Question

U-235 is fissile and will fission when absorbing a neutron. My understanding is, that this split is instant. It will (most likely) split into two smaller daughter nuclei, with a combined smaller binding energy than before the split. It will also release 3 neutrons.

Why doesn't U-235 just split right away into two smaller daughter nuclei and only release 2 neutrons?

Answer 1

U-235 does not spontaneously split as it requires a neutron to become unstable U-236, which then fissions into two smaller nuclei, releasing two or three neutrons. This variability in the number of neutrons ensures the possibility of a sustained nuclear chain reaction, which is essential for nuclear power or bombs.

Step-by-step explanation:

The question is concerned with why U-235 does not spontaneously split into two smaller daughter nuclei, releasing only two neutrons instead of the three that are typically observed in nuclear fission. In the fission process of U-235, after absorbing a neutron, the nucleus becomes U-236, which is highly unstable.

This nucleus then splits into two smaller, not necessarily equal, daughter nuclei and releases a varying number of neutrons, typically two or three. This variability is due to the different possible combinations of the daughter nuclei and the fact that uranium-235 fission does not produce a single unique outcome but a distribution of different pairs of fission products.

The phrase 'nuclear fission chain reaction' describes the process where the neutrons released from one fission event can go on to induce additional fissions, potentially resulting in a self-sustaining chain reaction. This is crucial for both controlled nuclear power generation and uncontrolled explosive scenarios.

The production of two or three neutrons per fission event is critical, as it ensures that at least some of the released neutrons will encounter other U-235 nuclei to maintain the chain reaction, despite some neutrons being lost or absorbed by other isotopes such as uranium-238.