Critical mass of selected elements and isotopes to sustain a nuclear chain reaction

In nuclear physics, an element's critical mass refers to the smallest quantity of fissile material needed in order to sustain a nuclear chain reaction, such as the controlled reactions observed in nuclear reactors or the uncontrolled reactions used in nuclear weapons. Critical masses are subject to various factors, such as its measurements, density, and chemical composition. A sphere is considered the optimum shape for fission, as it will allow the largest number of reactions using the smallest or lightest amount of material.UraniumUranium-235 is the only naturally-occurring fissile material found on earth, but it makes up just 0.72 percent of all naturally occurring uranium, while almost all the rest is made up of uranium-238. For this reason, uranium can be enriched by separating these isotopes and then creating new compositions with a higher share of 235U. A concentration of 20 percent or more of 235U is considered "highly enriched" or "weapons grade", although most nuclear weapons using this material had a concentration of 80 percent or more. Hypothetically, a sphere of 100 percent uranium-235 would have a critical mass of 49 kilograms.Synthetic elementsUnlike uranium, the rest of the elements on this list either do not occur naturally on earth, or occur in such small trace amounts that it is almost impossible to collect. Because of this, these elements must be synthetically created using a nuclear reactor or in a particle accelerator, or they may be a by-product of an atomic explosion.

在核物理学领域，某一元素的关键质量指的是维持核链式反应所需的最小可裂变材料量，例如在核反应堆中观察到的受控反应或在核武器中使用的失控反应。关键质量受多种因素影响，如测量值、密度和化学成分。球体被认为是裂变的最优形状，因为它能够在使用最小或最轻材料的情况下实现最多的反应。铀元素中，铀-235是地球上唯一自然存在的可裂变材料，但其仅占所有天然铀的0.72%，而其余部分几乎全部为铀-238。因此，可以通过分离这些同位素并创建含有更高比例235U的新组成物来对铀进行浓缩。235U的浓度达到20%或更高被认为为“高度浓缩”或“武器级”，尽管大多数使用这种材料的核武器其浓度达到80%或更高。假设100%纯铀-235的球体，其关键质量为49千克。人工合成元素与铀不同，本列表中的其余元素要么在地球上不自然存在，要么以极小的痕量存在，几乎无法收集。因此，这些元素必须通过核反应堆或粒子加速器人工合成，或者可能是原子爆炸的副产品。