Nuclear power plants are clean, efficient and silent. But they face complex safety issues. There is the danger of radiation leaks. They are vulnerable to terrorist attacks. And perhaps most vexing of all, they produce large amounts of waste in the form of spent uranium fuel, which remains dangerously radioactive for thousands or perhaps millions of years.  How and where to safely store that waste has been a central question in the decades-old debate over nuclear energy.  Producer Zulima Palacio explores the issue.