Main results of German risk study, Phase B, in the light of methodological problems

The German Risk Study, Phase B, which has been performed for a 1,300 MWe KWU-type pressurized water reactor, is a level 2 probabilistic safety assessment (PSA). In the study, 32 initiating events have been considered. An expected plant damage state frequency of about 3x10-5 per year has been calculated. In a plant damage state core meltdown can only be prevented by means of accident management procedures. The probability is high (about 98%) that in a plant damage state the primary system will not be depressurized. Core meltdown under high system pressure would endanger the containment integrity immediately. In most plant damage states accident management procedures are possible which bring the plant in a safe state or, at least, reduce the pressure in the primary system before the pressure vessel fails. On a preliminary basis, a success probability of 99% for accident management procedures (secondary or primary side bleed and feed) has been estimated. This results in an expected core melt frequency of about 4x10-6 per year. The expected frequency of high pressure core meltdown is about 6x10-7 per year. Loads on, and failure modes of, the containment during a core melt accident as well as release fractions after containment failure have been analyzed. The study did, however, refrain from quantifying the probability of containment failure modes. A main uncertainty in this area is the probability of containment failure due to hydrogen burning. The results of the study point out, that the risk can be reduced significantly if containment failure due to hydrogen burning can be reliably prevented