The potential hazard resulting from the accidental release of the fission products accumulated in a reactor enters into both design and site selection. This hazard is associated principally with the resultant internal exposure which is greatest from isotopes with long half-lives. A useful index of a reactor's potential hazard is the accumulated number of activity quantities each capable of 25 rem total bone exposure. This hazard index is sensitive to the relative composition of these fission products and is given by the empirical equation: HI = 112.5 P0.37 (Pt)0.63, in which P is the power in watts at which the reactor is operated for t days. This equation, valid over the time range of principal interest (30 to 1000 days), permits comparison of potential hazard for constant burnup (the Pt product) and/or other design parameters.