E.4 Fission

Nuclear Fission: The splitting of a heavy atomic nucleus into two smaller nuclei (called fission fragments), releasing a large amount of energy and typically two or three neutrons.

Nuclear fission is the process at the heart of both nuclear power stations and nuclear weapons. When a sufficiently heavy nucleus , such as uranium-235 or plutonium-239 , splits, the products have a combined mass less than the original nucleus. This "missing" mass is converted directly into energy via Einstein's mass-energy equivalence:

$E=\Delta m{c}^2$

There are two types of fission:

• Spontaneous fission , occurs naturally without any external trigger. Uranium-238 can undergo spontaneous fission, but it is extremely rare and slow.
• Induced fission , triggered when a neutron is absorbed by a fissile nucleus, making it unstable enough to split. This is the type used in nuclear reactors.

The most commonly studied example of induced fission is that of uranium-235. When a slow (thermal) neutron is absorbed, the nucleus becomes highly unstable uranium-236, which rapidly splits:

${}_0^{1}n+{}_{92}^{235}U\to {}_{92}^{236}U\to {}_{56}^{144}Ba+{}_{36}^{89}Kr+3{}_0^{1}n$

Let's check that this reaction is balanced:

• Nucleon number (A): $1+235=144+89+3(1)=236$ ✓
• Proton number (Z): $0+92=56+36+3(0)=92$ ✓

Key points about this reaction:

• One neutron is absorbed, but three neutrons are released , this is what makes a chain reaction possible.
• The fission fragments (Ba-144 and Kr-89) are not unique; fission produces a distribution of possible fragment pairs, not just one fixed outcome.
• The fragments are themselves radioactive and undergo further beta and gamma decay.