Exposure & Dosimetry at TMI-2

Radiation Exposure & Dosimetry at Three Mile Island Accident

"Backgrounder on the Three Mile Island Accident: Health Effects". U.S. Nuclear Regulatory Commission. Retrieved January 13, 2018.

“...The approximately 2 million people around TMI-2 during the accident are estimated to have received an average radiation dose of only about 1 millirem above the usual background dose. To put this into context, exposure from a chest X-ray is about 6 millirem and the area's natural radioactive background dose is about 100–125 millirem per year .... The accident's maximum dose to a person at the site boundary would have been less than 100 milligrams [sic: millirem] above the background.... Comprehensive investigations and assessments by several well-respected organizations, such as Columbia University and the University of Pittsburgh, have concluded that in spite of serious damage to the reactor, the actual release had negligible effects on the physical health of individuals or the environment.”

"Backgrounder on Biological Effects of Radiation". U.S. Nuclear Regulatory Commission. March 2017. Retrieved October 26, 2021.

"On average, a U.S. resident receives an annual radiation exposure from natural sources of about 310 millirem [3.1 mSv]. Radon [²²²Rn] and thoron [²²⁰Rn, a thorium isotope] account for two-thirds of this exposure [2 mSv]. Cosmic, terrestrial, and internal radiation account for the rest [1 mSv]. Man-made sources of radiation ... contribute roughly 310 mrem more …. Other medical procedures [such as a CAT scan] make up another 150 mrem or so each year. Some consumer products ... contribute about 10 mrem per year…. [T]he average annual U.S. radiation dose [is] 620 mrem [independent of medical exposure]."

Walker, J. Samuel (2004). Three Mile Island: A Nuclear Crisis in Historical Perspective. Berkeley, California: University of California Press. p. 231. ISBN 0-520-23940-7. Retrieved October 25, 2021.

“Another significant question that the cleanup of the containment building raised was why more radioactive iodine had not escaped from the plant. Although the accident discharged up to 13 million curies of radioactive noble gases [²²²Rn & ²²⁰Rn] to the environment, it released very little of the much more hazardous iodine-131 [¹³¹I]. A curie is a unit of measurement formerly used to [quantify the amount, or measure the activity] of radioactive substances. Of the estimated 64 million curies of ¹³¹I in the core at the time of the accident, less than 20 curies leaked to the atmosphere. This was a far smaller release of ¹³¹I than reactor experts had postulated in projecting the consequences of a severe reactor accident before Three Mile Island. Researchers found that most of the ¹³¹I in the core had combined with other elements to form compounds that dissolved in water [which remained inside the reactor building] or had attached to metal surfaces in the containment building. Under the conditions in the core and the reactor building, the iodine did not remain in a gaseous state long enough to escape from the plant into the environment.”

Notes: Emphasis in bold and commentary in [] added by SM Carr

        100 millirem (mrem) is equivalent to 1 milliSievert (mSv) in SI units. In the empirical LNT (Linear No Threshold] model used by many health physicists, an added dose of 100 mSv to an individual increases that individual's risk of death from cancer by 0.4%. The LNT model is contrasted by the LDDR (Low Dose, Dose Rate) model, which argues that extremely low added doses on the order of 1 mSv or less have little or no effect on added cancer rates. Epidemiologists suggest that psychological and societal effects for populations exposed to such low doses may be more serious than the radiation itself. It is known for example that many people in the vicinity of TMI-2 suffered from PTSD, underwent spontaneous abortions, chose elective abortions, refused to have more children, and (or) have been "shunned" as somehow "contaminated" as a result of the accident.

        Average annual exposure of a US resident is 620 mrem (~ 6 mSv), of which 2 mSv is from Radon gas, 1 mSv from environmental background, and 3 mSv is from man-made artificial sources including fallout from atmospheric weapons testing. Exposure from cosmic rays is increased at higher altitudes (e.g., Denver versus sea-level) and during high-altitude airplane travel. Medical procedures such as a CAT scan may add as much as 1.5 mSv.

        The NRC Backgrounder above estimated that 2 million people (2 x 10⁶) in the vicinity of the TMI-2 accident received ~ 1 mrem @ [0.01 mSv], and those at the 'site boundary' no more than 100 mrem [1 mSv]. If accepted for such low doses, the LNT model would then predict an added cancer death risk for the population of 2 x 10⁶ x 0.004% = 2 x 10⁶ x 4 x 10^-3 x 10^-2 = 8 x 10¹ = 80 added deaths, mostly from thyroid cancer. Epidemiological studies have not in general detected such excess deaths. It may be noted that the population within 20 miles of TMI-2 at the time of the accident was < 650,000, such that epidemiological estimates inclusive of a more widely dispersed population of 2 million exposed to even smaller doses would arguably tend to underestimate any biological effect.