It has been some time since the accident and a number of comprehensive studies have been performed. I think it is worthwhile to start with one of the most authoritative articles by the Journal Nature, Fukushima’s doses tallied — Studies indicate minimal health risks from radiation in the aftermath of Japan’s nuclear disaster (23 May 2012):
Few people will develop cancer as a consequence of being exposed to the radioactive material that spewed from Japan’s Fukushima Daiichi nuclear power plant last year — and those who do will never know for sure what caused their disease. These conclusions are based on two comprehensive, independent assessments of the radiation doses received by Japanese citizens, as well as by the thousands of workers who battled to bring the shattered nuclear reactors under control.
The first report, seen exclusively by Nature, was produced by a subcommittee of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) in Vienna, and covers a wide swathe of issues related to all aspects of the accident. The second, a draft of which has been seen by Nature, comes from the World Health Organization (WHO) in Geneva, Switzerland, and estimates doses received by the general public in the first year after the accident. Both reports will be discussed at UNSCEAR’s annual meeting in Vienna this week.
The Chernobyl incident is now well documented, see e.g. Wikipedia's article "Chernobyl Disaster". Fukushima has only just begun being studied, and much of the information still derives from contemporaneous articles (to which this article links to many).
At the time of this writing, it seems reasonable to conclude that the Fukushima incident will have a fraction of the impact of Chernobyl, based on the analysis below. In particular:
- The amount of radiation released by Fukushima is a fraction of Chernobyl;
- The spread of the radiation from Fukushima is unlikely to hit highly populated areas like Chernobyl's radiation did;
- The types of isotopes released by Fukushima are not as dangerous as those released by Chernobyl; and
- The reported effects of Fukushima are significantly less than Chernobyl's reported effects;
- Comprehensive studies indicate that the amount of cancer developed by Fukushima may actually be at or less than background rates for cancer.
The impact of Fukushima could be substantially greater in one area compared to Chernobyl: post traumatic stress disorder. From the Nature article noted above:
A far greater health risk may come from the psychological stress created by the earthquake, tsunami and nuclear disaster. After Chernobyl, evacuees were more likely to experience post-traumatic stress disorder (PTSD) than the population as a whole, according to Evelyn Bromet, a psychiatric epidemiologist at the State University of New York, Stony Brook. The risk may be even greater at Fukushima. “I’ve never seen PTSD questionnaires like this,” she says of a survey being conducted by Fukushima Medical University. People are “utterly fearful and deeply angry. There’s nobody that they trust any more for information.”
There remains the possibility for Fukushima to become a significantly worse disaster if an earthquake occurs, as per noted scientist and host of the Nature of Things, David Suzuki:
"Three out of the four plants were destroyed in the earthquake and in the tsunami. The fourth one has been so badly damaged that the fear is, if there's another earthquake of a seven or above that, that building will go and then all hell breaks loose.
"And the probability of a seven or above earthquake in the next three years is over 95 per cent."
... "I have seen a paper which says that if in fact the fourth plant goes under in an earthquake and those rods are exposed, it's bye bye Japan and everybody on the west coast of North America should evacuate," he said.
It is not clear what paper Dr. Suzuki is referring to.
Here is the analysis:
Amount of radiation by absorbed energy
The amount of radiation by absorbed energy in millisieverts per hours at Fukushima seems to have been a fraction of about a fiftieth (1/50th) of Chernobyl's at what seem to be comparable water sources.
Sample recorded levels during Chernobyl (See: JPRS Report Economic Affairs, "Chernobyl Notebook" by G. Medvedev June 1989):
- Vicinity of reactor core: 300,000 mSv/h
- Water in Level +25 feedwater room: 50,000 mSv/h
Highest reported level during Fukushima accident: 4,000 mSv/h reported as the level at a pool of water in the turbine room of reactor two. (See: The Guardian, "Japan doubles Fukushima radiation leak estimate", June 7, 2011)
Spread of the radiation geographically
The spread of radiation from the Japan incident over human habitats is significantly less than that of Chernobyl because much of the radiation from Japan dispersed over the Pacific ocean.
From Wikipedia's article "Chernobyl Diaster effects"
The explosion at the power station and subsequent fires inside the remains of the reactor provoked a radioactive cloud which drifted not only over Russia, Belarus and Ukraine, but also over the European part of Turkey, Greece, Moldova, Romania, Bulgaria, Lithuania, Finland, Denmark, Norway, Sweden, Austria, Hungary, Czechoslovakia, Yugoslavia, Poland, Estonia, Switzerland, Germany, Italy, Ireland, France (including Corsica4), Canada5 and the United Kingdom (UK).6
Much of the Fukushima radiation has dispersed over the Pacific Ocean. (See: Christian Science Monitor, "Fukushima raised to level 7 the same category as Chernobyl but Chernobyl had10 to 100 times more radiation", April 12, 2011)
Amount and types of radioisotopes
While parts of the fuel rods and graphite particles were ejected into the atmosphere at Chernobyl, only iodine, cesium and Xenon-133 have been noted in the media reports about releases from Fukushima.
While some reports of plutonium have been reported in the soil, greater amounts of plutonium were discovered in Japanese soil after overseas nuclear testing. (See: The Guardian, "Japan doubles Fukushima radiation leak estimate", 7 June, 2011)
Iodine released has been at a fraction of that of Chernobyl of about one fourteenth (1/14th) to one eight (1/8th). The measured amounts are: Fukushima: 770,000 terabecquerels of iodine-131 (The Guardian, "Japan doubles Fukushima radiation leak estimate", June 7, 2011); Chernobyl: 5.2 million terabecquerels (See: The Globe and Mail article "Japan Haunted by spectre of Chernobyl").
Cesium-134 and -137
A paper, A. Stohl, P. Seibert, G. Wotawa, D. Arnold, J. F. Burkhart, S. Eckhardt, C. Tapia, A. Vargas, and T. J. Yasunari “Xenon-133 and caesium-137 releases into the atmosphere from the Fukushima Dai-ichi nuclear power plant: determination of the source term, atmospheric dispersion, and deposition”, Atmos. Chem. Phys. Discuss., 11, 28319–28394, 2011, states "For [cesium 137], the inversion results give a total emission of 35.8 (23.3–50.1) PBq, or about 42 % of the estimated Chernobyl emission" (the "Stohl paper").
By this estimate Fukushima has released 40% of the radioactive caesium of Chernobyl. This is significant, and by this metric Fukushima is not on par with Chernobyl, strictly speaking, but it is in the same order of magnitude. As well, Chernobyl released many much more dangerous particles, namely radioactive isotopes of plutonium and uranium.
According to the Stohl paper, under the metric of Xenon-133 release, Fukushima exceeds Chernobyl:
Total a posteriori 133 Xe emissions are 16.7 EBq, one third more than the a priori value
of 12.6 EBq (which is equal to the estimated inventory) and 2.5 times the estimated
Chernobyl source term of 6.5 EBq
The authors note that their measurements of Xenon-133 exceed inventory, and "Emissions cannot exceed 100 % of the inventory, so this may indicate that our inversion overestimates the emissions." The estimate may be revised over time.
Xenon-133 has a very short half-life in the body, of "a few minutes", so it doesn't stay in the body very long. There's a well referenced post on the UC Berkeley Department of Nuclear Engineering on the effects of Xenon, which concludes:
So the bottom line is: Xe-133 does not stay in the body very long, and it's not very dangerous even if it's in the air around you.
So while Xenon-133 has been released from Fukushima in amounts greater than Chernobyl, and Xenon-133 is a radioactive isotope, the effects on human health of Xenon-133 even in amounts released from Fukushima are likely a fraction of the effects of the releases of more dangerous isotopes from Chernobyl (i.e. radioactive isotopes of plutonium, uranium, caesium, iodine).
Strontium 90 has been noted to be seeping into the bedrock near Fukushima, by the BBC and NBC. The latter states:
Tokyo Electric Power (Tepco), ..., said nearly 30 times the permitted level of the radioactive isotope was discovered in a well dug last month outside the turbine hall of Reactor No.2.
The company said it had not detected any rise in the levels of Strontium-90 in sea water, and that it believed the substance was trapped during the initial 2011 nuclear fallout.
It seems that not a great deal of analysis has been done on amount and effect of the Strontium.
The article True facts about Ocean Radiation and the Fukushima Disaster notes:
The leaking groundwater contains strontium and tritium which are more problematic than Cesium-137. But it sounds like strontium accumulates in bones and is only problem if you eat small fish with the bones in, like sardines (and it will only affect sardines caught near Japan since they don’t travel far).
From the Wikipedia article on the Chernobyl disaster remarking on the radioisotopes released from Chernobyl:
The release of radioisotopes from the nuclear fuel was largely controlled by their boiling points, and the majority of the radioactivity present in the core was retained in the reactor.
- All of the noble gases, including krypton and xenon, contained within the reactor were released immediately into the atmosphere by the first steam explosion.
- About 1760 PBq or 400 kg of I-131, 55% of the radioactive iodine in the reactor, was released, as a mixture of vapor, solid particles, and organic iodine compounds.
Caesium (85 PBq Cs-137) and tellurium were released in aerosol form.
- An early estimate for fuel material released to the environment was 3 ± 1.5%; this was later revised to 3.5 ± 0.5%. This corresponds to the atmospheric emission of 6 t of fragmented fuel.
Two sizes of particles were released: small particles of 0.3 to 1.5 micrometers (aerodynamic diameter) and large particles of 10 micrometers. The large particles contained about 80% to 90% of the released nonvolatile radioisotopes zirconium-95, niobium-95, lanthanum-140, cerium-144 and the transuranic elements, including neptunium, plutonium and the minor actinides, embedded in a uranium oxide matrix.
Reported health effects
The actual effects of Chernobyl are somewhat contested and vary dramatically. At the least, it is acknowledged that 237 people reported acute radiation sickness, with 31 deaths within 3 months. Some claim the deaths may be as high as 200,000 to 900,000. (See: Wikipedia's article: "Chernobyl disaster")
21 workers have reportedly been affected by minor radiation sickness. (See: Nuclear crises: How do Fukushima and Chernobyl compare?).
In the wake of the earthquake and tsunami it may be difficult to find accurate recordings of any effects of the nuclear incident alone.
Note regarding iodine: Thyroid cancer, caused by radioactive iodine, has been considered by some to be one of the main causes of death from Chernobyl. Iodine tablets are being distributed in Japan that prevent thyroid cancer. (See: Japan to Distribute Iodine Tablets Near Nuclear Plant, Newser.com, March 12, 2011; See also Wikipedia's article "Chernobyl disaster" at Assessing the diasaster's effects on human health). It's also noteworthy that thyroid cancer is treatable with very high success.
There were reports of [Caesium in baby milk]:
... tests found up to 30.8 becquerels of caesium per kilo of Meiji Step powdered milk.
The milk was recalled; there was no indication of the amount of distribution before the discovery. The distribution is noted to be limited to Japan only.
Around 70 sailors aboard the USS Reagan are making a claim for compensation for exposure to radiation.
Update — January 2014
A number of other recent resources have cropped up, including:
There are some reasonably comprehensive articles on the comparison, including: