Since shutdown , Politecnico di Milano managed L-54M nuclear research reactor according to deferred dismantling strategy. Recently, decommissioning activities have been started with preliminary radiological characterization, even though a more extended campaign would be required. In this work, a Monte Carlo N-Particle 3D model of the reactor has been proficiently developed to estimate graphite stack activation. To verify the model accuracy, several simulated criticality data have been satisfactorily compared with experimental ones, validating this computational approach as valid support to forthcoming radiological characterization campaign.

A new model for in-flight ice accretion is presented for both rime and glaze conditions. The model is based on the\ud local, exact solution of the unsteady Stefan problem for the temperature profiles within the ice layer in glaze\ud conditions. The new model moves from Myers’s formulation, and it includes an unsteady description of the heat\ud diffusion problem within the ice layer. Moreover, the local value of the air temperature outside the boundary layer is\ud used to compute convective heat fluxes, in place of the constant freestream temperature value considered in Myers’s\ud model. A source term is introduced to take into account mass transfer at the boundary separating rime and glaze\ud regions. The model was implemented in the ice accretion software PoliMIce to perform numerical simulations of inflight\ud ice accretion over two-dimensional airfoils in both rime and glaze ice regimes. The open-source computational\ud fluid dynamics software OpenFOAM was used to compute the aerodynamic flowfield and to reconstruct water\ud droplet trajectories. Numerical results suggest that the modifications introduced with respect to the original Myers\ud model improve significantly the accuracy of the predicted ice shapes for the considered test cases. The introduction of\ud the local value of air temperature was found to be essential for the formation of the well-known two-horn ice shape,\ud due to the occurrence of a local glaze to rime transition. The diverse contributions to the heat fluxes are discussed for\ud both the proposed and the Myers models

Monte Carlo integrated approach to radiological characterization for nuclear facilities decommissioning In the last decades, hundreds of nuclear reactors have been shutdown and have experienced decommissioning. This is also the case of L-54M Politecnico di Milano nuclear research reactor. Since shutdown in 1979, the plant has been managed following deferred dismantling strategy. Recently, preliminary radiological characterizations have been launched, even though a more extended campaign is necessary to implement facility decommissioning and restore unrestricted reuse status. In the framework of the IAEA collaborative research project on irradiated GRAphite Processing Approaches (GRAPA), a general radiochemistry and Monte Carlo integrated approach has been developed to study materials activation and support radiological characterization campaign. This integrated method, thanks to its general principles, could be applied to any nuclear reactor or facility undergoing decommissioning, thus helping to reduce the characterization efforts and, possibly, the associated costs.

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Since its shutdown in 1979, L-54M nuclear research reactor of Politecnico di Milano has been kept in safe storage configuration. In view of forthcoming decommissioning activities, many characterization activities have been performed on its matrices.Regarding the L-54M graphite stack, a neutron activation model was developed and validated by a graphite sampling and preliminary radiological characterization campaign focused on the most significant gamma-emitting radionuclides.Even if slightly underestimating the measured values, between -5% and -49% for 152 Eu, between -41% and -67% for 154 Eu, the simulated activity concentrations promisingly reproduce the experimental radial profile.