South Korea began its civil nuclear development program following its ratification of the Nuclear Non-Proliferation Treaty (NPT) in April 1975. Initially, Korea was wholly dependent upon the United States for nuclear power technology, and Korea's first units were built as turnkey plants constructed by U.S. firms. Over the span of several decades, Korea gradually localized nuclear power technology, and presently, it is rapidly emerging as one of the world's major vendors of civil nuclear technologies.