Problem with k-eff

Hello all,

I am simulating target irradiation for medical isotope production, via a rabbit tube design in a reactor, and observing the changes to k-eff when different materials enter the reactor core. For the reference case, I have the rabbit tube system present in the top right corner of my simulation without any target material present. In the model, I have the same rabbit tube system, but there is a solid cylinder of zircalloy-2 present. In the second case, I was expecting the k-eff to not change at all, due to the neutron transparency of the zirc. However, I did see a decrease in the k-eff of about 0.003. I was wondering if someone could go over my scripts to see if there is anything wrong with the way I set up my simulations.

First configuration
No (7.6 KB)

Second configuration (7.6 KB)

Thank you in advance,


Hi @alvinolay. I took a look at your models and I don’t see anything obviously wrong. Plotting the geometries seems to confirm what you say: the capsule model just has an extra tube of zircalloy. As a quick check, you could change the material of that rabbit tube from zircalloy to air and see if you get the same keff as the “no target” model. Also, you may want to make sure that the uncertainty on your keff values is low enough that a comparison between the two values is statistically significant.