Nuclear fission
Nuclear fission splits a heavy nucleus into two main fragments, usually with several neutrons and a large energy release. The deeper reason for the energy release is not simply that the nucleus “breaks”: the products are, on average, more tightly bound per nucleon.
From neutron capture to two fragments
A fissile nucleus such as uranium-235 can absorb a neutron and form an excited compound nucleus. The excitation can make the nucleus deform until it separates into two main fragments. Fission does not produce one unique pair of daughters; there is a distribution of possible fragment masses.
Where roughly 200 MeV comes from
Medium-mass nuclei sit higher on the binding-energy-per-nucleon curve than very heavy nuclei. After fission, the final products therefore have slightly less total rest mass than the initial system. The mass difference appears as energy according to Q = Δmc². A typical uranium-235 fission releases on the order of 200 MeV, mostly as kinetic energy of the heavy fragments, with additional energy in neutrons, gamma rays and later radioactive decays.
Why neutrons can sustain a chain
Several neutrons are commonly emitted in a fission event. Some escape or are absorbed without causing fission; others may trigger a new fission. The effective multiplication factor k summarizes the average generation-to-generation balance: below 1 the chain decreases, at 1 it is steady on average, and above 1 it grows. “Critical” is therefore a mathematical statement about this balance, not a synonym for an explosion.
Prompt and delayed components
Most fission neutrons appear essentially at once, but a small fraction arise later from radioactive fission products. Those delayed neutrons are important in controlled reactor physics because they lengthen the response timescale. The products themselves are often neutron-rich and continue to undergo beta and gamma decays after the original fission event.
Worked examples
1. Check a schematic equation
Solution
For ²³⁵U + n → ¹⁴¹Ba + ⁹²Kr + 3n, mass numbers give 236 on both sides and atomic numbers give 92 on both sides. The equation therefore passes the basic A and Z conservation check.
2. Interpret k
Solution
If k = 0.97, each generation produces fewer successful successor fissions on average, so the chain reaction dies away.
3. Locate the energy source
Solution
The energy is not created by “breaking a bond.” It comes from the final products having lower total rest mass and greater binding per nucleon than the initial system.