A Review of the Dose Reconstruction Program of the Defense Threat Reduction Agency (2003) / Chapter Skim
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IV. Process of Dose Reconstruction in NTPR Program
IV. Process of Dose Reconstruction in NTPR Program
Pages 65-123
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From page 65... ...
and in 32 CFR Part 218, the goal of the NTPR program is to obtain upper-bound estimates of dose to atomic veterans, consistent with the policy of giving the veterans the benefit of the doubt in estimating their doses (see Section I.C.3.2~. More specifically, the goal is to obtain at least a 95th percentile upper bound of possible doses, taking into account uncertainties in estimating dose.
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From page 66... ...
However, only a single estimate of dose is reported when a beta dose to the skin or lens of the eye or an internal dose from intakes of radionuclides is calculated, and this estimate is intended to be at least a 95th percentile upper bound.1 IV.A EXPOSURE SCENARIOS Development of exposure scenarios for participants in the nuclear-weapons testing program involves consideration of assumptions about the locations of the participants of concern, their activities at those locations, and the time spent at each location and assumptions about the radiation environment at the assumed locations of the participants during the time spent at those locations (see Section I.C.2.1~. Approaches to development of exposure scenarios used in the NTPR program are described in the standard operating procedures (DTRA, 1997)
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From page 67... ...
An example is participants in units who observed detonations at the NTS from trenches close to ground zero (see, for example, Figure IV.A.1~. This one-size-fits-all strategy assigns the same dose to everyone in a given unit, with an upper bound assigned to allow for uncertainty in the estimated dose.
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From page 68... ...
IV.A.2 Individualized Dose Reconstructions If a participant was involved in unusual activities, an individualized dose reconstruction is required. In such instances, there may have been complete or nearly complete badging during the entire time of participation.2 If so, and if the issue and turn-in dates for the badges of record are complete and cover the veteran' s entire time at the site, the badge readings are simply summed, and their variances are combined with a method called quadrature, in which the variance (error)
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From page 69... ...
For most participants, the reconstructed gamma and neutron dose from external exposure is based on a generic dose reconstruction performed for their particular units' activities during a given test series, modified as appropriate to conform to a participant's duties and period of exposure. It is important to note that the method used in the NTPR program to estimate external doses changed over time as shown below (Schaeffer, 2001a)
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From page 70... ...
The method currently used in the NTPR program to estimate the most likely external gamma dose based on film-badge dosimetry, most likely gamma dose based on a scientific dose reconstruction, most likely neutron dose, and upper-bound beta dose to skin or lens of the eye are discussed in the following paragraphs. The method used to estimate upper bounds of gamma and neutron doses is discussed in Section IV.E.2.
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From page 71... ...
, suspect film-badge data are discarded in favor of a scientific dose reconstruction. In a 1989 report, a committee of the National Research Council reviewed the method used in the NTPR program to analyze film-badge data (NRC, 1989~.
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From page 72... ...
This procedure is used because that dose presumably would have been included in the mission-badge reading. IV.B.2 External Gamma-Dose Estimation Based on Dose Reconstruction Because only a fraction of participants were issued film badges during the earlier test series and the time when badges were worn often covered only a portion of the time of potential exposure, methods have been developed to recon3The bias correction from exposure in air in roentgens to a whole-body equivalent dose in rem, which is approximately a factor of 0.7, was applied to film-badge data in 1990 as recommended by the 1989 National Research Council report, but was rescinded by the NTPR program in 1992.
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From page 73... ...
On the basis of those unit dose reconstructions, the NTPR program assigned a generic dose to all participants in the units. Unless a formal dose reconstruction is requested as a consequence of a VA claim or a specific participant request to the Defense Threat Reduction Agency, the participant' s dose of record is generally based on either a film-badge measurement or the assigned average dose for the participant's unit.
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From page 74... ...
TABLE IV.B. 1 Summary of unit dose reconstruction for maneuver troops at Operation UPSHOT-KNOTHOLE (Edwards et al., 1985~; BCT = Battalion Combat Team.
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IV PROCESS OF DOSE RECONSTRUCTION IN NTPR PROGRAM 75 FIGURE IV.B.2 Troops leaving a trench shortly after a detonation. FIGURE IV.B.3 Army personnel examining equipment damaged during a nuclear detonation.
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From page 76... ...
The total time-integrated dose is estimated from the initial exposure rates and estimated or measured decay rates. Tables of daily doses received by sailors were constructed for each ship participating in tests and for participants on various islands.
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From page 77... ...
The 1946 CROSSROADS test series was one of the largest with respect to the number of participants and potential exposures. More than 36,000 persons on
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The NTPR program has constructed curves of daily doses for each support and target ship (Weitz et al., 1982, Volumes 2 and 3) on the basis of monitoring data and decay extrapolations.
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From page 79... ...
IV.B.3 Estimation of Neutron Doses Estimates of dose due to external exposure to neutrons produced in a detonation (mainly for some close-in observers at NTS shots and crews of cloud-sampling aircraft) are based on calculations with radiation-transport models.
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From page 80... ...
The dose to any organ or tissue from external exposure to neutrons is assumed to be the same as the equivalent dose to the whole body. IV.B.4 Estimation of Beta Dose to Skin and Lens of the Eye Energetic electrons are emitted by most fission and activation products and were an intrinsic component of the radiation to which atomic veterans were exposed.
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From page 81... ...
The fundamentals of the method for estimating external dose to the skin and eye from standing on contaminated ground are summarized as follows: · Beta dose to the skin or lens of the eye from external sources is accrued with the gamma dose from radioactive fallout, contamination, or neutron-induced radionuclides. As a result, the beta dose is proportional to the gamma dose, and its relative magnitude can be expressed by a beta-to-gamma dose ratio.
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From page 82... ...
Thus, beta-to-gamma dose ratios of activation products are small because most of the activation products are deeper in the soil than the range of the emitted beta particles. Dose to skin is reduced by clothing, and Barss (2000)
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From page 83... ...
In an effort to "high-side" the total skin or eye dose, the beta-to-gamma dose ratio is applied to the upper-bound gamma dose from exposure to fallout or other beta-radiation field to calculate beta dose, and the upper-bound gamma dose from all sources is then added to the beta dose. IV.B.4.2 External Beta Dose from Immersion in Contaminated Air or Water Immersion in a descending fallout-debris cloud was a less frequent circumstance than exposure to fallout after deposition on the ground or other surface.
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From page 84... ...
, the film-badge gamma dose is a 4In the NTPR program, external dose rates per unit concentration of radionuclides in a source region are referred to as dose conversion factors. In this report, however, they are referred to as dose coefficients to conform to the terminology currently used in internal dosimetry by the International Commission on Radiological Protection (ICRP, 1991a)
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From page 85... ...
highly inaccurate indicator of skin dose, so beta-to-gamma ratios are not appropriate for such applications. As in the method used for surface deposition and exposure in air, the beta energy spectrum due to radioactive material on the surface of the skin can be determined as a function of time after detonation, and this allows beta doses to be directly calculated by using dose coefficients from Kocher and Eckerman (1987~.
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From page 86... ...
Estimation of internal doses to atomic veterans is inherently more difficult than estimation of external doses. External doses usually can be estimated directly on the basis of measurements of external exposure with film badges worn by participants or measurements of external exposure rates at various locations and times in a participant's exposure environment with field instruments (see Sections IV.B.1 and IV.B.2~.
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From page 87... ...
Section IV.C.3 discusses methods used to estimate ingestion doses to atomic veterans in rare cases in which ingestion is considered, and Section IV.C.4 discusses the possibility of internal doses due to absorption through the skin or open wounds. The committee's evaluation of methods used in the NTPR program to estimate internal doses to atomic veterans and to account for uncertainty in the estimates is presented in Section V.C.
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From page 88... ...
Therefore, indirect methods of estimating them must be used in dose reconstructions. Methods and data used in the NTPR program to estimate airborne concentrations of radionuclides in each 6In the NTPR program, the quantity DF is referred to as a "dose conversion factor." In the present report, however, this quantity is referred to as a "dose coefficient" to conform to the terminology currently used by the International Commission on Radiological Protection (ICRP, 1991a)
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From page 89... ...
Renormalization is based either on measurements of total photon exposure in air above the surface in roentgens (R) with film badges worn by participants who were present in the fallout field at known times and locations or on measurements of photon exposure rates (R hat)
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From page 90... ...
Mathematically, the activity concentration on the surface is (SAIL x I, where I is the exposure rate and SAII is the reciprocal of the calculated exposure rate per unit concentration on the surface. Exposure rates per unit concentration of radionuclides on the surface are calculated on the basis of known energies and intensities of photons emitted by the radionuclides assumed to be present in fallout, an assumption that radionuclides are distributed uniformly on a surface of infinite extent, and theoretical considerations of photon transport from the source region on the surface to the assumed height of a film badge or field instrument above the surface and the resulting exposure in air, taking into account the shielding effect of ground roughness.9 An important condition in applying this method is that exposures measured with film badges or field instruments must have been due primarily to deposited fallout.
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From page 91... ...
In some dose reconstructions for participants at a particular shot, estimates of inhalation dose due to resuspension of deposited fallout also take into account the presence of fallout that was deposited in the same area after previous shots. Inhalation doses due to resuspension of previously deposited fallout are estimated with the same methods described above, with radioactive decay since the times of the previous shots taken into account.
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From page 92... ...
Further discussion that focuses on the intention of the NTPR program that the method should result in overestimates of airborne concentrations of radionuclides in resuspended fallout is presented in Section IV.C.2.1.7. IV.C.2.1.2 Fractionation of radionuclides The term fractionation refers to the chemical and physical separation of radionuclides produced in a detonation.
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From page 93... ...
Separation of noble gases from other materials begins as soon as liquid droplets form and continues as the droplets solidify and begin to fall to Earth. As noted previously, this type of fractionation is taken into account by the NTPR program in estimating concentrations of radionuclides in fallout deposited on the ground or other surfaces at the NTS and in the Pacific.
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From page 94... ...
On the basis of a review of available data and the use of subjective judgment, different resuspension factors are used in the NTPR program to estimate airborne concentrations of radionuclides in various exposure scenarios. Resuspension factors commonly used in dose reconstructions at the NTS (Barrett et al., 1986)
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From page 95... ...
But the resuspension factor often is reduced by a factor of 10, compared with the standard assumption of 10-5 m-i used in many dose reconstructions, or to zero in cases of exposure to aged fallout that was deposited well before the time of exposure; this is especially the case on residence islands in the Pacific. TABLE IV.C.2 Resuspension Factors Normally Assumed for Various Activities of Atomic Veterans in Contaminated Areas at NTSa Resuspension Category Activity factor (my)
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From page 96... ...
. Thus, in general, judgment has been applied by the NTPR program in estimating resuspension factors used in dose reconstructions for particular exposure scenarios that involve previously deposited fallout.
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From page 97... ...
Again, the intent of the NTPR program has been to select resuspension factors that result in overestimates of airborne concentrations of activation products in soil to which participants at the NTS were exposed. IV.C.2.1.5 Scenario involving inhalation of descending fallout Airborne concentrations of radionuclides in descending fallout are estimated on the basis of estimates of concentrations on the ground or other surfaces that resulted from deposition of the fallout.
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From page 98... ...
This roundabout method is necessitated by the lack of data on concentrations of radionuclides in descending fallout. In this scenario, concentrations of radionuclides deposited on the ground generally are based on measurements of external photon exposure rates at particular locations and times with field instruments rather than measurements of total external exposure with film badges worn by participants who were exposed to descending fallout (Barrett et al., 1986~.
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From page 99... ...
Values of AAII generally are different from calculated values of SAII used to estimate inhalation doses from resuspended or descending fallout, and they are calculated based on an assumption that the atmospheric cloud is uniformly contaminated and infinite in extent. An effective resuspension factor is not used in this scenario, because the quantity I is a measured photon exposure rate due to airborne radionuclides rather than radionuclides deposited on a surface.
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From page 100... ...
Especially in scenarios involving inhalation of resuspended or descending fallout, a key assumption in the methods used to calculate inhalation dose is that there is no fractionation of radionuclides other than removal of noble gases; these two scenarios are the most important in inhalation exposures of many atomic veterans. As emphasized at the beginning of Chapter IV, the goal of the NTPR program is to obtain estimates of dose to atomic veterans that are upper bounds (at least 95% confidence limits)
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From page 101... ...
. All inhalation dose coefficients used in dose reconstructions are calculated with two kinds of models: · Biokinetic models that describe the deposition, retention, translocation, and absorption of inhaled radionuclides in the respiratory tract or ingested radionuclides in the gastrointestinal (GI)
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From page 102... ...
The committee is not aware of any written policy in the NTPR program to assist an analyst in choosing between ICRP and ORNL dose coefficients for use in dose reconstructions for atomic veterans; both sets of data are included in the FIIDOS computer code (Egbert et al., 1985) and are readily available.
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From page 103... ...
. In some dose reconstructions, however, inhalation dose coefficients for large, essentially nonrespirable particles based on an assumed AMAD of 20 ,um are used, especially when this choice results in higher doses to the organ or tissue in which a cancer in an atomic veteran occurred.
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From page 104... ...
The lower particle size is sometimes used in cases of resuspension of deposited fallout, especially when fallout was deposited well before the time of exposure, on the basis of an assumption that large particles in fallout were reduced in size by weathering. Thus, with the exceptions noted above, the standard practice in the NTPR program is to choose the particle size that gives the higher estimate of inhalation dose to the organ or tissue of concern (Schaeffer, 2002b)
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From page 105... ...
Inhalation dose coefficients that apply to radionuclides in oxide form are used in all dose reconstructions (Egbert et al., 1985) , on the basis of the expected insolubility of radioactive materials in fallout and the insolubility of oxide forms of many radionuclides compared with other chemical forms.
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From page 106... ...
In using these dose coefficients, only the assumed particle size could result in substantial overestimates of inhalation dose to particular organs or tissues. Thus, as emphasized in Section IV.C.2.1.7, the NTPR program, in its effort to obtain "high-sided" estimates of inhalation dose, relies primarily on assumptions that are intended to result in overestimates of airborne concentrations of radionuclides to which participants were exposed, especially assumptions about resuspension factors that are applied to deposited fallout or neutron-activation products in surface soil.
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From page 107... ...
does not include consideration of intakes of radionuclides by absorption through the skin or an open wound. Therefore, the NTPR program presumably has not considered those intake pathways in dose reconstructions for atomic veterans.
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From page 108... ...
To estimate inhalation doses, the surface activity was estimated from the measured photon exposure above ground (see Section IV.C.2.1~. A "high-sided" resuspension factor of 1 o-4 m-i was used in the vicinity of ground zero to account for the relatively high resuspension caused by mechanical activities, and a resuspension factor of 10-5 m-i was used at locations in fallout fields away from ground zero.
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From page 110... ...
Doses assigned in individual dose reconstructions often are substantially lower than upper bounds obtained in the generic assessment. IV.E METHODS OF ESTIMATING OR ACCOUNTING FOR UNCERTAINTY The following sections consider the methods used in the NTPR program to estimate or otherwise take into account uncertainty in exposure scenarios and in estimates of external and internal dose.
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From page 111... ...
The upper bound is intended to be a 95th percentile, meaning that if one calculates the distribution of possible doses for the participant, the true dose is expected to be lower than the assigned upper bound in 95% of cases with an identical exposure scenario. A1though some uncertainties were estimated in early unit reports, particularly if the dose was based primarily on a unit dose reconstruction, the NTPR program did not generally report upper bounds of total external dose to the veteran or the VA before 1992.
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From page 112... ...
IV.E.2.2 Upper-Bound Estimates in Unit Dose Reconstructions Unit dose reconstructions usually estimate the uncertainty in the average or "best-estimate" dose to a unit or subunit on the basis of estimated uncertainties in measured exposure rates, estimated decay rates, estimated positions of troops versus time, shielding, the fraction of the day spent indoors versus outdoors, and so on. The estimated 95th percentile upper bound is assumed to appropriately reflect the upper-bound dose to each individual in the unit (Goetz et al., 1981~.
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From page 113... ...
IV.E.2.2.1 Upper bounds in unit dose reconstructions at the NTS The generic unit dose reconstructions discussed in Section IV.B.2 generally also included estimates of uncertainty. As stated in Barrett et al.
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From page 114... ...
estimates generally are asserted to be "high-sided." However, the estimated uncertainty ranges for various parameters vary from shot to shot and among units, depending on the available information and specific exposure scenario. IV.E.2.2.2 Upper bounds in unit dose reconstructions at Pacific test sites The Pacific-test-site unit dose uncertainties are usually based on the estimated coefficient of variation (CV)
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From page 115... ...
are random, an estimated average upper bound for participants exposed over an entire test series will be only about 10-20% greater than the central estimate (Thomas et al., 1982; 1984~. IV.E.2.3 Upper-Bound Estimates of Neutron Dose An upper bound of the dose from external exposure to neutrons is also estimated in the relevant unit dose reports and is based on the estimated uncertainty in the transport-calculated exposure and the uncertainty in the shielding correction.
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From page 116... ...
IV.E.3.1 Exposure to Contaminated Ground As noted in Section IV.B.4.1, beta dose to the skin or lens of the eye from external sources is accrued simultaneously with gamma dose from radioactive fallout, contamination, or neutron-induced radionuclides. As a result, the beta dose is proportional to the gamma dose, and its magnitude can be mathematically expressed by a beta-to-gamma dose ratio.
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From page 117... ...
IV.E.3.2 Immersion in Contaminated Air or Water As noted in Section IV.B.4.2, beta doses from immersion in contaminated air or water are calculated by using dose coefficients, durations of exposure, and composite beta-spectrum radiation energies associated with a reconstructed gamma exposure or film-badge reading. The calculated beta dose is added to the upper-bound gamma dose for the corresponding period.
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From page 118... ...
Thus, the treatment of uncertainty in estimated internal doses differs from the approach to addressing uncertainty in estimated doses from external exposure to photons. As discussed in Section IV.E.2, dose reconstructions for individual veterans often provide an estimated upper bound of the external photon dose, especially if the veteran filed a claim for compensation.
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From page 119... ...
IV.F ESTIMATES OF TOTAL DOSE AND UNCERTAINTY FOR INDIVIDUAL PARTICIPANTS Although many participants have received a dose assessment from the NTPR program based on film-badge data in their medical records or their unit's generic dose reconstruction, VA may request a formal dose reconstruction from DTRA to evaluate a claim for compensation (see Section III.B)
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From page 120... ...
Again, it considered to be a "high-sided" estimate, and no additional upper bound is reported. Although the NTPR program does not combine external and inhalation dose estimates to estimate the total and upper-bound doses to a specific organ, the VA practice is to sum the reported external-dose upper bound (if an upper bound is provided)
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From page 121... ...
IV.G DOCUMENTATION AND QUALITY ASSURANCE IV.G.1 Documentation of Dose Reconstructions The documentation of dose reconstructions for individuals required by the NTPR program is specified in the standard operating procedures (SOPs)
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From page 122... ...
The SOP indicates what constitutes a quality dose reconstruction and directs review for conformity with the SOP's procedures, and appropriateness and responsiveness to the correspondence or request received by the NTPR program. The DTRA Program Manager conducts the final review/approval." The committee notes that the SOPs (DTRA, 1997)
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From page 123... ...
The statement of work included in the DTRA solicitation for NTPR program support indeed contains the following requirement for quality assurance: "The contractor shall provide quality assurance monitoring for the NTPR Program in the areas of database management, dose assessment, and veteran assistance." As stated by DTRA (Schaeffer, 2002e) : "In response to the solicitation, JAYCOR/ SAIC submitted a technical proposal that specified quality assurance measures in the program task areas of database management, radiation exposure assessment, and veteran assistance." The committee did not have the opportunity to review the technical proposal submitted by JAYCOR and SAIC and consequently did not see the specified QA measures.
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