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VERIFICATION SCIENCE The noble releases from Fukushima Some implications and reflections

by Anders AND Anders Axelsson Ringbom

The Fukushima Daiichi nuclear power plant after it was struck by the tsunami on 11 March 2011. Photo courtesy of Air Photo Service Co. Ltd., Japan.

The tsunami caused by the large contained in the nuclear reactor releases and their aftermath can underwater earthquake close to the was released. After a few weeks the prompt several interesting points of north-east coast of Japan on 11 entire northern hemisphere contained discussion regarding the design of March 2011 resulted in large releases radioxenon concentrations about 1,000 the Comprehensive Nuclear-Test-Ban of radionuclides from the Fukushima times above the normal background Treaty (CTBT) monitoring network, Daiichi nuclear power plant in the level2. It should be pointed out that the incorporated technologies, the days that followed. Particle-borne atmospheric radioxenon concentrations use of the data and future lines of radionuclides like Cs-137 and I-131, as at that level are still insignificant from a development for technology and data well as like Xe-133 human health perspective. use. In this article we will address some (see fact box on page 30) were released of these issues, focusing in particular on into the Earth’s atmosphere. Based on The estimated total release of the noble gas component of the IMS. measurements from the Preparatory Xe-133 exceeded 1019 Bq3. This is Commission for the Comprehensive one of the largest radioxenon releases How noble differ Nuclear-Test-Ban Treaty Organization’s in history, even exceeding the release from other radionuclides International Monitoring System (IMS) in from Chernobyl in 1986. As a source combination with atmospheric transport of noble gases, the Fukushima accident The key property that makes noble modelling (ATM)1, several research corresponds to an atmospheric nuclear gases different from other radionuclide groups have estimated that all of the explosion of 1 megaton. The design releases is the fact that they do not criterion for the IMS is 1 kiloton , react chemically with surrounding ______or 1,000 times less. Nevertheless, material. This means, for instance, [1]Atmospheric Transport Modelling (ATM) consideration of the Fukushima that they are difficult to contain in an is the calculation of the travel and underground nuclear test and will not dispersion of radionuclides released into the atmosphere, using meteorological ______be washed down by precipitation. The data. This calculation can be performed [2]On a global scale, the background is dominated latter property was also well illustrated in two ways: by releases from medical production by the Fukushima accident, resulting •As backtracking ATM, which identifies facilities, locally, the background can be the area from which radionuclides may strongly influenced by releases from nuclear in a very even distribution of Xe-133 have been released, calculated from the reactors and from hospitals. observed by the IMS network a few location where they were observed. [3]A Becquerel (Bq) is the amount of radioactive weeks after the accident. All stations • As forward ATM, which identifies material in which 1 decays every second. where radionuclides may travel from 1 milli-Becquerel (mBq) = 10**-3 Bq in the northern hemisphere measured their known point of release. 1 micro-Becquerel (uBq) = 10**-6 Bq very similar concentrations. This can

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CTBTO SPECTRUM 20 | July 2013 is now complete and equipped with these very sensitive measurement systems. A common way to represent the measurement capability of this network is to estimate the average number of stations that would detect a nominal explosion-size release of radionuclides from each point on the planet. This notion of coverage indeed captures an essential feature of the network. However, in judging how well the radionuclide network fulfills its part of the CTBT verification mission, one also needs to consider under which circumstances detections are most likely to be useful. These include possibilities for determining the source location and time of an event and the ability to distinguish it from detections that are not relevant to the CTBT, such as noble gas emissions by the radiopharmaceutical industry.

From this point of view, the releases from Fukushima constituted an enormous, if temporary, increase in background. To what extent was the effectiveness of the noble gas network hampered? The question is interesting both on a measurement level and on a data evaluation level. In fact, the noble gas detection systems responded very well to the impact of the Fukushima radiation releases. Apart from the very first samples measured at the IMS station in Takasaki, Japan, Figure 1: (the station located closest to the Approximate distribution of the atmospheric activity concentration of the noble gas isotope 133Xe and the particle-borne radionuclide 137Cs on 30 March 2011, almost three weeks after the massive earthquake in Japan. The maps were obtained by Fukushima Daiichi power plant which interpolating data collected by the IMS radionuclide network. Note the homogenous distribution of xenon in the upper panel. The was hit by the largest concentrations maps illustrate only the main features of the activity concentration, and should not be used to draw any quantitative conclusions for any specific geographic locations. In particular, the xenon concentration did not reach as far south as illustrated by the upper map. of radionuclides), it was possible to use the data generated by the station to obtain accurate concentration values – even at such high levels – after appropriate corrections had been made. be compared to the distribution of a from Fukushima. The development particle-borne nuclide like Cs-137, of the measurement systems that On a measurement level, the strong which had a more inhomogeneous spatial made this possible started some 15 global signature of the Fukushima activity distribution (see Figure 1). years ago within the framework of releases reminds us of the need to the so-called International Noble overcome the so-called “memory effect” Key role of noble gas Gas Experiment (INGE) which was by which the measurement of a sample detection systems set up in 1999 to test the measuring in systems using plastic of radioactive noble gases in the detectors such as the SAUNA and ARIX Had the event occurred ten years atmosphere (see page 31). Of the 80 systems (see page 31) containing a earlier, before the IMS was equipped IMS radionuclide stations foreseen in high amount of radioxenon activity will with noble gas detection systems, it the Treaty, 40 are to have additional degrade the sensitivity of subsequent would have been impossible to obtain a noble gas detection capabilities. More measurements for some time. Recent global picture of the noble gas releases than 75% of the noble gas network advances in the surface treatment

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CTBTO SPECTRUM 20 | JUly 2013 of detector cells (i.e. reducing the amount of radioactivity retained by the detector from previous measurements) promise to radically shorten or totally eliminate this recovery time following the measurement of a sample with a high concentration.

Knowledge of radioxenon background levels is essential

On a network response level, the experience highlights the need to understand and discriminate against background from non-explosion radioxenon sources such as nuclear power plants and radio- pharmaceutical production facilities. The latter type of background source constitutes the greater problem, both in terms of released amounts (obviously, Fukushima is an exception) and, more seriously, released signatures: the short irradiations used in medical isotope production can produce radioxenon with isotopic ratios similar to those produced in a nuclear explosion. The lies partly in improved source location to screen out known background sources, and partly in mapping and understanding Map of Japan showing the Fukushima Daiichi Nuclear Power Plant. specific background sources as well as generic background in various identify nuclear explosions as part one nuclide to another. Thus, regions of the world. The use of of a global network of monitoring simultaneously detecting and mobile or transportable radioxenon stations incorporating a number of quantifying two or more radioxenon detection equipment in various technologies. The optimal use of isotopes will be considerably more campaigns around the world for this data from all relevant technologies valuable than the simple detection purpose has been highly successful embodies the concept of data fusion, of only one. There are examples and is expected to continue to yield which is increasingly discussed today. of detections of other radioxenon valuable knowledge to improve It is evident that while seismic signals sources only a few weeks after the background discrimination. would be the preferred primary Fukushima accident, which could be source of location information identified despite high background The next generation of pertaining to an event, the usefulness concentrations. However, sensitivity detection systems of radionuclide detections in is not the only parameter of interest. demonstrating the nuclear character Given an ambient concentration of The experience that is being of that event depends on how radionuclides around a station, the accumulated from operating well the source region of detected more air that is sampled, the better a global network of sampling radionuclides can be defined. the detection sensitivity, but sampling stations can now be used to design more air takes more time. However, a next generation of detection Obviously, the sensitivity the time resolution of a measurement systems, which will also be more for the detection of nuclides of directly impacts the source location specialized for the intended interest is important. Source effectiveness of ATM. The 12- or mission and incorporate a more timing and source discrimination 24-hour sampling periods of the holistic view of that mission. are typically obtained from ratios systems deployed in the network may The overall task is to detect and of concentrations measured in not in fact represent the optimum

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CTBTO SPECTRUM 20 | July 2013 balance between detection sensitivity radioxenon measurement systems and time resolution from the point of deployed by the IMS, even though view of source location. the systems themselves exceed the initial sensitivity specifications. The How to augment operation of national monitoring network stations contributing data to the CTBTO’s International Data Centre The most favourable case for using (IDC) in Vienna and National Data ATM to estimate a well-defined Centres should be encouraged as a source region is when several way to augment network density. network stations at different locations detect radionuclides from In the context of the Fukushima the same source event. However, releases, it is important to remember there is no guarantee that this that the IMS radionuclide network will be possible if the releases are is not designed as a radiological from a nuclear explosion with a emergency system. However, unlike yield in the kiloton range. This national networks, the IMS network is due to the of is a globally integrated radionuclide

ARIX noble gas system Biographical notes

FACT BOX

Caesium-137 (C s -137): Cs-137 has a half-life of 30.1 years. This is the most common radioactive form of and is produced by nuclear fission. Cs-137 is one of the major radionuclides in spent nuclear fuel and radioactive wastes associated with the operation of nuclear reactors and fuel reprocessing plants. Large amounts of Cs-137 and other radioactive isotopes were released into the environment by atmospheric nuclear weapon tests between 1945 and 1980. Cs-137 did not occur in before nuclear weapon testing began.

IODINE-131 (I-131): I-131 has a half-life of 8.0 days. I-131 is a radioactive isotope released into the environment mostly in gaseous form as a result of the atmospheric testing of nuclear weapons and accidents that have occurred at nuclear power plants (e.g. the Anders Axelsson Anders Ringbom Chernobyl nuclear power plant in 1986 is a Senior Scientist at the Swedish is the Deputy Research Director at the and the Fukushima power plant in March Defence Research Agency, Swedish Defence Research Agency, 2011). It was a significant contributor to Division of Defence & Security Division of Defence & Security Systems the effects on human health from Systems and Technology. He is and Technology. He is a technical advisor atmospheric nuclear weapon testing and from the . manager of the Swedish National Data to the Swedish Government and Swedish Centre for Comprehensive Nuclear- representative at the CTBTO’s Working Test-Ban Treaty monitoring. Dr on verification issues. Dr Ringbom XENON is a in gaseous form. It Axelsson has previously worked at the was the main developer of the noble gas is one of the noble gases which is inert Preparatory Commission for the system SAUNA and has developed and rarely reacts with other chemicals. Comprehensive Nuclear-Test-Ban equipment and analysis techniques for Several of its radioactive isotopes, of Treaty Organization with on-site radioxenon detection used by the which one of the isotopes is xenon-133 inspection noble gas equipment and at CTBTO. In 2006 he conducted a (Xe-133), are short-lived and typical of the International Atomic Energy measurement in cooperation with South technological processes and are therefore Agency with environmental sampling Korea that detected radioxenon from the measured to detect clandestine data for international safeguards. first nuclear test in North Korea. underground nuclear explosions.

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CTBTO SPECTRUM 20 | JUly 2013 monitoring system, the only one of its kind. The global nature of the IMS The different ‘noble gas’ systems used t o detect radionuclides in the a tmosphere network enabled the global tracking of radionuclides released in the Fukushima incident, and made the Radioactive xenon (radioxenon) isotopes data available to the governments of are produced in abundance by a nuclear all CTBTO Member States. The IMS test. Four of them are used in CTBT network clearly has some special monitoring: Xe-131m, Xe-133, Xe-133m, and qualities which make the data Xe-135. They have a half-life of between valuable. The extent to which the 9.10 hours and 11.9 days (the half-life of data can be made public is governed any given nuclide is the time required for by both practical and political one half of the sample to decay). Although constraints. On the practical side, Xe-131m has the longest half-life of these SPALAX noble gas system only data which have been properly four isotopes, its yield is the smallest. With evaluated in the context of a possible a half-life of 5.2 days, xenon-133 (Xe-133) is radiological emergency should be in most circumstances the most abundant SAUNA was developed by the Swedish released to the public. On the political of the isotopes. With suitable equipment, Defence Research Agency and made side, the operation of the IMS is a it is therefore possible to detect radioxenon commercially available in 2004. SAUNA collaborative endeavour among all isotopes days or even weeks after their samples up to 15m3 of air during a 12 Member States. It is in some ways an release and at great distances from their hour sampling interval, thus producing intrusive verification measure, with source. two samples per day. The system uses a very sensitive monitoring stations measurement technique called ‘beta-gamma located on the territory of a large The CTBTO uses three noble gas coincidence’. number of States. In order to preserve systems to detect radioxenon. Forty of long-term support for the CTBT the CTBTO’s network of 80 radionuclide ARIX was developed and commercialized verification regime and the continued stations are being equipped with one of by the Khlopin Institute in the availability of data, decisions on the these special systems designed to detect Russian Federation. ARIX samples up to 15m3 handling of the monitoring data need noble gases: of ambient air and extracts xenon from it to be made with due regard for the in 12-hour cycles This system produces two different perceptions that Member • The Swedish Automatic Unit for samples per day and also uses the beta-gamma States may have of their national Noble Gas Acquisition (SAUNA) coincidence technique. interests in this respect. • Le Système de Prélèvements et d’Analyse en Ligne d’Air pour SPALAX was developed by the French A wealth of data helping quantifier le Xénon (SPALAX) Commissariat à l'énergie atomique. This to increase knowledge • The Analyzer of Xenon equipment samples up to 75m3 of air Radioisotopes (ARIX). (See below continuously in 24-hour cycles. At the The build-up of the noble gas system for more information). end of each collection cycle and after final component of the IMS has been very purification, the xenon gas is transferred into successful. New technology that many How radioxenon is isolated a High Purity detector counting doubted would work 15 years ago system. has been developed and implemented The systems work by continuously and in a short time. The new systems automatically separating xenon from The role of the analyst provide us with a vast amount of data ambient air using a purification device that results in new knowledge every that contains charcoal. Contaminants Analysts need to know which radionuclides day. The tragic Fukushima accident such as dust as well as air constituents occur naturally and which are man-made. It reminded us how important the global like , humidity, CO2 and is imperative to know which radionuclides are detection network can be beyond its are all removed during this process. generated by a nuclear explosion since xenon primary mission of verification. We Radioactive levels in the isolated xenon radionuclides also enter the atmosphere from now have to start gathering ideas on are then measured in a radiation counting other man-made sources. Analysts must also how to use this new knowledge in device. The resulting spectrum is sent be familiar with the quantities and ratios in a broader context in order to fulfill to the International Data Centre (IDC) which these nuclides are produced during a the CTBT verification mission to the in Vienna on a daily basis for analysis. nuclear explosion. Based on the analysis by maximum extent possible, and to The noble gas systems that are currently National Data Centres and the IDC, Member further investigate the possibilities deployed were developed in the late States can then determine whether or not the that exist for using the system for 1990s and have been tested under the sample suggests that a nuclear explosion has other purposes as well. International Noble Gas Experiment. indeed taken place.

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CTBTO SPECTRUM 20 | July 2013