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A. Bertrand Brill |
The aftermath of the Chernobyl accident is still being played out. Only one of the four RBMK Charcoal moderated plants is still operating. The second was recently shut down and the third has not been restarted, and the fourth is now encased in a shielded structure that is deteriorating and allowing water to get into inside which is of concern to people, especially those in the area. With financial assistance from the United States and the European Community, Ukrainian officials have agreed to shut the facilities by the end of this year. Their economy is struggling, and how they will replace the 20% of their power that comes from the reactor complex in the short run, is not clear.
Since my last note, the health consequences have not changed in kind. There is a continuing increase in thyroid cancer in those children exposed to what we believe to be high levels of radioactive nuclides that concentrate in the thyroid. The highest incidence is in children who were under age 5 at the time of the accident. The estimated dose to the thyroid is not well established since the amount of milk drunk, and the levels of radioactivity contained are not known. The average thyroid dose estimated for exposed children with thyroid cancer in a recent case-control study was in the range of 35 cGy. Many epidemiology studies of persons, mostly adults, who received doses two to more than 1000 fold higher did not show a significantly increased incidence of thyroid cancer. The general consensus is that a greatly increased radiation sensitivity of the thyroid gland in very young children accounts for the discrepant findings.
However, there are serious reservations about the magnitude of the dose. It has been assumed that I-131 is the major source of thyroid dose, but there are other short-lived radionuclides of iodine that are less well accounted for. These would have been most important via inhalation since going through the pasture, cow, and commercial milk delivery system leads to a very large loss in the potential amount that could have come through drinking commercial milk. Intake through leafy vegetables could have contributed, but the major source of the highest doses was probably from drinking milk from one’s own cow or goat. Since all radioactive iodine isotopes, except I-129, have decayed long since the accident, ongoing attempts are being made to reevaluate the amount and spatial distribution of iodine releases based on analysis of soil samples using high sensitivity accelerator mass spectrometry methods. Samples have been taken from soil in different regions. These data are being analyzed in the hope that further refinements in the regional distribution of dose will be forthcoming. The pattern of distribution of cases closely follows the pattern of wind and deposition that is well chronicled for long-lived Cs-137. However, different wind directions at different times could lead to significant differences in the radioiodine and cesium deposition patterns. Dosimetry investigations are a continuing activity, and may help to improve our understanding of the risk coefficients from radioactive iodine in different age groups. Also, it may be that continuing studies of the relative hazard of internal emitters and external beams of radiation (which were previously used in treating benign conditions in young children) may be forthcoming, given improved dosimetry and continued efforts to study and provide health care for the affected population.
So far, there is no evidence of other health effects, not even leukemia in the liquidators; the persons brought in to manage the aftermath of the accident, including the environmental clean up. The total body dose they received as measured by film badges was predominantly under 25 cGy, but some received doses estimated up to about four times this. These doses were delivered at different rates, depending on when and where they worked, and how long they were there. Since the data on the Japanese A-bomb survivors was from acute ultra-high dose rate exposures, it is important for radiation protection purposes to know how the hazards from low and high dose rate exposures differ, as standards for the workplace are properly based on low dose rate exposures.
The work continues, and has a strong humanitarian component. As things change, I will update you by interval reports.
Randy Brill can be reached at the Radiology Department, Medical Center, R-1302-MCN, Vanderbilt University, Nashville, TN 37232-2675; Phone (615) 322-3190; Fax: (615) 322-3764; E-Mail: aaron.brill@vanderbilt.edu