A secondary containment cap apparatus is provided for securing to a primary container containing a hazardous fluid, the primary container having a primary container wall with a primary container hazardous fluid passing port and a primary container wall exterior surface, the secondary containment cap apparatus including a cap wall defining a concave cap interior and a cap abutment port, the cap abutment port having a cap port rim, the cap abutment port and cap port rim being configured to fit closely against and follow the primary container wall in sealing relation with the primary container wall exterior surface and thereby to enclose a region of the primary container wall exterior surface to provide secondary containment over the region of the primary container wall exterior surface; a mechanism for controlled release of the fluid through the cap wall; and a mechanism for securing the cap wall to the primary container so that the cap port rim sealingly abuts the primary container wall exterior surface, such as a weld or tensioned cables.