 # Unit transformation in a fixed source problem

Hello all,

I would like to consult the way I am transforming the flux units given by OpenMC to the conventional flux units in a fixed source model.

As I mentioned in a previous post I am solving a shielding problem. Instead of simulating the fuel assemblies I am using the neutron and photon spectra and I am defining two point sources. I am working in 2D, then I determined the area of each mesh element (A in the equation below). I have the source intensity and the active fuel length. I would like to know if the equation below is correct to obtain the flux in “particles/cm2 s”. Maria

1 Like

Hi Maria. When you tally `flux` in OpenMC, the units will be [particle-cm/source]. Normally, multiplying by a source intensity in [source/sec] would give you [particle-cm/sec]. In your case, because you have a 2D model, your source intensity should really be given as [source/cm-sec], so multiplying by that then gives units of [particle/sec]. Dividing by the area of the tally in question gets you the conventional [particle/cm²-sec] units.

Hi @paulromano. Thanks for you reply. Just one thing, when you say “area of the tally in question” you mean the area of each mesh element?

Maria

Yes, that’s correct – the area corresponding to the tally result for a specific mesh element.

Ok. Thank you @paulromano

Hello all,

I am interested in getting the neutron and photon fluxes as a function of energy on a specific surface (infinite cylinder parallel to z-axis). With the flux values, I want to determine the dose. I tallied the flux values using particle, energy, and surface filters. I am trying to understand the results I am getting, so, my questions are:

Do the obtained fluxes consider the particles crossing the whole lateral surface of the cylinder? In this situation, how the flux units can be transformed?

@MaryAAZ Currently, surface filters only work in conjunction with the `current` score. It looks like OpenMC inadvertently lets you specify a flux score, but it is not giving you a surface flux (in fact, I’m not sure what value it is giving you in this case). The lack of a true surface flux tally is a longstanding issue.