In establishing our final standards, we
have considered that the level of
uncertainty increases as the time period
covered by DOE’s performance assessment increases.3 Therefore, it is
reasonable for us to consider how the
compliance standard itself might also
need to change. Specifically, we do not
believe that extending the 10,000-year
individual-protection standard of 15
mrem/yr to apply for 1 million years
adequately accounts for the
considerations outlined above or
represents a reasonable test of the
disposal system (more extensive
discussion of uncertainty in
performance assessments is in section
III.A.4 of this document, ‘‘How Did We
Consider Uncertainty and Reasonable
Expectation?’’); see also 66 FR 32098.
We turned back to the international
technical literature for advice regarding
appropriate points of comparison for
doses projected over hundreds of
thousands of years. A number of sources
suggested that natural sources of
radioactivity would provide an
appropriate benchmark for such
comparisons. In exploring this approach
further, we found that the variation in
background radiation across the United
States covered a wide range (from
roughly 100 mrem/yr to 1 rem/yr),
primarily because of local variation in
radon exposures. We chose for our
proposal a level of 350 mrem/yr, which
is close to a widely-cited estimate of 300
mrem/yr for the national average
background radiation exposure (NAS
Report Table 2–1), but specifically
represented the difference between
estimated background levels in
Amargosa Valley and the State of
Colorado. This level was proposed for
both the individual-protection and
human-intrusion standards as offering
both a reasonable level of protection and
a sound basis for regulatory decisionmaking
when exposures are projected to
occur hundreds of thousands of years
into the future. Selecting such a level
would also provide an indication that
exposures incurred by the RMEI in the
far future from the combination of
natural background radiation and
releases from the Yucca Mountain
disposal system would not exceed
exposures incurred by residents of other
parts of the country today from natural
sources alone. Today’s final rule adopts
a more stringent standard that is not
derived from an analysis of background
radiation, as explained in sections
III.A.1 (‘‘What is the Peak Dose Standard
Between 10,000 and 1 Million Years
After Disposal?’’) and III.A.5 (‘‘How Did
We Consider Background Radiation in
Developing The Peak Dose Standard?’’)
of this document.
Uncertainty in long-term projections
also influenced our proposal. Given the
probabilistic nature of performance
assessments, it is possible that some
combinations of parameter values will
result in very high doses, even if such
combinations have an extremely low
probability of occurring. Although there
may be only a few results that are very
high, extreme results have the potential
to exert a strong influence on the
arithmetic mean, which could make the
mean less representative of all
performance projections. This
possibility may be increased by the
introduction of additional, and possible
excessive, conservatisms as a way to
account for uncertainties. We expressed
a preference for a statistical measure
that would not be strongly affected by
either very high- or low-end estimates,
believing it appropriate to focus on the
‘‘central tendency’’ of the distribution,
where the bulk of the results might be
expected to be found. We proposed the
median of the distribution as being most
representative of central tendency.
Because it is always located at the point
where half the distribution is higher and
half lower, the median depends only on
the relative nature of the distribution,
rather than the absolute calculated
values. Given our concerns about
specifying a peak dose compliance
value against which performance would
be judged for a period up to 1 million
years, we believed the median might
also provide a reasonable test of longterm
performance. Today’s final rule
departs from the proposal by adopting
the arithmetic mean as the statistical
measure of compliance to be applied at
all times, as explained in section III.A.9
of this document (‘‘How Will NRC Judge
Compliance?’’).
Our consideration of FEPs also was
affected to some extent by uncertainty,
as well as by conclusions of the NAS
committee. In our proposal, the overall
probability threshold for inclusion of
FEPs remained the same as in the 2001
rule, which we believe provides a very
inclusive initial screen that captures
both major and minor factors potentially
affecting performance. Uncertainty
plays a role in the sense that very
gradual or infrequent processes and
events may begin to influence
performance only at times in the
hundreds of thousands of years, when
the overall uncertainty of assessments is
increasing. The additional uncertainty
introduced by these