It is shown that in a loss of coolant accident of a nuclear power plant the amount of a specified nuclide amoung fission products released to atmosphere from the post incident device system consisting of an N-fold multiple barrier can be approximately estimated by the simple formula
QL=α_mQ₀K₀K₁K₂………KN,
where
K_n=α_nβ_n/λ+β_n+ξ_n, (n=0, 1, 2, ………, N)
is a F. P. release reduction factor associated with the n-th barrier, Q₀ represents core F. P. inventory referred to the specified nuclide which has a decay constant λ, α_m is the core melt fraction, α_n is the F. P. removal fraction due to plate out or absorption in at the n-th barrier, β_n is the leak rate from the n-th barrier, and ξ_n is the F. P. removal rate due to containment spray, air recirculation system or natural steam condasation phenomena inside of the n-th barrier.
Utilizing this formula, functions of various post incident devices proposed actually for light water moderated power reactors in U.S.A. are surveyed with respect to the radioactive iodine release reduction effects.