Background: Causes of the accidents
The following is not meant to be a comprehensive accounting of the accidents but to provide basic information about what is known about events at the NPP sites.
On April 26, 1986 in response to a technical test in Unit 4 at the Chernobyl nuclear power plant (NPP) "xenon-poisoning" of the reactor occurred. This was misinterpreted by staff at the NPP and actions led to a power excursion that destroyed the RBMK-1000 reactor. There was at least one steam explosion and the graphite moderators of the reactor ignited and burned for 10 days resulting in very high dose rates from short-lived radionuclides close by and further radionuclide releases over time.
Beginning on March 11, 2011 the Fukushima NPP lost power due to a magnitude 9.0 earthquake and tsunami and lost capacity to operate emergency cooling systems to remove heat from the reactors and spent fuel pools at the site. Fuel meltdowns and redox reactions between water and fuel cladding at elevated temperatures led to venting and massive oxy-hydrogen gas explosions which seriously damaged units 1, 2, 3 and 4. Significant releases of radionuclides to the atmosphere and directly to the coastal ocean occurred.
Amounts of radionuclides released
Steinhauser and colleagues report that the most cited estimate of the Chernobyl source term is ~5300 PBq (PBq = PetaBq = 10^15 Bq) excluding nobel gases. The source term for Fukushima is evolving as more and more measurements are made in the environment. In this paper Steinhauser and colleagues estimate the total activity released to the atmosphere to be 520 PBq with a range of 340-800 PBq. A compendium of most likely source term estimates for various radionuclides for both accidents are reported with references in Table 1 of the study shown below.
Comparing land areas contaminated and evacuation zones
Ultimately, the exclusion zone around Chernobyl was established at 4300 square kilometers while as of this publication the exclusion zone around Fukushima is ~ 600 km^2. Maps below indicate the spatial extent and degree of contamination of land as determined by soil 137-Cs (half life 30 years) activities for Chernobyl and Fukushima.
Surface contamination with 137-Cs in Europe after the Chernobyl nuclear accident.
Estimated total deposition of radiocesium after the Fukushima nuclear accident, approximately half of which is 137Cs
The take-home message of these figures is that the areas showing contamination >185,000 Bq per meter squared owing to Chernobyl is ~29,000 km^2 while for Fukushima the are is ~1,700 km^2.
According to Steinhauser and colleagues the environmental consequences of Chernobyl exceed the Fukushima accident by almost any measure. While 4 reactor complexes were involved at Fukushima and only one at Chernobyl, the total releases of radionuclides appear to be an order of magnitude lower for Fukushima. This is consistent with other summaries presented thus far through this blog. The contamination levels in air, soil and rainwater are correspondingly lower in almost every case from the Fukushima event when compared to Chernobyl. It is expected then that the radiological health consequences of the Fukushima disaster will not exceed those of Chernobyl. This reflects both the amount of radionuclides broadcast to the environment as well as the effectiveness of food safety campaigns keeping contaminated foodstuffs off the market in Japan where such safety measures were slow to be implemented post-Chernobyl and exposed the public to much higher doses of ionizing radiation. Indeed, projected radiation-induced casualties and cancers in the Japanese public are not likely to be detected by epidemiological studies.
Here I provide data presented in the Tables in the paper for those that are interested in the specific comparisons of the accidents
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