Abstract: A method for making a storage vault includes a steel inner tank entombed within a reinforced concrete outer tank. The inner tank is covered with a thermal insulating spacer material and then wrapped with a sheet of liquid-impervious material to trap leaks from the inner tank. The spacer layer is made of a material which liquifies upon contact with the liquid stored within the inner tank. This permits the spacer layer to fill the space between the inner and outer tanks while permitting any leak to collect at a liquid collection region between the two. The leaked liquid collection region can be monitored through a leak detector tube. Since the covering of the inner tank with the spacer layer substantially fills the space between the inner and outer tanks, pouring the concrete outer tank is simplified and thermal insulation is maximized.