IMPROVED DETERMINATION OF TORNADO DAMAGE FREQUENCY

International safety regulations require the consideration of tornado impact on nuclear facilities. Existing guidelines contain values for damage zone size, wind speed ranges and occurrence frequencies of tornadoes. However, the direct application of these data might lead to an overestimation of the frequency of strong tornadoes at a specific site. Documented observations of tornado damage and measurements of tornado wind speeds clearly show that the damage caused by a violent tornado is not uniform, but differs with the most severe damage occurring in a small core zone and less severe damage in a large region around that zone. On this basis, an approach for a more realistic determination of tornado occurrence frequencies had been developed in a former publication of the authors.In this paper, an improved generic model for the tornado damage zone sizes depending on tornado strength and the local damage distribution within the damage zone is developed. This model is based on U.S. data from a very comprehensive research project of the U.S. National Institute of Standards and Technology (NIST). The model is applied for two examples with regional tornado data from Switzerland and Germany to estimate the frequency with which a site is hit by a tornado with a specific local intensity. The results are conservative as the damage zone size model is predominantly based on data from the region with the most devastating tornadoes worldwide. Further research should be devoted to improving the statistical basis for tornado frequencies and analysing tornado characteristics outside the USA.