long-term consequences for both health as well as the environment. so, what did chernobyl look like back then? does it give us a picture of what we're dealing with now? >> reporter: it's called the sarcophagus, a massive concrete structure built to contain the worst nuclear power plant disaster the world has ever known. an explosion in ukraine in 1986 showed us just how high the stakes could be in producing nuclear power. >> it is unconscionable in this -- with this tragedy there that they are not coming forth in a new spirit of frankness and openness. >> reporter: the numbers from that disaster are staggering. according to the world health organization, 240,000 cleanup workers received high levels of exposure in their attempts to contain the radioactive debris. 28 of them died that year. and public health experts believe that as many as 4,000 of those exposed to the disaster died later of cancers linked to chernobyl.