suggesting that these FEPs
may have less than a 50% chance of
occurring within the entire history of
the Earth. Indeed, the volcanic rocks
comprising Yucca Mountain and its
surroundings are only on the order of
10–12 million years old (
∼
10
7
years). In
determining the probability of particular
FEPs, the geologic record at the site is
the source of information to identify
what FEPs have occurred at the site in
the past and may occur in the future
(through the period of geologic
stability). Since the host rock formations
at the site are only about 10 million
years old, an annual probability cut-off
of 10
¥
10
would mean that probability
estimates for some FEPs would have to
be made in spite of the fact that there
is no evidence for their occurrence at
the site in the past. As it is, the 10
¥
8
probability threshold presents a
significant challenge to characterize
FEPs with some degree of confidence,
given the limits of today’s science and
technology. ICRP makes a similar point
in its 2007 recommendations: ‘‘The use
of probability assessment is limited by
the extent that unlikely events can be
forecast. In circumstances where
accidents can occur as a result of a wide
spectrum of initiating events, caution
should be exercised over any estimate of
overall probabilities because of the
serious uncertainty of predicting the
existence of all the unlikely initiating
events.’’ (Publication 103, Docket No.
EPA–HQ–OAR–2005–0083–0423,
paragraph 269) (Note that this
discussion is in the context of
‘‘potential’’ exposures, which include
releases that may occur in the far future
from disposal facilities. Therefore, the
term ‘‘accidents’’ should not be taken as
limited to operational activities.)
Overall, we believe events with a lower
annual probability than 10
¥
8
would
introduce speculation beyond what is
appropriate to define a reasonable test of
disposal system performance.
We also received comments stating
that maintaining the probability
screening criteria for the extended
compliance period undermines our
arguments for increasing uncertainty. To
the contrary, we believe the physical
meaning of the probability threshold
(0.01% chance of occurrence within
10,000 years, but a 1% chance within 1
million years) appropriately
incorporates the concept of uncertainty
increasing with time, while still
applying a substantially conservative
screening criterion.