I am confused to simulate and model the gamma scanning facility for fuel burn up reactor measurement using openmc. The purpose of this simulation is to obtain the value of self attenuation, collimation effect, and detector response as correction factors for gamma scanning measurement results. Self attenuation is caused by the absorption of gamma rays from the fission product Cs137 by the fuel itself before it reaches the collimator and is measured by the gamma detector. The next challenge is to apply the collimator and detector response to gamma rays from fuels that have experienced self attenuation. I don’t know any examples to simulate these conditions, starting from what to look for and how to simulate it in OpenMC. I hope a lot with the answers given. Thank you 
Hi Dila,
You’re question is very broad, and it will all depend on your assumptions and how accurate you require the results. For example, you could simulate an entire reactor within OpenMC, run a depletion simulation until a certain point, and then simulate the fuel at the burnup desired, measuring using tallies the flux of gammas through the pinhole into your collimator. This would account for self attenuation, and you could even measure this effect by tallying the gamma emission around the energy of Cs-137 and then measuring the energy at the collimator entrance.
This would probably be the most accurate, but would require significant model building and significant simulation time if you don’t have access to a lot of computer cores. Another method would be to simulate a single bundle/assembly with reflective conditions to make it critical, and then run the burnup sim on that instead, which would be significantly faster but clearly less accurate. Lastly, you could just simulate a isotropic source shaped like a fuel bundle/assembly with an output selected by you based on real life studies completed on fuel. If you use data from fuel inside the reactor you would need to model the shielding in between the fuel bundle/assembly and the collimator. This would be the quickest method, though you are relying on the accuracy of the studies. For actually measuring the collimator effects, you need to model the collimator, measure the flux of gammas at the entrance of the collimator, and then measure the flux leaving. The ratio of flux reaching the collimater and the flux leaving will give the efficiency. The resolution can be measured from recording the gamma events on the detector volume at the end of the collimator and plotting them with counts over energy and measuring the FWHM. As for the detector response I am not familiar with actually determining the intrinsic efficiency from a MC simulation, though you could assume a specific type of detector design (HPGe for ex) and use it’s efficiency at different energies and apply that to your spectrum. If you give more information on the problem you’re solving it might be helpful to decide what might be best. Let me know if you have any other questions.
Thanks a lot for your answer and the information! I will try to work on that