The Gesellschaft für Anlagen- und Reaktorsicherheit gGmbH (GRS) has published a brochure on the situation in Fukushima providing an overview of what took place during the accident at the Fukushima Daiichi nuclear power plant and the emergency control measures that followed. GRS has also published an overview of the current status of the work on the plant site nine years after the nuclear accident on its website.
The corium formed during the meltdown consists of a mixture of different materials, in particular nuclear fuel, molten fuel assembly structures and possibly also molten concrete from the bottom of the containment vessel.
The nuclear fuel and the corium in Units 1 to 3 are cooled by fed-in water. According to current knowledge the nuclear fuel and the corium do not present any immediate risk of triggering another chain reaction.
At the time of the accident, all nuclear fuel assemblies from the reactor pressure vessel (RPV) in Unit 4 were in the spent fuel pool, thus preventing a core meltdown in that RPV.
During the first week, mobile pumps were used to feed cooling water into Units 1 to 3. Initially, sea water was also used; however, the cooling of all three units was switched to the feed-in of fresh water via electric pumps later on. Since the end of June 2011, the newly built water treatment facility (Advanced Liquid Processing System - ALPS) has been providing the water for cooling Units 1 to 3, and an ‘open’ cooling cycle has been in use.
According to current knowledge, the nuclear fuel and the corium in Units 1 to 3 do not present any immediate risk of triggering another nuclear chain reaction. Factors including constant temperatures and the composition of the radioactive material released within the reactor vessel support this assessment. The xenon-135 concentration (a gaseous fission product with a short half-life period) in all three units is below the criticality limit value of 1 bq/cm³ stated by the operator Tokyo Electric Power Holdings, Inc. (TEPCO). If necessary, a boric acid mixture can be pumped into each unit to prevent nuclear fission reactions. Measurements for the reactors in Units 1 to 3, including the feed-in volumes of the individual vessels, temperatures, pressures and noble gas concentrations, are published on the TEPCO website on a daily basis.
Radiolysis and corrosion processes still give rise to the production of hydrogen. In order to prevent the formation of an explosive hydrogen-oxygen mixture, Tokyo Electric Power Holdings, Inc. (TEPCO) is constantly feeding nitrogen into the RPVs of Units 1 to 3 which, in combination with the ventilation system, creates a slight overpressure in the containment. While this prevents a possible hydrogen explosion, the elevated internal pressure in the RPVs increases the risk of releases. In Unit 2, TEPCO tested reducing the overpressure in the containment by half by adjusting the feed-in of nitrogen in autumn 2018. According to TEPCO, this test did not lead to any significant changes in the system values and therefore the reduced feed-in has been maintained from December 2018 onwards.
Unit 1: Unit 1 was examined in 2015. The reactor core was almost completely displaced out of the RPV and there is molten material at the bottom of the containment. By comparing the examination results with existing construction documentation, it could be determined that the RPV and the containment are still in their original position and that there are no nuclear assembly components larger than 1 m in the RPV. The examination results confirm the results of the accident analyses. During the accident, a major part of the nuclear fuel was displaced from the RPV to the lower part of the containment and began reacting with the concrete structures, resulting in a solidified mixture of concrete, metal structural material and nuclear fuel. In March 2017, results showed that, among other things, there was also molten material outside the control rod drive room. A material sample was taken from the bottom of the containment. No major damage was detected at the outer wall of the control rod drive room and other structures.
To facilitate further inspections of the inner part of the containment, on 8 April 2019 work started to gain access via an existing personnel airlock. The personnel airlock was drilled open for this. Measures to prevent the outflow of radioactive dusts from the containment included reducing the overpressure in the containment.
Unit 2: According to the accident analysis for Unit 2, only small amounts of corium were formed at the bottom of the containment during the accident. The major part of the molten nuclear fuel and the molten structural material are stated to have remained in the RPV and re-solidified. There are approximately 160 tonnes of material at the bottom of the RPV, 20 to 50 tonnes in their original core position and 70 to 100 tonnes of structural material in the area above it. There may also be smaller amounts in the outer area of the RPV. The results of the examination confirm the results of the accident analyses.
No major damage was detected at the inner wall of the control rod drive room, which is located below the RPV, nor any molten cable runs or metal support struts of the handling system for control rod drives. There are also deposits in higher areas which presumably have dripped down various structures. No outflow of molten material from the control rod drive room via its access was detected. On 13 February 2019, a robot was deployed in the containment of Unit 2 to facilitate further planning of the dismantling work. The robot was equipped with two gripping claws, a swivelling camera, a thermometer, a dose meter and LED lights; it was said to be capable of lifting up to 2 kilograms. The robot was introduced into the containment with the help of a rod and then lowered into the lower part of the control rod drive room. It was sent to six different positions to test the plasticity of the deposits. Tokyo Electric Power Holdings, Inc. (TEPCO) reported that the deposits were malleable and could be grasped with the gripping claws at five of the positions. This meant that the malleable material could be removed with a similar mechanism. TEPCO published a video of the examination of Unit 2 in February 2019 at the link TEPCO - Examination of Unit 2 in February 2019 (in Japanese). Another sample for additional laboratory tests was to be taken with the help of another robot in the second half of 2019.
Block 3: According to official Japanese statements relating to Unit 3, part of the molten core of Unit 3 is still in the RPV and corium has moved to the bottom of the containment.
In September 2017, a scan showed that a major part of the nuclear fuel was outside the RPV. TEPCO assumes, on the basis of the scan images, that there were no large accumulations of contaminated and very dense material in the RPV as is the case in Unit 2.
The deployment of a robot in the containment in July 2017 provided images of the control rod drives. The underwater images clearly showed parts of the control rod drives. In addition, the scans showed numerous damaged and/or molten structural building elements in the control rod drive room. Pieces of a control rod guide tube from the inside of the reactor with a diameter of several centimetres could be seen on the scans, which indicated a correspondingly sized opening in the RPV. In the control rod drive room corium was detected close to the access to the room, however, no inspection of the access itself was possible. Therefore, the possibility could not be excluded that part of the corium flowed out of the control rod drive room via the access. No major damage at the inner wall of the control rod drive room was detected.
Examinations and considerations are still ongoing with regard to the exact procedures for a potential recovery of the nuclear fuel, which will also depend on the results of inspections of Units 1 to 3. Several methods are being examined which assume different scenarios - from complete flooding of the containment to partial flooding to no flooding at all. At present, partial flooding of the containment with water during the recovery process is favoured, together with a lateral access at the bottom of the containment in order to recover the corium there. A recovery from above with a flooded containment is assessed to be difficult, because this would require the leakages of the containment to be sealed beforehand. It is assumed that, in addition to the leakages detected, there are also small cracks which as yet have not been detected. This assumption is based on the cracks found in Units 5 and 6 after sealing work was carried out. The general timeline for the dismantling work spans 30 to 40 years.
The molten core debris of Unit 2 is to be recovered first, as Tokyo Electric Power Holdings, Inc. (TEPCO), according to its own statements, knows most about the situation in Unit 2. The decision was based among other things on the results of the exploration rounds of various robots inside the containment (for example camera recordings and debris samples), which have been more extensive than in the other units so far, and the working environment as a whole (access to the containment is possible from outside, the reactor building was not damaged by a hydrogen explosion, working inside is possible for a limited time). In a next step, TEPCO plans to test using a robot arm to remove molten core debris via a lateral containment penetration. Should the method prove to be successful, the recovery of the core melt fragments could begin in 2021.
At present, additional contaminated water is produced on the plant site each day mainly due to groundwater inflow into the reactor buildings and precipitation. This water passes through ALPS (Advanced Liquid Processing Systems) which filter radionuclides from the water. However, tritium cannot be filtered out. The water is stored in tanks on the plant premises.
As of 23 January 2020, approximately 1.2 million cubic metres of water were stored on the plant premises in around 1,000 tanks. Tokyo Electric Power Holdings, Inc. (TEPCO) expects all tanks to be full in summer 2022, thus reaching maximum storage capacity. Space has been reserved on the plant premises to store corium and fuel elements in interim storage facilities. This space could be used for the storage of water, however, this would hamper the decommissioning activities. The relocation of water from the leakage prone flange-type tanks to welded tanks started in March 2014 and was concluded on 27 March 2019.
In autumn 2018, it became public knowledge that some of the water declared by TEPCO to be "tritium water" and stored in the tanks, still contained significant quantities of radioactive substances such as iodine, caesium and strontium. This water had already been filtered by ALPS (Advanced Liquid Processing Systems) which treats the contaminated water so that it meets the requirements for discharge into the environment.
In early February 2020, the Japanese government informed foreign diplomats about the options being considered for the disposal of the tritium water stored in the tanks. According to this information, a group of experts appointed by the Ministry of Economics METI had rejected the options of "geosphere injection", "hydrogen release" and "solidification with underground burial" on the basis of its scientific assessment as they were too many unresolved questions which impaired their practical implementation due to regulations, technology and time. Therefore the priority is to further investigate into the two options "controlled discharge into the sea" and "vapour release", as they are assessed to be technically feasible. The option "controlled discharge into the sea" is said to offer several advantages. Water containing tritium from nuclear power plants or nuclear fuel processing plants is already being discharged into the sea all over the world providing extensive operational experience with this option for a regular discharge of comparable waste water. Before discharge, the water from the tanks is to pass through ALPS again to ensure that it complies with the legal limit values. Following the treatment, an independent verification of the radionuclide concentrations is planned. Furthermore, there will be dilution with sea water before the actual discharge into the sea. Due to relatively stable sea currents, predicting the dispersion of the radionuclides in the sea is less complex than for the option "vapour release" with discharges into the air, which is influenced by meteorological conditions. This also holds true for the technical monitoring of the radionuclide dispersion in the sea. In addition, advantages relating to society’s acceptance both nationally and internationally are expected (for example due to no direct inhalation taking place, no exposure from precipitation, fewer impacts on flora and living organisms).
There have been no major leakages from the tanks and no contamination spread due to rainwater since 2015.
The seaside impermeable wall, a watertight barrier, directly in front of the cooling-water inlet structures and the cooling-water outlets of Units 1 to 4 was completed in October 2015. It consists of 594 steel pipes (diameter around 1.1 meters (m), length: 30 m), which were rammed into the sea floor about 20 m deep. The wall is approximately 780 m long, almost 520 m of which are in front of the cooling-water inlet area and around 265 m of which are in the open sea in front of the cooling-water outlet. According to Tokyo Electric Power Holdings, Inc. (TEPCO) publications, the impermeable wall allowed the reduction of the radioactivity concentration within the port area.
Various measures were taken to reduce the volume of contaminated water, including:
TEPCO measurements show that the inflow of ground water into the buildings of Units 1 to 4 was reduced by starting the operation of the sub-drain and groundwater drain pumps. This together with the full start of operation of the frozen soil wall in March 2018 allowed reduction of the inflow of ground water and rainwater. In fiscal year 2015 (FY - 1 April to 31 March) the average inflow was 270 m³/day; in FY 2016 it was 200 m³/day; in FY 2017, 140 m³/day; in FY 2018, 100m³/day; and in FY 2019, 140 m³/day. However, these volumes depend on precipitation. Plans have been made to reduce the inflow of ground and rainwater even further. The targeted figure is less than 150 m³/day for 2020 and less than 100 m³/day for 2025.
The dismantling of the units generates various kinds of radioactive waste which Tokyo Electric Power Holdings, Inc. (TEPCO) stores on the plant premises. Parts of it are also being treated on site. Handling of the waste and its storage must be managed to ensure that the additional local dose rate at the outer fence of the premises does not exceed the officially stipulated value of 1 mSv/year.
Different storage plans are used depending on the radiation impact of the specific kinds of waste. Solid waste such as concrete and metal parts from the clearing operations in and around the buildings with dose rates of 1- 30 mSv/hour are stored in tents or containers. However, radioactive residues filtered out of contaminated water by ALPS are being stored in HIC (high integrity containers), which are temporarily stored in shielded concrete containers on the plant premises in a special area. Undamaged fuel assemblies from Unit 4 are being stored in the "joint storage pool" for decay storage (for around three to five years). Once the fuel assemblies from Units 1 to 3 are recovered from their storage pools, major parts of them are also to be stored there. Following decay storage, the fuel assemblies will be transferred to dry storage, where fuel assemblies that were unloaded before the accident are already being stored.
In order to protect the buildings against future tsunamis, Tokyo Electric Power Holdings, Inc. (TEPCO) started building an L-shaped concrete wall on top of the outer quay wall in September 2019, increasing its height by 11 metres. The wall will be 600 m long and will be completed by the end of the 3rd quarter of 2020. Initially, a provisional barrier 2.4 to 4.2 m in height was erected. In addition, specific measures were taken to protect the individual reactor buildings against the ingress of water.
In order to prevent the Mega-Float, an engineless barge that stores water, from breaking away from the quay wall and causing damage to the premises, it was to be transferred to the cooling-water inlet pool for Units 1 to 4 and moored there. In May 2019, the Mega-Float was moved there for initial cleaning of the ballast water and decontamination. At the open end of the cooling-water inlet pool towards the sea, a grounding mound will be created so that the Mega-Float can be grounded in front of the groundwater barrier structure and anchored there. According to current planning, the work should be completed by the 3rd quarter of 2020.
Tokyo Electric Power Holdings, Inc. (TEPCO) plans the dismantling of the unsafe upper half of the joint vent stack of Units 1 and 2 (height: 120 m). In July 2011, a local dose rate measurement at one of the ducts into the vent stack was approximately 10 Sv/hr one metre above ground. This duct was used for pressure relief of the containment of Unit 1 during the accident. It is assumed that this has led to the accumulation of a larger quantity of radioactive material in the duct. In order to review the planned schedule for the demolition work, in April 2019 dose rate measurements on the inside and outside of the vent stack und visual inspections with the help of a camera were carried out. The start of the dismantling work was postponed several times and finally commenced in August 2019. Work is to be completed in May 2020.
On 9 January 2019, a steel plate fell down from the joint vent stack of Units 3 and 4. The area around the vent stack was then cordoned off in order to prevent any injury to people. Similar measures were introduced at all four vent stacks on the plant premises.
In December 2013, the decision was made to decommission Units 5 and 6 as well. Before that, these units were being used as a model test site for researching and developing inspection and recovery techniques for Units 1 to 4. This also included training and testing new techniques. Tokyo Electric Power Holdings, Inc. (TEPCO) carried out these activities in cooperation with research institutes such as IRID (International Research Institute for Nuclear Decommissioning) and several manufacturers. The transfer of the fuel assemblies from the reactor core to the spent fuel storage pool was completed in 2015 in Unit 5 and in 2013 in Unit 6.
Information on examinations, inspections and actions taken in Germany following the nuclear disaster in Japan are available.
The radioactive substances (radionuclides) released into the atmosphere as a result of the Fukushima reactor accident were carried by the wind and deposited in the oceans or on the earth’s surface at local, regional and global levels. The distribution of the different radioactive substances depended largely on the moment of their release and the prevailing weather conditions, such as wind and precipitation. During the weeks following the accident, people were exposed to radiation both by inhalation and externally as a result of airborne radioactive substances; later this exposure was limited to radionuclides deposited on the ground and the intake of radionuclides through the food chain.
Following the Fukushima reactor accident, higher concentrations of airborne iodine-131 and caesium-134/137 were recorded for about a month in Germany. However, the concentrations measured were so low that they posed no health risk to the population and the environment in Germany and Europe. Resulting total population radiation exposure was just a few millionths of a millisievert (mSv), which is considered extremely low when compared with the average radiation exposure of an individual due to natural radionuclides present in the environment (2.1 mSv/year). Following their release, these primarily highly volatile radionuclides were dispersed by air throughout the entire Northern hemisphere. The concentration of radioactive substances in the air decreased with increasing distance from the site of the accident due to natural dilution. The concentration of radioactive substances was also reduced by part of the radionuclides being washed out by precipitation on the way, and by the natural process of nuclear decay, which particularly applies to the short-lived radionuclides iodine-131 and tellurium-132, which made up much of the initial dose at the beginning of the accident in Japan. Due to these dilution processes, in Europe only very low levels of airborne radioactivity reached Germany which could only be detected through elaborate trace analysis methods. On 23 March 2011, iodine-131 and caesium-134/137 were first detected in air samples taken at trace measuring centres in Brunswick, Potsdam and Offenbach; by the next day similar readings were recorded at the Schauinsland trace analysis laboratory near Freiburg. The highest concentrations of radioactivity were reached approximately one week later; at the four measuring centres, these amounted to a few thousandths of a becquerel per cubic metre air (Bq/m³) for iodine-131, and a few ten thousandths for caesium-137. After that, the radioactivity concentrations of the radionuclides decreased continuously. By the end of May 2011, all readings had returned to pre-Fukushima reactor disaster levels. By way of comparison, outdoor concentrations of the naturally occurring noble gas radon and its progenies which are bound to airborne dust amount to several becquerels per cubic metre of air (Bq/m3), and indoor concentrations to an average of some 50 Bq/m3. These values fluctuate depending on the location, since concentrations of radon are influenced by the geological properties of the parent rock and prevailing weather conditions.
Travel and safety information for Japan are available at the website of the Federal Foreign Office.
Owing to the great distance from Japan and the associated dilution of the radioactive releases on their way to Europe, only very small amounts actually reached Germany. It was possible to record minimal traces of iodine and caesium in the atmosphere, but only due to the extreme sensitivity of the measuring equipment. Once the radioactive cloud had passed over Germany, the Max Rubner Institute in Kiel (coordinating office for soil, vegetation, feed and foodstuffs of plant and animal origin) conducted detailed analyses of representative environmental media in addition to its routine environmental monitoring activities. Milk and winter leek samples were tested in order to investigate different food chains. Between 31 March 2011 and 11 April 2011, iodine-131 was found in some milk samples with concentrations between five and 12 mBq per litre. After 14 April 2011, milk readings dropped to below the detection threshold of 2 mBq per litre. No radioactive caesium was recorded in any of the milk samples. Between 31 March 2011 and 26 April 2011, iodine-131 concentrations between 110 and 550 mBq per kg were measured in winter leek samples. Caesium was only detected in two leek samples. At 40 and 60 mBq per kg respectively, these readings only just exceeded the detection threshold. The readings taken were so low that even with a high personal consumption of milk and vegetables, no health risk could be said to exist.
Foods imported from Japan are tested for radioactivity by the relevant food regulatory authorities of the federal states. The legal basis for this are special EU provisions governing the import of feed and food originating in Japan following the accident at the Fukushima nuclear power station as of the 25 March 2011 (based on Regulation (EC) No 178/2002 laying down the general principles and requirements of food laws). The first special provisions entered into force with Commission Implementing Regulation (EU) No 297/2011 of 25 March 2011. In April 2011, Commission Implementation Regulation (EU) No 351/2011 stipulated that the threshold values in place in Japan shall apply. As of then, these special provisions are reviewed on a regular basis by the EU Commission in cooperation with the member states; if necessary, they are adjusted according to the results of food inspections carried out in Japan.
At present, Commission Implementing Regulation (EU) No 2019/1787 of 24 October 2019 applies amending Implementing Regulation (EU) 2016/6 imposing special conditions governing the import of feed and food originating in or consigned from Japan. Pursuant to the regulation, specific feed and food from some Japanese regions, where increased radioactivity values are still being measured in individual cases, may only be imported to Germany following testing and certification in Japan. Before such feed and food may be imported to the EU, a certificate must be presented confirming that the products comply with the stipulated limit values in Japan and the EU. Should, contrary to expectations, contaminated foods be detected at the EU’s external borders, these will be rejected thus preventing them from reaching the European market. In Germany, the risk of coming into contact with foods contaminated with radioactive substances imported from Japan is extremely low. Controls prescribed for agricultural products imported from Japan have detected only very few cases where contamination was actually measurable.
Further information, in particular about the results of food inspections, are available at the website of the Federal Ministry of Food and Agriculture (BMEL).
It can be assumed that when travelling to Japan, the exposure to radiation during the long-distance flight from Europe to Japan will be significantly higher than that during a stay in Japan. When planning a longer stay in the Eastern areas of the Fukushima prefecture, it is recommended to ask local authorities beforehand about possible local restrictions.
No increased radiation levels are to be expected in Japan’s neighbouring countries (including Korea, China, Russia, the Philippines). Even in the regions of Russia directly to the north of Japan, where traces of airborne radioactivity were detected shortly after the accident, no increase in the dose rate could be determined recently. Food radioactivity monitoring conducted in Japan’s neighbouring countries showed that import samples exceeded the maximum values permitted in only a very few individual cases out of thousands of measurements.
In Germany, as in the other European countries, current preparedness plans for nuclear power plant accidents and protective measures for the general public are based on experience gained from the accident at Chernobyl. The course of events in Fukushima was characterised by releases over a longer period of time. In this respect, it differs from the general assumptions for measures for incidents, which are characterised by a longer lead time prior to release or a release taking place over a comparatively short period of time. These are emergency response measures (remaining indoors, potassium iodide as a thyroid blocker, temporary or long-term evacuation). The technical authorities and institutions involved in emergency response to accidents in nuclear power plants (Federal Office for Radiation Protection (BfS), Gesellschaft für Anlagen- und Reaktorsicherheit gGmbH (GRS), Commission on Radiological Protection (SSK)) analysed whether the existing preparedness planning has any weak points and what precisely these may be, using a complex procedure. The results of these analyses are part of the "Basic recommendations for emergency preparedness in the vicinity of nuclear installations" adopted by the SSK. The most important changes include the significant enlargement of the planning zones for emergency response measures (evacuation up to 20 km, staying indoors up to 100 km, potassium iodide as a thyroid blocker for adults under the age of 45 years up to 100 km, for children, young people and pregnant women nationwide). In addition, experience in Fukushima has shown that the "stay indoors" measure can only be maintained for a short period of time.
The Federal Office for Radiation Protection (BfS), an executive agency under the Federal Ministry for the Environment (BMU), operates an integrated monitoring and information system (IMIS) for the comprehensive monitoring of radioactivity in the environment in Germany. The nationwide system includes some 1,800 around-the-clock monitoring stations. This means a detailed picture of the radiological situation in Germany is available at all times. Specific details about any region are available at the website "ODL-INFO, Radioaktivität in Deutschland". In addition, there are 50 monitoring stations measuring airborne radioactivity concentrations. Moreover, more than 60 federal and federal state-run laboratories regularly conduct sample analyses of food and feed, plants, soil and waste. The BMU reports on these issues in its annual report on environmental radioactivity, "Umweltradioaktivität und Strahlenbelastung", which is published on the websites of the BMU and the BfS.