremendous amount of energy we associate with fission, following Einstein's famous equation E=mc².

In summary, elements like Uranium-235 and Plutonium-239 are uniquely suited for nuclear fission because their specific nuclear structures (especially their odd neutron count) allow them to absorb low-energy neutrons, become highly unstable, and split, reliably releasing enough new neutrons to sustain a chain reaction. Other heavy elements lack this specific instability with slow neutrons and tend to absorb them without fissioning, or require much higher energy input to split.