emilmammadzada said:
How does the number of fuel pins or fuel volume affect it?
One has to define a set of technical limits for the fuel, usually set by the peak power location, the peak channel enthalpy gain/rise, fuel rod (or particle) internal pressure, peak fuel temperature (one has to retain some margin to melting), peak cladding temperature (determined by limiting creep and/or corrosion/material loss), limiting heat flux, . . . , and dimensional stability (addressing creep, swelling, and physical distortion of the fuel elements and structure). Translating that into.a core design comes from experience. One has to similate normal operation, anticipated operational occurrences (AOOs), and a variety of transiens, and beyond design basis events (severe accidents). The fuel should retain fission products to the extent possible, the core must remain coolable (mostly to removed decay heat), and the reactor must be able to shutdown in a subcritical configuration upon demand (i.e., the fission process (chain reaction) ceases).
Like large commercial nuclear reactors, SMRs could be smaller LWRs (based on PWR or BWR technology), graphite moderated (gas, liquid metal, or molten salt cooled), molten salt fueled (MSRs), or liquid metal fast reactors.
I did a compact 300 MW fast reactor design that used (U,Pu)N cladding in a W-Mo-Re alloy, with a core exit temperature of 1300°C. The core was cooled by liquid Li. The core size was on the order of 1 m height and slightly more than 1 m in diameter. I did a similar design recently, but using liquid Pb, or Pb-Bi, but more conventional core design.
Fuel cycle management (batch sizes, enrichments and enrichment distributions), cycle length (annual, 18-month, 24 month, . . . ), maximum burnup of the fuel assembly and peak rod (or pellet), and how many times an assembly (or fuel element) will be returned to the core.
Small cores generally have a higher fraction of neutron leakage from the core compared to large cores. One can somewhat mitigate neutron leakage with an appropriate 'reflector', e.g., low enriched, or natural, or even depleted U) fuel.