Potential Source Apportionment and Meteorological Conditions Involved in Airborne 131I Detections in January/February 2017 in Europe.

Traces of particulate radioactive iodine (131I) were detected in the European atmosphere in January/February 2017. Concentrations of this nuclear fission product were very low, ranging 0.1 to 10 µBq m-3 except at one location in western Russia where they reached up to several mBq m-3. Detections have been reported continuously over an 8-week period by about 30 monitoring stations. We examine possible emission source apportionments and rank them considering their expected contribution in terms of orders of magnitude from typical routine releases: radiopharmaceutical production units > sewage sludge incinerators > nuclear power plants > spontaneous fission of uranium in soil. Inverse modeling simulations indicate that the widespread detections of 131I resulted from the combination of multiple source releases. Among them, those from radiopharmaceutical production units remain the most likely. One of them is located in Western Russia and its estimated source term complies with authorized limits. Other existing sources related to 131I use (medical purposes or sewage sludge incineration) can explain detections on a rather local scale. As an enhancing factor, the prevailing wintertime meteorological situations marked by strong temperature inversions led to poor dispersion conditions that resulted in higher concentrations exceeding usual detection limits in use within the informal Ring of Five (Ro5) monitoring network.

Third international challenge to model the medium- to long-range transport of radioxenon to four Comprehensive Nuclear-Test-Ban Treaty monitoring stations.

In 2015 and 2016, atmospheric transport modeling challenges were conducted in the context of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) verification, however, with a more limited scope with respect to emission inventories, simulation period and number of relevant samples (i.e., those above the Minimum Detectable Concentration (MDC)) involved. Therefore, a more comprehensive atmospheric transport modeling challenge was organized in 2019. Stack release data of Xe-133 were provided by the Institut National des Radioéléments/IRE (Belgium) and the Canadian Nuclear Laboratories/CNL (Canada) and accounted for in the simulations over a three (mandatory) or six (optional) months period. Best estimate emissions of additional facilities (radiopharmaceutical production and nuclear research facilities, commercial reactors or relevant research reactors) of the Northern Hemisphere were included as well. Model results were compared with observed atmospheric activity concentrations at four International Monitoring System (IMS) stations located in Europe and North America with overall considerable influence of IRE and/or CNL emissions for evaluation of the participants' runs. Participants were prompted to work with controlled and harmonized model set-ups to make runs more comparable, but also to increase diversity. It was found that using the stack emissions of IRE and CNL with daily resolution does not lead to better results than disaggregating annual emissions of these two facilities taken from the literature if an overall score for all stations covering all valid observed samples is considered. A moderate benefit of roughly 10% is visible in statistical scores for samples influenced by IRE and/or CNL to at least 50% and there can be considerable benefit for individual samples. Effects of transport errors, not properly characterized remaining emitters and long IMS sampling times (12-24 h) undoubtedly are in contrast to and reduce the benefit of high-quality IRE and CNL stack data. Complementary best estimates for remaining emitters push the scores up by 18% compared to just considering IRE and CNL emissions alone. Despite the efforts undertaken the full multi-model ensemble built is highly redundant. An ensemble based on a few arbitrary runs is sufficient to model the Xe-133 background at the stations investigated. The effective ensemble size is below five. An optimized ensemble at each station has on average slightly higher skill compared to the full ensemble. However, the improvement (maximum of 20% and minimum of 3% in RMSE) in skill is likely being too small for being exploited for an independent period.

Migration of noble gas tracers at the site of an underground nuclear explosion at the Nevada National Security Site.

As part of an underground gas migration study, two radioactive noble gases (37Ar and 127Xe) and two stable tracer gases (SF6 and PFDMCH) were injected into a historic nuclear explosion test chimney and allowed to migrate naturally. The purpose of this experiment was to provide a bounding case (natural transport) for the flow of radioactive noble gases following an underground nuclear explosion. To accomplish this, soil gas samples were collected from a series of boreholes and a range of depths from the shallow subsurface (3 m) to deeper levels (~160 m) over a period of eleven months. These samples have provided insights into the development and evolution of the subsurface plume and constrained the relative migration rates of the radioactive and stable gas species in the case when the driving pressure from the cavity is low. Analysis of the samples concluded that the stable tracer SF6 was consistently enriched in the subsurface samples relative to the radiotracer 127Xe, but the ratios of SF6 and 37Ar remained similar throughout the samples.

Measurements of Argon-39 at the U20az underground nuclear explosion site.

Pacific Northwest National Laboratory reports on the detection of 39Ar at the location of an underground nuclear explosion on the Nevada Nuclear Security Site. The presence of 39Ar was not anticipated at the outset of the experimental campaign but results from this work demonstrated that it is present, along with 37Ar and 85Kr in the subsurface at the site of an underground nuclear explosion. Our analysis showed that by using state-of-the-art technology optimized for radioargon measurements, it was difficult to distinguish 39Ar from the fission product 85Kr. Proportional counters are currently used for high-sensitivity measurement of 37Ar and 39Ar. Physical and chemical separation processes are used to separate argon from air or soil gas, yielding pure argon with contaminant gases reduced to the parts-per-million level or below. However, even with purification at these levels, the beta decay signature of 85Kr can be mistaken for that of 39Ar, and the presence of either isotope increases the measurement background level for the measurement of 37Ar. Measured values for the 39Ar measured at the site ranged from 36,000 milli- Becquerel/standard-cubic-meter-of-air (mBq/SCM) for shallow bore holes to 997,000 mBq/SCM from the rubble chimney from the underground nuclear explosion.

Development of a low-level (37)Ar calibration standard.

Argon-37 is an environmental signature of an underground nuclear explosion. Producing and quantifying low-level (37)Ar standards is an important step in the development of sensitive field measurement instruments. This paper describes progress at Pacific Northwest National Laboratory in developing a process to generate and quantify low-level (37)Ar standards, which can be used to calibrate sensitive field systems at activities consistent with soil background levels. This paper presents a discussion of the measurement analysis, along with assumptions and uncertainty estimates.

Detection and analysis of xenon isotopes for the comprehensive nuclear-test-ban treaty international monitoring system.

The use of the xenon isotopes for detection of nuclear explosions is of great interest for monitoring compliance with the comprehensive nuclear-test-ban treaty (CTBT). Recently, the automated radioxenon sampler-analyzer (ARSA) was tested at the Institute for Atmospheric Radioactivity (IAR) in Freiburg, Germany to ascertain its use for the CTBT by comparing its results to laboratory-based analyses, determining its detection sensitivity and analyzing its results in light of historical xenon isotope levels and known reactor operations in the area. Xe-133 was detected nearly every day throughout the test at activity concentrations ranging between approximately 0.1 mBq/m3 to as high as 120 mBq/m3. Xe-133m and 135Xe were also detected occasionally during the test at concentrations of less than 1 to a few mBq/m3.

Intercomparison experiments of systems for the measurement of xenon radionuclides in the atmosphere.

Radioactive xenon monitoring is one of the main technologies used for the detection of underground nuclear explosions. Precise and reliable measurements of (131m)Xe, (133g)Xe, (133m)Xe, and (135g)Xe are required as part of the International Monitoring System for compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). For the first time, simultaneous testing of four highly sensitive and automated fieldable radioxenon measurement systems has been performed and compared to established laboratory techniques. In addition to an intercomparison of radioxenon monitoring equipment of different design, this paper also presents a set of more than 2000 measurements of activity concentrations of radioactive xenon made in the city of Freiburg, Germany in 2000. The intercomparison experiment showed, that the results from the newly developed systems agree with each other and the equipment fulfills the fundamental requirements for their use in the verification regime of the CTBT. For 24-h measurements, concentrations as low as 0.1 mBqm(-3) were measured for atmospheric samples ranging in size from 10 to 80 m(3). The (133)Xe activity concentrations detected in the ambient air ranged from below 1 mBqm(-3) to above 100 mBqm(-3).

Potential impact of releases from a new molybdenum-99 production facility on regional measurements of airborne xenon isotopes.

The monitoring of the radioactive xenon isotopes (131m)Xe, (133)Xe, (133m)Xe, and (135)Xe is important for the detection of nuclear explosions. While backgrounds of the xenon isotopes are short-lived, they are constantly replenished from activities dominated by the fission-based production of (99)Mo used for medical procedures. At present, one of the most critical locations on earth for the monitoring of nuclear explosions is the Korean peninsula where the Democratic People's Republic of Korea (DPRK) has announced that it conducted three nuclear tests between 2006 and 2013. This paper explores the backgrounds that would be caused by the medium to large scale production of (99)Mo in the region of the Korean peninsula.

Radioxenon detections in the CTBT international monitoring system likely related to the announced nuclear test in North Korea on February 12, 2013.

Observations made in April 2013 of the radioxenon isotopes (133)Xe and (131m)Xe at measurement stations in Japan and Russia, belonging to the International Monitoring System for verification of the Comprehensive Nuclear-Test-Ban Treaty, are unique with respect to the measurement history of these stations. Comparison of measured data with calculated isotopic ratios as well as analysis using atmospheric transport modeling indicate that it is likely that the xenon measured was created in the underground nuclear test conducted by North Korea on February 12, 2013, and released 7-8 weeks later. More than one release is required to explain all observations. The (131m)Xe source terms for each release were calculated to 0.7 TBq, corresponding to about 1-10% of the total xenon inventory for a 10 kt explosion, depending on fractionation and release scenario. The observed ratios could not be used to obtain any information regarding the fissile material that was used in the test.

Source term estimation of radioxenon released from the Fukushima Dai-ichi nuclear reactors using measured air concentrations and atmospheric transport modeling.

Systems designed to monitor airborne radionuclides released from underground nuclear explosions detected radioactive fallout across the northern hemisphere resulting from the Fukushima Dai-ichi Nuclear Power Plant accident in March 2011. Sampling data from multiple International Modeling System locations are combined with atmospheric transport modeling to estimate the magnitude and time sequence of releases of (133)Xe. Modeled dilution factors at five different detection locations were combined with 57 atmospheric concentration measurements of (133)Xe taken from March 18 to March 23 to estimate the source term. This analysis suggests that 92% of the 1.24 × 10(19) Bq of (133)Xe present in the three operating reactors at the time of the earthquake was released to the atmosphere over a 3 d period. An uncertainty analysis bounds the release estimates to 54-129% of available (133)Xe inventory.

The workshop on signatures of medical and industrial isotope production - WOSMIP; Strassoldo, Italy, 1-3 July 2009.

Radiopharmaceuticals make contributions of inestimable value to medical practice. With growing demand new technologies are being developed and applied worldwide. Most diagnostic procedures rely on (99m)Tc and the use of uranium targets in reactors is currently the favored method of production, with 95% of the necessary (99)Mo parent currently being produced by four major global suppliers. Coincidentally there are growing concerns for nuclear security and proliferation. New disarmament treaties such as the Comprehensive Nuclear-Test-Ban Treaty (CTBT) are coming into effect and treaty compliance-verification monitoring is gaining momentum. Radioxenon emissions (isotopes Xe-131, 133, 133m and 135) from radiopharmaceutical production facilities are of concern in this context because radioxenon is a highly sensitive tracer for detecting nuclear explosions. There exists, therefore, a potential for confusing source attribution, with emissions from radiopharmaceutical-production facilities regularly being detected in treaty compliance-verification networks. The CTBT radioxenon network currently under installation is highly sensitive with detection limits approaching 0.1 mBq/m³ and, depending on transport conditions and background, able to detect industrial release signatures from sites thousands of kilometers away. The method currently employed to distinguish between industrial and military radioxenon sources involves plots of isotope ratios (133m)Xe/(131m)Xe versus (135)Xe/(133)Xe, but source attribution can be ambiguous. Through the WOSMIP Workshop the environmental monitoring community is gaining a better understanding of the complexities of the processes at production facilities, and the production community is recognizing the impact their operations have on monitoring systems and their goal of nuclear non-proliferation. Further collaboration and discussion are needed, together with advances in Xe trapping technology and monitoring systems. Such initiatives will help in addressing the dichotomy which exists between expanding production and improving monitoring sensitivity, with the ultimate aim of enabling unambiguous distinction between different nuclide signatures.

Analysis of data from sensitive U.S. monitoring stations for the Fukushima Dai-ichi nuclear reactor accident.

The March 11, 2011 9.0 magnitude undersea megathrust earthquake off the coast of Japan and subsequent tsunami waves triggered a major nuclear event at the Fukushima Dai-ichi nuclear power station. At the time of the event, units 1, 2, and 3 were operating and units 4, 5, and 6 were in a shutdown condition for maintenance. Loss of cooling capacity to the plants along with structural damage caused by the earthquake and tsunami resulted in a breach of the nuclear fuel integrity and release of radioactive fission products to the environment. Fission products started to arrive in the United States via atmospheric transport on March 15, 2011 and peaked by March 23, 2011. Atmospheric activity concentrations of (131)I reached levels of 3.0×10(-2) Bqm(-3) in Melbourne, FL. The noble gas (133)Xe reached atmospheric activity concentrations in Ashland, KS of 17 Bqm(-3). While these levels are not health concerns, they were well above the detection capability of the radionuclide monitoring systems within the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty.

Elevated radioxenon detected remotely following the Fukushima nuclear accident.

We report on the first measurements of short-lived gaseous fission products detected outside of Japan following the Fukushima nuclear releases, which occurred after a 9.0 magnitude earthquake and tsunami on March 11, 2011. The measurements were conducted at the Pacific Northwest National Laboratory (PNNL), (46°16'47″N, 119°16'53″W) located more than 7000 km from the emission point in Fukushima Japan (37°25'17″N, 141°1'57″E). First detections of (133)Xe were made starting early March 16, only four days following the earthquake. Maximum concentrations of (133)Xe were in excess of 40 Bq/m(3), which is more than ×40,000 the average concentration of this isotope is this part of the United States.

Alternative treaty monitoring approaches using ultra-low background measurement technology.

The International Monitoring System (IMS) of the Comprehensive Test Ban Treaty includes a network of stations and laboratories for collection and analysis of radioactive aerosols. Alternative approaches to IMS operations are considered as a method of enhancing treaty verification. Ultra-low background (ULB) detection promises the possibility of improvements to IMS minimum detectable activities (MDAs) well below the current approach, requiring MDA < or = 30 microBq/m(3) of air for (140)Ba, or about 10(6) fissions per daily sample.