events, and this standard's methodology has been developed in a conservative manner.
Therefore, the DOE has provided the 1E-6/y screening value for aircraft crash impact
accidents in terms of design basis considerations. DOE-STD-3009-94 also states that
the use of this cutoff frequency represents a unique case, based on established NRC
precedents for human-caused external events, such as aircraft crash.
There are no universally accepted definitions of either risk acceptance criteria or
screening values. It is impossible to use a "zero risk" philosophy because, short of
terminating a program, there are always residual risks. In fact, terminating a program
could actually result in higher risks because of the spreading of risks from one program
area to another. Because of these and other difficulties, risk frequency screening as
used in this standard has, as its primary objective, efficient resource allocation. The
screened out scenarios are considered safe enough, for the purpose of this standard,
that additional resources do not have to be expended in further analyzing them.
However, those postulated accident scenarios that are screened out because they are
located in the "risk acceptance" region of the risk curve or matrix can still be evaluated
The FDA, EPA, DOE, NRC, and ANSI (References 1 through 8) have documented
precedents that exclude events from further analysis if they have postulated accident
frequencies less than 1E-6/y. DOE has further restricted this interpretation to apply only
to external events, such as aircraft crash impact accident sequences. Only the FDA and
EPA have attempted to codify a quantitative "incredible" cutoff frequency. The FDA
assumed that "one in a million" is considered safe enough in terms of developing cancer.
In the 1990 revised National Contingency Plan, EPA used accident frequencies of 1E-6/y
as a point of departure, below which regulatory consideration is not warranted, and
recognized that the acceptable risk frequency range could in fact be several orders of
This standard uses 1E-6/y as a benchmark for resource allocation. In other words, when
postulated aircraft crash impact frequencies fall below the 1E-6/y cutoff frequency, this
standard implies that, for the purpose of resource allocation, these scenarios can be
regarded as "safe enough," and no further resource expenditures are necessary for