Fire Safety Science Digital Archive

Previous attempts to predict the distance from an atomic explosion at which wood would be ignited have been based on the assumption that the radiation is constant for a period of 3 secs. after which it falls rapidly to zero. This is very far from the truth as will be seen on reference to Figure 1 which shows the probably variation of intensity with time at a distance of 1 km from the explosion. Although the average intensity used in the previous calculations was derived from the time integral of this curve it was felt that the non uniformity of the radiation with time would affect the results significantly and that the probably discrepancy should be determined. In the past the distance of ignition has been found by determining experimentally the intensity required to ignite various species of wood in three seconds and then computing the distance from an atomic explosion at which this intensity would occur under various atmospheric conditions. This direct approach is not suitable when account has to be taken of an incident intensity which is varying with time and an alternative method has to be devised. It is well known that the diffusion of heat in materials may be represented by the current flowing inside suitably designed electrical networks and this analogy has been exploited in the solution of the problem. The problem divides itself into two parts; first to determine the intensity of radiation required to bring the surface of the wood temperature for spontaneous ignition and second to determine the distance from the explosion at which these intensities will be encountered.