Review of the Hanford Thyroid Disease Study Draft Final Report (2000) / Chapter Skim
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4 Evaluation of Dosimetric Methods and Results
4 Evaluation of Dosimetric Methods and Results
Pages 61-85
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From page 61... ...
The fuel rods were "cooled" for various periods to let the short-lived fission products decay and then carried to separation plants, where the plutonium was ultimately reclaimed and the remaining ill{ effluent was a byproduct of the chemical separation procedures. The quantities of Lit released to the air from the separation processes used to obtain plutonium from the uranium fuel rods of the reactors at the Hanford Atomic Product Operations were estimated, and meteorologic information and environmental-transfer models were used to assess the environmental concentrations of ]
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From page 62... ...
in the HTDS Draft Final Report, the thyroid doses were calculated on the basis of information provided by the HEDR project. It is the subcommittee's understanding that the HEDR project estimated the daily environmental concentrations of ]
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From page 63... ...
It is recommended that the HEDR project develop a single document in which the methods, assumptions, coefficients, and shape and magnitude of the coefficient uncertainties are summarized, the nature of the results provided to the HTDS is clearly descnbed (with their strengths and wealmesses) , and the ways in which the HTDS made use of the HEDR results to estimate individual thyroid doses are described in detail.
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From page 64... ...
The dose assessment has been found, on the whole, to be structurally sound for the estimation of thyroid doses to representative, hypothetical persons, but minor errors have been found and doubts have been raised about the validity of the results for particular environmental conditions and for the estimation of thyroid doses to specific people. In its letter report dated January 25, 1999, this subcommittee recommended that .
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From page 65... ...
The DESCARTES code was operated with a daily time step for 1944-1949 and with a monthly time step for 1950-1957 (Farns and others, 1994; Price, 19941. The thyroid dose via ingestion is due primarily to the consumption of milk produced by cows that fed on pasture grass.
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From page 66... ...
(DCF) ' where Dm = thyroid dose, Gy, ST = source term, Bq, VEG = concentration of i31} in pasture grass at time of fallout, Bq kg-~, TVeg = mean time of residence of 13lT on pasture grass, d, PI = pasture intake by cows, kg d-~, Em = transfer coefficient of AT to cow's milk, d L-~, MDF = milk-distribution factor (which tales into account the fact that milk consumed might not be of local · ~ ongm, MCR = milk-consumption rate by subject, L d-l, and DCF = thyroid dose coefficient, Gy Bq~l.
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From page 67... ...
The HEDR project inadvertently used the medians instead of the arithmetic means of the monthly source terms for the air-concentration and ground-deposition calculations. · The HEDR project did not propagate the source-term uncertainties to air concentrations, ground deposition, and doses.
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From page 68... ...
efficiency on investigations conducted at fuel-reprocessing plant WAK in Karisruhe, Germany, and concluded that the HEDR project had overestimated the efficiency of emission-control equipment for Gil. They also concluded that the cooling time (time elapsed between fuel discharge from the reactor and fuel dissolution in the chemical plant)
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From page 69... ...
The derivation of the 13~{ concentration in pasture grass Tom the ground-deposition densities makes use of a relationship that is valid only for dry deposition processes. It is not clear whether that relationship was used when precipitation occurred.
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From page 70... ...
It would be helpful to know how the uncertainties and the correlations in the distribution of milk were taken into account by the HTDS. Milk-consumption rate: The HEDR project selected milk-consumption rates for representative people in 12 age and sex categories (Anderson and others, 1993; Fares and others, 1994)
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From page 71... ...
That is caused by the variation of the mass of the thyroid gland as a function of age; the uptake of ill} from blood by the thyroid and the time of retention of }3~} in the thyroid do not vary substantially with age. The central estimates of the thyroid dose coefficients that are used by HEDR and the HTDS are taken from the international Commission on Radiological Protection BURP.
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From page 72... ...
Similarly, the historical measurements that are the most interesting for testing the validity of the calculated inhalation doses are the thyroid burdens and the ground-level air concentrations of silt. Thyroid burdens Nearly 7,900 thyroid burdens of Hanford workers are available for the period June 1945-August 1946 (Napier and others, ~ 9941.
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From page 73... ...
In particular, pasture grass and milk at two farms~alled Farm A and Farm B were measured regularly, and thyroid counts were taken on two children who were consuming milk from a family cow at Farm B The thyroid doses that were calculated for the two children with the HEDR models are in very good agreement with those derived from the thyroid burdens: there is a difference of only about 20% between the "measured" doses and the medians of the calculated doses.
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From page 74... ...
It would have been valuable to investigate how the calculated concentrations agree with the measured values. Concentrations in vegetation Historical measurements of 13~{ in vegetation at the Hanford site were available to the HEDR project for the period beginning in the middle of ~ 945 and continuing to the present.
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From page 75... ...
have reviewed the HEDR dosimetry and claimed that thyroid doses were substantially underestimated by the HEDR project. However, the divergences that they identified occurred during years when 13~t releases were rather small, and their findings and HEDR results differed by only about a factor of I.3 in total releases.
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From page 76... ...
In contrast, the HEDR project might have overestimated the transfer coefficient from cow's intake to milk by a factor of 2, and some of the validation comparisons also suggest an overestimation of doses by the HEDR calculations. The effect of dose overestimation could have implications for the interpretation of HTDS results because overestimation would mean that the statistical power of the study was also overestimated; this would imply that the negative results were not as definitive as the investigators asserted.
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From page 77... ...
DOSE UNCERTAINTIES Considerable efforts were made by the HEDR project to mode! the uncertainties attached to estimated thyroid doses.
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From page 78... ...
There might be adequate explanations for these discrepancies that do not appear in the publications or in the Draft Final Report. The omission and the lack of clarity of what the HEDR project dice constitute inadequacies in communication and perhaps in scientific methods.
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From page 79... ...
The authors of the report should explain why GSD values of less than 2 are found for many HTDS thyroid-dose estimates. It is to be noted that the GSDs of the thyroid doses calculated by the HEDR project are 2 or greater (Harris and others, 1996~.
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From page 80... ...
In the absence of such a document, however, it seems that the uncertainties in the thyroid doses were underestimated by the HEDR project and consequently by the HTDS. ~ FROM THE NEVADA TEST SITE AND GLOBAL FALLOUT During the 1950s and early 1960s, two other environmental sources of i3~{ contributed to the thyroid doses received by the populations around Hanford: the nuclear-weapons tests carried out at the NTS, mainly in 1952, 1953, 1955, and 1957; and the nuclear-weapons tests conducted outside the lower 48 states of the United States, which gave rise to so-called global fallout, mainly in ~ 9 5 4, ~ 9 5 6, ~ 9 5 7, ~ 95 8, ~ 96 I, and ~ 962.
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From page 81... ...
For the purposes of this review, a crude assessment of the thyroid doses resulting from global fallout near Hanford has been prepared in order to obtain preliminary evidence as to whether global fallout is likely to be an important confounding variable in the analyses pertaining to Hanford fallout and thyroid disease. Details of the assessment are provided in appendix C
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From page 82... ...
depositions in 1957, 195S, 1961, and 1962 were then estimated from the 89Sr depositions at sites in the western United States and the measured monthly rainfall in counties in the Hanford area. The ratios of the deposition in WalIa WalIa and Stevens counties to deposition in Franklin, Adams, and Benton counties were used to estimate the milk concentrations in those counties in 1961 and 1962 from those measured in milk near Hanford.
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From page 83... ...
in theory, global fallout is a potential confounding variable in the analyses of thyroid-disease risk posed by Hanford fallout. But in reality, the degree to which global fallout could confound the analytic results appears to be limited.
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From page 84... ...
Nevertheless, it is not very likely that medical radiation exposure would be a confounding variable, in that the frequency and intensity of such exposures would have to be correlated with the magnitude of the Hanford IT doses for confounding to occur. The thyroid doses from the medical radiation exposures asked about would probably vary appreciably among medical procedures and radiological practices.
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From page 85... ...
the reported medical diagnostic and therapeutic radiation exposures for each thyroidcancer case, and this is informative.
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