Environmental hazards of decommissioned nuclear power plants

Lack of practical experience on decommissioning nuclear power stations
Waste of resources in obsolete nuclear power plants
Excessive cost of decommissioning nuclear reactors
The ultimate waste from a nuclear power plant is the plant itself. A commercial reactor can only operate for thirty to forty years. When it stops operating, 15 to 20% of its contents remain radioactive. A closed-down reactor could be even more dangerous than one that is operating, because many of the security devices may have been removed. Dismantling a reactor is the only way to guarantee safety, but no one is certain if dismantling is feasible.
When a plant closes, it is first mothballed (welded shut with steel), then entombed (completely encased in concrete), and eventually dismantled. Thus far, twenty prototype reactors have been shut down; only five have gone beyond mothballing, and not even one of these relatively small reactors has been dismantled. Because old reactors are so radioactive, the entire dismantling process must be done by remote control and could cost as much as $100 million. While a reactor is mothballed or entombed, it must be guarded for safety. Leaks are likely, and the reactor remains radioactive for up to 250,000 years.
By the turn of the century, one hundred reactors around the world may have shut down, and by 2020 all 233 commercial reactors in operation in 1979 may be ready for the graveyard.
(E) Emanations of other problems