Numerical simulations were carried out to model the propagation of an airwave from the fireball that passed over Chelyabinsk (Russia) on 15 February 2013. The airburst of the Chelyabinsk meteoroid occurred due to its catastrophic fragmentation in the atmosphere. Simulations of the space-time distribution of energy deposition during the airburst were done using a novel fragmentation model based on dimensionality considerations and analogy to the fission chain reaction in fissile materials. To get an estimate of the airburst energy, observed values of the airwave arrival times to different populated localities were retrieved from video records available on the Internet. The calculated arrival times agree well with the observed values for all the localities. Energy deposition in the atmosphere obtained from observations of the airwave arrival times was found to be 460 ± 60 kt in trinitrotoluene (TNT) equivalent. We also obtained an independent estimate for the deposited energy,450 þ200 À160 kt TNT from detecting the air increment velocity due to the wave passage in Chelyabinsk. Assuming that the energy of about 90 kt TNT was irradiated in the form of visible light and infrared radiation, as registered with optical sensors [Yeomans and Chodas, 2013], one can value the total energy release to be about 550 kt TNT which is in agreement with previous estimates from infrasound registration and from optical sensors data. The overpressure amplitude and its positive phase duration in the airwave that reached the city of Chelyabinsk were calculated to be about 2 kPa and 10 s accordingly.

Transportation of radioactive materials is obviously accompanied by a certain risk. A model for risk assessment of emergency situations and terrorist attacks may be useful for choosing possible routes and for comparing the various defence strategies. In particular, risk assessment is crucial for safe transportation of excess weapons-grade plutonium arising from the removal of plutonium from military employment. A fuzzy-probabilistic model for risk assessment of railway transportation has been developed taking into account the different natures of risk-affecting parameters (probabilistic and not probabilistic but fuzzy). Fuzzy set theory methods as well as standard methods of probability theory have been used for quantitative risk assessment. Information-preserving transformations are applied to realise the correct aggregation of probabilistic and fuzzy parameters. Estimations have also been made of the inhalation doses resulting from possible accidents during plutonium transportation. The obtained data show the scale of possible consequences that may arise from plutonium transportation accidents.