Hello everyone!
I’m new here, hope to be describing my problem correctly. I created a fuel pin code with four cylindrical sections (fuel, gap, cladding and moderator), I am having a problem with the simulation because gap section (name is helium gap) looks like this in the card (I know the description is faulty and I could just modify the script but please keep reading 
)
<geometry>
<cell id="69" material="59" name="Fuel Cell" region="-102" universe="29"/>
<cell id="70" material="62" name="Helium Gap" region="102 -103" universe="29"/>
<cell id="71" material="60" name="Cladding Cell" region="103 -104" universe="29"/>
<cell id="72" material="61" name="Moderator Cell" region="104" universe="29"/>
<cell fill="29" id="73" region="105 -106 107 -108" universe="30"/>
<surface coeffs="0.0 0.0 4.6955" id="102" type="z-cylinder"/>
<surface coeffs="0.0 0.0 4.6955" id="103" type="z-cylinder"/>
<surface coeffs="0.0 0.0 5.3685" id="104" type="z-cylinder"/>
<surface boundary="reflective" coeffs="-7.2135" id="105" name="minimum x" type="x-plane"/>
<surface boundary="reflective" coeffs="7.2135" id="106" name="maximum x" type="x-plane"/>
<surface boundary="reflective" coeffs="-7.2135" id="107" name="minimum y" type="y-plane"/>
<surface boundary="reflective" coeffs="7.2135" id="108" name="maximum y" type="y-plane"/>
</geometry>
Given this description I thought that the ‘helium gap’ region wouldn’t be present (since starting and ending cylinders are the same), but when I plot the geometry the region where there should be the moderator is filled with helium and the region with cladding is filled with moderator, how is it possible?
I would thank you very much if you could help me understand!