What does the term ‘critical mass’ refer to in nuclear physics?

The term 'critical mass' in nuclear physics specifically refers to the minimum amount of fissile material necessary to initiate and sustain a nuclear chain reaction. When a sufficient quantity of fissile material, such as uranium-235 or plutonium-239, is brought together, it can lead to a self-sustaining process in which the neutrons released during fission events go on to cause additional fission events. This concept is fundamental to the design and operation of nuclear reactors, as it determines the smallest volume of material that can lead to a runaway reaction, or alternatively, be controlled for energy production.

Understanding critical mass is crucial in nuclear engineering because it involves many variables, including the type of fissile material, its purity, shape, and surrounding environment. This understanding helps engineers ensure that systems are designed safely, maintaining the reaction below critical mass in reactor control scenarios or achieving it in weapons design contexts.