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OCR for page 161
APPENDIX E
PERSONAL SUPPLEMENTARY STATEMENT
OF THOMAS H. PIGFORD
INTRODUCTION AND SUMMARY
This supplementary statement clarifies two alternative methods of
calculating racliation exposures to people in the far future. They are the
exposure scenarios involving the "probabilistic critical group" describeci in
Appendix C and the "subsistence-farmer critical group" Ascribed in
Appenciix D. Both exposure scenarios involve critical groups, as
recommended by the International Commission on Radiation Protection
(ICRP). ICRP also recommends that the critical group include the person
at highest exposure. The objective is to ensure that if the individual at
calculated maximum exposure is suitably protected, no other individual
doses wit} be unacceptably high [ICRP, 1985ab].
~ believe that this objective can be reasonably met if exposures and
risks are calculated using the subsistence-farmer scenario and if the
calculated risks meet the Standard's performance criterion. The
subsistence-farmer is the indiviclual at calculated maximum risk. Thus, the
subsistence-farmer approach is conservative and bounding. Its use
represents wide national anti international consensus for safety assessment
when characteristics of exposer! populations are not known. In contrast,
the probabilistic critical-group calculation is based on arbitrary choices of
reference populations, is not well defined, is not mathematically valid, ant!
is subject to manipulation. It could leac] to much lower calculated doses
and risks. There is no indication, however, that this country needs to adopt
a calculational approach that is so much more permissive than current
national and international practice. Its adoption wouIci undermine
confidence in the adequacy of public health protection and jeopardize
future success of the Yucca Mountain project.
A policy decision common to exposure scenarios in Appendices C
en c} D of the Report is that future humans will have cliets ant] foocI-water
intake similar to that of people now living in the vicinity. In both exposure
161
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162
YUCCA MOUNTAIN STANDARDS
scenarios, calculations are to be made for future people who cio not have
extreme sensitivity to radiation, who have the same response to radiation
as present people, and who do not have abnormal cliets. This
Supplementary Statement speaks of calculating maximum and average
closes ant! risks to such future humans, not to persons who may be at
greater risk because of unusual diets or unusual sensitivity to radiation.
COMMENTS AND EXPLANATION
1.
Among the many possible exposure scenarios, the subsistence-
farmer exposure scenario is the most conservative. It is
bounding. All future people will be protected if the calculated
subsistence-farmer dose/risk meets a prescribed safety limit.
Future humans can be exposed to radiation by drinking well water
containing radionuclides and consuming food grown from that
contaminated well water. 2 In addition to assuming diets and food-water
Calculated concentrations of radionuclides in ground water are a function of
location and time. Exposure calculations translate these concentrations into
estimates of dose and risk to fixture people. The method of exposure calculation
is the "exposure scenario"; it is sometimes called the "biosphere scenario".
2 The Committee is also concerned with the persons exposed to "the highest
concentration of radiation in the environment". The environment includes air,
water, and soil. The radiation in that environment consists of photons, free
electrons, and alpha particles from radioactive decay of radionuclides. The
"concentrations" of such radiation are rarely calculated, but could be deduced
from calculated radiation fluxes. Evidently the Committee has in mind possible
exposure from external radiation, such as doses to the skin from swimming in
contaminated water or from being immersed in contaminated air. However,
studies presented to the Committee show that such doses and risks from external
radiation in the environment are minor compared to doses and risks from
inhalation and ingestion of radionuclides that may be released to the
(continued...)
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APPENDIX E --STAT~ENT OF THONGS H. PlGFORD
163
intake typical of that of present humans, it is also necessary to assume how
much of the lifetime intake of food and water is affected by water
contaminated with radioactivity, as well as how near the withdrawal well is to
Me repository. These "human activity" assumptions are most difficult to deal
with.
Future people are deemed to be suitably protected if their calculated
lifetime radiation doses and risks are less than a prescribed dose or risk limit.
The calculational method should be constructed! so that if the person receiving
the calculated maximum dose is suitably protected, then all future people with
similar cliet and close response will also be protected [ICRP, 1985ab]. To
ensure such protection we should assume conservatively that some future
-individuals are subsistence farmers who use contaminated grounc! water for
Winking and for growing their food over their entire lifetime.3 To ensure that
no future person receives a greater lifetime dose, we assume that the water
used by the subsistence farmer is extracted from the location of maximum
concentration in ground water.
The subsistence farmer calculation is the most conservative for the
type of people assumed for dose/risk calculations. It is bounding. It is
patterned from the widespread practice, current and historical, of calculating
close and risk to maximally exposed individuals where the exposure habits of
real people cannot be specified or calculated. It is also the most stringent
exposure scenario.
(continued)
environment from a geologic repository tNapier e' al., 1988~.
3 Large uncertainties in the calculation of radionuclide concentrations in the
geosphere mean that calculated doses and risks to the subsistence farmers will also
be extremely uncertain. Consequently, dose/risk estimates will be little affected
whether all or only a "substantial portion" of the subsistence farmer's intake of
water and food is contaminated by the extracted ground water.
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164
YUCCA MOUNTAIN STANDARDS
2. There is international consensus to calculate doses and risks
for subsistence-farmers in determining compliance with a
safety limit for geologic disposal. There is no such consensus
for the probabilistic critical group proposed by the Committee.
There is consiclerable precedence, in the U.S. and abroad, for
basing dose en c! risk predictions on a subsistence farmer, or on a critical
group that includes that subsistence farmer, as defined above.4 Projects for
high level waste clisposal in the UK, Sweden, Finiand,-Canada, and
Switzerland follow similar practices [Barraclough et al., 1992; Charles et
al., 1990; Vieno et al., 1992; Davis et al., 1993~. Switzeriand's geologic
clisposal project defines the critical group as a self-sustaining agricultural
community located in the areats) of the highest potential concentration.
Switzerianc! assumes that no food anti water are obtained from outside
sources tSwitzeriand, 1985, 1994; van Dorp, 19941.
In discussing the choice of critical groups and exposure scenarios
for long-term waste management, UK's National Radiological Protection
Board (NRPB) [Barraclough et al., ~ 992] states:
".... it is appropriate to use hypothetical critical groups. For the
purposes of solid waste disposal assessments, these are assumed
to exist, at any given time in the future, at the place where the
relevant environmental concentrations are highest, and to have
habits such that their exposure is representative of the highest
exposures which might reasonably be expected."
and, for long-term estimates of racliation dose and risk, Barraclough et al.,
state:
" the 'reference community' replaces the critical group, and is
locater} so as to be representative of individuals exposer! to the
greatest risk, at the point of highest relevant environmental
concentrations The reference community should normally
comprise 'typical' subsistence farmers, i.e., perhaps a few families
who produce a range of food to feed themselves."
4 Many ofthese projects adopt the term "maximally exposed individual" instead
of the "subsistence farmer". The dose/risk assumptions are the same.
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APPENDIX E - STA TEMENT OF THOMAS H. PIGFORD 165
Likewise, the U.S. Yucca Mountain project estimates radiation
doses to future indivicluals on the basis of conservative subsistence farmers
whose entire foot} ant! water are contaminated with radionuclides from the
proposed repository [Anclrews et al., 1994; Wilson et al., 19941. The
GENII code [Napier et al., 1988; Leigh, et al., 1993] is used to define the
biosphere scenario and to calculate doses to subsistence farmers.
The U.S. Nuclear Regulatory Commission (USNRC) calculates
radiation closes to future individuals who could be affected by geologic
disposal fMcCartin et al., 1994; Neel, 1995:1 To calculate future human
exposures, USNRC assumes a hypothetical farm family of three persons
who obtain all their drinking water from a contaminateci well. Well water
is used to grow a large portion of the family's vegetables, fruits and grains.
All of the family's beef ant! milk is obtained from farm animals fed on
vegetation irrigated by contaminates! well water Napier, et al., 1988~. The
assumed farm family's well is not restricted to the location of the present
populations. Well depth and withdrawal rate are not constrained by present
practice in the vicinity of Yucca Mountain. These assumptions meet the
criteria for the conservative subsistence farmer described above. They
meet the ICRP criteria for calculating doses for geologic disposal rNeel,
1995].
There are numerous other relevant examples. The U.S. WIPP
project to dispose of transuranic waste in becIdeci salt calculates radiation
doses based on a biosphere scenario that is the equivalent of the
conservative subsistence-farmer approach. They use the GENII code
[Napier, et al., 1988; Leigh, et al., 1993:1 to calculate individual doses once
concentrations in water have been estimated. The estimated doses can be
converter} to risks by using the dose-risk conversion factors. Sangria
National Laboratories recently user} the subsistence-farmer calculation to
evaluate doses and risks from DOE-owned spent fuel emplaces} in a tuff
repository [Rechard, 19953. DOE's Hanford Environmental Dose
Reconstruction Project Harris, 1994ab] allows variants of the subsistence
farmer approach to calculate doses when occupancy factors and locations
of actual exposed people are not sufficiently known. When the location,
occupancy, and foot] source of real people cannot be iclentified, as in
specifying a generically safe level in drinking water or in calculating long
s No one in the present population lives nearer than 20 miles from Yucca
Mountain.
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166
YUCCA MOUNTAIN STANDARDS
term performance of geologic disposal, dose/risk estimates are based on the
more conservative approach involving the hypothetical maximally exposed
individual.
Thus, adopting the subsistence-farmer approach is the consensus
among the several geologic disposal projects in other countries and in the
U.S., including the USNRC plans for calculating individual closes for a
high-level waste repository. It is adopted to calculate closes when actual
location and habits of potentially exposer! people are not known.
On the other hand, the Committee has iclentiiled no reference
wherein the kind of probabilistic exposure analysis of future human
activities, as proposed in Appendix C, has been adoptecl for geologic
clisposal.
The reference population for the Committee's probabilistic
exposure can be chosen arbitrarily.
The Committee's probabilistic exposure calculations are to be
baser] on extrapolation of location ant! habits of an arbitrarily selectee}
reference population. The Committee acknowledges (cf. Appendix C) that
the selection of the reference population for probabilistic analysis would
be arbitrary. The population might be present inhabitants in the vicinity,
inhabitants in some adjacent area, or inhabitants of an entirely different
community6, or inhabitants of a hypothetical future population. It could
evidently be any population of the past, present, or future. The Committee
would! only require sufficient parameters to enable a calculation to be
made.
The Committee illustrates the probabilistic method by adopting an
arbitrary reference population consisting of those people living 20 or more
miles away from Yucca Mountain.7
6 It has been suggested by proponents of the Appendix C approach that the
population of Las Vegas could be a suitable reference population instead of the
population in the region surrounding Yucca Mountain.
7 No people now live nearer than 20 miles from Yucca Mountain because the
nearer land is publicly owned.
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APPENDIXE - STATEMENT OF THOMAS H. PIGFORD 167
The subsistence-farmer calculation of dose and risk fulfills
recommendations of the International Commission on
lladiological Protection (ICRP), the probabilistic critical-
group calculation does not.
The International Commission on Racliological Protection (ICRP)
endorses calculating the average dose to a homogenous~ critical group.
The group shouicl include the person at highest exposure and risk. {CRP's
critical-group concept has been useful in evaluating the safety of operating
facilities, where habits of the present population at risk can be included in
the analysis of closes ant! risks.
However, because the habits and population at risk in the far future
are not known, ICRP recommends (see "Radiation Protection Principles for
the Disposal of Solid Radioactive Waste", ICRP-46 [ICRP, 1985al):
"When an actual group cannot be defineci, a hypothetical group or
representative incliviclual should! be considered! who, clue to
location and time, wouic! receive the greatest dose. The habits and
characteristics of the group should be based upon present
knowledge using cautious. but reasonable. assumptions. For
example, the critical group coup! be the group of people who
might live in an area near a repository and whose water would be
obtained from a nearby groundwater aquifer. Because the actual
doses in the entire population will constitute a distribution for
which the critical group represents the extreme, this procedure is
intended to ensure that no inclivi~iual doses are unacceptably high."
Emphasis added]
ICRP-43 also endorses the single hypothetical individual when dealing
with conditions far in the future:
"In an extreme case it may be convenient to define the critical
group in terms of a single hypothetical indivi(lual, for example
when clearing with conditions well in the future which cannot be
characterized in cietail" fICRP, 1984b]. [Emphasis added.]
ICRP recommends that the group include the most exposed individual and that
there be no more than a tenfold variation in exposure within the critical group.
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168
YUCCA MOUNTAIN STANDERS
On the basis of the above quotes from {CRP, I concur with UK's NRPB ant!
others that the subsistence farmer is the appropriate-single hypothetical
individual to be considered] for close and risk calculations for the distant
future. The cliet and close response of the subsistence farmer are to be
based on present knowledge. as recommended by ICRP. It is cautious and
reasonable that there can exist in the future a farmer whose food intake is
largely that grown in contaminated water. Because the subsistence-farmer
calculation is bounding, it represents the extreme of the actual doses in the
entire population. Protecting the subsistence farmer will ensure that no
individual doses are unacceptably high. [Emphasis shows connection to
ICRP-46 and ICRP-43 recommendations.]
Those wishing to identify a critical group can imagine a group that
would include the subsistence farmer, subject to ICRP's homogeneity
criterion that the dose or risk to individuals within the group should vary
no more than tenfold.9
The full-time subsistence farmer, who receives no food and water
from noncontaminated sources, is obviously the bouncing scenario. We
assign a probability of unity that he can exist. Some part-time farmers will
be included in the data for the Committee's probabilistic analysis, because
they exist now in the Amorgosa Valley. However, because the
Committee's method is expected to synthesize a continuous probabilistic
ciistribution function of occupancy ant! exposure to radiation, the full-time
subsistence farmer will not be found on that distribution. Speculation that
the Committee's probabilistic approach will yield the full-time subsistence
farmer as the individual with maximum exposure is not valid. Methods of
Appendices C and D do not converge.
9 The Committee makes much of the claim that the probabilistic exposure
scenario of Appendix C can predict the dose/risk variation within the calculated
critical group, so that the average dose within the group can be calculated.
However, the ratio of maximum to average dose/risk must lie between one and
ten, if the critical group meets ICRP's homogeneity criterion. An assumed linear
variation results in a ratio of two, as assumed in the subsistence-farmer
approach. I have already noted that the large uncertainties in calculating
geosphere performance, together with the additional uncertainties inherent in the
Committee's proposed probabilistic exposure calculations, do not justify such
attempts to refine the ratio beyond that assumed above. Again, calculated
exposures from the probabilistic scenario are of questionable validity, whereas
the subsistence-farmer results are conservative and bounding.
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APPENDIX E - STATEMENT OF THOMAS H. PIGFORD 169
The probabilistic approach can yield a maximum value of the
close/risk calculated by that method. However, that maximum is not the
maximum to which future people can be exposed. It is not bounding.
Although the probabilistic approach may suffice for those who desire a
self-consistent calculational exercise as a matter of policy, it cannot fulfill
the desired goal that "if the indiviclual at calculated maximum risk is
suitably protected, all other inclividuals will also be protected."
The Committee justifies its probabilistic scenario on {CRP's use of
the words "basecl upon present knowleclge". By attempting to extrapolate
data on the present nearby population to predict probabilities of location,
number, and exposure of future people, the Committee overextends its use
of present knowledge. The Committee's probabilistic approach is neither
"cautious" nor "reasonable". It can lead incorrectly to low values of
calculated doses and risks to a group selected as "the critical group". The
mmittee's probabilistic procedure cannot ensure that no individual closes
^~ ~~ ~ rid ~
are unacceptabiv high. It does not fulfill the recommendations of 1c
quoted above. (see Comments 6 and 71.
According to the Committee, probabilities of habits and behavior
of future humans can be clerived from data on any arbitrarily chosen
reference population, whether past, future, hypothetical, or present. The
Committee adopts the present population only to illustrate the probabilistic
method. However, past, immature, or hypothetical reference populations could
not provide the kind of "present-knowledge" human data that the
Committee claims must be used to satisfy ICRP's recommendation.
Therefore, the Committee's definition of reference population does not
satisfy the Committee's interpretation of {CRP guidance concerning use of
"present knowledge" for establishing a critical group.
The Committee does not claim that its probabilistic exposure
scenario can predict the habits of future generations; it only presents what
is said to be a self-consistent calculation of individual risks based on
assumer} extrapolation from an arbitrary reference population. Even if
correctly formulated, the Committee's probabilistic approach can tell us
nothing about whether a subsistence farmer family can and will exist
during any of the thousands of generations when people can be at
significant risk. Common sense tells us that it is not reasonable to assume
that the probability that a subsistence-farmer will not exist cluring one of
the many thousands of future generations is necessarily low. The
subsistence farmer is the bounding scenario for calculating doses and risks
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170
YUCCA MOUNTAIN STANDARDS
to the types of people who, by policy, are to be protected. Therefore,
protecting a critical group that includes the subsistence farmer Is
necessarily the only cautious and reasonable approach that will fulfill
ICRP's goal of ensuring that no indivi(lual doses are unacceptably high.
Clearly, the Committee's less stringent probabilistic approach cannot
ensure that no individual doses are unacceptabiv high.
The Committee wishes to avoic! calculating dose/risk to a single
individual or to a family of subsistence farmers as adopted by NRPB and
USNRC (see Comment 2~. The Committee does not explain why. As
quoted above, ICRP-46 accepts a "representative indiviclual" for
calculation, and {CAP-43 endorses the single hypothetical individual when
clearing with conditions far in the future:
The Committee's argument against the subsistence farmer appears
in the following statement in Chapter 2 of the Committee's report:
''... we believe that a reasonable en c! practicable objective is to
protect the vast majority of members of the public while also
ensuring that the decision on the acceptability of a repository is not
prejudiced by the risks imposed on a very small number of
individuals with unusual habits or sensitivities. The situation to be
.
avoidecI, therefore, is an extreme case definer! by unreasonable
assumptions regarding the factors affecting dose and risk, while
meeting the objectives of protecting the vast majority of the
public." [From Chapter 2, emphasis added]
The objectives are laudable, but the Committee and others [EPRI, ~994]
infer that it is necessary to calculate doses and risks to groups of future
people rather than to an indiviclual such as a subsistence farmer,
contradicting {CRP tICRP 1984,19853.
The Committee infers, in the above quote, that it is the subsistence
farmer (or maximally exposed individual) who is to be ruled out because
of "unusual habits or sensitivities." The Electric Power Research Institute
(EPRI) reaches a similar conclusion and so states. The Committee and
EPRI have apparently adopted words by UK's NRPB:
"The purpose of the critical group concept ....is to ensure that the
vast majority of members of the public do not receive
unacceptable exposures, whilst at the same time ensuring that
OCR for page 171
APPENDIX E
STATEMENTOF THOMAS H. PIGFO~ 171
decisions as to the acceptability or otherwise of a practice are not
prejudiced by a very small number of individuals with unusual
habits." [Barraclough, et al., ~ 992]
Both the Committee and EPR] have taken the NRPB words out of context
and have misinterpreted NRPB. As is apparent from the full quotes of
NRPB (see Comment 2), the individuals with "unusual habits" whom
NRPB refers to are those with unusual sensitivities to radiation and with
unusual diets.~° It is a mistake to assume that the NRPB statement about
"a very small number of individuals" refers to the subsistence farmer,
because NRPB endorses the use of the subsistence farmer.
Because the Committee's probabilistic approach cannot predict the
actual habits of future people, and because it will predict lower doses ant!
risks than would be calculated for a subsistence farmer, there will be no
way of knowing whether the Committee's objective to protect the vast
majority of members of the public will be fulfilled.
There is consensus that the subsistence-farmer approach is
consistent with the critical-group concept.
The USNRC adopts a critical group that consists of a subsistence-
farmer family of three people [McCartin, et al., 19941. According to Neel
t1995] this is the "reference-man" concept developed by ICRP. Nee} also
states that a similar approach has been taken by a working group within
BlOMOVS, the international Biospheric Model Validation Study, for
making long term assessments of dose. BlOMOVS is a cooperative effort
by selected members of the international nuclear community to cievelop
and test models designed to quantify the transfer and big-accumulation of
raclionuclides in the environment.
A Some precedence for excluding such individuals arises from UK's recent
Sizewell Inquiry, which concerned a proposal to construct a new operating
facility that could affect existing populations. A study of present population
revealed that several individuals subsisted almost entirely on clams obtained in
the vicinity. Because of the unusual diet, UK did not include those individuals
in its analysis of the critical group.
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176
YUCCA MOUNTAIN STANDARDS
Arbitrary assumptions could result in low probabilities of exposure
or to a conclusion that a less stringent calculation of doses and risks is
warranted. For example, one such assumption is that the future population
could be large in number but confined to present population boundaries,
effectively imposing a 20-mile exclusion distance.
Another such
assumption is that, if not confined to present boundaries, future populations
would use wells no deeper than used by the present population 20 or more
miles away, so future people nearer the repository would have to import
foot! and water producer! farther from Yucca Mountain. Such assumptions
would certainly result in low probabilities ant! lower calculated doses and
risks. The assumptions are arbitrary and not clefensible.
One might argue that the benefits of the arid climate and present
low population near Yucca Mountain will be lost if closes ant} risks are
calculates] for individuals exposed to radioactivity extracted from wells.
However, there are advantages and disadvantages. The arid climate ant!
lack of flowing surface water may invite people to use water extracted
from wells. At other sites flowing surface water may dilute the
contaminated ground water before it is user} by humans pNRC, 1983~.
However, at least two projects in other countries are calculating doses/risks
to subsistence farmers who are assumed to use contaminated ground water
clirectly, similar to what would occur at Yucca Mountain. These projects
expect that they can meet performance goals similar to those suggested in
this study.
There is no evidence that would! justify adopting a calculational
method for Yucca Mountain compliance that is less stringent than the
subsistence-farmer method adopted in other countries. The recent
individual dose/risk calculations for the proposed Yucca Mountain
repository are preliminary. They involve many conservative and
unrealistic assumptions about engineering features. The hydrogeological,
environmental, and engineering-clesign features of Yucca Mountain do not
suggest that a less stringent calculational approach is necessary. Indeed,
there are many features that can favor long-term performance.~4
i4 A repository in unsaturated tuff at Yucca Mountain may have much greater
dilution of many radionuclides than repositories in those other countries that
calculate doses from using ground water contaminated by waste buried in
saturated rock. For radionuclides whose release Tom waste solids is limited by
(continued...)
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APPENDIX E - STATEMENTOF THOlL4SH. PIGFORD 177
If a less stringent approach were justified, it would be far better to
adopt a less restrictive value of the close/risk limit than to adopt a
probabilistic exposure calculation that will be so difficult to clefenci. The
probabilistic exposure approach is neither cautious nor reasonable. It
cannot ensure that no future individual will receive an unacceptable dose
or risk.
Calculational techniques described in Appendix C are not
mathematically valid. They can be manipulated to produce
even lower calculated doses/risks.
The Committee proposes to establish full distributions, with
respect to space and time, of numbers of future populations and of their
water ant! food sources in the area surrounding Yucca Mountain. The
surrounding area is to be clivideti into subareas. Each subarea can be
arbitrarily large and can contain as many people as one chooses. Baser} on
the assumed and extrapolated probabilities of location and living habits of
future people, ant] using calculates! concentrations of contaminants in
ground! water, closes anti risks to individuals in each subarea are to be
calculated.~5 The arithmetic average of all individual doses/risks in a
(...continuecl)
solubility, the release rate from the solid waste will be far less for the
unsaturated repository, because of the low infiltration rate of ground water in the
unsaturated zone. Contaminants in this infiltration flow will be highly diluted
when they reach the underlying aquifer. Water flow past waste packages in
saturated rock will be far greater, as will the release rate of such radionuclides
to ground water. It would be premature to conclude that Yucca Mountain would
be at a disadvantage relative to other repositories. There is no basis for
proposing a less-stringent calculation of doses and risks for Yucca Mountain.
s The Committee's probabilistic method will yield calculated individual doses and
risks that will depend on the population density and number of people in a
subarea. The Committee has not explained how the growth in population is to
be predicted; how the probabilistic distributions of number of people with
respect to location and time, together with probabilistic distributions of
parameters of occupancy, food source, etc., can result in a map of potential farm
(continuecI...)
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178
YUCCA MOUNTAIN STANDARDS
subarea is to be calculated. The subarea that is calculated to have the
highest average dose/risk, together with aciditional subareas in which the
average subarea risk is greater than or equal to one tenth of the risk in the
subarea with maximum average risk, is said to clefine a critical subgroup.
The average subgroup risk is said to be calculated as the arithmetic mean
of the average risks of the selected subareas. The process is repeated for
many different samplings of parameters that affect the probabilistic
distributions, to produce new values of the critical-subgroup risks. The
critical-group risk is said to be the arithmetic average of all calculates!
critical-subgroup risks. (see Appendix C)
However, the Committee's interpretation of {CRP would require
calculating closes/risks for individuals over a large area, properly utilizing
the many probability distribution functions of the geosphere and biosphere
to calculate probabilistic distributions and expected values of
consequences, selecting the inclivicluals whose risks are within the top ten
percent, and calculating the average risk of that critical group. This
method is mathematically inconsistent with the Committee's proposed
subarea/subgroup method. It would be fortuitous if the two methods were
to produce the same result. The subarea method! will tend to calculate
lower doses and risks.
The Committee's subarea method will not necessarily yielc} a
critical group that inclucles the inclividual at maximum exposure ant} risk.
That individual may be locater! in a subarea wherein are many indivicluals
at much lower exposure. The subarea size and boundaries are arbitrary.
There could result so low an arithmetic average dose for that entire subarea
that it would not be selected for calculating the critical group. The
(...continued)
density or water use; how many such maps will have to be generated and how
they are to be used in conjunction with the many equivalent maps of sampled
plume concentration; how population changes from the many expected cycles
of climate change are to be calculated; how the expected values of consequence
to individuals at various times and locations are to be obtained without
simultaneously sampling distribution functions of geosphere performance and
biosphere performance; and how the probability distribution functions are to be
generated if any of the other arbitrary reference populations suggested by the
Committee are adopted.
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APPENDAGE - STATEMENT OF THOMAS H. PIGFO~ 179
resulting "critical group" would not meet the ICRP criterion that the
individual of greatest exposure should be included.
Further, to achieve a lower calculated average dose in a subarea,
one would need only to move the outer boundaries of the subarea farther
from Yucca Mountain, to add more people exposed to lower closes.
Appliecl to all subareas, arithmetic average doses would decrease, as would
the average dose for the calculated "critical group." The repository would
appear to be safer! The calculated critical-group doses and risks wouIc! be
much lower than those for a critical group that includes a subsistence
farmer. Or, to lower the calculates! risk, a different reference population
could be selected. The calculates! lower closes ant! risks would be obtained
with an illusion of safety, but with a serious loss of credibility.
X. Calculated uncertainties in terms of confidence levels should
be used to test compliance.
Large uncertainties are inherent in predictions of the transport of
radionuclides to the environment far into the future. Even larger
uncertainties wouIc} be introduced by the probabilistic approach based on
current-population data. The Committee does not discuss how information
on uncertainties is to be conveyer! and used in compliance determinations.
The performance measure of risk recommended by the Committee
is the expected value of the probabilistic distribution of consequences. The
Committee recommends that the expected value be compared directly to
the risk limit to determine compliance. However, uncertainty should be
considered in determining compliance. The expected value (or mean
value) conveys nothing about uncertainty. Basing compliance on the
expected-value comparison is not sufficient.
A technique commonly used to convey uncertainty is to express the
"confidence range" of the result. UK's NRPB illustrates presentation of the
results in terms of the 95 percent confidence level. This states a range of
values of dose or risk, such that 9S percent of the possible values of the
distribution are calculated to fall within that range. NRPB then compares
that range with a close or risk limit [Barraclough et al., 1992] . Effectively,
the upper value of the range becomes the close or risk value for determining
compliance. Methods of calculating confidence levels are well
documentecl.
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180
YUCCA MOUNTAIN STANDARDS
Presenting 90 or 95 percent confidence levels is done extensively
for the geologic disposal projects in Sweden ant! Finland. It is a technique
commonly used in the U.S., particularly when the results are important to
public understanding and acceptance [e.g., Farris et al., 1994abl.
9.
The Yucca Mountain project needs a soundly based standard
for performance assessment and compliance. The U.S.
program needs to share the benefits of an international
approach towards developing standards and technology for
geologic disposal.
A standard and regulatory guidance to ensure public health ant!
safety in the long-term for geologic disposal must include both a regulatory
limit as well as guidance on assumptions of habits of future individuals and
population groups to be adopted in calculating those individual closes ant}
risks. ~ agree with and support the Committee's recommendation that the
measure of performance best suited to assure public health and safety for
the long term is the dose and risk to future individuals. That measure was
adopted by the National Research Council's Waste Isolation Systems
Committee (WISP) [NRC, 1983], after review and analysis of the release
limits then proposer! by EPA, and was subsequently incorporates] in EPA's
standard, 40 CFR 191. The WISP Pane! concluded that individual dose is
a traditional and sound measure in assessing public-health protection. It
was also noted that most, or possibly all, other countries undertaking
geologic disposal use individual dose (or individual risk) as a performance
measure. Adopting the same performance measure as other countries
wouIcI provide a framework for interchanging and sharing information with
other countries on the developing technology for geologic disposal. The
technical approach to design and performance analysis, for the purpose of
ensuring long-term safety, depends greatly on the performance criterion
that is adopted.
~6 I agree that individual risk is better than dose as a measure of performance,
because it allows for possible fixture changes in the dose/risk conversion factor.
As has already been explained in the Panel's report, calculated values of
radiation dose would include probabilistic analysis of uncertainty and
probabilities, if calculable, of being exposed to the radiation.
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APPENDIXE-STATEMENTOFTHOM~4SH. PIGFORD 181
The EPA release-limit standard has now been set aside for Yucca
Mountain after considerable effort has been expended in designing for
compliance with that standard. Adopting a performance measure based on
indiviclual dose and risk is an important step towards cleveloping a standard
that has a clear basis for protection of public health. The international
consensus favoring individual dose/risk is likely to ensure unclerstanding
and support of its adequacy for protecting public health. Both the technical
community and the general public can be reasonably expected to see the
virtues in individual dose/risk as a performance measure.
However, acceptance ofthe use of inclividual dose/risk for ensuring
safety cannot be expected if methods of calculating doses and methods of
assessing compliance are not visibly sound, suitably conservative and
understandable. Selecting an exposure scenario to be used in calculating
long-term doses is a crucial step that can greatly affect the magnitude of
calculated inclividual doses and risks. If calculated risks to the bounding
subsistence farmer are found be within compliance limits, then no future
inclividual doses would be unacceptably high.~7 In contrast, the
probabilistic exposure calculation is too vaguely defined, subject to too
many arbitrary and unconservative policy decisions and subject to too
many questions of valiclity to meet any reasonable test of acceptability,
once the shortcomings of that approach have been sufficiently understood.
Aclopting the probabilistic exposure calculation would again put
the U.S. repository program on a course divergent from that in other
countries. One must expect continued questioning, by the scientific
community, by the public, and by geologic programs in other countries, of
why the U.S. finds it necessary to adopt such a unconservative approach to
regulating geologic clisposal. The U.S. program needs to share the benefits
of an international approach towards developing standards and technology
for geologic disposal, including how to calculate individual doses and risks
for compliance determination.
The U.S. geologic disposal program needs a standard, including
regulatory guidance, that can be clearly implemented and that can be
expected to survive challenges. Serious challenges are likely to arise many
years hence when an application is finally submitted to the regulatory
agency for licensing determination. By that time an enormous investment
of public and electric-utility funds will have been expended in the
i7 See Comment 4.
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YUCCA MOUNTAIN STANDARDS
development of repository technology and in the performance analysis to
assure compliance with the new performance standard. Of the total funds
expended, most will have been to develop technological and geosphere
information, to produce designs of engineering barriers that can assure
safety, to produce calculations of individual risk for determining
compliance, and for administration and services. The cost of constructing
the repository is expected to be small in comparison. Therefore, it is
essential that the new regulatory standard and guidance be on firm ground
so that this enormous effort, measured in money and time, is not wasted.
Adopting an individual dose/risk standard is a step in that direction.
Adopting the probabilistic exposure calculation, however, would leave the
U.S. program vulnerable to future challenge on grounds of reasonable
assurance of safety.
I advocate an approach that ensures that all individuals are suitably
protected, that is based on sound science and logic, and that does not
compromise scientific validity and credibility under the aegis of policy.
Adopting the unconservative probabilistic exposure scenario will
undermine public confidence. The scientific community and the public
will find it difficult to understand why the Committee endorses the
probabilistic exposure scenario that is demonstrably less stringent in
protecting public health than the subsistence-farmer approach, the approach
that has been adopted for geologic disposal projects in other countries and
in the U.S.
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Barraclough, I. M., S. F. Mobbs, and J. R. Cooper, "Radiological
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Radioactive Wastes," Documents of the National Radiation
Protection Board (NRPB), 3, No. 3, 1992.
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APPENDIX E
-
STATEMENTOFTHOMASH. PIGFORD 183
Charles, D., and G. M. Smith, "Project 90 Conversion of Releases From
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Davis, P. A., R. Zach, M. E. Stephens, B. D. Amiro, G. A. Bird, I. A. K.
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184
YUCCA MOUNTAIN STANDARDS
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Falconer, N. J. Grant, I. R. Johnson, J. M. Matuszek, R. R.
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APPENDAGE - STATEMENT OF THO - S H. PIGFO~ 185
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A. Meijer, D. E. Morris, "Total System Performance
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Representative terms from entire chapter:
subsistence farmer
