A general solution for optimum design of a radiaton chemical reaction vessel having an internal uniform triangular array of long, thin γ-ray sources is derived. The dependence of chemical production rate on amount and distribution of radioactive material and on size and shape of vessel is accounted for. Values for two general design parameters (vessel efficiency, ψ, and unit cell efficiency, µ) as a function of the vessel diameter and source spacing are given and include radiation buildup. The rate equation expressed as a power law of the radiation intensity is combined with information on the dependence of cost of reactor vessel on volume and pressure. The total cost of source material and vessels is then minimized to determine optimum size and number of vessels and the number of curies of radiation. The rate and cost equations are applied to the radiation polymerization of ethylene. By the methods outlined here it is possible to determine the parameters of an optimum irradiation assembly. The dimensions of the vessel and source array and the quantity of radioactive source material necessary for a given rate of production are determined for the minimum cost condition.