The Disposition Dilemma Controlling the Release of Solid Materials from Nuclear Regulatory Commission-Licensed Facilities (2002) / Chapter Skim
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5 Review of Methodology for Dose Analysis
5 Review of Methodology for Dose Analysis
Pages 80-114
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From page 80... ...
In addition, the committee met with knowledgeable experts involved in preparing the reports, to clarify specific issues, particularly the reasons why dose factors differ between reports. Because one of the reports, the draft report NUREG-1640, Radiological Assessments for Clearance of Equipment and Materials from Nuclear Facilities (USNRC, 1998b)
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From page 81... ...
convert a primary dose standard into secondary activity standards that can be used by licensees to ensure compliance with the primary standard (see Box 5-1~. The major compilations of these dose factors are listed in Table 5-1, along with the scientific bodies that have reviewed the underlying technical analyses.
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From page 82... ...
on on ~ F _ .____ FICURE 5-1 Pouts ~ ~bicb ~cbn~1 in~rm~on Ed judgment can idiom ~lem~ing decisions related to cle~=ce of sligbOy radioactive solid mogul NO~: Circles indict policy decisions. Rectangles indict ~cbuic~1 con~ibuUons.
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From page 83... ...
. In some cases, effective dose equivalent is used, which accounts for both the relative biological effectiveness of different types of radioactivity and the differing sensitivity of organs to cancer mortality.
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From page 84... ...
Specifically, the semiquantitative uncertainty analysis described in Chapter 10 showed that, depending on choice of input parameters, normalized doses could be higher by a factor of 5-50 or lower by a factor of 100-500, i.e., they favored more protective levels. The NUREG1640 draft shows a distribution of dose factors based on Monte Carlo simulations of the aggregate uncertainty resulting from uncertainties in the component estimates.
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From page 85... ...
REVIEW OF METHODOLOGY FOR DOSE ANALYSIS Population Group I and / Critical Group (drivers) Population groups 85 \ ~opulatiop/G roup I I $\~:X / (workers)
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From page 86... ...
The risk assessment methodology is largely state of the art. Critical groups are chosen by assuming a policy of clearance, although information in the appendixes may be sufficient to allow choices of other critical groups to support derivation of dose factors for possible conditional clearance policies.
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From page 87... ...
If one takes the median of the distribution, then 50 percent of the dose factors are below and 50 percent above. Choosing the median as the de minims value for use in clearance or conditional clearance standards, however, would leave the decision maker without the higher degree of assurance that the dose to the critical group is below the ordinary dose standard as when higher percentile-valued dose factors are chosen (e.g., 90th percentile)
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From page 88... ...
Of the studies listed in Table 5-1, only draft NUREG-1640 includes a formal uncertainty analysis that reduces the amount of professional judgment required in assigning uncertainty bands to dose factors. Excellent discussions of formal uncertainty analysis can be found in other USNRC documents (e.g., USNRC, 1995)
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From page 89... ...
Therefore, from a scientific perspective, the committee does not believe it is cost-effective to repeat the work done in draft NUREG-1640. The committee believes that once the remaining questions about and limitations in draft NUREG-1640 are addressed, either in the final version of the report or in follow-up reports, the USNRC and interested parties will have a sound technical basis for evaluating the health impacts, measurement issues, and implementability of various primary dose standards and the unavoidable uncertainties involved in risk estimates.
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From page 90... ...
Using several important assumptions, the study estimated both doses to an individual member and collective doses to the entire group for several critical groups. The most important assumption was that the proposed exemption from regulatory control would apply only to scrap metal ingots coming out of a licensed smelter, thus ensuring a radionuclide content in the scrap of no more than 17.5 ppm uranium and 5 ppm technetium.
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From page 91... ...
ENVIRONMENTAL PROTECTION AGENCY DOCUMENTS ON DOSE FACTORS The Environmental Protection Agency (EPA) Technical Support Document (TSD)
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From page 92... ...
(That is, in 90 percent of cases, use of the calculated dose factor will result in a dose to a member of the critical group that is at or below the primary dose standard.6) The National Council on Radiation Protection and Measurements (NCRP, 1998)
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From page 93... ...
The document tabulates derived screening levels, above background, for the clearance of SRSM or items containing surface or volume activity concentrations of radioactive materials. These screening levels are derived by applying dose factors to the primary dose standard.
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From page 94... ...
The IAEA dose factors are contained in Safety Practice No. 111-P-l.l, Application of Exemption Principles to the Recycle and Reuse of Materials from Nuclear Facilities (IAEA, 1992~.
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From page 95... ...
In deriving nuclide specific clearance levels, the EC reports use a collective dose standard of 1 person-sievert (person-Sv) per year and a derived dose stan{OThe authors did consider what they called "pessimistic" assessments in developing dose factors and clearance values.
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From page 96... ...
With respect to the dose factors selected in the IAEA and EC reports however, the NUREG-1640 values are on average about 5 to 14 times higher and hence would allow less activity to be released on average given the same primary dose standard. For particular radionuclides and particular critical groups, the disagreement between the U.S.
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From page 97... ...
for the same primary dose standard than will the dose factors from the IAEA and EC studies. In other 1lFor instance, the committee looked at the GSD of the ratio of NUREG mean dose factors to those computed by the EPA and the EC (volume-contaminated metals)
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From page 98... ...
Considered in part Not considered To determine which parameters contribute most to uncertainty None NOTE: NA = not applicable; NARM = naturally occurring and accelerator-produced radioactive material; NORM = naturally occurring radioactive material.
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From page 99... ...
a Member of critical group Traceable for volumecontamination factors Yesb In part None Implicit Not considered Not considered None None 52 NA Most conservative of dose factors from range of studies considered reasonable All solids 10 ,uSv/yr; higher on a caseby-case basis. Unspecified Not traceable Qualitative discussion No Assessed on an overall basis, not nuclide by nuclide Implicit Not considered Not considered None None dDose calculations that result in higher percentile-valued dose factors are more conservative.
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From page 100... ...
This allows the decision maker to compensate for the lack of complete knowledge in the dose analysis and thus increase confidence that the dose to the critical group will be below the primary dose standard. For example, the value for the dose factor can be set to the 95th percentile in the distribution of values for that dose factor rather than the median.
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From page 101... ...
, like the choice of a primary dose standard, is a matter of policy that cannot be decided by scientists through analysis or facts alone. For instance, policy makers could decide to choose dose factors closer to the median of the distribution of dose factors forgoing the additional margin of protection afforded when a higher percentile-valued dose factor is selected because they consider a 1 mrem/yr dose to be too far below background to be of concern.
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From page 102... ...
Despite the use of bounding assumptions, considerable uncertainty remains in some scenarios, particularly when it comes to predicting the behavior of radioactive materials leaching from landfills. Analysts often add margins of protection to components of a dose factor calculation because information about a parameter is lacking or because the analyst is seeking greater generality for the analysis.
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From page 103... ...
For example, leachate can be sampled from representative landfills, or concentrations of radioactivity in sample pieces of recycled steel can be checked, to ensure that the model assumed in calculating dose factors reasonably represents reality, with an adequate margin of protection. The model, and the dose factors calculated from it, should be updated if the primary dose standard is being exceeded or even if key assumptions in the model are clearly inadequate.l4 The IAEA encourages this type of retrospective review, including the "testing of radioactive consumer products on the market" (IAEA, 1988, p.
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From page 104... ...
General Limitations of the Reviewed Studies Failure to Consider Uncertainties Associated with Implementation of a Primary Dose Standard Dose factors as estimated to date are useful theoretical tools. However, they have practical value only within a specific implementation protocol, where such a protocol can introduce uncertainties into dose estimates tied to primary dose standards.
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From page 105... ...
For example, if the policy choice for selecting dose factors is to maintain a 95 percentile level, then the implicit percentile level of a rounded activity standard should be at least 95 percent. Alternatively, information such as the implied confidence level after rounding should be presented with the proposed activity standards so that policy makers understand the implications of adopting a policy of rounding the activity standards.
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From page 106... ...
, which suggests that this type of human error is not likely to have a significant impact on dose factor estimates. USNRC staff were not able to provide the study committee with the frequency at which exit monitors at licensed facilities were triggered by shipments on their way to final disposition, following clearance based on Regulatory Guide 1.86, a license provision, or approved by case-by-case review.
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From page 107... ...
These differences can lead to inconsistencies between clearance policies adopted in different countries. However, in discussing transnational consistency of dose factors and derived secondary clearance standards, two types of consistency must be distinguished.
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From page 108... ...
Issue 3: Sensitivity Analysis The uncertainty analysis was reasonable, but since the study uses a Monte Carlo analysis, the committee wondered why a set of sensitivity analyses was not carried out. Sensitivity analyses can be misconstrued as uncertainty ranges, but the committee believes that they can be constructive.
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From page 109... ...
several waste disposal sites in the same region) ." Issue 7: Resuspension of Contamination The draft document has only limited consideration of resuspension of surface contamination into the air.
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From page 110... ...
Issue X: Collective Dose Draft NUREG-1640 has no consideration of collective dose. The EC and the IAEA have a two-part primary dose standard, 10 ,uSv/yr for an individual and 1 person-Sv/yr for the collective dose to the population.
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From page 111... ...
The authors consider (justifiably) that the total amount of curies released and stored affects the estimation of cumulative doses more than the estimates of critical doses (i.e., the individual doses on which dose factor selection is based)
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From page 112... ...
. With such a dose factor in hand, a primary dose standard can be converted to obtain a secondary clearance standard in terms of radionuclide activity, which 19''Based on assumed good health physics practices at NRC licensed facilities, removable surface contamination has been removed during decontamination procedures prior to final survey and clearance" (USNRC, 1998b, p.
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From page 113... ...
The uncertainty in dose factor estimates is a key technical issue. When an uncertainty has been estimated, a quantitative determination can be made of the likelihood that the dose to an individual in the critical group will be below the primary dose standard.
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From page 114... ...
The dose factors developed in draft NUREG-1640 should not be used to derive clearance standards for categories of SRSM other than those considered in the draft NUREG-1640, without first assessing the appropriateness of the underlying scenarios. Some of the dose factors developed in draft NUREG1640 are likely to require modification when applied to other mixtures of radionuclides (e.g., mixtures in which transuranics dominate)
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