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| Fig. 1: Map of the Chernobyl Exclusion zone and accompanying Radiation Level information from 1996. [2] (Source: Wikimedia Commons) |
One interesting issue that did not seem to be discussed as much as I would have liked in the wake of the Fukushima disaster was was the magnitude of the leaks compared to natural background radiation. Background energy is interesting because presumably having radiation levels below it doesn't tend to matter. If your cell phone rings in a quiet room, the sound energy in the room is appreciably increased. However if your cell phone rings at rock concert, the sound energy is unlikely to change at all compared to the background noise.
It seems that nobody would call you crazy and cancer destined for moving to the Rocky Mountains, yet the background radiation is 3.2 times as high as in gulf coast states, but that then begs the question how bad is a place like Chernobyl? [1] Of course its radioactive, but is spending a vacation there worse than going to college in Colorado? Or traveling across the country every week for work? The magnitudes are hard to know yet are clearly important. We will try to get a feel for how much radiation a place like Chernobyl has some years after the disaster.
Looking at the map of the Chernobyl Exclusion Zone we see a region of about 50 × 50 miles as the primary area of radiation fallout around the reactor. [2] There is another 50 by 100 mile zone of sporadic hot spots as well. There isn't data on how bad the worst areas are, but the lower threshold limit of the worst spots is 40 Ci/ km2 with most of that being from Cs-137. Lets assume the average in the bad areas is twice that, at 80 Ci/km2.
Cs-137 gives off gamma and beta radiation in its decay and we will assume a magnitude typical for nuclear decays of about 1MeV. The power flux is
| 80 Ci/km2 × 10-6 km2/m2 × 3.7 × 1010 Decays/(Ci Second) |
| × 1.17 × 106 eV/decay × 1.602 × 10-19 joules/eV = 4.7 × 10-7 joules/(m2 sec) |
If we assume a human is about 80 kg, and has a cross secant of about a meter squared, we can calculate the rate of Grays (Joules of ionizing energy per kg of mass) absorbed per second :
The Q for finding Sieverts (the same unit as the Gray but scaled to include biological factors depending on the type of radiation) from Grays is 1 for beta and gamma decay, so this is then also our value for 5.8 × 10-9 Sieverts/second as well.
The average American background radiation level is 0.098 nSv/s due to natural sources. [3] This level of radiation in the bad spots around Chernobyl is about 59 times as much as the average US background level.
Fifty nine times is a lot, but it is surprisingly low given that this is the worst nuclear disaster in our history. Its also only about 18 times worse than the radiation found naturally in the Rocky Mountains, and only 10 times worse than the highest levels found naturally in an area in southwest india (0.012 - 0.584 nGy/s). [4]
If we broaden the search for worst-case to include smaller pockets of radiation, this level of 5.8 nGy/s is only 60% higher than the natural level of radiation 3.8-4.1 nGy/s found on few beaches in Brazil due to radioactivity in the sand. [5] Granted nobody is arguing these levels of radiation are good, but for the worst disaster in nuclear history, it is surprising to find anything comparable at all in terms of background radiation.
© David Christensen. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. Jagger, "Natural Background Radiation and Cancer Death in Rocky Mountain States and Gulf Coast States," Health Phys. 75, 4 (1998).
[2] UNSCEAR, Sources and Effects of Ionizing Radiation, Vol II, Annex J (United Nations Publications, 2000).
[3] D.A. Schauer and O. W. Linton, "NCRP Report No. 160, Ionizing Radiation Exposure of the Population of the United States, Medical Exposure - Are We Doing Less With More, and Is There a Role For Health Physicists?," Health Phys. 97, 1 (2009).
[4] A. P. Radhakrishna et al., "A New Natural Background Radiation Area on the Southwest Coast of India," Health Phys. 65, 390 (1993).
[5] D. C. Vasconcelos et al., "Natural Radioactivity in Extreme South of Bahia, Brazil Using Gamma-Ray Spectrometry," Associação Brasileira de Energia Nuclear, 27 Sep 09.
