Since the early 1990s, Ukrainian officials have been working on a plan to replace it, finally launching a project called the New Safe Confinement, a 300-foot steel arch that will enclose and seal off the reactor for the next 100 years. Its cost is estimated at $1.4 billion, to be paid largely by donor nations. The project, originally scheduled to be finished in 2005, has been beset by delays and financing shortfalls.