Even though half-lives of hundreds or thousands of years would be relatively long for superheavy elements, they are far too short for any such nuclides to exist primordially on Earth. Additionally, instability of nuclei intermediate between primordial actinides (Th, U, and U) and the island of stability may inhibit production of nuclei within the island in r -process nucleosynthesis. Various models suggest that spontaneous fission will be the dominant decay mode of nuclei with A > 280, and that neutron-induced or beta-delayed fission —respectively neutron capture and beta decay immediately followed by fission—will become the primary reaction channels. As a result, beta decay towards the island of stability may only occur within a very narrow path or may be entirely blocked by fission, thus precluding the synthesis of nuclides within the island. The non-observation of superheavy nuclides such as Hs and Fl in nature is thought to be a consequence of a low yield in the r -process resulting from this mechanism, as well as half-lives too short to allow measurable quantities to persist in nature. Various studies utilizing accelerator mass spectroscopy and crystal scintillators have reported upper limits of the natural abundance of such long-lived superheavy nuclei on the order of 10 relative to their stable homologs.