sentences:  It also is quite difficult to ensure that such a chain reaction consumes a significant fraction of the fuel before the device flies apart.The procurement of a nuclear fuel is also more difficult than it might seem, since sufficiently unstable substances for this process do not currently occur naturally on Earth in suitable amounts.One isotope of uranium, namely uranium-235, is naturally occurring and sufficiently unstable, but it is always found mixed with the more stable isotope uranium-238.