Does anyone know what the source strength should be set to when doing a fixed source simulation? Or can someone explain to me what the source strength is exactly?
OpenMC, like most Monte Carlo radiation transport codes, will provide results that are normalized to a single source particle. For example, neutron fluence may be reported in neutrons/cm^2/source_particle. The user is required to multiply these results by the known strength of their source, often in post-processing. For example, if you have a fixed source problem in which you know there are 10^12 neutrons born per second, the above results would be multiplied by 10^12 source_particles/s to get a neutron flux of n/cm^2/s.
This brings us to the source strength that can be defined as part of the OpenMC input. This is only meaningful as a relative source strength in a problem where you define more than one contribution to your fixed source. For example, if you had two fixed sources with different strengths - say, 1E15 and 3E15 - you could define two fixed source in OpenMC with strengths of 1E15 and 3E15. Your results would still be reported in a normalized fashion, and you would need to multiply those results by 4E15 (the total source strength) to get physically meaningful units.
(Note: in this example, you could also define the source strengths as 1 and 3, or as 2 and 6, or any combination that has a ratio of 1:3)
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