rushthezeppelin
TGT Addict
There would be far less force applied than you would feel on earth. In space the blast can't create a shockwave. It would probably be pretty inefficient TBH. The only force pushing the spacecraft forward would come from the act of pushing the nuke out, and whatever particles from the explosion hit the umbrella. Unlike earth where the blast throws out a pressure wave in the atmosphere.
Actually if done right it's actually a super efficient propulsion method. Really the only downside is you can't use it near an inhabited planet. https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion