Letters to the Editor

Diablo risks unknown

The Tribune reported that a state-appointed independent panel of scientists has concluded that PG&E has not been duly conservative in evaluating the risk earthquakes pose to the Diablo Canyon nuclear power plant.

This illustrates a larger pattern of negligence by PG&E in estimating the reactor’s risks. The utility company and the Nuclear Regulatory Commission use mathematical models, called Probabilistic Risk Assessments , or PRAs, to investigate reactor safety. Not only the probability of a major earthquake, but myriad other variables, such as the durability of construction and materials, human error and wind patterns, must be taken into account in estimating the possibility and consequences of a major reactor accident. The known variables cannot be estimated accurately, and there are many more unknown variables.

Neither PG&E nor the NRC accept the final risk estimates of their own PRAs as accurate. Rather, these models are merely used for what is called sensitivity analysis — an exploratory procedure for estimating the relative or comparative influence of different variables on estimates.

PG&E does not know what the risk of a Fukushima-type accident happening at Diablo Canyon is. If pressed in public hearings, it would have to admit this.