Abstract: 1. The method of rendering immobile the movable or potentially movable poons of the casing-enclosed activation element of an explosive device which comprises preparing a sufficient quantity of a prepolymeric material in a pressure vessel by reacting an arylene diisocyanate with an amount of a fatty acid triglyceride having a hydroxyl number not materially less than 49, such that the ratio of the number of hydroxyl groups contained in the fatty acid triglyceride to the number of isocyanate groups in the arylene diisocyanate is greater than 0.45 to 2 but not more than 0.95 to 2, mixing with the prepolymeric material water in the ratio of one mole of water per two isocyanate groups, and connecting the pressure vessel to the interior of the casing-enclosed activation element whereby the material in the pressure vessel foams and expands, the pressure of the forming foam causing its injection therein filling all voids in the casing-enclosed activation element and allowing the material to cure to a rigid foam.