Heat Load measurement

I want to calculate wall heat load on first wall and vacuum vassel of a fusion reactor. I want to tally fast neutrons (applying energy filter for tallying 1e5 to 20e6 energy range) at those cells and then multiply it by 17.6×10^6×0.8×1.6×10^-19×1000×source_rate for heat load in W/m^2. Is the method correct or wrong? If correct any other better way to calculate the parameter?