Inverse modelling-based reconstruction of the Chernobyl source term available for long-range transport

Abstract

Abstract. The reconstruction of the Chernobyl accident source term has been previously carried out using core inventories, but also back and forth confrontations between model simulations and activity concentration or deposited activity measurements. The approach presented in this paper is based on inverse modelling techniques. It relies both on the activity concentration measurements and on the adjoint of a chemistry-transport model. The location of the release is assumed to be known, and one is looking for a source term available for long-range transport that depends both on time and altitude. The method relies on the maximum entropy on the mean principle and exploits source positivity. The inversion results are mainly sensitive to two tuning parameters, a mass scale and the scale of the prior errors in the inversion. To overcome this hardship, we resort to the statistical L-curve method to estimate balanced values for these two parameters. Once this is done, many of the retrieved features of the source are robust within a reasonable range of parameter values. Our results favour the acknowledged three-step scenario, with a strong initial release (26 to 27 April), followed by a weak emission period of four days (28 April-1 May) and again a release, longer but less intense than the initial one (2 May-6 May). The retrieved quantities of iodine-131, caesium-134 and caesium-137 that have been released are in good agreement with the latest reported estimations. Yet, a stronger apportionment of the total released activity is ascribed to the first period and less to the third one. Finer chronological details are obtained, such as a sequence of eruptive episodes in the first two days, likely related to the modulation of the boundary layer diurnal cycle. In addition, the first two-day release surges are found to have effectively reached an altitude up to the top of the domain (5000 m).

Inverse modelling of atmospheric tracers: non-Gaussian methods and second-order sensitivity analysis

“…The source was correctly localised (ETEX-I and ETEX-II), and the total released mass was correctly estimated up to a few percents (ETEX-I). In the case of the Chernobyl accident, it was also applied to the reconstruction of radionuclides available for long-range transport with a strong sanitary impact (Davoine and Bocquet, 2007). In both applications, this approach performed significantly better than the optimisation with a cost function based on a quadratic criterion (that is to say based on Gaussian prior laws for the source and the modelled errors).…”

“…The model was driven by the 40-year re-analysis ECMWF meteorological data (wind fields, and fields necessary to the diagnostic of the vertical turbulent diffusion coefficient K z ). Since the number of observations is much lower than the number of variables to invert, the rows of H are computed using POLAIR3D in an (approximate) adjoint mode using a technique that has now been vastly advertised in this context (Issartel and Baverel, 2003;Hourdin and Talagrand, 2006;Davoine and Bocquet, 2007 Two types of prior and therefore two types of reconstruction will serve as illustrations throughout the second-order analysis of Sect. 5.…”

Data assimilation in atmospheric chemistry models: current status and future prospects for coupled chemistry meteorology models

“…Paradoxically, they have slowly percolated in air quality data assimilation, where they should be crucial given the uncertainty in most forcings or the sparsity of observations for in situ concentration measurements. The error covariance matrices can be parameterized with a restricted set of hyper-parameters, and those hyperparameters can be estimated through maximum-likelihood or L-curve tests (Ménard et al, 2000;Davoine and Bocquet, 2007;Elbern et al, 2007). Alternatively, with sufficient data, the whole structure of the error covariance matrices in the observation space can be diagnosed using consistency matrix identities; see for example Schwinger and Elbern (2010), who applied the approach of Desroziers et al (2005) to a stratospheric chemistry 4D-Var system.…”

Radionuclide Disperse To Environtment From Pwr Reactor On Severe Accident Condition Using Maccs Program

“…The analysis was performed for the dispersion coefficient of 8 th fission product groups radionuclides are release from the NPP are the noble gas, lanthanides, precious metals, halogens, alkali metals, tellurium, cerium, strontium and barium groups. Calculations using removable fraction of radionuclides in PWR reactors Westinghouse is calculate using MELCOR code [10,11].…”