Reactor Core Rod Assembly: Hexagonal Arrangement Benefits?

[Andronicus1717]

Why are all of the reactor core rod assemblies I see in an overall hexagonal arrangement with rods of uniform cross section (I assume due to modularity and mass production benefits)? Could not some abstract and variable rod cross-sections produce a more efficient reactor?

 

[Astronuc]

One has to trade-off thermo-mechanical contraints and fuel performance, heat transfer considerations, fluid flow considerations, and manufacturability (cost) considerations against the ideal nuclear considerations.

Commercial LWR fuel manufacturing is a batch/lot based process. The ceramic UO₂ pellets, which are the first barrier to retain fission products, are produced by chemcially converting UF₆ to UO₂ powder, which is then pressed into small right circular cylinders (pellets), which are then sintered and ground to specified dimension. The ceramic pellets are then inserted into a Zr-alloy seamless tubular cladding, which is then seal welded with machined Zr-alloy bar stock. That forms the tube. In the US, Asia and Europe, LWR (PWR and BWR) fuel is fabricated in square-pitch lattices, as opposed the VVER (Russian LWR) fuel which uses a triangular pitch.

Guide tubes (Zr-ally), into which the control rods (or burnable poison assemblies in unrodded locations) are inserted, are attached to the spacer grids, into which the fuel rods are inserted. The bottom of the guide tube is mechanically fastened to the bottom nozzle (or tie plate), and the top of the guide tubes are mechanically fastened to the top (upper) nozzle (tie plate). PWR fuel assembly nozzles (tie-plates) are built of stainless-steel (304SS or a derivative) forgings or precision castings, which involve machining and welding. BWR tie-plates are cast (usually CF-3).

The hexagonal (VVER) lattice does have some benefit.