On March 11, when news of the terrible events at the Fukushima Daiichi nuclear power plant began to emerge, so did the contrasts: Will this be another Chernobyl? How does this compare with Three Mile Island? Discussions have focused on the communications, as well as technical, differences between these accidents: specifically, how to avoid another Chernobyl or Three Mile Island in terms of disseminating inaccurate information.
Since the disaster, the Bulletin's experts have reflected on the necessity for journalists and experts to work together to ensure that the most accurate information is delivered to the public. Robert Socolow wrote, "... we scientists have only one job right now -- to help governments, journalists, students, and the man and woman on the street understand in what strange ways we have changed their world." Richard Wilson echoed this: "… one conversation with a reporter is not enough to make a difference in the world. A scientist who wants to engage the public must keep reaching out, correcting mistakes, and pointing reporters in the right direction -- even though you never know exactly where they will go, or what approach will be most effective."
Over the upcoming weeks, the Bulletin will feature experts who will explore what we, the public and media, don't know about Fukushima Daiichi. We hope this will educate and inspire both citizens and journalists as they reflect on the devastating tragedy.
The release of radionuclides from the Fukushima Daiichi nuclear reactors in Japan is the largest accidental release into ocean waters in history. While it is entirely possible that some of the consequences of this release may be problematic with respect to safe consumption of some seafood items, much of this concern stems from the public's fears about the unknown in general and fear of radioactivity specifically. Public health issues are not the only concern; it is possible, although not likely, that there are also consequences for fish and other marine life in waters closest to the discharge point.
The Japanese have issued periodic reports recording measurements of the radioactive isotopes that are of greatest concern in a nuclear power accident: iodine 131, cesium 134, and cesium 137 -- which have half-lives of 8 days, 2 years, and 30 years, respectively. These measurements have been taken in the power plant's discharge canals (which normally convey heated water from the plant's steam generators to the ocean), in the ocean near the damaged reactors, and offshore to a distance of 30 kilometers (about 19 miles).
Our knowledge of the concentrations of these isotopes at greater distances from shore, and at different depths, is extremely limited at present. Moreover, there is very little that has been reported regarding the concentrations of these radioactive isotopes in marine sediments or in resident marine organisms.
There have been only scant reports of other radionuclides that were released from Fukushima since the earthquake on March 11. It is possible that other radionuclides such as strontium 90, ruthenium 106, cerium 144, and possibly transuranic radionuclides such as plutonium 239, may have also been released into coastal waters. It is noteworthy, but not surprising, that none of the reports on radionuclide concentrations in marine ecosystems near Fukushima have yet been published in the peer-reviewed scientific literature.
Each of the released radionuclides has its own characteristic reactivity for sediments and for bioaccumulation in marine organisms. The more easily a radionuclide is assimilated into marine animals, the more likely it is to accumulate in the marine food chain and potentially be eaten by humans. Animals can also package unassimilated radionuclides in fecal material that sinks rapidly and transports the radionuclides to deeper waters, thereby shortening their residence times in the water column.
Without direct measurements of radionuclides in the ocean waters east of Japan, it is impossible to assess which isotopes are most abundant and which may pose risks to marine life or human consumers. Based on studies of previous releases into other coastal waters -- for example, intentional discharges from the Sellafield nuclear reprocessing plant in the United Kingdom, and accidental releases associated with the Chernobyl disaster -- it is likely that naturally occurring radionuclides, such as potassium 40 and carbon 14, will dominate the radioactivity in marine organisms. For example, fish muscle tissue normally contains potassium 40 levels of about 93 millibecquerels per gram (wet weight), carbon 14 of about 15 millibecquerels per gram, polonium 210 up to 5.2 millibecquerels per gram, and about 1 millibecquerels per gram each of uranium 238 and rubidium 87. By comparison, fish exposed to Sellafield effluent in the Irish Sea display cesium 137 concentrations of less than 2 millibecquerels per gram. Among the anthropogenic radionuclides of interest, cesium concentrates principally in muscle, while strontium and transuranic elements (such as plutonium and americium) concentrate in bone.
It is anticipated that for most locations the doses to humans from naturally occurring radionuclides, even for avid seafood consumers, will exceed those from the radionuclides released by the Fukushima accident. Near Sellafield, for example, doses to humans from naturally occurring radionuclides in seafood caught in the Irish Sea are about an order of magnitude higher than doses from artificial radionuclides. Total collective dose rates from natural radionuclides via marine pathways on a global basis are four orders of magnitude higher than collective doses from Chernobyl radionuclides. Even in the Baltic and Black Seas, the marine waters most contaminated by Chernobyl fallout, natural radionuclides provide a much larger collective dose to seafood consumers than do Chernobyl radionuclides.
Once the concentration of specific radionuclides is determined in fish and other animals off the coast of Japan, it will be possible to compare the doses from anthropogenic and natural radionuclides there. If Fukushima radionuclides are concentrated in the sediment and groundwater near the site, these may remain a long-term source of radionuclides to bottom-dwelling organisms and coastal waters.
To address some of the unknowns regarding the dispersal and risks of radionuclides, we undertook a cruise in waters east of Japan from June 4 to 19, to assess the concentrations and spread of Fukushima-generated radionuclides in the northwestern Pacific. We collected a wide range of dissolved and particulate radionuclide samples in water, and in planktonic organisms and small fish. In addition, we took measurements of basic water properties and ocean currents that will be used to model dispersion rates and transport from the accident site to the Pacific Ocean. Background cesium 137 concentrations, primarily from prior weapons testing, will be used as a baseline against which new measurements of this isotope will be compared. We will use ratios of cesium 134 to cesium 137 to help quantify the radioactive cesium released from Fukushima, because the only source of cesium 134 in the Pacific today is the Fukushima release.
Many radionuclides sink to the ocean bottom faster than cesium does, because they bind more readily with settling particles. By measuring the ratio of plutonium to cesium in the surface ocean over distance and time, for example, we can study the geochemical residence time of plutonium. Given the short half-life of iodine 131, we anticipate that offshore waters and most marine organisms will have only traces of this isotope, unless there are new releases from Fukushima.
Last month's cruise included international collaboration by many labs and scientists, measuring a variety of radionuclides. By this process we will obtain a more comprehensive understanding of radionuclide levels, and reach greater international agreement on these levels, so we can more fully assess the impacts to marine life and answer the many questions regarding the magnitude, transport, and fate of Fukushima radionuclides in the Pacific.
Radiation can't seem to stay out of the news. We worry about people over-exposed to CT scans, and people getting medically unnecessary x-rays. We worry about dirty bombs. Some even worry about whole-body airport scanners. Now, sadly, it's the Japanese nuclear accident: a few brave folks inside the reactor getting significant radiation exposures; and many worried people near the Fukushima Daiichi nuclear power plant, in the rest of Japan, and worldwide getting exceedingly small radiation doses. Just how worried should we be? What do we really know?
We know that the primary long-term concern after low-dose radiation exposure is cancer -- maybe not the only concern, but certainly the primary concern. Higher doses mean higher individual cancer risks; lower doses mean lower individual cancer risks; and very low doses mean very low individual cancer risks.
We know that the radiation doses to everyone outside the Fukushima Daiichi plant are small, and thus the radiation-related cancer risks to any individual as a result of the Fukushima disaster are small -- small near the evacuation zone in Fukushima, very small in the rest of Japan, and miniscule in other countries. So the radiation risks for any individual outside the evacuation zone are not big enough to change our plans, not big enough to stop drinking the water, or eating the food.
We know that the first phase of the situation in Japan is slowly ending. Major airborne releases of radioactivity are probably in the past (but there are no guarantees), and we know that the amount of radioactivity in the air has been steadily falling since the end of March. In fact we were extraordinarily lucky that the winds near the Fukushima Daiichi plant blew fairly consistently offshore in the crucial periods in March and April, taking most of the airborne releases over the Pacific Ocean, and not toward inhabited areas of Japan. The result is that the radiation doses to people in and around Fukushima have been low, and the radiation doses to people further away, such as Tokyo and beyond, have been extremely low. The wind was really our friend.
But the second phase of the accident scenario, a much longer-lived and potentially more significant chapter, is now beginning. The two main radioactive isotopes that we worry most about are iodine and cesium. Radioactive iodine has a half-life of only eight days, so in a couple of months almost all of the released radioactive iodine should be gone. Radioactive cesium is another story. Its half-life of 30 years means that most of the cesium released from the Fukushima power plants will be with us for generations. Most of this radioactive cesium will end up in the Pacific Ocean and will be enormously diluted in the 700 quintillion liters of water there. But some of the cesium will end up on dry land, in our food, and in our water -- and there it will stay, at very low levels, literally for generations.
Should this worry us? We know that the extra individual cancer risks from this long-term exposure will be very small indeed. Most of us have about a 40 percent chance of getting cancer at some point in our lives, and the radiation dose from the extra radioactive cesium in the food supply will not significantly increase our individual cancer risks.
But there's another way we can and should think about the risk: not from the perspective of individuals, but from the perspective of the entire population. A tiny extra risk to a few people is one thing. But here we have a potential tiny extra risk to millions or even billions of people. Think of buying a lottery ticket -- just like the millions of other people who buy a ticket, your chances of winning are miniscule. Yet among these millions of lottery players, a few people will certainly win; we just can't predict who they will be. Likewise, will there be some extra cancers among the very large numbers of people exposed to extremely small radiation risks? It's likely, but we really don't know for sure.
Does this really matter? Surely it does. All of these uncertainties make it hard to frame a debate about the future of nuclear power in Western countries. We are clearly at a fork in the road regarding nuclear power, where we will either have to replace many of our aging reactors, or move away from nuclear power entirely. To make rational decisions about these momentous questions -- not to mention what we should do about the rapid increase in medical imaging such as CT scans, or even the new airport X-ray scanners -- we need to understand the risks of low doses of radiation with a great deal more certainty. Otherwise the debates will be framed around the extreme positions of "radiation is universally dangerous" and "low doses of radiation pose no risk." Neither is true.
More than a month has passed since the one-two punch of an earthquake and tsunami added a third dimension to the tragedy in Japan: a major nuclear crisis at the Fukushima Daiichi nuclear power station. The situation remains serious, and radioactivity continues to be released. Yet media reports of the disaster have become sporadic, reduced to headlines on the news ticker at the bottom of television screens, as the world's attention turns to other events. Over the next year, the impact of the Fukushima disaster on the public's perception of nuclear power will evolve, with advocates portraying the event as an opportunity to make an indispensable source of energy safer, and critics characterizing it as a final indictment of the dangers of nuclear energy. As this debate develops, the public would be well served by answers to a few simple but critical questions.
In the United States, the Nuclear Regulatory Commission (NRC) and the American Nuclear Society (ANS) will conduct safety reviews of US nuclear facilities. Both the NRC and the ANS have already expressed confidence in the safety of nuclear power plants in the United States, even before completing their reviews. The NRC will carry out two assessments: A near-term review, due by mid-July, will focus on operating reactors and spent fuel pools in the United States, and a longer-term review will deal with broader technical and policy issues. The ANS has created a Special Commission on Fukushima Daiichi that will examine the technical aspects of the event to "help policymakers and the public better understand [the disaster's] consequences and its lessons for the US nuclear industry." The quality of these reviews, and the depth to which they probe our understanding of safety, will be important to the final judgment on the benefits and risks of nuclear power.
Critical questions. As with all learning experiences, the usefulness of these reviews depends on the questions that are raised and addressed. The first step in such reviews should be to pose critical questions that will guide the review toward fundamental issues. The questions can be simple, but the answers must be honest and fully accessible to an educated and concerned public. One question in particular demands attention: Why was the actual event in Japan, an earthquake and tsunami, so different from the "credible" event that was expected?
From our perspective as geoscientists, this is the most important question because the definition of the credible event provides the basis against which a nuclear power plant is designed. In the case of the Fukushima Daiichi power station, the magnitude of the earthquake (9.0 on the Richter scale, or M9) and subsequent tsunami (with a reported wave height of 14 meters) exceeded the credible event on which the nuclear power plant's design was based. The site has six nuclear reactors; three of them were operating at the time of the quake and successfully shut down in response to the ground shaking. Nevertheless, the power station and its spent fuel storage pools were overwhelmed by an event that had not been planned for -- a "larger-than-expected" tsunami wave, leading to a sequence of catastrophic failures.
Some experts have since described the tsunami as a "rare" or "exceptional" event that was entirely out of the range of reasonable or credible expectation. But shallow, offshore earthquakes can cause tsunamis, and the height of the tsunami at Daiichi was certainly not unexpected for a 9.0 magnitude earthquake. In addition, there have been three 9.0 magnitude earthquakes during the past decade: Indonesia in 2004, Chile in 2010, and now Japan in 2011. The fact that such earthquakes occur infrequently over historical periods does not explain why the Fukushima nuclear power plant was not designed to withstand this type of geologic event.
Reconsider the definition of "credible." From a geologic perspective, the earthquake and its great magnitude should not have been a surprise. Ten years ago, Japanese earth scientists, led by Koji Minoura at Tohoku University in Sendai, described a major earthquake and tsunami that happened in July 869 and was recorded in an historical document. This event, which is also clearly recorded in the coastal sediment of the Sendai plain, extended inland about four kilometers from the coast. Based on even older tsunami deposits that go back some 3,000 years, Minoura and his colleagues suggested a 1,000-year recurrence interval for large-scale earthquakes and tsunamis in Japan and presciently published their results in the Journal of Natural Disaster Science.
Their results and conclusions did not go unnoticed. Based on the Minoura et al. paper, Yukinobu Okamura, the director of Japan's Active Fault and Earthquake Research Center, raised the possibility that a large tsunami could damage the Fukushima Daiichi plant. The plant operator, Tokyo Electric Power Company, dismissed these warnings.
The essential question is: Why were these very clear results and reasonable concerns not included in an updated safety assessment? This question is not meant to point fingers or establish blame, but rather to understand why critical information was excluded from the safety analysis. The oldest nuclear reactor at Daiichi was built 40 years ago; the intervening years have seen remarkable progress and insight into Japan's tectonic setting, but this new information seems not to have raised much alarm or concern.
Put the risk in a broader perspective. Part of the explanation may lie in the inherent difficulty of reducing geologic data and interpretation to a form that is amenable to the probabilistic risk assessment (PRA) methodology. Probabilistic risk assessments focus on determining the risk that a credible event may pose to a specific nuclear power plant. From this narrow perspective, the risk of a very large tsunami hitting a specific nuclear power plant along the coast of Japan may be very low; however, in a broader geologic context, the risk may be very different.
Nuclear power is vital to Japan's electricity production and future energy plans. Consider that 54 nuclear power reactors are operating in Japan, with a dozen more planned or under construction. Nuclear reactors generate about 25 percent of Japan's electricity, and the country's energy policy calls for doubling the number of reactors -- to generate 50 percent of the nation's electricity. Most of Japan's reactors are or will be located on the coast. Imagine Japan in 2030, with some 100 reactors -- approximately 50 of them along the eastern coast. These reactors, which may be re-licensed or replaced over time, are likely to be essential parts of Japan's energy landscape for the next several hundred years.
This picture of nuclear power in Japan takes on special significance when it is put into the geologic context of the Japanese islands: Japan sits on the western edge of the Pacific Ring of Fire, a distinct boundary between oceanic and continental plates where some 90 percent of the world's earthquakes occur. Along the eastern coast of Japan, two tectonic plates are colliding. The rate of convergence (8.3 centimeters per year) is relentless, driving the Pacific plate to plunge westward beneath the Eurasian plate to form a subduction zone along the Japan Trench. When the interface between these two plates suddenly ruptures during an earthquake, a huge volume of water can be displaced—forming a tsunami. Importantly, long periods without major earthquakes may be the precursors to large-scale events in which the accumulated strain from hundreds of years of convergence is suddenly released in a single earthquake.
When the tectonic setting and the distribution of nuclear power plants along the east coast of Japan are taken into account, the probability that a major earthquake and tsunami will strike a nuclear power plant somewhere along the coast increases significantly, particularly when viewed over periods of hundreds of years. Regions of low seismicity may deserve special attention and concern. Distinguished Japanese geoscientist Hiroo Kanamori and colleagues noted in the Geophysical Journal International in 2010:
This [seismic] behaviour raises another important question regarding tsunami potential of subduction zones where no large historical event has been documented (e.g., a segment of subduction zone south of Sanriku in Japan). Because most seismic and tsunami hazard mitigation measures heavily rely on the past experience, such "quiet" subduction zones tend to receive less attention, but slow accumulation of strain in such subduction zones can lead to extremely serious, though infrequent, tsunami hazard, and special attention needs to be paid to such possibilities.
Clearly, assumptions about the probability and intensity of credible events drive the results of a risk analysis. Unfortunately, probabilistic risk assessments have failed to incorporate all of the geologic data available -- and to interpret this information in a broad context. As a result, the risk assessment for the nuclear power station at Fukushima Daiichi underestimated the probability and intensity of the actual geologic event. The definition of credible events used for reactor design must consider Japan's tectonic setting and its past history of major earthquakes and tsunamis.
