**6. Summary**

Previous Probabilistic Risk Assessments estimated the probability of a core melt accident to be in the range of one in several 10,000 to one in several 100,000 reactor-years. The real core melt accidents in the past happened with a probability of one in 3,700 years. Much more frequent than anticipated before. Thus, a world with 443 reactors has to expect 2.99 core melt accidents within the next 25 years, a country like the USA with 103 reactors 0.7 core melt accidents.

*The Guardian* data showed that incidents and accidents happen with a probability of approximately 0.001 = 1 × 10<sup>−</sup><sup>3</sup> per reactor-year. The data are consistent with no learning effect on the side of the plant operators. The second investigation based on Sovacool's data shows a decrease of the accident rate from 0.010 = 10 × 10<sup>−</sup><sup>3</sup> per reactor-year in 1963 to 0.004 = 4 × 10<sup>−</sup><sup>3</sup> in 2010. There is also some indication of a stronger learning effect until the beginning of the 1960s, although this is not statistically significant. Between 1963 and 2010, the operating experience increased from 96 to 14,704 reactor-years. So, while operating experience increased by a factor of over 150, the probability of a minor or severe accident at a reactor decreased by merely a factor of 2.5.

It might be interesting to compare the last results with the empirical core melt probability of 1/3700 = 0.27 × 10<sup>−</sup><sup>3</sup> . Depending on the dataset, a core melt accident is only 37 times (*The Guardian* data) or 15 times rarer than other accidents or incidents. Regarding the possible outcomes of a core melt accidents, these differences seem to be unexpectedly low and might indicate that the datasets used do not contain all incidents and accidents that happened in the past.

This guess finds support in an article by Phillip A. Greenberg. 'Between 1990 and 1992 the US Nuclear Regulatory Commission received more than 6600 "Licensee Event Reports" because US nuclear plants failed to operate as designed and 107 reports because of significant events (including safety system malfunctions and unplanned and immediate reactor shutdowns)' [18].

Our work shows the possibility of studying learning effects within the nuclear industry. But more detailed results require more analysis and more information from reactor operators and regulators. But this is difficult on an international scale because of the restrictive information policy of the IAEA.
