I am manually post-processing some tallies on cell geometries and am curious if my hand calculations are correct. More importantly, I am trying to confirm whether I am applying my source particles, areas, etc. correctly to get my results into desired units. This project involves a fast neutron source from an ITER-DT spectra. Here are a couple examples:
Neutron Flux
My tally output from my cell is nominally 0.151489 (The units from the OpenMC tallies page are (n*(cm^2))/sp. My starting number of particles for the run is 100,000. I have a 0.59874 leakage rate, which means that the number of particles to use is 100,000- (0.59874*100,000) = 401260 particles.
Run time = 19456.5 sec
Particles/sec = (401260 particles/19456.5 sec) = 20.62344 particles/sec
Using this number of starting particles and time gives:
My heating tally output is in the units of ev/sp. the nominal value from my cell tally is (2.86E+03 ev)/sp. I would like my heating value in Joules (J). The calculation is as follows:
My tritium production tally output is in the units of (n,t)/sp. the nominal value from my cell tally is 1.97E-03 ((n,t))/sp. I would like to know the number of tritium atoms produced. The calculation is as follows:
atoms of T = 1.97E-03 ((n,t))/sp * 401260 particles = 7.904E2 tritium atoms.
Are these calculations correct? I have never post processed these by hand and have a sinking feeling (especially surrounding my heating values) that they are too low. Please let me know your thoughts.
I am not an expert so take the following with a grain of salt.
My understanding of OpenMC is that the defintion for the source (sp in your examples) used to normalize tallies has nothing to do with your number of simulated particles.
OpenMC seems to behave differently if you have 1 or several sources.
In the case of only 1 source in your model :
source.strength is not defined : by default, it’s 1. You’ll have to multiply your tally results by the actual intensity of your source (total activity in Bq for instance)
source.strength is defined : you have nothing to do: tally results will directly be in the right unit, assuming your source strength is correct.
In the case of several sources in your model :
source.strength is the relative strength of that source over the sum of all source.strength. You’ll have t o multiply your tally results by the sum of all intensities of every source you defined.
I only have this one source, and have made it a monodirectional planar source based on the energies and fluxes of this spectra. I do not have source.strength defined.
After doing some reading, I find a wide variety of DT n/s values, ranging from 1E14 to 5E20. I Assume 1E14 n/s is a conservative starting point to post process.
Utilizing this value greatly simplifies my calculations and means that I was drastically undercounting my normalized results. Utilizing 1E14 gives: