Hi, I have problems with normalizing flux and reaction rate tallies. I have a model of a fuel assembly. Every time I used these tallies I specified a 700 bin energy filter. The goal is to get a reaction rate distribution over energy for nuclide production through the reaction rate tally and the product of the flux tally and cross-section data over all bins.

I first filled the instrumentation tube with water and tallied the flux (with a cell filter). I used the normalization that’s provided here: 8. Specifying Tallies — OpenMC Documentation. The Power/heating part is clear to me but the documentation dictates to divide by volume. I asked some colleagues and checked previous discourses. I’ve gotten mixed answers about whether it’s the whole bounding box volume, the fuel volume, or the volume of the tallied cell. I decided on the fuel volume since it’s what my colleague always uses and got the flux values. I used these values and calculated the reaction rate (multiplying the products with precalculated number densities and checked the units). This is supposed to be an overestimate.

I then filled the instrumentation tube with the material containing the target nuclide and encased it in aluminium, leaving a sliver of water. I used a material filter and tracked the wanted reaction on only the significant nuclide in each example. I ran this for 5 different nuclides and the shapes seem to match up nicely with the result from before. I normalized these reaction rates by multiplying them with Power/heating, which is the same as in other questions I found here. The result of this is 2 orders of magnitude bigger than the first before even multiplying by the number densities. This partially makes sense to me since the documentation says that the reaction rates are automatically multiplied by the number densities. Despite this, the results don’t line up. I’ve even looked at the flux distributions and they are similar to the one in the water-filled example in shape and value (these are of course a little smaller).

Since the models are completely the same except for the tweaks to the geometry in the instrumentation tube and tallies (even the heating values match up), the only thing that might be wrong in my mind is the normalization or me not completely understanding what the reaction rate tally does.

Hi Enej, welcome to the community.

Regarding the tally normalization, from what I understand in your case, you use a source strength that came from fuel assembly power/energy per fission tally (either from kappa fission or fission q recoverable) source/sec. That’s similar to what I usually did, but then you use fuel volume to get the flux value (n/cm2-sec) when flux-tallying the instrumentation tube on the center of your fuel assembly.

I think for getting flux (n/cm2-sec), you need to use the volume of cell that you have tallied (the instrumentation tube) so the flux tally which has a unit of (#-cm/source) can be multiplied by source (source/sec) and divided by tallied cell volume (cm3) and you get the average neutron flux on your tallied cell n/cm2-sec.

Then regarding the other score, i.e. reaction rate: fission, absorption, capture, etc. the tally unit itself has a unit as it already described on the URL you sent before, i.e. fission score has a unit of fission/source. So I think you only need to multiply these tally values with the source and you get your reaction rate in reaction/sec.

So all tally has been integrated to its volume, same as their equation Sig.rx(macroscopic cross sections) × flux × corresponding cell volume, besides that it has been normalized to the source strength which came from the fuel assembly power in your case.

You also can check it from your power tally i.e. if you have tallied kappa fission on all your fuel pins individually, or as you want it to be, then the total fuel assembly power will be the same as the sum of individual pin power, or #fuel pin × 1 fuel pin power if you think you have almost similar pin power fission rate and kappa.

So, if you want to know the reaction rate on the instrumentation tube cell, you can try to tally and then multiply it by source, and you get your rx. rate in rx./sec.

I hope it could help you

Thank you for the help Wahid.

This was partially a case of reading and not totally comprehending what the User Guide was telling me. I tried normalizing with respective cell volumes and it was a bit off. The biggest part was a mistake in conversion from barns to cm^2 in the first case (multiplying by 10e-24 instead of 1e-24). Fixing that solved the issue completely.

Again Thank you for spelling out the obvious clearly and concisely.