There is no universally accepted design criteria for fission reactors launched into orbit, mostly because until recently only the U.S. and the Soviet Union had the capability and both countries employed vastly different design and operation philosophies. Most of the existing standards in the west are likely to be based on the U.S. DOE policy for the use of radioisotope systems which looks at statistical failure models and their effects on radioisotope dispersal and their effects on the health of affected populations.
Fission reactors have some aspects that make launch inherently safer, and in some aspects more dangerous. When launched, fission reactors have yet to fission a single atom and therefore have no radioactive fission products built up, making dispersal of fuel a lesser health concern. However, there is a concern that enriched fuel materials could land in countries attempting to acquire nuclear weapons. Also, the majority of earth is covered by water, which is a great neutron reflector and also softens the neutron spectrum to a region where an unintended criticality event becomes more likely. Finally, to date there has been little work to define a minimum safe orbit for fission operations. Obviously, the primary concern is that radiation could have negative effects on manned space operations, but to be honest, any gamma, neutron or beta radiation, would be negligible to what already exists due to our own star and other stars. The bigger concern, is how it could affect scientific instruments in orbit.
While this is a lengthy response, the quick answer is that there are few existing guidelines, because at present nobody is launching nuclear reactors. However, there are a few areas of concern that have to be addressed when procedures are put in place.