 There was an effort by the US government to put an exception into the 1963 treaty to allow for the use of nuclear propulsion for spaceflight but Soviet fears about military applications kept the exception out of the treaty.This limitation would affect only the US, Russia, and the United Kingdom.It would also violate the Comprehensive Nuclear-Test-Ban Treaty which has been signed by the United States and China as well as the de facto moratorium on nuclear testing that the declared nuclear powers have imposed since the 1990s.