One of the major problems of nuclear energy is the inability of scientists to discover a safe way to dispose of the radioactive wastes which occur throughout the nuclear process. Many of these wastes remain dangerously active for tens of thousands of years, while others have a life span closer to a quarter of a million years. Various methods have been used to date, but all have revealed weaknesses, forcing scientists to continue their search. The nuclear process involves several stages, with the danger of radioactivity constantly present. Fuel for nuclear reactors comes from uranium ore, which, when mined, spontaneously produces radioactive substances as by-products. This characteristic of uranium, ore went undetected for a long time, resulting in the deaths, due to cancer, of hundreds of uranium miners. The United States attempted to bury much of its radioactive waste material in containers made of steel covered in concrete and capable of holding a million gallons. For a long time it was believed that the nuclear waste problem had been solved, until some of these tanks leaked, allowing the radioactive wastes to seep into the environment. Canada presently stores its nuclear waste in underwater tanks, with the long-term effects largely unknown. However, plans are under consideration for above-ground storage of spent fuel from re- actors. These plans include the building of three vast concrete containers, which would be two stories high and approximately the length and width of two football fields. Other suggestions include enclosing the waste in glass blocks and storing them in underground caverns, or placing hot containers in the Antarctic region, where they would melt the ice, thereby sinking down about a mile. This idea has since been abandoned because of the possible adverse effect on the ice sheets. It is implied in the passage that the primary difficulty in seeking a safe way to dispose of nuclear wastes is caused by______.