Sadly, nuclear engineering prepares you for a career in nuclear engineering but not one in cosmology. But it really depends on what your program was like as an undergraduate. If it required you to take advanced courses in math and physics, then you may have some of the skills you need to enter a graduate school in astronomy. However, you should be forewarned that astronomy is a very competitive program with few openings at the graduate level. These few spots are filled by students who have concentrated specifically in physics and astronomy and not in applied areas such as chemistry, engineering and the like. It is a great pity, but it is essentially true that the program you elect to take in your first year can make a big difference in your choices 4 years later. If you live in a country where your colleges and universities do not have strong physics programs, or astronomy departments, this can make your options even fewer unless you are willing to leave your native country.