An assessment is made of the containment margin available in the Fast Flux Test Facility to mitigate the consequences of a postulated failure of in-vessel post-accident heat removal following a hypothetical core disruptive accident. The consequences of a number of assumed meltdown configurations (both in-vessel and ex-vessel) are assessed using the CACECO (CAvity, CEll, COntainment) containment analysis computer code together with currently available melt front penetration models. The sensitivity of the accident scenarios to a number of crucial assumptions is established by scoping studies. It is concluded from both the in-vessel and ex-vessel analyses that sodium vapor combustion is a major source of reactor containment building (RCB) pressurization. The conditions (a combination of sodiumconcrete reaction, pool size, and decay heat level) that most rapidly bring the sodium to boiling, together with those that enhance mass transfer of sodium vapor to the RCB, are the ones that most significantly affect the pressure response.