Some criteria for the walls surrounding the protected room have been already described in the previous sections. Additional particular information is shown in Fig. 2 and described below. These walls are composed of the following layers, listed below from inside to outside:

• A steel box (14) with essentially vertical and reasonably tight walls.
• A thermal shield composed of at least one row of tightly spaced risers (15Y), normally filled by water at ambient temperature. In case of a fire outside the protected room, water (possibly with some fraction of steam) flows upwards through the risers.
• A barrier (16) made of appropriate solid materials according to the valid rules.
• All above barriers must incorporate a system of locks, allowing the movements of persons and materials across them at various scenarios of operation or of fire without impair the essential objectives of the protected room.