Unit cell conversion and shape of fuel pellet

[oksuz_]

As you may know, below is the conversion process of square unit cell into circular shape in order to calculate the thermal utilization factor (f) for a lattice.

As far as I know, it is really difficult to make a simulation for which a lattice is constructed as a circular fuel and square moderator in shape or the other way around.

My question is why fuel pellets are not manufactured as rectangular prism. Is there some kind of drawback?

Thank you in advance.

 

[DoItForYourself]

Hello,

In my opinion, the shape of fuel pellets has to do mainly with the combustion process.

What do you think?

 

[oksuz_]

For the same volume of cylindrical and cubic pellets, the surface area is bigger for cylindrical shape. So, neutrons see more fuel in that design but I am not sure tough.

 

[DoItForYourself]

I agree, the surface area is bigger in the case of cylindrical pellets. The surface area of the solid plays an important role in reaction's rate.

However, I do not know whether the manufacturing of cubic pellets is more difficult than the manufacturing of cylindrical ones.

 

[Astronuc]

However, I do not know whether the manufacturing of cubic pellets is more difficult than the manufacturing of cylindrical ones.

Mechanically, it would be a disadvantage to manufacture square pellets, and similarly, square tubes would produce high stresses in the corners. The fuel rods of LWR fuel can develop relatively high internal gas pressure.

As far as I know, it is really difficult to make a simulation for which a lattice is constructed as a circular fuel and square moderator in shape or the other way around.

The square moderator simply reflects the square or rectilinear array in which cylindrical fuel rods are arranged. VVER fuel and Fast Reactor fuel are arranged in hexagonal arrays, or triangular lattices.

Some gas-cooled graphite moderated reactors used fuel dispersed in hexagonal fuel blocks with cylindrical cooling channels, while pebble bed reactors use spherical fuel elements that do not necessarily have a regular array.