Reassessment of Computational Tools for the Modelling of Heat Transfer in a Molten UO2 Pool in Natural Convection

This work, performed within the ESFR-SMART H2020 European project, is part of a larger framework intending to reassess the modelling of heat transfer in molten pools on SCARABEE available experimental results. This paper presents simulation results of the in-pile BF1 test, performed within the SCARABEE program, using ASTEC, SIMMER III and SIMMER V simulation tools as well as comparison with its available experimental data. This program was performed in the 80's in the frame of the Safety Assessment studies of Superphenix sodium-cooled reactor. This test was dedicated to verify the stability of a molten UO2 pool under decay heat conditions within natural convection and the long-term resilience of the peripheral fuel crust. The pool was generated in a stainless steel crucible by a progressive heating (six power plateaus) of a fuel pellet stack. For the benchmark purposes, only the molten pool behavior at the last power plateau (largest pool and highest fuel temperatures) was investigated. Experimental data such as the axial profile of radial heat fluxes and heat transfer from the pool to the surrounding inter-assembly coolant or the peripheral fuel crust thickness were used for the reassessment of the simulation tools. In addition, other variables of interest not measured during the test, such as the radial and axial velocities in the pool, were also benchmarked. Finally, a critical analysis of the correlations and models used in the simulation tools for the BF1 test modelling is also provided in the paper.

Development and Processing of Thermal-Hydraulic Model for GFR 2400 Fast Reactor Design and NESTLE Coupled Transient Code

The paper investigates the influence of the used thermal-hydraulic approximations on the coupled calculations of Gas-cooled Fast Reactor design (hereby GFR 2400). The NESTLE code is used as coupled simulation tool and solves the multigroup neutron diffusion equation by the finite difference method that is internally coupled with a thermal-hydraulic sub-channel code. The in-house developed TEMPIN code and the CFD code FLUENT (from ANSYS code system) are used to prepare the thermal-hydraulic data for the GFR 2400 calculations. The TEMPIN code solves the steady state heat balance equation with flowing coolant in triangular lattice cell together with temperature dependent thermal-hydraulic properties of the fuel, cladding and coolant. Based on the calculated fuel bundle temperature distributions by the TEMPIN code, the thermal-hydraulic material properties (approximations) suitable for the NESTLE coupled code are processed for the GFR 2400 design. The influence of the constant and radial heat generation term within the fuel pin is studied within the paper. The performance of the NESTLE code with thermal-hydraulic approximations processed by both (TEMPIN and FLUENT) methods are compared with the findings of the GoFastR project. Moreover, both the thermal-hydraulic approximations were compared for one steady state and one transient state, related to the rapid withdrawal of one control rod assembly from the core. Changes in thermal-hydraulic distributions are described and visualized in the paper.

Modeling of the nitrous oxide synthesis in a microchannel reactor: the effect of parameters on the temperature regimes and output

The study deals with the synthesis of nitrous oxide via selective oxidation of ammonia in a microreactor (MCR), which is a metal disk with cylindrical channels filled with the manganese-bismuth oxide catalyst. The proposed 3D mathematical model of MCR takes into account axial and radial heat and mass transfer, catalytic reactions and related changes of the reaction mixture volume, heat exchange between the disk and channels, and thermal conductivity of the disk. Parameters providing the maximum output of nitrous oxide were determined with allowance for restrictions on the temperature in MCR channels. The highest efficiency of the nitrous oxide synthesis is achieved at a temperature of the outer edge of reactor 370 °С and an inlet concentration of ammonia 20 vol.%. The output per unit catalyst volume in MCR is approximately 1.5 times higher as compared to a tubular reactor; the maximum temperature corresponds to the optimal one, which provides the best selectivity of the process with respect to nitrous oxide.

On the evolution equation with a dynamic Hardy-type potential

Motivated by the celebrated paper of Baras and Goldstein (1984), we study the heat equation with a dynamic Hardy-type singular potential. In particular, we are interested in the case where the singular point moves in time. Under appropriate conditions on the potential and initial value, we show the existence, non-existence and uniqueness of solutions, and obtain a sharp lower and upper bound near the singular point. Proofs are given by using solutions of the radial heat equation, some precise estimates for an equivalent integral equation and the comparison principle.

A perturbative approach to the time-dependent Karmarkar condition

In this work we employ a perturbative approach to study the gravitational collapse of a shear-free radiating star. The collapse proceeds from an initial static core satisfying the time-independent Karmarkar condition and degenerates into a quasi-static regime with the generation of energy in the form of a radial heat flux. The time-dependent Karmarkar condition is solved together with the boundary condition to yield the full gravitational behaviour of the star. Our model is subjected to rigorous regularity, causality and stability tests.