Nuclear? What Type Of Nuclear?

Hypothetically, there are 900 types of nuclear reactors:

The enormous difficulty of choosing a proper path for reactor development is easily seen by estimating the number of conceivable reactor types. With  

• 3 fissionable fuels [²³³U, ²³⁵U, ²³⁹Pu], 
• 2 fertile materials [²³²Th & ²³⁸U],  
• 3 neutron energy ranges [slow or thermal, epithermal or resonance, & fast], 
• at least 5 coolant types [2 waters {light & heavy}, sodium, CO₂, He, & air], 
• 5 moderators [light & heavy water, graphite, beryllium, & beryllium oxide], 
• and 2 general categories of geometrical arrangement (heterogeneous and homogeneous), 

there are 900 possible combinations! [3 x 2 x 3 x 5 x 5 x 2 = 900]

Of course not all of these are sensible; for example a fast reactor could hardly be cooled with H₂O. Even so, there are probably at least 100 combinations which are not obviously unfeasible.

Plus there are liquid salt and liquid lead as possible coolants of the future. Which raises the hypothetical 900 to hypothetical 1260. At least.