Pitfalls and troubleshooting
Things to check in case of suspicious results
Unresolved resonance probability table sampling
Unresolved resonance probability table sampling is not used by default. This method is needed to account for self-shielding effects in the unresolved resonance region. The effect is usually not significant in thermal systems, but can lead to noticeable discrepancies in fast-spectrum systems.
Probability table sampling is switched on using the set ures option.
Mass vs. atomic densities
The densities and compositions of materials can be entered in atomic or mass units (see the input syntax of the material card), and using incorrect units may lead to unexpected results. The difference may not be clearly noticeable if the material consists nuclides with similar atomic weight, but if there are both light and heavy nuclides the results can be off by tens of percent.
In particular, using incorrect units for isotopes in water causes a major distortion in the flux spectrum in LWR calculations. In burnup calculation this results in completely unexpected depletion rate of 235U and build-up rate of 239Pu.