In space (actual space, not a planetary surface), it's difficult for nuclear to dissipate its prodigious amounts of waste heat. Solar, in contrast, can be made much lighter than on Earth, as there isn't wind and rain to deal with. And of course in general the Sun is available 100% of the time in space, further improving the economics.
But in any case, the argument wasn't that solar was necessarily better, it was that nuclear wasn't necessary. Solar can provide energy basically anywhere, with power beaming, even out into interstellar space.
MSR and many other fluid-fuel reactors run hot and it makes radiators small: https://www.flickr.com/photos/oakridgelab/31537153147/
Stefan–Boltzmann law: Radiation heat transfer is proportional to 4th power of temperature.
This does not rule out use of solar energy.
> During interstallar journey there is time delay in communication, so it may take hours to adjust the power beam when spacecraft moves a little bit
The vehicle tracks the beam, not vice versa.
> Fusion is like solar energy just near to us and MSR will be eventually replaced by fusion reactors.
Fusion is ridiculous when examined closely. It's like fission, only far worse from an engineering and cost perspective.
> Stefan–Boltzmann law: Radiation heat transfer is proportional to 4th power of temperature.
Which means waste heat from a reactor in space has to be radiated at higher temperature than for a reactor here on Earth, which means nuclear is worse off there than here. In contrast, solar works better in space, as I argued upthread.