Mass change in depletion runs

I did a depletion run on a reflected molten salt sphere just to get an idea of fission product buildup and mass changes. I have two what are probably silly questions:

1. The mass decreases by much more than the amount predicted by energy generation. By that I mean if you take the power level * time to get energy, then divide that by c^2 to get mass decrement, the amount predicted by that is much less than the amount predicted by OpenMC. In the specific instance I am doing, power=3E9, t=480 x 60 x 60 seconds (20 days), and actual mass loss in the depletion results file is 5211 grams, when e=mc^2 would predict about 58 grams. What am I missing, or what is OpenMC â€ślosingâ€ť?
2. What is the easiest way to get the relative fission product poison strength? What I am thinking is simply the product of number of atoms times microscopic cross section of absorption for each isotope in the burn material, but I donâ€™t know the best way to get the microscopic cross section of absorption for say Xe135.
Thank you for any insight.
Brian

Hi Brian, welcome back to the community.
For the first question, can you describe how you calculate the spent fuel mass? Because it should be the total mass of all isotopes in your fuel material that could be a heavy metal either trans uranium, actinide, and all the fission products and its daughter isotopes. Or if you can get the whole spent fuel mass density and subtract this value from the initial mass density (you already know since it is being used as input) and then multiply by fuel volume, I think it could do the math.

For the second question, if you want to tally the absorption reaction rate on your core model (flux weighted) for a specific isotope or a group of isotopes, I think you can try `Tally.nuclides` attribute. But since I havenâ€™t used that feature, you can try the documentation or wait for other members
https://docs.openmc.org/en/stable/usersguide/tallies.html

For the change in mass, I am simply comparing the material.get_mass() function for the original fuel material, and then the depleted materials (from the last increment of the depletion_results.h5 file) get_mass() function.

Thanks for the help. Brian

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