ML20003F868
| ML20003F868 | |
| Person / Time | |
|---|---|
| Issue date: | 01/16/1981 |
| From: | Minogue R NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| To: | Jennifer Davis, Harold Denton NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS), Office of Nuclear Reactor Regulation |
| References | |
| RIL-110, NUDOCS 8104230785 | |
| Download: ML20003F868 (3) | |
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-J M MEMORANDUM FOR: Harold R. Denton, Director Office of Nuclear Reactor Regulation John G. Davis, Director Office of Nuclear Material Safety and Safeguards FROM:
Robert B. Minogue, Director Office of Nuclear Regulatory Research
SUBJECT:
RESEARCH INFORMATION LETTER # 110 - RELATIVE HAZARD OF RADI0 IODINE AS A FUNCTION OF RADIATION QUALITY AND AGE AT EXPOSURE Introduction and Summary This memorandum transmits the results of completed research on factors affecting the risks from radiciodine exposures.
Although it had been suspected that short-lived radioiodines produced more biological damage than the same dose from iodine-131, definitive studies were lacking.
Similarly, the irmature thyroid gland was known to be more radiosensitive than that of the adult, but quantitative comparisons could not be made.
In an effccc to improve consequence assessments for accidents involving release of volatile radiciodines, research was initiated to provide data on the biological effectiveness of different radiciodines and on the radiosensitivity of the thyroid gland as a function of age.
This work was performed by the Laboratory for Energy-Related Health Research of the University of California at Davis under the direction of the Environmental Effects Research Branch of the Office Nuclear Regulatory Research. The final report entitled, " Relative Hazard of Radioiodine as a Function of
( A) Radiation Quality and (B) Age at Exposure," NUREG/CR-1228, has been transmitted to your staff.
Methodoloqy For the study of the comparative effects of short-lived radiciodines, the index of biological effect chosen was the ability of the thyroid gland to enlarge in response to goitrogenic stimulation via thyroid-stimulating hormone from the anterior pituitary gland. Female Sprague-Dawley rats were injected intraperitoneally with single, graded doses of either I-132 or I-131 in sterile saline. After 4 weeks, irradiated and unirradiated control rats were randomly divided into two groups:
(1) those maintained on standard laboratory chow and (2) those on the same diet but containing 0.1 percent propylthiouracil (PTU), a goitrogen. Animals were maintained on these diets for 4 weeks and then sacrificed. Thyroid glands were removed, B1C4230'l Y
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a Harold R. Denton John G. Davis weighed and prepared for histopathologic examination. A three-way (radioiodine, goitrogen, dose) analysis of variance was performed on the thyroid weights and the percentage weight increase was determined as:
Gr - Cr x 100 where G-C Gr is the mean weight of goitrogen-stimulated, irradiated thyroid glands, G is the mean weight of goitrogen-stimulated, unirradiattd thyroid glands, Cr is the mean weight of unstimulated, irradiated thyroid
- glands, C is the mean weight of unstimulated, unirradiated thyroid glands.
For the study of the differences in age-related radiosensitivity of the thyroid gland to radioiodine exposure, fetal, neonatal, weanling, and adult guinea pigs were exposed to single, graded doses of I-131.
The guinea pig was chosen as tne experimental model because of its lengthy gestation period (9 weeks) and because its thyroid begins to function relatively early in fetal life. At approximately 100 days of age, or 100 days after dosing in the case of adults, each animal was given a single microcurie of I-131 and sacrificed 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> later. The animals were weighed, and the radioactivity in the thyroids was measured.
Analyses were performed on thyroidal uptakes of I-131 expressed as a percent of the injected dose per gram of thyroid.
Results Thyroid gland weights were fairly consistent among the groups of rats which remained on control food after irradiation, but they showed a wide range of response to the goitrogen treatment. Thyroids of animals which received the highest doses demonstrated the least ability to respond
- to the effects of PTU. The effectiveness of the two radioiodines was compared by plotting the relative thyroid weight increase versus the radiation dose to the thyroid gland. From this plot, the 50 percent suppression of thyroid growth was determined to result from 280 rads of I-132 and 2500 rads of I-131, indicating a higher effectiveness for I-132 by a factor of nine.
Tracer uptakes of I-131 by thyroids of guinea pigs demonstrated differences in I-131 concentration and retention. At 100 days of age, or 100 days after dosing in the case of adults, uptakes of I-131 and weights of thyroid glands decreased with increasing radiatien exposure.
. Doses to reduce the percent uptake of I-131 to half the unirradiated values were determined to be 13, 34,14, and 19.5 kilorads for fetuses, newborns,
Harold R. Denton John G. Davis weanlings, and adults, respectively.
This indicates that fetuses and weanlings are about one and a half times as radiosensitive as adults.
Future Work All work on this project has been completed.
Conclusions and Recommendations The study of the influence of radiation quality ccnfirmed the appropriateness of the factor of 10 used for the increased effectiveness of short-lived radiciodines. Since the magnitude of this difference is similar to that observed in animals for X-rays and I-131, application of X-ray results as surrogates for short-lived radiciodines is appropriate.
The study of the influence of age at exposure confirmed the greater thyroidal radiosensitivity of fetuses and weanlings compared to adults.
In addition, it quantified the risk factor difference as less than an order of magnitude.
As a result of these studies, more accurate estimates of the consequences of accidental releases of radiciodines can be made.
We reccmmend that the relative hazard factors determined by this study be used in risk assessment calculations required for reactor safety or fuel cycle facility licensing.
For further information on these studies, please centact Dr. Judith D. Foulke at 427-4353.
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4'Nto ert B. Minogue, Director Office of Nuclear Regulatory Research e
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