Since there is no specific exam for the nuclear engineering discipline provided for the NCEES's FE Exam, nuclear engineers must choose another exam in order to begin their credentialing towards being a professional engineer.
The most popular choices for nuclear engineers are the Mechanical or Other Disciplines exams. While the Mechanical exam is probably most aligned with the a nuclear engineers academic background, the topics are a little more in depth. Because of this, some nuclear engineers choose the Other Disciplines exam for its breadth of coverage and shallower topic depth. The Other Disciplines is a good general engineering exam and is often chosen as an "easier to study for" option for chemical engineers.
Please also see the American Nuclear Society's FE exam page for useful information.