A secondary containment cap apparatus for a turret of a hazardous fluid primary container having a primary container wall with a primary container turret opening having a turret opening perimeter wall, the secondary containment cap apparatus including a valve mounting plate extending over and covering the turret opening in sealing relation with the perimeter wall, the valve mounting plate having at least one valve port fitted with a fluid passing container valve; a containment cap including a cap wall with a concave cap interior and a cap abutment port, the cap abutment port having a cap abutment port rim, the containment cap being secured onto the valve mounting plate so that the container cap covers and encloses the fluid passing container valve, the cap abutment port rim being sealingly retained to the valve mounting plate; and a cap attachment structure securing the valve mounting plate to the perimeter wall.