Differences in methodologies used for externality assessment. Why are the numbers different?

During the last few years, externalities related to power production technologies have been calculated making use of different methodologies. The external costs may turn out to be very different for the same fuel cycle depending on the methodology thathas been used to assess the externalities. The report gives a review of different valuation issues, which are used in different externality studies and focuses on why the numbers often are different for the same fuel cycle, using different methodologiesfor assessment of the externalities. The review of externality valuation focuses in this report on the assessment of environmental externalities. Importance has been attached to health effects, as these are the dominating effects in the external costs.Other effects are only mentioned on a superior level. The report points out different parameters, which are important to consider when externalities estimated for the same fuel cycle in different studies are compared. 8 studies have been chosen forfurther analysis and comparison in order to show the variation in external costs. The comparison shows the importance of possessing knowledge of which kind of methodologies have been used, which impacts are included etc. to explain why the numbers vary somuch in different studies for the same fuel cycle. As an example a comparison of the impacts and damage costs related to air emissions has been made for three studies using different methodologies. The external costs are estimated for the same referenceplant using the dispersion models, dose-response functions, impacts and monetary values from the three studies. The estimates from the three studies are compared two and two, and a more detailed analysis is performed in relation to human health, which isthe dominating impact in all externality studies.