Impact of Environmental Backgrounds on Atmospheric Monitoring of Nuclear Explosions

Abstract

Radionuclide monitoring for nuclear explosions includes measuring radioactive aerosol and noble gas concentrations in the atmosphere. The International Monitoring System (IMS) of the Comprehensive Nuclear Test-Ban Treaty has made such measurements for decades, revealing much about how atmospheric radioactivity impacts the sensitivity of the network. For example, civilian emissions of radioiodine make a substantial regional impact, but a minor global impact, while civilian radioxenon emissions create major regional and complex global impacts. The impacts are strongly influenced by the minimum releases anticipated to be interesting. The original design of the IMS anticipated relatively large releases, and the current IMS network substantially meets or exceeds the sensitivity needed to detect those levels. Much lower signal levels can be motivated from historical tests. Using a release that corresponds roughly to a one-ton equivalent of fission in the atmosphere rather than the design level of one-kiloton equivalent, the network detection probabilities for ¹⁴⁰Ba and ¹³¹I are quite good (~ 75%) and for ¹³³Xe is still considerable (~ 45%). Using measured and simulated background concentrations, various possible desired signal levels, and an innovative anomaly threshold, maps of sensitivity and a station ranking are developed for IMS radionuclide stations. These provide a strong motivation for additional experimentation to learn about sources and the potential plusses of new technology.

Appendix 1: Additional Information about Detection Limits and Anomalous Background Levels

The operational data used to assign the detection limits for the four types of aerosol samplers in operation in the IMS are summarized in Table 9. Data to determine the average MDC values are obtained for 2 years (2019–2020) if the system regularly reported data. Two years is long enough to capture both daily and seasonal variations in ²¹²Pb backgrounds. Some stations were not in operation or collected a limited number of samples in 2019–2020, so the data period for them was extended to cover 2012–2020. Starting the data collection in 2012 means the ¹³¹I released from the Fukushima nuclear power plants following the 2011 earthquake would have decayed away (Biegalski et al., 2012). In a desire to only include data where the sampler was operating properly, the following screening criteria were implemented on reviewed radionuclide reports (RRR): (a) the sample had a spectrum category of 1, 2, 3, 4, or 5, (b) the air flow-rate quality flag was ‘PASS’, (c) the acquisition time was between 20 and 28 h, and (d) the ²¹²Pb concentration was less than 400,000 µBq/m³. For the 2012–2020 time period, approximately 200,000 samples met the screening criteria. The ¹³¹I concentration data in Table 9 are for samples taken between January 1, 2012 and February 15, 2021.

Table 9 Information on the detection limits and ¹³¹I detections used in the network performance analyses

Three types of noble gas samplers were deployed in the IMS at the time these calculations were performed. The SAUNA (Ringbom et al., 2003) has a 0.2 mBq/m³ MDC for ¹³³Xe using 12-h samples. The next-generation Swedish system (Ringbom et al., 2017), denoted by SAUNA III, uses a 6 h collection period and began operation at IMS station RN63 in Stockholm, Sweden in the middle of 2021. The SAUNA III system is not used in this analysis. The SPALAX (Fontaine et al., 2004) has a 0.15 mBq/m³ MDC for ¹³³Xe using 24-h samples. The ARIX (Dubasov et al., 2005) has a 0.5 mBq/m³ MDC for ¹³³Xe using 12-h samples. Fortunately, the noble gas concentrations do not depend on the ²¹²Pb background. For this analysis, all noble gas samplers were assumed to have a 12-h collection period. The stations with a SPALAX sampler were assumed to have a detection limit of 0.15 for all samples.

Currently, only 40 of the radionuclide sampling locations in the treaty have noble gas samplers. That number is reduced to 39 because RN35 does not have specified coordinates in the treaty. Thus, some assumptions are required to model a 79-station noble gas network. The entries in Table 10 are based on three assignment rules for stations where noble gas systems are not currently installed: 1) If the same country operates a noble gas sampler at another location in the IMS, the same equipment is used. These noble gas sampler assignments are denoted by using ‘()’, for example, (SPALAX), 2) All other assignments use a SAUNA system, denoted by [SAUNA]. The SAUNA and SPALAX have nearly the same MDC for ¹³³Xe and the 12 h collection cycle matches with the assumptions in the underlying ATM analysis. New generation noble gas samplers under development (Haas et al., 2017; Ringbom et al., 2017; Topin et al., 2020) have lower detection levels and shorter sample collection periods than current systems. Although they are expected to have better network performance that the currently deployed systems, this work uses only the existing deployed sampler technologies.

Determining the anomalous detection level as the 95th percentile of detections is complicated by the fact that sampling data do not exist for many of the noble gas stations used in this analysis. Thus, the 95th percentile is based on modeling nominal release quantities for medical isotope production facilities and operating nuclear power plant complexes. We provide summary results in Table 10 followed by additional data on the source terms and a brief discussion of the modeling approach. The average fraction of the modeled concentrations due to medical isotope production facilities over the 2 years is also provided in Table 10. In most cases, the detections are dominated by MIPF releases.

Table 10 Information on the sampler type assignments and the ¹³³Xe anomaly limits (mBq/m³) that will be compared with the sampler MDC

The average daily release values of ¹³³Xe (Bq) for the 12 medical isotope production facilities used in this study are provided in Table 11. The release rates were compiled from published sources over the last 10 years, with no attempt to modify the published values in response to changing global production levels of ^99mTc. However, the facility at Chalk River, Canada, has ceased operations and was not included in this study. In some cases, such as PINSTECH in Pakistan, the release rate was estimated using announced production rates of ^99mTc, combined with basic knowledge of the type of separations technology used in the facility.

Table 11 Daily release rates of ¹³³Xe (Bq) used for each medical isotope production facility

Daily release estimates were developed for the nuclear power plants in the online Power Reactor Information System (IAEA-PRIS, 2019) that were operating in 2019. These nuclear power plants are operating at 181 different locations. All release rates were set to 4.67 × 10⁹ Bq/d per reactor, which is derived from the combined continuous and batch releases (arithmetic average) in Table 4 of Kalinowski and Tuma (2009). Releases at each location accounted for the number of operating reactors. Many of the locations have only one reactor, but others have multiple reactors. For example, the Qinshan complex in China has 7 operating reactors.

The atmospheric transport runs in the Hysplit code (Stein et al., 2015) that propagate facility emissions of ¹³³Xe used archived meteorological data for 2015 and 2016 on a 0.5° spacing and 3-h time step (GDAS0P5, 2020). Each run modeled plume movement for 10 days after release and saved concentration data for each hour on a global 0.25° grid. The concentration data were then interpolated to individual sampler locations and aggregated to the sampler collection time periods.

A comparison of modeled versus measured ¹³³Xe values for 2019 at the IMS station RN38 in Takasaki, Japan, is provided in Fig. 13. The model predicts more values below about 0.25 mBq/m³ than are measured, but the cumulative frequency of predictions and measured values is quite close for values of 0.35 or higher. The sampled data are not censored below the detection limit of approximately 0.25 mBq/m³. The 95th percentiles agree quite nicely, providing evidence that the 95th percentile anomaly level based on the modeled concentrations is reasonable.

Fig. 13

Comparison of measured ¹³³Xe concentrations for 2019 at RN38 (Takasaki, Japan) with modeled concentrations based on nominal releases from nuclear power plants and medical isotope production facilities