Radioactive decay
Sustainable design requires a complete assessment of a design in place and time. Some impacts may not occur until centuries in the future. For example, the extent to which we decide to use nuclear power to generate electricity is a sustainable design decision. The radioactive wastes may have half-lives of hundreds of thousands of years. That is, it will take all these years for half of the radioactive isotopes to decay. Radioactive decay is the spontaneous transformation of one element into another. This occurs by irreversibly changing the number of protons in the nucleus. Thus, sustainable designs of such enterprises must consider highly uncertain futures. For example, even if we properly place warning signs about these hazardous wastes, we do not know if the English language will be understood.