Description

Several papers/reports in the literature have documented that the limited PRNG stride in Monte Carlo codes can negatively impact results. For example, see the paper by Hakim and Fynan as well as the recent technical report by @cjosey. Currently, OpenMC uses a fixed stride of 152,917:

For some problems (highly scattering media, deep penetration fixed source with variance reduction, subcritical multiplication with k close to 1), the stride in OpenMC would be easily exceeded, which calls into question the reliability of the results. At a minimum, we should allow the stride to be user-configurable so that for these types of problems, one can at least choose a longer stride.

Other Considerations

Since the total period of the PRNG is fixed, picking a longer stride means that the effective period between source particles is reduced. Longer term, for very large problems we probably ought to adopt a PRNG with a longer period to handle problems with lots of source particles.

Compatibility

We can make the stride user-configurable while maintaining the default value, which means there won't be any compatibility issues or changes in test results.