Use of a neutron poison in a lattice

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1

Good afternoon,

Through your help, I have been able to run a number of experiments looking at the concentration of Uranium and Plutonium in a lattice to get a keff = 1. So great stuff, thanks.

I have hit an issue with my second experiment looking at the impact of a neutron poison on this configuration. This runs through fine without any error but the answer does not look correct compared to my expectations.

In my lattice the central 10 pins now contain boric acid. This reduces the keff, but when this is compared to the central 10 pins containing water this has the same keff. indicating that water is having an equal effect. That the two values are identical (not just similar) with identical errors makes me doubt what I have done. I have tried to sense check this a number of times - both in terms of materials / tallies / settings but I can’t see what I’ve done wrong.

Please can you have a quick overview of my XML scripts and point out if there are any obvious failings (below are a few details - attached the full batch)?

In Geometry I have altered from boric acid to water by altering the following from material=“5” to material=“3”

In materials:

Thanks so much for you time.

Mark

Boric Acid help.zip (8.53 KB)

2

Further to this note - I’ve replaced all water in the simulation with Boric Acid and receive the same Keff result and error. This points to a likely error in settings / tallies but I ca’t see what this is.

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3

One thing I noticed right off the bat – for materials 1 and 2, you’re using density=“sum” which indicates that the values that appear for each nuclide should be in units of atom/b-cm. However, the values you have listed (on the order of 1e18 – 1e22) look like they are atom/cm^3. You should multiply those values by 1e-24 so that they are really atom/b-cm and see if that changes anything.

4

This has been solved from advice from Paul.

Solution:For materials if you’re using density=“sum” which indicates that the values that appear for each nuclide should be in units of atom/b-cm, I had atom/cm3.

Onward.