K-effective fluctuation with beryllium nuclide addition in higher than room temperature

User Support
1

Hi,
I’ve a model that uses beryllium as a reflector material. I’ve run the same model in MCNP as well. With increasing temperature for beryllium, I noticed a large fluctuation for combined k-effective calculation. The calculation at the room temperature agrees with MCNP calculation, but deviates from the MCNP calculated k-effective as with increasing temperature for beryllium whereas MCNP calculated k-effective remains the same. Has anyone encountered this problem before or would happen to know why openmc k-effective would fluctuate too much for beryllium at higher than room temperature?

2

I would check to make sure you are using the same cross section library as your MCNP model and that the cross sections are interpolating properly for temperature variations.

3

Yes we are using the same cross section. We built our own cross section library based off of endfb7.1 and tested the same library for both MCNP and openmc. And still we see the fluctuation for openmc as we deviate from room temperature, but MCNP keeps at a consistent value for k_effective.

4

Hi openmc users
Since you are comparing the openmc simulation with mcnp, have you tried to use the h5 files nuclear data from these sites that were produced by converting ACE files generated by the Nuclear Data Team at LANL for their MCNP code?

Also, sometimes, I forget to set the s(alpha, beta) in my openmc model, have you checked your s(alpha, beta) if it is also being used in your mcnp model?
Did you use the coupled neutron photon transport for your mcnp model? You need to activate the coupled n p transport in openmc settings when needed.

I don’t think that you have some geometry overlaps in your model, but you can check that with the geometry debug feature of openmc.

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