Note: I have a tooth-ache and such, making it hard to focus, so if anything's off or something, just lemme know. I normally proof-read my stuff but I can't focus completely right now and summarized a lot more.

The in game reactors, look more like what you'd see in a passive system like an RTG (Radioisotope Thermoelectric Generator), which uses a passive heat generation to convert heat into energy. These produce extremely low volumes of energy in the watts range. Real world reactors, the big power plants and such, use the heat of the fuel to heat water which turns to steam which turns turbines. This heat is far greater than that in an RTG and thus needs active cooling or it will meltdown and initiate a thermonuclear explosion.

The game's reactors however, one the size of a basketball, produce insane amounts of energy, 0.5MW of energy, thats 500KW of energy. That electric heater you use, thats 1.5KW of energy and its the highest power device you'll likely be using in your home, next to your fridge which occasionally draws up to 2KW if its an old model, and your AC which can draw considerable power too. Now if you think about it, the most a house will usually draw, the average home in America, is 3 kilowatts. That means, that basketball can power 100+ homes all on its own. Given that its storage capacity is 125L, and 1L lasts 2 hours at maximum output, thats a basketball that can power 100+ homes for 250 hours or 10 days at max output. Longer if less.

Thats not even counting the heat output. That thing would be a ball of molten slag before it even lasted an hour. Cooling is an essential step if you want to produce grand volumes of power. Cooling becomes that much harder in space, as there's no atmosphere left to expel the heat to. Even if you could pipe heat away from the reactor using pipes inside the ship walls, all you'll be doing is turning your ship into a giant oven. The captain will yell at you from the cockpit as the room temperature approaches 400C. Assuming the wires haven't melted yet.

In the game world, 1kg of uranium produces 1MWh of power.
In the real world, 1kg of uranium-235 can produce 24GWh of power under perfect conditions.

In the game world, uranium is deleted.
In the real world, uranium becomes depleted uranium, a highly radioactive waste product.

In the game world, there is no heat management. No risk of meltdown.
In the real world, real engineers go through a lot of trouble to manage the heat and flow systems to keep reactors properly cooled and safe to operate.

In the game world, uranium ore just gets purified into an ingot and tossed in like wood burning.
In the real world, uranium is processed, then enriched, then fabricated into fuel rods.

Nuclear Submarines use small reactors!
Why, yes, yes they do. Pressurized Water Reactors, PWRs. These use heat to heat water in an enclosed system, and because the water is highly pressurized, it cannot turn into steam. It flows through a heat exchanger, which heats up water in the other pipe network which vaporizes the water into steam which is then forced through turbines to generate energy. This closed system keeps everything contained so that it can go long periods without needing refueling and its capable of going 20 years without refueling.

These reactors produce around 10-40MW and are 50-150 cubic meters in size.

Small Modular Reactors; Gas Cooled Reactor!
These are small reactors that are cooled by gasses like helium or carbon dioxide. These use pebble beds or prismatic blocks of uranium oxides or other fuels, encased in ceramics for stability. The helium (which doesn't absorb radiation), flows through the reactor absorbing heat and passing it through a heat exchanger which heats up water into steam to be pushed through the turbines.

These reactors produce around 15-60MW and are 150-250 cubic meters in size.

MSRs, Molten Salt Reactors!
Uranium or Thorium is mixed in with melted salt, dissolved together in a liquid state. The salt acts as a fuel carrier and a coolant. This salt is transferred through a loop which transfers its heat to a second salt loop, and goes through the whole turbine thing same as the above systems. YAY! I'm sensing a pattern here!

These reactors produce around 3-60MW and are 100-300 cubic meters in size.

Liquid Metal Fast Reactors (LMFRs)!
These reactors use liquid metal as coolant, such as sodium or lead-bismuth. They can be used as breeder reactors which can make more fuel. These can operate at high temperatures safely and be shut down without much risk. They are portable for ships at sea, small size low outputs, with larger ones aiming much higher for land production.

These reactors produce around 3-90MW and are 100-200 cubic meters in size.

Now place any of these reactors on a space ship or space station. No land, no atmosphere, nothing to transfer heat away from the ship. How do you get rid of heat now? Through radiation, or radiators. Very large ones. There's very little of anything in the void of space to make contact with these panels in order to dissipate the heat, which causes the heat to just build up. You need to impact particles of some kind in order to radiate the heat away. The ISS has this problem, and its in high atmo barely making contact with the atmosphere so it has far more radiative capability than something deeper in space and its radiators are massive, just for the tiny amount of heat they generate! Computers, equipment, and your own body, generate heat. And the ISS needs a huge array just to handle that. How big an array do you think it'd take for a nuclear reactor running at 900C?

Lets not forget that when you reach space, there's no gravity anymore. Many of these reactors use designs that use gravity to push fluid through their pipes. In space, you'd need to incorporate pumps into the designs to keep the liquids flowing properly. Liquid metals may behave oddly inside micro gravity or no gravity, causing various issues in the flow behavior too which may cause reactors to stall and meltdown.

In space, there's nothing to stop radiation from going where it wants to go. You need shielding around crew quarters, crew work areas, and computer systems. Basically you'd need to shield your entire reactor area away from the rest of the ship or not only will the crew die out, but so will the computers as the constant radioactive particles on them will bit flip random things and cause lots of program errors.

No I didn't, I was saving the best for last. The type of thing that should be in the game! RTGs are simple, passive power generation devices. No moving parts, no coolants, no refueling. Its a long term always on battery that can last decades, up to a century depending on design.

MHW-RTG was used in Voyager 1 and 2, producing about 150 watts per RTG using Plutonium as its fuel source. With a decay rate lasting around 80 years, typing from memory so I'm not gonna be specific here, it can power the space probes through a person's entire life span.

The largest cluster of RTGs used in space was for the Cassini-Huygens and Galileo Probes, producing 885 watts at launch and nearly 620 watts by the end of its mission. Not even one kilowatt. These all fit within roughly 0.25 cubic meters total.

Now this would of course, require the game to redesign its power needs systems, to make everything use less power.

Small Grid Small Reactor: 0.5m³ size, 0.5MW output.
Small Grid Large Reactor: 1.5m³ size, 14.75MW output.
Large Grid Small Reactor: 2.5m³ size, 15MW output.
Large Grid Large Reactor: 7.5m³ size, 300MW output.
1 kilogram of uranium produces 1MWh of power.