there was also the hurdle of developing mass-efficient radiation shielding for the open-cycle systems; most prototypes used the [usually] liquid-hydrogen fuel or water produced by the onboard fuel cells simply because there wasn't a cost-effective way to actually have solid shielding without adding tremendously to the aggregated payload (remember that the main advantage of nuclear rockets besides the higher exhaust velocity is a lower gross liftoff mass) and thus the delta-vee needed for a trans-planetary injection on a realistic timetable. of course, as these consumables were, well, consumed, there went the shielding for the crew and sensitive payloads. a closed-cycle "nuclear lightbulb" propulsion system would've been even more efficient in terms of both effective exhaust velocity and payload mass, in addition to generating far more electrical power than fuel cells, and the quartz-window containment system would've been much lighter than the layers of graphite and lead needed to shield the crew from radiation.
