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Thank you for this information
and insight. I am sure that you did not set the policies/procedures described,
so please do not take my comments as personal criticism.
I am very dismayed by the
thought a lot of our tax money being squandered in following assessments
that are largely contrived, arbitrary, subjective, bureaucratic nonsense
intended to give the impression that science is being done. I somehow can't help
but believe that such operations start out with what is considered to
be a politically acceptable conclusion and then develop assumptions and
calculations to obtain that conclusion. It may look good, but it isn't
science!
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Sent: Saturday, April 06, 2002 4:36
PM
Subject: Re: MRFA
This
is for your information -- please do not take it as an argument for or against
anything. It is to inform RADSAFERs. briefly, how RAM transportation
accident risks are analyzed at present, and what the reasoning is behind
this analysis. Clearly, since there have been no accidents with Type B
casks that involve releases, we do not have direct evidence of what would
happen in an accident like that. However, as in any risky endeavor
(which any transportation is) we are frequently required
to make some projections or estimates of risk. The method, which
I outline below, was pioneered in NUREG-0170 (1977)and is, to my mind, a clever way to obviate arguments about specific accident
scenarios. Please excuse the length of this post, but it may answer
some questions.
Accident releases, and consequent radiation dose
risks, for accidents whose severity exceeds that of 10 CFR Part 71 Subpart E
are estimated as follows:
1. Consider the entire universe of
possible accident scenarios. From general truck and rail accident data,
better than 99% of the possible accidents have no impact on the cargo at
all, and would have no release or loss of lead shielding. Current data
and studies indicate that at least 99.99% of possible accidents fall into this
category.
2. The remaining 0.001% are classified into classes
defined by two parameters: mechanical force exerted during the accident and
temperature if there is a fire. When accidents are defined by a scheme
like this, any scenario can be represented by a combination of mechanical
force and temperature, and one can dispense with conjuring up scenarios.
3. Each mechanical force/thermal stress class has an associated
conditional probability.
4. NUREG-0170 used engineering judgment to arrive at a matrix of
accidents. The 1987 Modal Study (Fischer et al,
NUREG/CR-4829) analyzed a large number of heavy truck and rail accidents
and developed an event tree and a two-dimensional matrix, with dimensions of
strain and temperature, and estimated releases under these conditions, using
cask designs and cask tests as a basis for the estimates.
NUREG/CR-6672 (Sprung, et al, 2000) re-examined the Modal Study,
and using more recently developed accident and cask test data as well as
finite element analysis, refined the Modal Study matrix, used impact speed and
temperature as the two parameters, and developed a matrix of 19 cells for
truck and 21 cells for rail, for four "generic" Type B casks. Each cell
has an associated conditional probability of occurrence and release fractions
for volatiles, Ru, CRUD, particulate matter, and inert gas. (I can send anyone
interested a copy of thes! e matrices electronically). The conditional
probabilities and release fractions may be found on pp. 7-73 to 7-76 of
NUREG/CR-6672. Slightly more than 99.99% of the accidents defined in this way
have neither release nor loss of lead shielding.
4. The accident
analysis then calculates the risk associated with each cell by
multiplying each release by the conditional probability of the
combination of impact speed and temperature that could have produced that
release. These conditional risks are then summed, multiplied by accident
frequency (e.g., from Saricks and Tompkins, 1999) and a "dose risk" calculated
from the resulting "release risk" by applying a dispersion and deposition
model, dose conversion factors, etc. The details of this calculation may
be found in the RADTRAN 5 Technical Manual (Neuhauser, Kanipe and Weiner,
2000) and in the Transportation Health and Safety Calculation Package of the
Yucca Mountain EIS.
5. All of the conditional probabilities of
accidents that would include a release are estimated at less than 1E-4; the
largest is the fire-only scenario, which has an estimated conditional
probability of about 6E-5. However, the matrix of accidents does
include at least one accident of conditional probability about 1E-7, and the
associated combination of impact speed and temperature may be considered
the "maximum reasonably foreseeable accident." Another way of
arriving at this accident is to make a plot of accident conditional
probabilities and see where the 1E-7 cutoff point is. At any rate, the
potential release and consequent dose can then be calculated
OK it's
not a perfect method, but in the absence of actual accident release
data, I think it's pretty good. And (please take note, Sandy!) we cannot
say that such accidents will never happen. There are combinations of
impact and temperature that can be estimated to be so unlikely that their
conditional probability is zero, but the method also considers conditional
probabilities of the order of 1E-14, 1E-10, etc.
Before anyone
launches into a broad-scale critique of this method, I suggest you familiarize
yourselves with the details, using the cited references.
What I have given is a necessarily brief overview.
However, I would certianly welcome suggestions for improving this
method, making it more realistic, and so on.
Finally, the limit of
"reasonably foreseeable" as 1E-7 is admittedly arbitrary, but is related to
the EPA credibility limit and is parropriat4e, considering the latter.
Ruth Weiner, Ph. D.
ruthweiner@aol.com
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