  Certain modeling assumptions underlying Probabilistic Risk Assessment (PRA)
allow a simple computation of core damage frequency (CDF). These assumptions
also guarantee that the time remaining until a core damage event follows an
exponential distribution having parameter value equal to that computed for the
CDF. While it is commonly understood that these modeling assumptions lead to an
approximate characterization of uncertainty, we offer a simple argument that
explains why the resulting exponential time until core damage distribution
under-estimates risk. Our explanation will first review operational physics
properties of hazard functions, and then offer a non-measure-theoretic argument
to reveal the the consequences of these properties for PRA. The conclusions
offered, here, hold for any possible operating history that respects the
underlying assumptions of PRA. Hence, the measure-theoretic constructs on
filtered probability spaces is unnecessary for our developments. We will then
conclude with a brief discussion that connects intuition with our analytical
development.
