Getting too low of a k-eff with a large leakage fraction

Hi there,

I just ran a simulation at 10,000, 1 million, and 2 million particles all receiving roughly the same k-eff of 0.86 when I’m expecting slightly above 1. The leakage fraction I’m getting is ~0.13 which feels huge.

Could this leakage fraction be the cause of the underestimated k-eff? If so, how can I fix that? My first thought was something to do with my vacuum boundary condition but I also wondered if my defined source could be causing issues.

Thank you!