One of the most important conclusions in the INSAG-12 report (INSAG, 1999a) is the upper frequency target of 10-4 or less per year for severe core damage. This principle is discussed under the heading 2.3 Technical Safety Objective, paragraphs 19 to 27, in the INSAG-12 document. The objective of this approach is to establish an upper bound recurrence frequency of severe core damage accidents that could lead to release of a large amount of radioactive material that could, if the containment structure were damaged consequent to the event, lead to severe consequences for human health.