Low altitude nuclear detonations generate EMPs on the ground; high altitude ones may generate enough of an EMP to take out higher altitude satellites. Also, radiation will be trapped by magnetic fields like the Van Allen belts, making those larger and more dangerous. There may also be fallout concerns, but I don't think those would be significant
Your question got me curious so I did some googling.
The current international agreement is (best I can tell) UNITED NATlONS: GENERAL ASSEMBLY RESOLUTION AND PRINCIPLES RELEVANT TO THE USE OF NUCLEAR POWER SOURCES IN OUTER SPACE [December 14, 1992]
> In order to mlnlmlze the quantity of radioactLve materzal in space and
the risks Involved, the use of nuclear power sources in outer space shall be
restrxcted to those space mLssions which cannot be operated by non-nuclear
energy sources in a reasonable way.
and
> (a) Nuclear reactors may be operated: (i) On interplanetary missions; (ii) In sufficiently high orbits as defined in paragraph 2 (h); (iii) In low-Earth orbits if they are stored in sufficiently high orbits after the operational part of their mission.
So it seems like nuclear reactors are restricted, but not completely banned. For an interplanetary mission to Jupiter, it would probably be allowed.
That's a non-binding UN resolution. There's treaties about nuclear weapons in space that have the force of law (ratified by US & USSR), but there's nothing for nuclear reactors AFAIK.