Why would it be so hard? If an arc welder can do it so can this. The mechanics are the same as an arc welder with feedable electrode except the electrode is now made of uranium or plutonium and the hot vapors are shot into this hypercritical reactor. More so handling standardized sticks of metal will not be hard. The cryogenics in space will be needed anyway for the superconductors on the nozzle, so you should not consider it mass. Keeping the liquid hydrogen cool in interstellar space won't be a problem either considering the temperature of interstellar space is below the boiling point of liquid hydrogen. If the tank mass was so much greater with liquid hydrogen then it would not be us in rocketry as it is today! Are you claiming we would launch this thing from earth, are you insane? It would be built in space, regardless if it were to operate on water, ammonia or solid/LH2, and it structure would only need to handle the g-loads of the engines thrust which would be a fraction of earth gravity. As for the tanks remember that many would be needed, they would be very long to reduce internal volume below criticality and have to be positioned far from each other as well, tanks with that much surface area and structure is a lot of wasted mass. Even assuming ammonia could be used the ISP would not be increased much. Based on figures from soild core nuclear rockets ammonia would only increase isp ~20% above the use of water, liquid hydrogen though allows for 100% increase in isp (doubling)! No, the atoms heat them selves, its a nuclear reaction, ideally no boosting liquid would be add and the ship would operate off the impulse of the fission fragments alone, but hydrogen is needed to provide moderation and keep the whole thing from melting. As a result its necessary addition will reduce ISP and the less that can be added the better. A modern atomic bomb can do beyond 50%, why can't this? To handle plasma of this temperature and density, I don't think so! ! Or solid state x-ray converters. And yet kerosene does it this as well but in space travel they have usually opted for hydrogen and lox for upper-stages because in space the extra ISP is worth it over the extra mass in tanking and plumbing. Yeaaaahh, you need to look up how much thrust can come from light, I want to see the numbers! It will be an extremely small pressure, and tracking won't be a problem a simple laser coms kit is all the ship will need. How silly, lets say I had a cherry bomb and could blow it up for thrust, now instead I keep it in a chamber and just us the light given off for thrust, dragging the weight of the fragmented cherry bomb with me, which is more efficient? The amount of radiative pressure you could get from the fission fragments is nothing compared to the pressure they them selves exert and as soon as they fission they become waste that must be jettisoned anyways, might as well jettison them with all the impulse they can provide.