Computing (effective) delayed neutron fraction

[paulromano]

Hi @Lorenzo_Mazzocco. Unfortunately there is no way to properly get the effective delayed neutron fraction from our basic tallies. If you’re willing to make a patch to the code, you can calculate an estimate of beta-effective using the “prompt method”. See the discussion on a previous topic:

k-Eigenvalue Filter

Right now we don’t have any way of doing this directly. I’ve had numerous people ask about this though, usually in the context of trying to estimate beta-effective. I’ve previously put together a patch which changes the behavior of the code to only use prompt neutrons. To use it, you would need to need to get the OpenMC git repository, apply the diff on the ‘develop’ branch, and compile/reinstall the code. Best, Paul

The prompt method is described in a paper by Meulekamp and var der Marck. The way it works is as follows. First, you would calculate k using a normal version of OpenMC, and then you’d calculate k using the patched version (call it k’), which gives you k-effective accounting only for prompt neutrons rather than prompt+delayed. The prompt method approximation is then βeff = 1 - k’ / k.