Nuclear Energy Institute

Abstract: Recent exptl. results on formation of ordered surface nanostructures of hexagonal symmetry on metals under the action of a series of scanned doubled collinear femtosecond pulses of orthogonal polarization and variable time delay are analyzed.It is demonstrated that the appearance of structures is related to the formation of a dynamic grating by the first pulse and excitation of two pairs of surface plasmon-polaritons by the second pulse in directions sym. with respect to its vector of polarization and forming an angle of π/3 with each other.This leads to the formation of corresponding dynamic gratings and fourfold degeneracy of the first grating.Three dynamic gratings sym. with respect to the surface normal form a residual surface relief of hexagonal symmetry that can be observed exptl.

This paper analyzes new exptl. results on the surface breakdown of semiconductors and thin metallic films by ultrashort pulses of plane-polarized terahertz radiation.It shows that the character of the observed breakdown is satisfactorily explained by the universal polariton model, tested with ultrashort pulses of visible and near-IR radiation interacting with condensed media.By using a nonlinear math. model of the formation of the spatial periods of a nanograting that are multiples of the wavelength of the exciting radiation, it is shown that the periods of the gratings thus formed are substantially shorter than the linear diffraction limit in the optical and terahertz regions.It is demonstrated that the diffraction limit can be overcome in nonlinear processes by which interference nanogratings are formed with normal and anomalous orientations.

The objective of this article is to study the activities and special characteristics of graphite and Fluental neutron moderator material in FiR1 TRIGA Mark II type research reactor in Finland.Quantifying the nuclide vector and total activities of decommissioning waste is a legal requirement and provides input to many other aspect of decommissioning project.Because earlier calculations of decommissioning waste nuclide inventories used partially assumed input data e.g. material compositions, measurement data is needed to validate the calculated results.Graphite and Fluental are especially important in long term final disposal of the waste, because graphite contains volatile radionuclides (¹⁴C, ³H, ³⁶Cl) that spread rapidly in final disposal conditions and the corrosion of aluminum in Fluental will result in hydrogen pressure generation, which can accelerate the diffusion of radionuclides in the bedrock.Moreover, these types of materials are not used in Finnish power reactors and their behavior in final disposal conditions have not been studied so thoroughly earlier.Results are used to validate the calculated estimates of total activities and the scaling matrix method that will be used to classify the packed decommissioning waste.Due to limited number of samples, numerical results still contain variations.However, developed methods are still valuable in future analyses and measured data also rules out unexpected activities.