The following exchange took place between Congressman and Chairman Ed Markey and the NRC at an NRC Authorization Hearing April 17, 1985:

Question 21: Chairman Markey: "What does the Commission and NRC staff believe the likelihood of a severe core melt accident to be in the next twenty years for those reactors now operating and those expected to operate during that time?"

Answer: "The Staff has available to guide its judgement on this matter close to two dozen plant and site-specific probabilistic risk assessments [PRAs]. The most complete and recent PRAs suggest core-melt frequencies in the range of 10 to the minus 3  per reactor year to 10 to the minus 4 per reactor year. A typical value is 3 times 10 to the minus 4, were this  the industry average, then in a population of 100 reactors operating over a period of 20 years, THE CRUDE CUMULATIVE  PROBABILITY OF SUCH AN ACCIDENT WOULD BE 45%.[OUR CAPS].

Commissioner Asselstine Adds the Following: " The PRA estimates given above have substantail uncertainties that span a factor of 10 above and below the reported values. I believe it is mandatory that consideration of these uncertainties be  factored into any application of such point estimates. Thus, the cumulative probability of a core meltdown accident in the  next 20 years, based only on PRA estimates and their uncertainties, ranges anywhere from 0.99 to 0.06."

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