Define nuclear fission, nuclear chain reaction, and critical mass.

Imagine setting off a line of dominoes. With just a flick, one tile knocks over the next, and the effect ripples through the entire line. A nuclear chain reaction works under a similar principle, but with atoms. When a heavy nucleus, like that of uranium-235, absorbs a neutron, it becomes unstable and splits in a process called nuclear fission. This fission produces additional neutrons which, in turn, can trigger more fissions in nearby nuclei.

It's a self-propagating series of events – one fission leads to another, and that to another, much like the falling dominoes. The scale, however, is incomparably greater. A single fission can release a huge amount of energy, which is why managers of nuclear reactions must be precise and cautious to avoid uncontrolled reactions. In nuclear reactors, the chain reaction is carefully regulated to ensure a steady production of energy. This is achieved by controlling the number of free neutrons and thus the rate of fission reactions.

If you've ever tried to start a fire, you know that if you have too few twigs or logs, the fire just won't catch. Critical mass is the nuclear equivalent of having just enough logs to keep a fire burning. Specifically, it's the minimal amount of fissile material required to sustain a nuclear chain reaction. Think of fissile material as the combustible wood that feeds the fire.

The term 'critical' refers to the balance point where the number of neutrons produced equals the number of neutrons lost. If the mass of the fissile material is below this critical threshold, the reaction will fizzle out because neutrons escape too quickly without causing further fissions. On the other hand, if there's more than the critical mass, the reaction can become self-sustaining or even escalate, which is useful in nuclear reactors and, regrettably, the principle behind nuclear bombs. The concept of critical mass is vital to understand for the safe and effective use of nuclear energy.

The actors of the nuclear stage are fissile materials. Just as specific roles in a play are not suitable for every actor, not all elements can undergo fission. Fissile materials have nuclei that can capture a neutron and then fission as part of a chain reaction. The most widely known fissile materials include isotopes such as uranium-235 and plutonium-239.

The criteria for an isotope to be considered fissile include the ability to sustain a nuclear reaction and the release of excess neutrons during fission. In a nuclear reactor, these materials must be enriched, meaning the percentage of fissile isotopes is increased to enable a sustained chain reaction. It's a careful balance: too little fissile material and the reaction stalls; too much and it can become uncontrollable. This makes understanding and managing fissile materials crucial for nuclear power and safety.