This discussion focuses on running MCNP 5 input files for flux calculations, specifically addressing issues related to the number of particles simulated (NPS). Users reported that increasing NPS beyond 1e9 leads to fatal errors, such as "entries must be integers," due to exceeding integer limits on systems with limited resources, like a laptop with 4 GB RAM. The conversation emphasizes that while increasing NPS can improve statistical accuracy, alternative methods such as variance reduction techniques should be considered for better results without overloading the system.
PREREQUISITESResearchers, physicists, and engineers involved in particle transport simulations, particularly those using MCNP 5 for flux calculations and seeking to optimize their simulation parameters.
Thanks a lot Alex,Alex A said:Most MCNP runs are time independent because most particle transport is time independent. Two neutrons passing close by do not 'see' one another so one neutron only exists at any one time per thread. Two electrons would affect each other in the real world but this is not simulated.
All the tallies will produce an answer that is per source particle. More source particles simulated will make the answer more trustworthy. The statistics on the numbers will be better as they get closer to the 'true' result.
To simulate 1e18 you could enter nps 1e6, and then multiply the tally at the end by 1e18. The same answer would be true for a source twice as strong, except you would multiply by twice as much. 1e6 might be enough for many problems but too few for some.
The ps in nps does not mean 'per second' btw. I am not sure what it actually stands for but it is the number of particle 'histories' (MCNP speak for seeing what happens to a source particle) to run before it stops. It has nothing to do with the strength of the source.
Just for fun, try writing that as 10000000000 not 1E10. It probably won't make any difference, but it might. I don't recall exactly what the limit on an integer is for MCNP 5, but I seem to recall it was bigger than 1E9.Salman Khan said:Can any one please explain if I want to run mcnp 5 input file of certain geometry for flux calculation on different surfaces. So far as I know If I increase the NPS (number of particles) it wll give more accurate result but when I increase NPS from 10e9, input file do not run and close within a second ?