How to calculate the raction rate by using MCNP

One way to do this is to calculate flux (f4) in the cell that you are interested, for example
f4:n 101 (were 101 is the cell number where you want to calculate flux)
If you would like to know the absorption reaction rates in that cell you then write the following tally multiplier, for example:
f4m:n -1 10 (-2:-6) this way you will calculate total absorption reaction rates (absorption+fission, since absorption (-2) in MCNP is just the capture+fission (-6)), 10 is the number of the material in that cell and -1 is the multiplier (atom density of thet material)...you can use -1 or enter the atom density of that material...
be careful using tallys, when you calculate flux there must always be 4 the last number...
you can use the following numbers...
f04, f14, f24, f34,...

ref click