Half-life is one of the basic technical terms in dealing with radioactive materials. Plutonium-239, for example, has a half-life of 24,100 years, meaning that it takes that long for its radioactivity to fall to half of its original level. To remind people that an immeasurable time is needed for plutonium-239's radiation to drop to a harmless level, Danish film director Michael Madsen used the title "Into Eternity" for his 2010 documentary film on the underground storage of nuclear waste in Finland.
Since the average life-expectancy of human beings is around 80 years at best, even 1,000 years is far too long a time for us to imagine what the world will be like then.
A former director in Japan who has produced excellent TV programs on Hiroshima, Chernobyl, etc., once made a metaphorical warning that the half-life of human memory is rather short. His warning hits the nail on the head as we tend to lose our knowledge of disastrous accidents even of a world scale in a short time.