Hi, you wrote about
pressure drop in fuel assemblies. But you have to specify, whether you mean
pressurized water reactors (single phase flow) or
boiling water reactors (two-phase flow).
In general, total fuel assembly pressure drop is formed by fuel bundle frictional drop (dependent on
relative roughness of fuel rods,
reynolds number,
hydraulic diameter etc.) and other pressure drops of structural elements (top and bottom nozzle, spacing grids or mixing grids).
In general, it is not so simple to calculate pressure drops in fuel assemblies (especially the spacing grids) and it belongs to key know-how of certain fuel manufacturer. Mostly, pressure drops are measured in experimental hydraulic loops, rather than calculated.
If you want to know more, specify your problem. Here are some valuable sources:
Frictional losses - single phase
Pressure drop - two-phase