Appendix C to Part 191 - Guidance for Implementation of Subpart B
40:27.0.1.1.2.5.1.1.3 : Appendix C
Appendix C to Part 191 - Guidance for Implementation of Subpart B
[Note: The supplemental information in this appendix is not an
integral part of 40 CFR part 191. Therefore, the implementing
agencies are not bound to follow this guidance. However, it is
included because it describes the Agency's assumptions regarding
the implementation of subpart B. This appendix will appear in the
Code of Federal Regulations.]
The Agency believes that the implementing agencies must
determine compliance with §§ 191.13, 191.15, and 191.16 of subpart
B by evaluating long-term predictions of disposal system
performance. Determining compliance with § 191.13 will also involve
predicting the likelihood of events and processes that may disturb
the disposal system. In making these various predictions, it will
be appropriate for the implementing agencies to make use of rather
complex computational models, analytical theories, and prevalent
expert judgment relevant to the numerical predictions. Substantial
uncertainties are likely to be encountered in making these
predictions. In fact, sole reliance on these numerical predictions
to determine compliance may not be appropriate; the implementing
agencies may choose to supplement such predictions with qualitative
judgments as well. Because the procedures for determining
compliance with subpart B have not been formulated and tested yet,
this appendix to the rule indicates the Agency's assumptions
regarding certain issues that may arise when implementing §§
191.13, 191.15, and 191.16. Most of this guidance applies to any
type of disposal system for the wastes covered by this rule.
However, several sections apply only to disposal in mined geologic
repositories and would be inappropriate for other types of disposal
systems.
Consideration of Total Disposal System. When predicting
disposal system performance, the Agency assumes that reasonable
projections of the protection expected from all of the engineered
and natural barriers of a disposal system will be considered.
Portions of the disposal system should not be disregarded, even if
projected performance is uncertain, except for portions of the
system that make negligible contributions to the overall isolation
provided by the disposal system.
Scope of Performance Assessments. Section 191.13 requires
the implementing agencies to evaluate compliance through
performance assessments as defined in § 191.12(q). The Agency
assumes that such performance assessments need not consider
categories of events or processes that are estimated to have less
than one chance in 10,000 of occurring over 10,000 years.
Furthermore, the performance assessments need not evaluate in
detail the releases from all events and processes estimated to have
a greater likelihood of occurrence. Some of these events and
processes may be omitted from the performance assessments if there
is a reasonable expectation that the remaining probability
distribution of cumulative releases would not be significantly
changed by such omissions.
Compliance with § 191.13. The Agency assumes that,
whenever practicable, the implementing agency will assemble all of
the results of the performance assessments to determine compliance
with § 191.13 into a “complementary cumulative distribution
function” that indicates the probability of exceeding various
levels of cumulative release. When the uncertainties in parameters
are considered in a performance assessment, the effects of the
uncertainties considered can be incorporated into a single such
distribution function for each disposal system considered. The
Agency assumes that a disposal system can be considered to be in
compliance with § 191.13 if this single distribution function meets
the requirements of § 191.13(a).
Compliance with §§ 191.15 and 191.16. When the
uncertainties in undisturbed performance of a disposal system are
considered, the implementing agencies need not require that a very
large percentage of the range of estimated radiation exposures or
radionuclide concentrations fall below limits established in §§
191.15 and 191.16, respectively. The Agency assumes that compliance
can be determined based upon “best estimate” predictions (e.g., the
mean or the median of the appropriate distribution, whichever is
higher).
Institutional Controls. To comply with § 191.14(a), the
implementing agency will assume that none of the active
institutional controls prevent or reduce radionuclide releases for
more than 100 years after disposal. However, the Federal Government
is committed to retaining ownership of all disposal sites for spent
nuclear fuel and high-level and transuranic radioactive wastes and
will establish appropriate markers and records, consistent with §
191.14(c). The Agency assumes that, as long as such passive
institutional controls endure and are understood, they: (1) Can be
effective in deterring systematic or persistent exploitation of
these disposal sites; and (2) can reduce the likelihood of
inadvertent, intermittent human intrusion to a degree to be
determined by the implementing agency. However, the Agency believes
that passive institutional controls can never be assumed to
eliminate the chance of inadvertent and intermittent human
intrusion into these disposal sites.
Consideration of Inadvertent Human Intrusion into Geologic
Repositories. The most speculative potential disruptions of a
mined geologic repository are those associated with inadvertent
human intrusion. Some types of intrusion would have virtually no
effect on a repository's containment of waste. On the other hand,
it is possible to conceive of intrusions (involving widespread
societal loss of knowledge regarding radioactive wastes) that could
result in major disruptions that no reasonable repository selection
or design precautions could alleviate. The Agency believes that the
most productive consideration of inadvertent intrusion concerns
those realistic possibilities that may be usefully mitigated by
repository design, site selection, or use of passive controls
(although passive institutional controls should not be assumed to
completely rule out the possibility of intrusion). Therefore,
inadvertent and intermittent intrusion by exploratory drilling for
resources (other than any provided by the disposal system itself)
can be the most severe intrusion scenario assumed by the
implementing agencies. Furthermore, the implementing agencies can
assume that passive institutional controls or the intruders' own
exploratory procedures are adequate for the intruders to soon
detect, or be warned of, the incompatibility of the area with their
activities.
Frequency and Severity of Inadvertent Human Intrusion into
Geologic Repositories. The implementing agencies should
consider the effects of each particular disposal system's site,
design, and passive institutional controls in judging the
likelihood and consequences of such inadvertent exploratory
drilling. However, the Agency assumes that the likelihood of such
inadvertent and intermittent drilling need not be taken to be
greater than 30 boreholes per square kilometer of repository area
per 10,000 years for geologic repositories in proximity to
sedimentary rock formations, or more than 3 boreholes per square
kilometer per 10,000 years for repositories in other geologic
formations. Furthermore, the Agency assumes that the consequences
of such inadvertent drilling need not be assumed to be more severe
than: (1) Direct release to the land surface of all the ground
water in the repository horizon that would promptly flow through
the newly created borehole to the surface due to natural
lithostatic pressure - or (if pumping would be required to raise
water to the surface) release of 200 cubic meters of ground water
pumped to the surface if that much water is readily available to be
pumped; and (2) creation of a ground water flow path with a
permeability typical of a borehole filled by the soil or gravel
that would normally settle into an open hole over time - not the
permeability of a carefully sealed borehole.
[50 FR 38084, Sept. 19, 1985. Redesignated and amended at 58 FR
66415, Dec. 20, 1993]