Reduced-Precision in One-Way Regions

[jtramm]

In this task, we will attempt to utilize reduced precision in “one-way” regions of code, i.e., data structures where data gathered from transport is accumulated but otherwise doesn’t impact transport itself. An example of this is user-specified tallies. The mean and sum-of-squares for tally information is tracked in these data structures but does not influence subsequent particle transport. In addition to reducing memory consumption, use of reduced precision in these data structures could decrease the parallel communication costs, leading to further benefits for scalability. Rigorous quantification of the effects on tally accuracy would be required to ensure that tally results produced by OpenMC are not changed significantly relative to the associated statistical variance inherent in the MC method itself