International challenge to predict the impact of radioxenon releases from medical isotope production on a comprehensive nuclear test ban treaty sampling station
Introduction
The International Monitoring System (IMS) is part of the verification regime for the Comprehensive Nuclear-Test-Ban-Treaty Organization (CTBTO, 2014). The verification regime is designed to detect nuclear explosions no matter where they occur on the earth. When complete, 80 of the IMS stations will have aerosol measurement systems sensitive enough to detect releases from nuclear explosions at great distances. At entry-into-force, half of the 80 stations will also have equipment that measures concentrations of four radioactive xenon isotopes (131mXe, 133Xe, 133mXe, and 135Xe) produced in a nuclear explosion, and following entry-into-force, a plan to add xenon monitoring capabilities to the other 40 stations will be reviewed (Comprehensive Nuclear-Test-Ban Treaty, 1996). An understanding of natural and man-made radionuclide backgrounds can also be used in accordance with the provisions of the treaty (such as event screening criteria in Annex 2 to the Protocol of the Treaty) for the effective implementation of the verification regime.
A number of studies of the release and transport of radioxenon from nuclear explosions, nuclear power plants, and medical isotope production facilities have been published (Becker et al., 2010, Eslinger et al., 2014, Hoffman et al., 2009, Kalinowski et al., 2008, Saey et al., 2010b, Wotawa et al., 2010, Wotawa et al., 2003, Zähringer et al., 2009). These studies confirm that fission-based production of 99Mo for medical purposes is the largest routine contributor of radioxenon to worldwide background levels. The 99Mo (half-life of 66 h) decays into 99mTc (half-life of 6 h) and the resulting 99mTc is used in approximately 30–40 million medical procedures per year (Peykov and Cameron, 2014) and the demand is expected to increase in the future.
A reduction in radioxenon releases to relatively low levels (Bowyer et al., 2013) has the potential to reduce background radioxenon to levels that don't significantly impact treaty verification activities. However, medical isotope production facilities meet regulatory release requirements and their releases don't pose public health risks, thus the operators have no financial incentive to reduce releases. Another way of mitigating the impact on treaty verification activities is to use stack monitoring data, if they are available, and atmospheric transport modeling. In the modeling context, one could attempt to model background sources accurately enough to subtract a background contribution from any sampled value. Given the uncertainties (source terms, modeling), simulated peaks may not accurately represent reality. Thus, alternately, when a xenon peak is observed, one could check whether the simulated background increases during the same period (synchronization in time). If that is the case, the observed peak could be linked to the rise of the radioxenon background.
Unfortunately, the details of the stack monitoring data needed, such as the time resolution, the accuracy, and whether or not local weather data are needed is not well known. There have been questions about whether stack data would be useful in a practical way at all, depending on the type of data made available and when it could be made available from a producer. To date, only one published study (Schöppner et al., 2013) has addressed the impacts the time resolution of stack monitoring data have on predicted concentrations at an IMS station location. The minimum source term resolution considered in that study was one day. Atmospheric modeling studies using inert tracers have been conducted since the early 1980s (Ferber et al., 1986, Gudiksen et al., 1984). This study addresses the difficult nuance of whether atmospheric models currently in wide use can yield information on the accuracy and timing of the source term data needed to faithfully reproduce sampling data.
This paper describes a challenge exercise formulated to start to answer some of these questions. Namely, to ascertain the level of agreement that can be achieved between atmospheric transport models using stack monitoring data and xenon isotopic concentration measurements at IMS stations. An evaluation criterion is used to measure the level of agreement. However, the real value of the exercise is in discussions resulting from the challenge without over-analyzing the evaluation criterion. The challenge is expected to spark discussions on what techniques are best, what gaps exist in our knowledge, and what type of data fidelity is needed from stack monitors. In general, this challenge will help inform the international treaty verification community of the status of the current capability.
The general approach of the exercise was to challenge atmospheric transport modeling groups to reproduce the time-history of 133Xe measurements at an IMS station using stack monitoring data from a medical isotope production facility. Participants received stack monitoring data that included the location, UTC date and time of releases, the measured activity concentrations of 133Xe in Bq m−3, an average stack flow rate (80,000 m3 h−1), and the height (m above ground level) of the release. All other data were gathered by the participants. Each participant used the atmospheric transport model and the associated meteorological data of their choice. The individuals participating in the challenge are identified in Table 1. Participants were asked not to use the IMS sampling data, if they had access to them, until after completing the modeling exercise.
Section snippets
Atmospheric transport models and meteorological data
The participants used several transport codes and several different sources for meteorological data. Several participants submitted results for more than one model. Some of the submissions were averages of other models or low and high resolution runs for the same model. Model metadata are provided in Table 2. Although the analysis considers all twenty six submissions, a subset of the submissions was selected to discuss common model characteristics. The reduced set of submissions is identified
Comparison measures
The purpose of this challenge was to ascertain the level of agreement one can achieve between simulated concentrations and IMS measurements using only the stack data and an atmospheric transport model, as might be expected for situations in which there was a detection of radioxenon at an IMS station and very little other information. Concentration estimates from this modeling exercise are expected to be quite variable (Draxler et al., 2015), thus it is useful to explore the general
Release and detection data
Participants in the modeling challenge received 133Xe stack emission data from the Institut des Radioéléments (IRE) radiopharmaceutical plant in Fleurus, Belgium. Releases from IRE have a measurable influence on 133Xe concentrations collected at DEX33 (Saey et al., 2010a) which is located 376 km from the IRE stack. The emission data covered the period 10 Nov 2013 through 8 Dec 2013. The measured concentration values for the stack data are based only on the 81 keV decay energy level and have an
Model comparison results
Thirteen participants submitted 26 solutions containing modeled concentrations of 133Xe at the sampler (DEX33) in Germany on the time periods used by the sampler. A plot of modeled concentrations for all 26 submissions and the concentrations at the sampler (black dots connected by a dotted line) is provided in Fig. 3. One submission had two predicted concentration values larger than 100 mBq m−3, but the upper limit on this plot partially obscures that fact. Some of the values were zero, thus
Discussion
The ranking and ensemble analysis in this paper suggests that combining multiple models may provide more accurate predicted concentrations than almost any single model. One ensemble selection technique was used in this paper. Further research is needed to identify optimal methods for selecting ensemble members, and those methods may depend on the nature of the transport problem. Although this exercise only addressed release and transport of a nondepositing noble gas, other radionuclides of
Acknowledgments
Participants in the atmospheric transport modeling challenge received 133Xe emission data from the Institut des Radioéléments (IRE) radiopharmaceutical plant in Fleurus, Belgium. IRE granted permission to use the data for the challenge.
The German national authority Bundesamt für Strahlenschutz (BfS) granted permission to use the 133Xe concentration data collected at the IMS noble gas sampler (DEX33) in Schauinsland, Germany for the challenge. Clemens Schlosser and Verena Heidmann of BfS
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