Fukushima Review

A special session on the geochemical aspects of the Fukushima nuclear power station crisis has been organised on Tuesday 16th August, in the main Congress Hall. Confirmed speakers and details of their talks are listed below.

Mitsuru Ebihara Mitsuru Ebihara, Tokyo Metropolitan University
The Role of the Geochemical Society of Japan for Mitigating the Fukushima Accident and its Aftermaths

09:30 on Tuesday 16th in Congress Hall

A huge earthquake (M 9.0) and coupled tsunami on March 11, 2011 yielded enormous damage in the area from north Kanto to whole Tohoku district. Besides direct disaster caused by earthquake and tsunami, equally serious accident happened at Fukushima Daiichi nuclear power plant of Tokyo Electric Power Company. By this accident, nuclear power reactors couldn’t been controlled properly, and eventually radioactive materials were released from the nuclear power reactors and dispersed over rather wide area. Consequently, agricultural materials, drinking water and food were contaminated with radioactive nuclides. It was urgent to figure out how radioactive materials were spread not only in the area close to the nuclear reactors but also over surrounding, rather wide district. Such information must release the uneasiness for public people and objectively predict the possible influence caused by radioactive material over agricultural products for people engaging in agriculture.

To grapple with this task promptly as well as effectively, scientists belonging to the Geochemical Society of Japan, the atmospheric and oceanic section of the Japan Geoscience Union and the Japan Society for Nuclear and Radiochemical Sciences were allied and presented a proposal for comprehensive and systematic survey of radioactive materials in the environment to the ministry of Education, Science, Culture and Technology (MEXT). In this proposal, aerosol, rain, soils and underground water samples are to be collected as systematically as possible and as wide as possible for the sampling area. Besides such an action, we discussed with nuclear physicists to work together in collecting soil samples in Fukushima prefecture systematically and measuring radioactivity precisely under controlled counting protocol, making contour maps of individual radionuclides. Such a collaborative project was eventually supported by the government and officially started in early June, being continued for three months. The data thus obtained are to be opened to the governmental sector as well as to the public including scientific communities in due course so that national and international academic societies can utilize these data for scientific discussion.

Jeroen Ritsema Jeroen Ritsema, University of Michigan
Seismological Investigations of the March 11, M9, Tohoku-Oki Earthquake

10:00 on Tuesday 16th in Congress Hall

On March 11, 2011, a shallow, M9 earthquake with an epicenter roughly 160 km offshore struck the Fukushima prefecture in Japan. The ensuing tsunami was responsible for 20,000 fatalities, widespread damage along the Honshu coast, and the flooding of the Fukushima Dai-ichi nuclear power station, a crisis that is still unfolding. Yet, from a seismological point of view, the response to the earthquake has been a success. Thanks to the enormous investment in seismological and geodetic infrastructure by scientists from Japan and around the world, the earthquake magnitude, faulting mechanism, depth, extent along the Japan Trench, and, hence, its tsunami-generation potential was fully understood within 30 minutes after the earthquake occurred.

The Tohoku-oki earthquake was a classic mega-thrust (i.e. reverse) earthquake. It ruptured a fault plane on the plate interface that is dipping 14 degrees to the west. The largest slip was concentrated at shallow depth near the trench which explains the exceptionally large tsunami. Every large earthquake holds surprises and Tohoku-oki was not an exception. Before March 11, the seismic record suggested that M7.5 to M8.2 earthquakes break small patches of the plate interface along the Japan Trench south of Sendai. Given this experience, the magnitude of the Tohoku-oki earthquake was unexpectedly high. Moreover, the Tohoku-oki earthquake was spatially compact. It ruptured only a 200x100 square km area of the plate interface. Just west of the trench, slip on the fault plane exceeded 60 m. This is the largest fault offset ever reported.

The Tohoku-oki earthquake has been analyzed by a large number of scientists around the world. An open-access special issue of the journal "Earth, Planets, and Space" appeared in June. Articles in this issue are already providing a detailed and remarkably consistent picture of the many complex facets of the Tohoku-oki earthquake. In this presentation, I will summarize these investigations and place the earthquake in a historical context.

Edward Blandford Edward Blandford, Stanford University
Deconstructing the Nuclear Accident at the Fukushima-Daiichi Plant: What Went Wrong and What are the Prospects for Recovery?

10:30 on Tuesday 16th in Congress Hall

The recent large-scale industrial accident at the Fukushima-Daiichi Nuclear Power Plant was the culmination of three inter-related factors: external natural hazard assessment and site preparation, the utility’s approach to risk management, and the fundamental reactor design. The reactor accident was initiated by a magnitude 9 earthquake followed by an even more damaging tsunami. However, it was othe inability to remove the decay heat in the reactor core that led to core meltdown and radioactive release. A review of the timeline of the major Fukushima accident sequences will be given. The plant first experienced a station blackout (i.e. loss of all offsite and onsite power) due to flooding of backup critical emergency cooling equipment. The lack of an ultimate heat sink led to the fuel overheating. Subsequently, the generation of hydrogen through steam oxidation of of the fuel cladding led to chemical explosions causing significant structural damage.

The focus of this talk is on the engineering aspects of the reactor accident and the prospects for local environmental recovery. Radionuclide measurements in space and time provide important evidence for the exact evolution of fuel damage leading to partial core melting in multiple units. A review of the spent nuclear fuel pools is given where isotopic water composition and visual inspection images provide important evidence for the condition of the spent nuclear fuel.

While it will be several months to a year before we will be in a position to learn most of the lessons from this tragdy, several conclusions about defensive design, mitigation actions, and emergency response have been drawn by international organizations.

While the public health impact appears to have been low, the economic and nearby environmental consequences are severe, There is no doubt that land restoration will take over a decade and perhaps much longer. A review is given of actions taken by the Japanese government for land recovery in areas such as decontaminating top soil and local farmland as well as highly radioactive water used during ‘feed and bleed’ cooling of the core.

Naohiro Yoshida Naohiro Yoshida, Tokyo Institute of Technology
A Preliminary Overview of Studies on Dispersals of Radionuclides from Fukushima Nuclear Power Plants

11:00 on Tuesday 16th in Congress Hall

The Tohoku Japan earthquake occurred on 11th March, 2011, accompanied by huge tsunami which attacked all coastal areas in northeastern Japan and caused the accdidents of Fukushima Daiichi nuclear power plant. Some nuclear reactors lost their control almost entirely or partly, and the fission products and feasible neutron activated nuclides in the reactors have been dispersing in environment.

To promote a study on disperse of the radionuclides in order to prevent further unnecessary dose for people, many scientists have gathered and started monitoring, mapping, and model simulation of disperse of radionuclides in local and large covering areas as well. This is a preliminary overview of studies on dispersals by many motivated collaborators.

Radionuclides are found to be strong tracers of atmospheric and oceanic movement with time stamps of some with their different half-lives to distinguish different emission phenomena, direct and/or rebound transport, and dry and/or wet deposition. The distribution of radionuclides in time and space is highly localized, which may be determined by their accumulation and diffusion because of distance, topography, wind direction, precipitation, surface runoff, marine gires and currents. Their occurrences, however, are sometimes difficult to be explained straightforward because of the unknown source processes and their strength, their physical and chemical forms in the atmosphere and the oceans, their transformation through chemistry, the size distribution of dusts on which they are absorbed, dry and/or wet deposition on the surface soil, and their chemistry and transport after deposition.

Some scientific programs have started and will continue before and even after the cease of the nuclear plant crisis, to obtain knowledge on the ongoing radioactive pollution, to understand and to predict their distributions, and for the unexpected similar future accidents. This is a great challenge for the scientists in this research field, geochemistry and not necessarily limited to isotope geochemists nor any scientists, but to human beings. Further international collaborations would be much appreciated in any fields of study with different time and space scales.

Masayoshi Yamamoto Masayoshi Yamamoto, Kanazawa University
Present Situation of Radioactive Contamination in Soil by the Fukushima Dai-Ichi Accident

11:30 on Tuesday 16th in Congress Hall

On 11 March 2011, A Great East Japan Earthquake of magunitude 9.0 happend, and genarated a series of large tsunami waves in the east coast of Japan. By thses events, Tokyo Electric Power Company (TEPCO) Fukushima Dai-ichi Nuclear Power Plants (units 1, 2, 3 and 4) suffered major damage, and significant amouts of radionuclides were released into the environment. Monitoring efforts performed so far have shown that dengeous contamination is mostly localized in a narrow band within 40 km of the plant, stretching to the northwest. The Japanese Government announced that the serious accident of TEPCO’s Fukushima Daiichi Nuclear Power Plant was ranked up to Level 7, the most serious accident according to INES, the same level as that in the case of Chernobyl Nuclear Power Plant in the Soviet Union on April 26, 1986.

To understand the components of radionuclides released, their levels and distributions, some soil core samples up to a depth of 30 cm were taken at the zone within 20-30 km from the plant in the end of March. The samples were cutted every 5 cm in depth. Another one soil sample (0-5 cm in depth) was also collected at the ca. 1.7 km point (outside the premises) west from of the palnt in April. These samples were subjected to the measurements of γ-ray emmitting radionuclides and Pu isotopes.

As a result, volataile radionuclides such as 131I,134,137Cs and so on were maily detected with various levels, showing serious contamination in the NW from the plant. The distribution refrected the role of wind pattern and rainfall in washing out radionuclides to the ground. A small amounts of Pu isotopes (239,240Pu and 238Pu) were detected for all of the surface soil (0-5 cm) from zone within 20-30km, but their activity ratios of 238Pu/239,240Pu were around 0.03, indicating the origin of global fallout Pu. Only one sample collected from 1.7 km close the plant showed a little higher ratio of ca 0.06. Thus, currently, Pu release was negligibly small and volatile nuclides 131I,134,137Cs, etc. were responsible for a large share of released radioactivity. There are some spots of higher contamination called “Hot Spot”.

Seiya Nagao Seiya Nagao, Kanazawa University
Measurement of Radioactivity of Aerosol at a few Sites in Japan after the Fukushima Daiichi Accident

12:00 on Tuesday 16th in Congress Hall

Nuclear accident at the Fukushima Daiichi Nuclear Power Plant (NPP) was occurred after the 2011 Tohoku Earthquake and Tsunami. About 370-630 PBq of radionuclides was released from the Fukushima Daiichi NPP, according to the estimation by Nuclear Safety Commission (NSC) of Japan and Nuclear and Industrial Safety Agency (NISA). To understand migration behaviour of radionuclides in atmosphere, we have to monitor radioactivities of 131I, 134Cs and 137Cs of aerosol samples in Japan temporally and spatially.

We have been collected aerosol samples using a high volume air sampler at Wajima and Nomi in Ishikawa Prefecture, Japan before the Fukushima Daiichi NPP accident. To realize transport processes of radionuclides from the Fukushima Daiichi, the filter samples collected at above sites, about 400 km far from the Fukushima Daiichi NPP, were measured for radioactivities of 131I, 134Cs and 137Cs using ultra-low background Ge detection system at the Ogoya Underground Laboratory of Kanazawa University.

The presentation will be reported for the radioactivities of 131I, 134Cs and 137Cs at the Ishikawa sites from March to May in 2011, before and after the Fukushima accident. We also show temporal variations in radioactivities of these radionuclides at a few sites in Japan, reported by Japanese scientists as preliminary data, and discuss the migration of these radionuclides in atmosphere on the basis of the spatial variations.

Anne Mathieu Anne Mathieu, IRSN, France
Atmospheric Dispersion of the Fukushima Effluents

14:00 on Tuesday 16th in Congress Hall

In case of an accidental situation involving radioactive material, government agencies have to provide, in support of the public authorities, a scientific estimation of the emission and of its consequences for human health and environment. The technical crisis centre of IRSN has a special focus on atmospheric dispersion since it’s the first contribution to radiation exposure. IRSN operates a complete modelling platform which links a release of radioactive material to its consequences. The evolution of atmospheric and ground activity is computed by using an estimation of the source term and the state of the atmosphere. Atmospheric dispersion is modelled using a Gaussian puff model at small scale, and a Eulerian long-range transport model at large scale.

Following the earthquake and tsunami that hit Japan on March 11, 2011, IRSN has monitored in real time, the state of the Dai-ichi nuclear power plant as well as simulated the atmospheric dispersion of estimated releases. The talk will present the atmospheric dispersion of the plume simulated through the Japanese territory as well as the American and European continents. The relevance of the simulations will be assessed by comparisons with measurements. Uncertainties about the source term and the meteorological conditions will be discussed. Finally, the use of atmospheric dispersion model, inverse modelling and data assimilation techniques to narrow uncertainties on the source term will be illustrated.

Mitsuo UEMATSU Mitsuo UEMATSU, University of Tokyo
Distribution of Radioactive Materials in Seawater of the North Pacific Ocean: Past and Present

14:30 on Tuesday 16th in Congress Hall

Prior to the Fukushima nuclear accident in March 2011, the main sources of anthropogenic radioactivity to the earth’s ocean and land were fallout from atmospheric nuclear weapon testing and releases from nuclear fuel reprocessing plants and from the Chernobyl accident in 1986.

The artificial radionuclides, 90Sr, 137Cs, 239,240Pu, and other transuranics were used for monitoring and study by many institutes and organizations in many countries. Before the Fukushima accident, 137Cs in the surface ocean had decreased by two orders of magnitude from its highest levels in the 1950s while the monthly deposition of 137Cs and 90Sr had decreased by five order of magnitude from its highest levels in the 1960s.

The Fukushima accident created 137Cs levels in the surface ocean off the Fukushima Nuclear Power Plant (NPP) site that were several orders of magnitutde higher than those prior to the accident. At the end of March 2011, 137Cs levels in the surface water around the water discharge port rose to 107 Bq m-3 and decreased to approximately 105 Bq m-3 by the end of April 2011. It was, however, observed radioactivity levels between 104 and 105 Bq m-3 during May 2011 that suggested the release from the Fukushima NPP had not yet stopped. In addition, 137Cs levels in the surface water 30 km offshore of the NPP site ranged from 100 Bq m-3 to 10,000 Bq m-3 in March and April 2011. Atmospheric deposition signals to the sea surface was also found from 22 March to 30 March at the same 30 km offshore sites.

It is important to note that we still do not have reliable estimates of either the total amount of radionuclides, including 137Cs, 134Cs and plutonium, released directly into the ocean, or the total amount of radionuclides deposited onto both the ocean and land surfaces through the atmosphere. The atmospheric deposition to the ocean surface might be larger than that deposited on land since a large portion of released radionuclides were transported over the ocean by the prevailing westerly wind. To fully evaluate the impact of the release, it is essential to develop comprehensive figures for radionuclide release that are based on both comprehensive field studies and model studies for land and the ocean.

Bernd Grambow Bernd Grambow, IN2P3, France
Geochemistry of the Long Term Evolution of the Used Nuclear Fuel/water Interaction

15:00 on Tuesday 16th in Congress Hall

Options for stabilizing the failed reactors in Fukushima are to create a sarcophage or to remove the fuel debris for safe geological disposal. In either case interaction of the molten and re-solidified damaged fuel (corium =UO2-ZrO2-Zr-Fe2O3..) will occur with natural waters leading to certain radionuclide release. Very little is known on the interaction of this material under natural geochemical conditions. In the absence of more pertinent data, and since in corium the UO2/ZrO2 seems to be a tetragonal solid solution, one may use the large knowledge from used fuel/water interaction to obtain some insight in potential radionuclide release controls.

A very large experimental and theoretical database has been collected since 30 years on used nuclear fuel/water interaction in the context of deep geological disposal options in clay rock, granite and salt. Current understanding and model uncertainties were recently assessed in the just finished European MICADO project. Detailed parametric studies of dissolution and radionuclide release behavior have allowed identifying key dissolution mechanism. The comparison of spent fuel behavior to that of natural uraninite as well as to pure UO2 and Pu doped UO2 gave important insight into radiation effects on fuel behavior over geological time. Two radionuclide release terms can be distinguished: a rapid release of a few percent of the inventories of volatile fission products such as 135Cs, 137Cs,131I and 129I from fracture surfaces and grain boundaries in the fuel and a much slower release governed by the dissolution of the fuel matrix. The latter process concerns the release characteristics of matrix bound radionuclides, mainly the actinides, Sr and the rare earth elements. Fuel surface oxidation states are of fundamental importance for fuel matrix dissolution. The gamma and alpha radiation fields may create local oxidizing conditions at the fuel surface, accelerating fuel dissolution. The radiolytical enhancement of fuel dissolution is counteracted in geological disposal by the presence of hydrogen. Estimated times needed under natural water conditions for complete fuel matrix dissolution are in the range of 10000 yr under oxic conditions and in the range of millions of years under reducing conditions.

The fraction of matrix bound radionuclides will be lower and the labile fraction will be higher in the fuel debris of Fukushima than in ordinary spent fuel since high temperatures lead to stronger volatilization. Even though reducing conditions might have prevailed in the initial stages of fuel melt-down in the accident (large oxygen potential buffer capacity of melted metal Zirconium and presence of hydrogen) the beneficial long-term effect of hydrogen is unlikely to occur in Fukushima debris since hydrogen is expected to escape and oxidizing conditions will prevail.