The Achilles’ heel of coal instead is the carbon price. Doubling the carbon price, for instance, from USD 30 per tonne of C02 to USD 60 per tonne would increase the total average cost of coal-produced power by 30%, more than doubling its variable cost in the process. This is reflected in the good security of supply performance of OECD countries with significant investments in nuclear energy during the 1970s and 1980s as measured in terms of the Simplified Supply and Demand Index (SSDI).14 Figure 4.5 from the NEA study on the security of energy supply shows how countries such as Finland, France, Sweden or the United Kingdom improved their security of supply situation during the past 40 years. While any composite indicator is the product of its defined inputs (which are explained in detail in NEA, 2010), it is clear that nuclear energy has very attractive security of supply characteristics.