Adding the engineering plant is the last step because we need to know everything else about the ship first. We need to pick a deltaV, a burn time over which we are going to spend that deltaV, and the peak power requirements of our other systems. Using all of this information we will calculate the size of the reactor we need to supply that much power. The difficult part is that the reactor can often be a significant portion of the mass of ship. So we play a little guessing game until two equations meet each other.

The Propellent Mass Flow is essentially how many kilograms of propellent are thrown through the engine per second. Take your deltaV and divide it by a burn time. A big, relatively slow, ship has a long burn time. A high performance racer a short burn time. For a frigate, I usually specify a 100 hour burn time. For something huge, like the Cézanne going out to Pluto, the burn time is measured in months.

MASS_FLOW_RATE = (TOTAL_PROPELLENT / BURN_TIME)

With the flow number in hand, we can then sort out the size of reactor that would be needed to accelerate that amount of propellent to that speed. We simplify if down to simple newton's law stuff. VE for our fictitious engine system is 2.10E+06 .

THRUST = MASS_FLOW_RATE * VE

In this design, the energy for the thrust comes directly from the nuclear reaction. The propellent is inert mass that we excite with the energy from this reaction and chuck out the back of the spacecraft. But this process is not 100%, so we scale the output we need by an efficiency factor. (We'll say efficiency is around 0.80)

REACTOR_OUTPUT = THRUST / THRUST_EFFICIENCY

There is also a Square/Cube Law that relates the reactor output to the surface area (and thus mass) and plasma volume. However it also takes energy to make energy. We factor in those losses with another efficiency factor. For this class of reactor we'll say we get twice the power out that we put in, so efficiency is 0.5

PLASMA_VOLUME = (REACTOR_OUTPUT / EFFICENCY) * PLASMA_CONSTANT

PLASMA_RADIUS = CUBE_ROOT(PLASMA_VOLUME * 3 / 4*PI)

SURFACE_AREA = 4*PI*PLASMA_RADIUS^2

REACTOR_MASS = SURFACE_AREA * REACTOR_MASS_CONSTANT

The reactor itself requires fuel. There are three basic reactor designs in common use:

This design uses pellets of Lithium Deuteride. The fuel is solid, and relatively easy to store. You just don't want to expose it to either water (it will explode). You also want to limit its exposure to hard x-rays or fast neutrons to while it is inside the reactor.

This design is powered by heavy water, and uses an exotic mechanism to generate Muons. The end result is that muon decay products transmute some of the deuterium into tritium. Larger vessels that can absorb the cost of the Muon generator select this design in order to avoid carrying large amounts of LID fuel.

Used in some high performance, short endurance craft. Fuel is stored as cryogenic deuterium and helium that are injected directly into the plasma chamber. The fusion the results doesn't breed neutrons, which eliminates the need for heavy shielding between the reactor compartment and the crew. The downside is that cryogenic deuterium and helium leak no matter the container they are stored in. This leads to fuel loss over time, with a handy side effect of hydrogen embrittlement to the tank material over time, as well as the structure around the tank.

The equation assumes a 1 megawatt reactor weighs 1000 kg, and the larger reactors have a smaller output/mass ration.

REACTOR_MASS = PEAK_OUTPUT * 0.01/SQRT(H8/1000000)

Note: At this point we are working in a realm of pure fantasy.