Ehhhhh . . . That's kind of misleading. While the risks of nuclear fuel dispersal is generally overblown (coal plants are hard to beat in this regard), there is not a zero risk of dispersal and there have been historic releases of fissile material.

In the 1970s, the Soviet Union's US-A radar imaging satellites (often known as RORSAT) depended on nuclear reactors to power them in lieu of RTGs and Solar panels. An unnamed launch failed to reach orbit in 1973 and resulted in the reactor entering the Pacific Ocean. Kosmos 954 and Kosmos 1402 had their cores reenter the atmosphere in 1978 and 1983 respectively, with the latter dispersing its debris over Canada. More recently, the Russia Federation's Mars 96 launch failure resulted in the reentry of its onboard plutonium-238.

The United States also had plutonium-238 enter the Earth's atmosphere during Apollo 13. As this material was originally intended to remain on the Moon to power surface instruments, it remained onboard since a landing attempt was aborted.

Only a small handful of launch providers can claim a 100% success rate, and nuclear reactors for man rated thermal rockets or electric rockets must inevitably much larger than any of those involved in the beforementioned incidents.

I also caution against people arguing that nuclear fission is the only way forward for space travel. There have been very encouraging developments in Solar electric propulsion in recent years, and nuclear power for spacecraft comes with engineering headaches that tend to be ignored more often than not. Nuclear reactors must have heavy shielding to protect the crew, and this shielding increases their already large minimum engine mass. Reactors generating electricity must also have significant mass dedicated to generating said electricity and then shedding the large amounts of waste heat produced as a result.

Moreover, nuclear reactors can only be reused so many times before they accumulate too many poisons and can no longer produce useful levels of energy. Once a reactor has also been used, they will also continue to generate radiation. This can make docking in particular very problematic, as economical shielding can only cover a certain 'cone' in front of the reactor and thus forcing other vehicles to enter via that safe zone once they've come within a certain distance of the nuclear-armed target.