I’ve developed a model for a whole experimental nuclear reactor core and wish to produce normalised plots of the neutron flux (cm^-2s^-1). The procedure, as I understand it, is
H’ = 1.602x10^-19 * H,
where H is the ‘heating-local’ tally, and H is the heating rate in J/source. The normalisation factor (f) is then
f = P/H’,
where P is the power in joules per second (75MWth → 75e6 joules per second). Finally,
phi’ = (f * phi)/V,
where phi’ is the neutron flux in physical units (cm^-2s^-1) and phi is the ‘flux‘ tally. My issue here is how to calculate V?
I’ve taken these flux values over a mesh (-150 to 150 in x and y and -147.5 to 147.5 in z) with a resolution of 400 in x and y, and 1 in z. Is V the volume of the mesh? the mesh cell? the reactor volume? For reference, I’m trying to make a contourf plot (x and y).
Thank you!
Hi Jupiter, welcome to the openmc community.
The volume being used to calculate the average flux (#n/cm2-sec) is the volume of the flux tally you defined. If you are tallying flux score over a cell, then the cell volume is the volume being used to divide the tally score×source (f) in your formula.
As you said that you use a mesh to tally your flux, then each mesh will have its corresponding flux score. Since the flux score on each mesh came from the mesh volume being used during flux tallying, the mesh volume is used to divide your flux score×f.
I am imagining that by having a blank box as a counting system, and as the openmc runs, we count how many neutrons get into our box caused by a specific neutron flux at each position inside the box. After that, I can average the neutron flux inside my box, which corresponds to my box size or volume and its position. By using a smaller box, the average flux being calculated became more specific to the position of that small box in my model.
Also, most of the time, I manually calculate the mesh volume from the mesh definition (lower left, upper right, mesh size), but I think the mesh filter of openmc has a built-in volume function, so you can double-check that against your manual calculation of mesh volume.