State Health Agency Employs
Crystal Ball to Protect Public from Radioactive Exposure
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APPLICATION: Pathway analysis for exposures to radioactive
materials
SUMMARY: When the Washington State Department of Health
calculates risk limits from significant exposure sources, they perform
uncertainty analysis to ensure an appropriate amount of conservatism
when predicting the desired endpoint.
RESULTS: The public is appropriately protected from exposure
to radioactive materials without undue burden on industry.
The Washington State Department of Health is a Nuclear Regulatory
Commission (NRC) agreement state. As an agreement state, it serves
as a local regulatory agency for Department of Defense sites, Department
of Energy sites, commercial power plants, low level waste sites,
uranium mines and mills, medical applications, and numerous other
users of radioactive materials throughout the state. Andrew H. Thatcher
is a Certified Health Physicist with the Division of Radiation Protection
at the Washington Department of Health. Thatcher uses Crystal Ball
to perform environmental pathway analysis for exposures to radioactive
materials.
While the dose or risk limits on radioactivity exposures to the
public from commercial sites will vary depending on the source of
exposure, these limits are generally single-point, or deterministic,
estimates applied to protect the maximally exposed individual (MEI)
or the average member of a critical group. For example, in the case
of a low-level radioactive waste facility, the exposure limit is
25 mrem/y. Deterministic estimates of the exposure to the MEI or
critical group rely heavily on judgments about the appropriate values
to use for input parameters into dose (or risk) calculations. One
of the dangers of using these deterministic estimates is that compounding
conservatism can result in dose estimates that greatly exceed the
intended endpoint.
In an effort to ensure an appropriate amount of conservatism when
predicting the desired endpoint, the state now performs uncertainty
analysis for significant exposure sources. Andrew Thatcher uses
Excel to model his pathway analysis because every dose calculation,
regardless of complexity, can be simplified into a series of input
parameters and formulas. With Crystal Ball, he can then enhance
his spreadsheets by applying range and distribution information
for the parameters of interest.
The pathway models Thatcher uses vary from a single pathway, such
as radon emanation into a home, to an entire pathway analysis for
all exposure routes to include inhalation, ingestion, and external
exposure from radioactive sources from all environmental media (soil,
water, air, food products). To develop his probabilistic variables,
he applies Crystal Ball's distribution fitting function, which can
analyze a data set and determine not only the best fit but also
the quality of the fit. After an initial simulation, he uses Crystal
Ball's sensitivity analysis to determine which input parameters
drive the predicted dose or risk. He can then direct his efforts
to refine ranges and distributions to only those parameters that
have the greatest impact on the pathway.
With the Monte Carlo or Latin Hypercube functions and the speed
of new computers, Thatcher can perform these detailed analyses in
relatively short order. The result is an output that provides a
subjective confidence bound on a desired value such as the 95% (the
MEI) and gives the public a much greater assurance that they are
being adequately protected from commercial sources of exposure.
"The benefit of Crystal Ball in a regulatory application is that
one can ensure that the public is appropriately protected without
being cumbersome to industry," Thatcher noted.
Thatcher's more recent applications of Crystal Ball have included
the use of the Two-dimensional Simulation tool, which separates
sources of uncertainty and variability in an analysis. The Two-dimensional
Simulation tool capability gives him the ability to easily follow
the guidance set forth in NCRP Commentary #14 (A Guide for Uncertainty
Analysis in Dose and Risk Assessments related to Environmental Contamination)
for developing subjective confidence intervals for a given dose
or risk estimate.
As for the future, Thatcher says, he plans to apply the software
to any locations where the current deterministic results indicate
that the exposure potential is near the allowable exposure limit
for the public or where the desire is to determine the most likely
dose or concentration. An example would be estimating the modal
radon emanation rate from uranium tailings piles or buried waste.
"One great feature of Crystal Ball is that the model developed with
it can be as complex or as simple as one desires, with value added
in either model, " he said.
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