Parameters Affecting k-effective

User Support
1

Hello everyone , What are the parameters that affect the k-effective (keff) in an OpenMC simulation?

2

There are tons of variables to play with in this context.

  • geometry
  • cross section lib
  • material composition
  • material temperature
  • BCs

etc

3

Thank you for your answer.
Could you please indicate which parameters I should try to modify first to increase keff?
For example:

  • Should I adjust the geometry (fuel region, moderator, or possible leakage)?
  • Could boundary conditions have a strong impact?
  • Should I check material density or enrichment?
    Which changes are usually the most effective to bring keff closer to 1.0 or more ?

Thank you in advance.

4

Hi,

This is a very open-ended question to which there isn’t one singular answer.

Changing the geometry (e.g., ratio of moderator to fuel) will impact the keff especially in a thermal reactor.

Boundary conditions may have a strong impact, depending on the size of the reactor. In larger reactors, the effect of the boundaries is smaller, and vice versa.

Material density and fuel U-235 enrichment also change reactivity. For example, if your fuel is denser usually reactivity goes up. However, if density goes up in other materials (e.g., structural material) then you have larger parasitic absorptions which bring reactivity down.

Normally, some of those things will be constraint. For example, you will have a certain type of fuel, which includes a specific density or enrichment. Then your geometry can be adjusted, e.g., by altering the fuel-to-moderator ratio. If you can add a reflector around the core, or make the core larger, that will also increase your reactivity.

I recommend first developing a theoretical understanding of what the keff parameter represents and how reactivity can be impacted by different things in a reactor.

5

Hi
Thanks for your clarification. It helped me understand that keff depends on multiple factors in the reactor.
I see now that parameters like geometry, moderator-to-fuel ratio, material properties, and enrichment all play a role. Also, neutron leakage and the presence of a reflector can significantly influence the results.
Based on this, I will focus on adjusting the geometry and material composition in my model and study their impact on keff.
Thank you for your guidance.