Airborne concentrations and chemical considerations of radioactive ruthenium from an undeclared major nuclear release in 2017

Abstract

In October 2017, most European countries reported unique atmospheric detections of aerosol-bound radioruthenium (¹⁰⁶Ru). The range of concentrations varied from some tenths of µBq·m^-3 to more than 150 mBq·m^-3 The widespread detection at such considerable (yet innocuous) levels suggested a considerable release. To compare activity reports of airborne ¹⁰⁶Ru with different sampling periods, concentrations were reconstructed based on the most probable plume presence duration at each location. Based on airborne concentration spreading and chemical considerations, it is possible to assume that the release occurred in the Southern Urals region (Russian Federation). The ¹⁰⁶Ru age was estimated to be about 2 years. It exhibited highly soluble and less soluble fractions in aqueous media, high radiopurity (lack of concomitant radionuclides), and volatility between 700 and 1,000 °C, thus suggesting a release at an advanced stage in the reprocessing of nuclear fuel. The amount and isotopic characteristics of the radioruthenium release may indicate a context with the production of a large ¹⁴⁴Ce source for a neutrino experiment.

Keywords: accidental release; environmental radioactivity; environmental release; nuclear forensics; ruthenium.

Fig. 1.

Compiled maximum ¹⁰⁶Ru airborne “uncorrected” activity concentrations (in mBq·m⁻³; sampling period in parentheses) in Europe.

Fig. 2.

Airborne ¹⁰⁶Ru concentrations (mBq·m⁻³) at Romanian locations (values have been attributed to the midsampling date of the composite samples). The connecting lines between data points are only meant to guide the eye.

Fig. 3.

Daily maps of above-LOD airborne ¹⁰⁶Ru (red dots) in Romania from September 28 to October 5, 2017. Gray dots indicate sampling locations with ¹⁰⁶Ru levels below the respective limits of detection at the given time.

Fig. 4.

(Left) Map of uncorrected average concentrations at European stations, and (Right) map of 7-d corrected average concentrations (based on average plume duration of 7 d at each location).

Fig. 5.

HYSPLIT-based 240-h backward trajectories ending at the Romanian monitoring station in Zimnicea (black star) (43.666 N, 25.666 E), every 3 h on September 30, 2017, from 2 AM to 11 PM UTC. The National Oceanic and Atmospheric Administration (NOAA) HYSPLIT model uses Global Data Assimilation System (GDAS) meteorological data. The Model Vertical Velocity was used as vertical motion calculation method. The green circle indicates the position of the Mayak industrial complex. The altitude of the air parcels is given in meters above ground level (AGL). The green circle in the altitude sections of the trajectories ending at 3 PM and 5 PM UTC (maps surrounded by red frames), respectively, indicates the time and altitude (approximately 500 m) the air parcels were in closest proximity to the Mayak area.

Fig. 6.

Age estimation of radioruthenium from various power reactor types (boiling water reactor, BWR; pressurized water reactor, PWR; Russian Water-Water-Energetic-Reactor, VVER) based on ¹⁰³Ru/¹⁰⁶Ru activity ratios of regular spent nuclear fuels (UO₂ and mixed oxide fuel, MOX) at the end of their fuel lifetimes. The light gray area represents the uncertainty of the ratio.

Fig. 7.

Volatility tests of ¹⁰⁶Ru trapped on an air filter from Vienna (2 replicates with 30 Bq ¹⁰⁶Ru pieces). The loss of mass is shown in green for comparison.