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EEG Uses Crystal Ball to Assess an
Inhalation Dose from a Radioactive Source
CUSTOMER OF THE MONTH (MAR., 2000)
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Dale Rucker is an environmental engineer for the Environmental
Evaluation Group (EEG), an independent technical oversight group
for the Department of Energy's (DOE) Waste Isolation Pilot Plant
(WIPP) project. The WIPP, a deep geologic repository for defense-generated
transuranic waste, is located in southeastern New Mexico. The EEG
performs independent technical analyses covering various aspects
of the WIPP, including reviewing the safety analysis report. The
safety analysis report is a DOE-required report for nuclear facilities
that documents all aspects of safety related to the handling of
radioactive materials.
As part of its responsibilities, the DOE performs safety analysis
studies to identify the potential for accidents such as an airborne
release of transuranic radioactive particles. This type of accident
would pose a significant inhalation hazard, so the DOE calculates
the maximum radiological dose from an accidentally released airborne
contaminant. The DOE analyses are conducted deterministically, and
the values are chosen based on standard DOE assumptions from various
handbooks and guides. For example, the calculated dose to a receptor
is simply the product of values representing worst case and typical
site conditions. The DOE models do not take into consideration actual
measurements of meteorological conditions or expected radioactivity
in the waste containers.
After the DOE safety studies have been completed, Rucker and the
EEG review the results and assess the potential radiological release
consequences. To ensure that no potential risk factors have been
overlooked, the EEG performs a complementary probabilistic analysis,
a procedure mandated by the International Council of Radiation Protection
(ICRP) (for details, see ICRP-64 - "Protection from Potential Exposure:
A Conceptual Framework." International Commission of Radiation Protection,
1994). Unlike the deterministic approach, a probabilistic assessment
of the inhalation dose calculations can consider a multitude of
independent scenarios. Single-value input parameters are replaced
by probability distributions that represent meaningful variations
in the range of conditions. These parameters are then randomly sampled
to simulate realistic accident scenarios.
For his probabilistic model, Rucker used Crystal Ball and Monte
Carlo simulation to establish the random sampling procedure. Probabilistic
variables (Crystal Ball "assumptions") included site-specific meteorological
conditions and varying quantities of radioactive sources within
the waste containers. He ran a total of 10,000 simulations, and
the forecasted doses were plotted to show 95%, 50%, and 5% dose
likelihood. The 95% dose likelihood showed that the DOE deterministic
doses were generally higher and therefore more conservative than
the probabilistic doses. This result was attributed to the effect
of smearing the high and low end values from the probability distribution
functions. The more conservative DOE assessment showed that the
release consequences may be over-represented by DOE, and that the
high doses attributed to accident scenarios are less likely to occur.
The Crystal Ball analysis also identified a potential weakness
of the deterministic approach for accidents involving a large number
of waste containers. The standard method used to assess the radioactive
source in an accident assumes that one container was loaded to its
administrative limit of 80 Ci, with the remaining containers at
8 CI While some accidents were assessed as having only one container,
with the average source of 80 CI, other accidents involved a larger
number of containers. In these multi-container cases, the deterministic
assessment showed a lower average source from each container than
was reflected in the probabilistic analysis. Based on this finding,
the EEG suggested to the DOE that a reassessment of the source term
values should be conducted in the evaluation of deterministic doses.
To learn more about the WIPP, you can visit http://www.wipp.carlsbad.nm.us/wipp.htm.
Mr. Rucker can be contacted at:
Environmental Evaluation Group
7007 Wyoming Blvd. NE, Suite F-2
Albuquerque, NM 87109
Tel: 505.828.1003
E-mail: druck@eeg.org
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