Flux tally odd behavior.

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1

Hello, I am designing an IAEA benchmark reactor for neutronic analysis with openmc. In my analysis, In my analysis, I am varying the fuel enrichment and observing the flux change. Since the reactor is an MTR reactor, there is a flux trap facility (filled with water) where I am calculating the flux in z-axis, So, my tally data (mash) is set to the width of x and y dimension of the flux trap and the z dimension is divided into 120 with 1cm each.

Now, As a general rule, increasing the enrichment will increase the effective multiplication factor (Keff). And my observation is showing the increase in Keff with higher enrichment. The issue is in Flux calculation. The flux seems to be decreasing in the flux trap facility with higher fuel enrichment.

I am not sure this is correct behavior or not, because, in my other previous design, the flux increases with higher fuel enrichment. As it should be or the flux I am calculation here is correct as an exception.

Any pointers where the issue might be will be very helpful. Thanks.

2

Hello, Good peoples, Can anyone give me a pointer on this? I am trying so many different lengths of the flux traps, I have three energy groups (0-0.625eV, 0.625-5531eV, 5531-10000000eV.

Here is the tally.xml contains

<?xml version='1.0' encoding='utf-8'?> 1 1 100 -7.75 -3.7 -50 7.4 7.4 1 1 0.0 0.625 0.625 5531.0 5531.0 10000000.0 1 2 flux 1 3 flux 1 4 flux

Any help is appreciated, any pointer, something. Thanks

3

Whether the flux in the trap should increase or decrease as a function of enrichment is a tough question to answer. It will depend on the specifics of your reactor. The best way to verify that your simulations are making good predictions is to compare against benchmark experiments of similar reactors. Try to find experimental results from a reactor that is similar to your design (same fuel isotope, same flux spectrum). If you can make simulations that accurately match the benchmark reactor then you can be more confident in the results for your reactor design.