Four factors formula

Hi Paul and Adam,
Happy new year 2014.

Out of curiosity, I wanted to reproduce the value of K_infinity from reaction rate tallies. I adopted the indc-usa-107 problem 1 from benchmarks-master examples.

Results: (see attached files)

The four factors formula underestimates K_infinity. While comparing to results of MCNP, I found that the reproduction factor η is strongly underestimated by OpenMC even if the results for ν Σ f are the same for both the two codes. Whereas the thermal utilization factor f produced by OpenMC is slightly higher than that of MCNP calculation.
It seems that OpenMC overestimates absorption rate (as it contains also fission rate). The fission rate is also slightly overestimated by OpenMC.
The flux and neutron spectrum produced by OpenMC and MCNP are in a very good agreement.
Please give a look at files attached to this message and tell me if I’m right.

Tarek El Bardouni

indc-usa-107.rar (297 KB)

I went through some of the spreadsheet and I think there may be a mistake in interpreting OpenMC tallies. I went ahead and added my own worksheet in the Excel file called ‘Herman’ and performed a calculation of the 4-factors. I just used thermal and fast energy groups (I combined your Epithermal and Fast tallies). Results look close to MCNP and I reproduced OpenMC k-infinity. I have also attached a PDF explaining my approach.

Please let me know if you have any questions,

-Bryan Herman

Comparison-MCNP-OpenMC-PROBLEM1.xlsx (968 KB)

4_factor_OpenMC.pdf (91.6 KB)

Thank you Herman for the clarification. I think everything will be fine.

Tarek El Bardouni