Atmospheric modeling and source reconstruction of radioactive ruthenium from an undeclared major release in 2017

Abstract

In October 2017 unusual 106Ru detections across most of Europe prompted the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) to analyze the event in order to locate the origin and identify the magnitude of the release. This paper presents the inverse modeling techniques used during the event to achieve this goal. The method is based on a variational approach and consists of using air concentration measurements with the ldX long-range dispersion model included in the IRSN’s C3X operational platform. The method made it possible to quickly identify the southern Urals as the most likely geographical origin of the release. Despite uncertainties regarding the starting date of the release, calculations show that it potentially began on 23 September, while most of the release was emitted on 26 September. Among the nuclear plants identified in the southern Urals, the Mayak complex is that from which the dispersion of the 106Ru plume is most consistent with observations. The reconstructed 106Ru source term from Mayak is ∼250 TBq. In total, it was found that for 72% of the measurements simulated and observed air concentration agreed within a factor of 5. In addition, the simulated deposition of 106Ru agrees with the observed deposition. Outside the southern Urals, the simulations indicate that areas with highest deposition values are located in southern Scandinavia and southeastern Bulgaria and are explained by rainfall events occurring while the plume was passing over.

On the model uncertainties in Bayesian source reconstruction using the emission inverse modelling system FREARtool v1.0 and the Lagrangian transport and dispersion model Flexpart v9.0.2

“…Therefore, one cannot simply predict beforehand how the posterior will change when the model error is changed. The resulting source location is in line with what previous studies found (Sørensen, 2018;Saunier et al, 2019;Bossew et al, 2019;.…”

“…Since no other fission products such as iodine and cesium were measured, a nuclear reactor accident can be excluded. Several studies using direct and inverse atmospheric transport modelling showed that a release in the region of the southern Ural mountains in Russia can best explain the observations (Sørensen, 2018;Saunier et al, 2019;Bossew et al, 2019;. Two major nuclear facilities are located in that area: the Research Institute of Atomic Reactors and the Mayak Production Association (Fig.…”

Non-natural ruthenium isotope ratios of the undeclared 2017 atmospheric release consistent with civilian nuclear activities

“…In any case, while pressurized water reactors of the VVER type are commonly used in many countries, there are comparatively very few operating reprocessing plants. Consistent with the recent reconstruction and air modeling of the origin of the radioruthenium cloud, 2,3,5 the Ru stable isotope signature points to the involvement of a reprocessing facility. Furthermore, the fission-generated Ru component in the air filter displays the signature of spent VVER nuclear fuel.…”

“…The characteristics of the release (e.g., lack of concomitant radionuclides) suggested that the source was a spent nuclear fuel reprocessing facility. The source term of the release was estimated at 250 TBq 106 Ru, and atmospheric modeling indicated that the cloud originated in the southern Urals in the Russian Federation 2,3 . This area hosts one of the largest nuclear facilities in the world, the Federal State Unitary Enterprise (FSUE) Production Association Mayak in Ozersk, Russia.…”

“…Currently, no country has assumed responsibility for this considerable release, which is likely the single-largest accidental release from civilian nuclear reprocessing 4 . Despite a large number of meteorological indications 3,[5][6][7][8] , Russian authorities and institutions have repeatedly denied any involvement of the Mayak facility in the release [9][10][11] . In their official statement 9 , the Rosatom State Nuclear Energy Corporation emphasized that there were not any incidents at any of the Rosatom sites during the period of September-October 2017.…”