Neutron activation and decay in openmc

Hello, everyone. I want to use openmc to simulate the neutron and photon dose after the reactor shutdown, it was supposed that the power is 60MW, I set the power=[6e7, 0] to simulate the flux in the process of depletion, and then I use the “openmc_simulation_n0.h5” to get the first time’s flux, it’s successful, but when I use the “openmc_simulation_n1.h5” or “openmc_simulation_n2.h5”, it shows that “invalid value encountered in divide” and the value is [nan].
Is it possible for openmc to simulate the “real flux” or “real dose” without normalization factor? I want to use openmc to replace the fispact, what can I do to achieve it? While the power is zero, does it mean the shutdown, and without power what I should do to calculate the normalization factor?