The leakage rate of gas through a crack of the concrete wall can be formulated by the folowing equations. f=W^3・(P^2_1-P^2_2)/(2・ρ_0・P_0・Q^2・T) f=a^^-(W)・(12・μ/ρ_0)/Q+ b(W) f=coefficient of friction Q=leakage rate W=crack width P_1=pressure at inflow side P_2=pressure at outflow side P_0=l.0 atm=1.01325×10^5(Pa) ρ_0=density of gas μ=viscosity of gas T=wall thickness a^^-(W) b(W)=experimental functions, functions of crack width Under the following conditions 0.8×10^5(Pa)≦P_1≦1.2×10^5(Pa) 0.8×10^5(Pa)≦P_2≦1.2×10^5(Pa) P_1-P_2≦0.2×10^5(Pa) Re=2・ρ_0・Q/μ≦10^2, the leakage rate can be estimated by the next equation. Q=a^^-(W)・(P_1-P_2)/(μ・T) a^^-(W)=experimental function, function of crack width In case that the crack width changes slowly along the surface of the wall and does not change across the wall section, the leakage rate can be calculated by simple integration.