## BWR Water Rods Geometry

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ianyounker
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### BWR Water Rods Geometry

I am modeling GE BWR 10x10 fuel assemblies. These assemblies contain 2 large water rods. How do I include these rods into the fuel assembly lattice? I only need to enter one number in the lattice to represent the water rod, however the large water rod takes the place of 4 fuel rods. Also, the water rod has a different pitch then the fuel rods. How do I position it correctly within the fuel assembly?

Jaakko Leppänen
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### Re: BWR Water Rods Geometry

You need to model them in the universe that contains the lattice. See:

http://virtual.vtt.fi/virtual/montecarlo/examples/BWR/

for an example with a single water rod.
- Jaakko

ianyounker
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### Re: BWR Water Rods Geometry

I was looking at your BWR input file as I am also trying to make a BWR model. I am struggling with my k-eff values (they are low and around 0.15) and I saw that yours are where they should be (over 1.0). Could you please explain why you did not "cap" the final sqc of your assembly with pz planes at the top in bottom? Is your core infinite in all directions? In my model i use reflective boundary conditions and my final sqc is capped with pz planes on top and bottom. The planes are filled with axial reflectors (composed of a mixture of steel, water, and void) on the top and bottom of the assembly. I believe my geometry and boundary conditions are causing my low k-eff. Any thoughts?

Jaakko Leppänen
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### Re: BWR Water Rods Geometry

Boundary conditions are handled in Serpent in a very specific way. To get reflections at the top and bottom, you'll need to model the outer boundary as a cuboid. There is something about creating different types of infinite geometries in Serpent manual Section 3.7.
- Jaakko

ianyounker
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### Re: BWR Water Rods Geometry

What type of reflections are you using in your BWR model? When use a sqc to contain your assembly and you use reflective boundary conditions are you creating an infinite core both radially and axially?

Jaakko Leppänen