I have a model with a 5cm aluminum sphere surrounded by a much larger lead sphere. I am simulating 10 MeV neutrons inside the aluminum sphere and am getting a normal flux result for the aluminum but a zero flux through the lead. I cant imagine that the aluminum absorbs all neutrons so i was hoping for any help on why this is happening. Could it be that the lead sphere is too big to record flux since it absorbs all the neutrons before they reach the outside? It is 100cm radius.