Question

How does a neutron striking an atom of uranium-235 initiate a fission reaction?

Answers

GPT-4o mini
The process of nuclear fission in uranium-235 (U-235) initiated by a neutron involves several key steps:

1. **Neutron Absorption**: When a free neutron strikes a U-235 nucleus, it can be absorbed, resulting in the formation of an unstable isotope of uranium, uranium-236 (U-236). This event is often enhanced if the neutron is slow-moving (thermal neutron), as U-235 is more likely to capture low-energy neutrons due to its cross-section properties.

2. **Nuclear Instability**: The addition of the neutron makes the U-236 nucleus unstable. This instability arises from the imbalance between the number of protons and neutrons in the nucleus. The U-236 nucleus tends to exist in an excited state, meaning it has excess energy.

3. **Nuclear Fission**: Eventually, the U-236 nucleus will undergo fission, which involves the nucleus splitting into two (or more) smaller nuclei, known as fission products. This process can happen via a variety of different pathways, but it generally results in the release of a couple of neutrons (typically 2 to 3), which are key to sustaining a nuclear chain reaction.

4. **Release of Energy**: The fission process releases a significant amount of energy, primarily due to the conversion of mass into energy as described by Einstein's equation \(E=mc^2\). The energy release occurs in the form of kinetic energy of the fission fragments, as well as other forms of radiation, including gamma rays.

5. **Chain Reaction**: The neutrons released during fission can then go on to collide with other U-235 nuclei, leading to further fission events. If there are enough U-235 nuclei and the conditions are right (like in a nuclear reactor), a self-sustaining chain reaction can occur. In a nuclear power reactor, this process is carefully controlled to maintain a steady rate of fission.

In summary, a neutron striking a U-235 atom can lead to fission through neutron absorption, resulting in an unstable nucleus that eventually splits into smaller nuclei, releasing energy and more neutrons, potentially continuing the chain reaction.