Why is my neutron leakage rate so high

User Support
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communication.ipynb (473.2 KB)

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It’s probably better to post a code block via ``` (and report the leakage value you are seeing) so that we can try to reproduce your simulation behavior. I will try to take a look at the python notebook, though.

Depending on the leakage value and system (geometry, BCs, materials), the result might make sense.

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It seems like ~20% leakage and you computed k = 0.97502 +/- 0.00095. What do you expect the leakage to be?

I didn’t run your model and generate plots, but I see vacuum boundary conditions at the lowest and highest z_plane, as well as in a z_cylinder (r_wall). Particles that cross here will contribute to leakage. Could you provide a diagram of your system and label all the vacuum BCs?

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Thank you for your answer, my expected leak rate should be less than 5%, but after I changed from a vacuum condition to a total reflection condition, the leak rate did not decrease significantly, and as a beginner, I really couldn’t figure out why.

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I tried to run your notebook as is, but there’s a few variables undefined that prevent it from executing. I’ll list the undefined variables

  • r_gasgap
  • hexagon_wall
  • gasgap1_cell2
  • fission_rate

From what I can tell, you are doing fairly normal things. Could you share some of the plots you made that show the geometry and label which surfaces are vacuum?

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communication.ipynb (478.7 KB)
Thank you for your answer. It was my fault, I re-uploaded my file and now it works smoothly, I changed the boundary conditions for the upper and lower Z-planes. Switching from a vacuum condition to a reflection condition, the neutron leakage rate decreased from 0.21 to 0.15, which did not meet my expectations.

QQ20240920-134845
QQ20240920-134845743Ă—189 3.88 KB

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If your leakage is still higher than expected, my guess is somewhere there is a surface with a vacuum BC where you might want a different boundary condition. I’ll try and look again next week.

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Thank you very much. I checked the correctness of my file using the “geometriy_debug=True” code and found no overlapping areas. For the setting of boundary conditions, I only set vacuum BC at the top, bottom, and outer periphery of the core. I have another question that I would like to discuss with you. When calculating the temperature feedback coefficient of fuel, increasing the fuel temperature, such as from 900K to 1200K, increases the reactivity of the core. In fact, this should be incorrect.

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I have found out why the neutron leakage rate is too high. Thank you for your answer, Professor. I will continue to move forward on the path of learning openmc.

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Glad you figured it out! And I am not a professor, just an OpenMC community member :slight_smile:

Do you mind sharing how you fixed it? I think it would be good so that others can see how to fix their problems if they have a similar issue. I am also curious where the particles were leaking from (where they weren’t supposed to be)

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haha, people who can guide others are professors.:smile: As the probability of neutron absorption increases, the leakage rate decreases accordingly. When I changed the upper and lower BC, the neutron leak rate did not decrease to what was expected. I re-read some of the literature and found that some of the cores were not arranged in the same way as mine, so I added a new ring of neutron-absorbing material around the periphery of the core, and found that the neutron leakage rate reached my expectations, and the neutrons leaked mainly from the radial to the outside of the reactor.

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