[en] The formation of combustible atmospheres during unrestricted core heatup accidents in High Temperature Gas-Cooled Reactors is being investigated, considering the effects of only partially mixed atmospheres. It is found that the previosly used assumption of complete mixing presents the more severe limit in most cases. In the few cases where higher loads were obtained, these were still below the containment building failure limits, and did apply only locally. Furthermore these cases required the invocation of even more remote failure scenarios. A qualitative discussion applying the above results to corresponding accidents at Fort St. Vrain is included