cant divide the tally value of epithermal to thermal capture reaction

User Support
1

I am trying to calculate the epithermal to thermal capture in U238.In order to find the ratio ,I am trying to divide the (n,gamma) score for U-238 in epithermal range by (n,gamma) reaction for U-238 in thermal range.I have tried this and several other things but no success

tallies_file = openmc.Tallies()
thermal = openmc.Tally(name=‘thermal capture’)
thermal.scores = [’(n,gamma)’]
thermal.nuclides = [‘U238’]
thermal.filters = [openmc.EnergyFilter([0.0, 0.625]),openmc.CellFilter([fuel_cell])]
tallies_file.append(thermal)

epithermal = openmc.Tally(name=‘epithermal capture’)
epithermal.scores = [’(n,gamma)’]
epithermal.nuclides = [‘U238’]
epithermal.filters = [openmc.EnergyFilter([0.625,100.0e3]),openmc.CellFilter([fuel_cell])]
tallies_file.append(epithermal)

sp = openmc.StatePoint(‘statepoint.100.h5’)
ep = sp.get_tally(name=‘epithermal capture’)
th = sp.get_tally(name=‘thermal capture’)
r = ep/th
print®

I get this when i do things like this

ValueError: Unable to get the bin index for Filter since "(0.625, 100000.0)" is not one of the bins

what am I doing wrong?Can you please show how its done?
thanks in advance

Untitled.ipynb (32.5 KB)

2

You’re close. You just need to get the mean values from the Tally objects rather than trying to do math on the Tally objects themselves:

r = ep.mean / th.mean

Also note that you can compute the same quantity using a single tally with multiple filter bins:

tallies_file = openmc.Tallies()
tally = openmc.Tally(name=‘U238 capture’)
tally.scores = [’(n,gamma)’]
tally.nuclides = [‘U238’]
tally.filters = [openmc.EnergyFilter([0.0, 0.625, 100.0e3]),openmc.CellFilter([fuel_cell])]
tallies_file.append(tally)
tallies_file.export_to_xml()

…

tally = sp.get_tally(name=‘U238 capture’)
r = tally.mean[1, 0, 0] / tally.mean[0, 0, 0]
print®

3

Why do I have to write ep.mean and th.mean ,sir? I mean when I am calculating reaction rate of fuel,it should give me an average because i am not doing any mesh or anything.

and you wrote tally.mean[1,0,0] and tally.mean[0,0,0]? Does it mean that the (n,gamma) reactions in the two energy bins are given output as a numpy array with shape (2, 1, 1)?
Thanks in advance,sir

4

The mean refers to taking an average over all the active batches from the simulation. Similarly, you can get ep.std_dev and th.std_dev (short for standard deviation) which gives estimates of the uncertainty of the tally.

And you’re correct on the numpy array. The mean and std_dev are output in the from of 3D numpy arrays. The first dimension corresponds to filter bins, energy filter bins in this case. If you specify multiple nuclides for a tally then you index their results with the second dimension of the numpy arrays. Multiple scores can be indexed on the third dimension.

5

Thanks ,sir