Development of innovative solutions suitable for generation III and IV nuclear reactors

This work has been especially focused on R&D issues related to innovative components and/or systems aimed at enhancing the safety features of nuclear reactor designs. The present work, aimed at the development of safety systems to be applied to nuclear reactors, has been mainly focused on systems characterized by an intensive use of passive devices. The activation and operation of passive safety systems are generally characterized by limited driving forces (e.g. temperature difference, density gradient, pressure drop, etc.). As a consequence, the design of passive systems is more sensitive to the system parameters. The passive system design is much more complicated since the driving force, which allows the system to be activated and/or to operate, has a limited intensity, comparable with other natural forces present into the system itself. The design of a passive system requires an optimization of the geometry and an accurate evaluation of the timing to be sure that the system complies with the needed safety requirements. The main effort, that has been done in the present research work, has been mainly focused on the identification of design solutions able to improve the plant safety features. Studies have been mainly focused on passive safety systems characterized by special features that allow it to limit the possible initiating failure events that would make the system unavailable when requested. Better solutions are usually characterized by very simplified systems (number of components as low as possible) coupled with very robust designs in which the main purpose is the minimization of the forces that could counterbalance the main driving force. The present study shows how, the proposed approach concerning the safety issues of nuclear reactors, thanks to the accidental condition prevention considered from the design stage, would make possible to avoid or to minimize the occurrence of accidental conditions. The philosophy that has been followed in the present work is that of the "safety by design"; criterion according to which the extreme simplification of the systems together with the use of natural laws to guarantee the safety functions make the whole system cheaper and safer

Development of innovative solutions suitable for generation III and IV nuclear reactors

This work has been especially focused on R&D issues related to innovative components and/or systems aimed at enhancing the safety features of nuclear reactor designs. The present work, aimed at the development of safety systems to be applied to nuclear reactors, has been mainly focused on systems characterized by an intensive use of passive devices. The activation and operation of passive safety systems are generally characterized by limited driving forces (e.g. temperature difference, density gradient, pressure drop, etc.). As a consequence, the design of passive systems is more sensitive to the system parameters. The passive system design is much more complicated since the driving force, which allows the system to be activated and/or to operate, has a limited intensity, comparable with other natural forces present into the system itself. The design of a passive system requires an optimization of the geometry and an accurate evaluation of the timing to be sure that the system complies with the needed safety requirements. The main effort, that has been done in the present research work, has been mainly focused on the identification of design solutions able to improve the plant safety features. Studies have been mainly focused on passive safety systems characterized by special features that allow it to limit the possible initiating failure events that would make the system unavailable when requested. Better solutions are usually characterized by very simplified systems (number of components as low as possible) coupled with very robust designs in which the main purpose is the minimization of the forces that could counterbalance the main driving force. The present study shows how, the proposed approach concerning the safety issues of nuclear reactors, thanks to the accidental condition prevention considered from the design stage, would make possible to avoid or to minimize the occurrence of accidental conditions. The philosophy that has been followed in the present work is that of the "safety by design"; criterion according to which the extreme simplification of the systems together with the use of natural laws to guarantee the safety functions make the whole system cheaper and safer

Supercritical carbon dioxide applications for energy conversion systems

In the present paper, the possibility of increasing the thermodynamic efficiency of an electric energy production plant, by using an advanced energy conversion system based on supercritical carbon dioxide (S-CO2) as working fluid, has been analyzed. Since the supercritical carbon dioxide cycles are being considered as a favorable candidate for the next generation of nuclear power plant energy conversion systems, a lead cooled fast reactor has been selected as reference in the present analyses. The main aim of the present study is to compare two different S-CO2 thermal cycles applied on the conversion system of a nuclear power plant. The reference Lead cooled Fast Reactor (LFR) used for the present analyses is the ALFRED reactor, which has a thermal power of 300 MW and it is considered the scaled down prototype of the industrial European Lead Fast Reactor (ELFR). Thermodynamic cycles selected for the present study are a Recompression Cycle and a Brayton Cycle with Regeneration. Each of them has been analyzed under several design conditions regarding the maximum pressure and the regeneration coefficient. Among different design conditions, the solution allowing the maximization of the overall efficiency has been identified. Thermodynamic analyses have been carried out with GateCycle™ v. 6.1.1, which is a General Electric software able to predict design and off-design performance of power plants

An anisotropic numerical model for thermal hydraulic analyses: application to liquid metal flow in fuel assemblies

A CFD analysis has been carried out to study the thermal–hydraulic behavior of liquid metal coolant in a fuel assembly of triangular lattice. In order to obtain fast and accurate results, the isotropic two-equation RANS approach is often used in nuclear engineering applications. A different approach is provided by Non-Linear Eddy Viscosity Models (NLEVM), which try to take into account anisotropic effects by a nonlinear formulation of the Reynolds stress tensor. This approach is very promising, as it results in a very good numerical behavior and in a potentially better fluid flow description than classical isotropic models. An Anisotropic Shear Stress Transport (ASST) model, implemented into a commercial software, has been applied in previous studies, showing very trustful results for a large variety of flows and applications. In the paper, the ASST model has been used to perform an analysis of the fluid flow inside the fuel assembly of the ALFRED lead cooled fast reactor. Then, a comparison between the results of wall-resolved conjugated heat transfer computations and the results of a decoupled analysis using a suitable thermal wall-function previously implemented into the solver has been performed and presented

Thermal-hydraulic analysis of an innovative decay heat removal system for lead-cooled fast reactors

Improvement of safety requirements in GEN IV reactors needs more reliable safety systems, among which the decay heat removal system (DHR) is one of the most important. Complying with the diversification criteria and based on pure passive and very reliable components, an additional DHR for the ALFRED reactor (Advanced Lead Fast Reactor European Demonstrator) has been proposed and its thermal-hydraulic performances are analyzed. It consists in a coupling of two innovative subsystems: the radiative-based direct heat exchanger (DHX), and the pool heat exchanger (PHX). Preliminary thermal-hydraulic analyses, by using RELAP5 and RELAP5-3D© computer programs, have been carried out showing that the whole system can safely operate, in natural circulation, for a long term. Sensitivity analyses for: the emissivity of the DHX surfaces, the PHX water heat transfer coefficient (HTC) and the lead HTC have been carried out. In addition, the effects of the density variation uncertainty on the results has been analyzed and compared. It allowed to assess the feasibility of the system and to evaluate the acceptable range of the studied parameters. A comparison of the results obtained with RELAP5 and RELAP5-3D© has been carried out and the analysis of the differences of the two codes for lead is presented. The features of the innovative DHR allow to match the decay heat removal performance with the trend of the reactor decay heat power after shutdown, minimizing at the same time the risk of lead freezing. This system, proposed for the diversification of the DHR in the LFRs, could be applicable in the other pool-type liquid metal fast reactors

Heat and mass transfer analogy applied to condensation in the presence of noncondensable gases inside inclined tubes

A theoretical and experimental investigation on steam condensation in presence of noncondensable gases within horizontal and inclined tubes is summarized in the present paper. A simple correlation mainly based on dimensionless numbers was derived and compared with previous formulations based on the diffusion layer model. The noncondensable gases presence during condensation is an important issue affecting the whole thermodynamic efficiency of the process, and for this reason highly investigated by many researchers. The experimental data obtained for condensation, inside horizontal or inclined tube (15°, 30° and 45°) with an internal diameter of 22 mm, of an air/steam mixture, at low mixture Reynolds numbers (< 6000), have been used to verify the present Heat and Mass Transfer Analogy (HMTA) formulation. In order to perform the heat and mass transfer analogy, the suction effect at the interface has been taken into account since it considerably affects temperature and concentration profiles and hence the transfer coefficients. The model of Chato is used for the condensate boundary layer since it has been identified to be the better model under the experimental condition. Finally, the experimental data have been compared with the theoretical Couette flow model with transpiration showing a quite good agreement

Condensation heat transfer coefficient with noncondensable gases inside near horizontal tubes

An experimental investigation on the role of noncondensable gases during condensation of steam inside horizontal/slightly inclined tubes is presented. In a condenser of a thermal desalination unit, the noncondensable gases flowing within the gas phase cause reduction of performance and efficiency. Many researchers have investigated in-tube condensation for horizontal heat exchangers, but very few works have been performed to study the condensation in slightly inclined tubes with noncondensable gases. Inclination of tubes may be requested to achieve suitable and reliable draining. Experiments were conducted in the following conditions: tube internal diameter 12.6 mm, 20 mm and 26.8 mm; tube indination 7 degrees; inlet noncondensable gas mass fraction omega(m)=5%-42%; inlet mixture Reynolds number Re-m,Re-in= 5000-20,000; local noncondensable gas mass fraction omega(m), = 5%-60%; local mixture Reynolds number Re-m=500-20,000; saturated steam at atmospheric pressure, gravity controlled with stratified flow regime, in which the condensate is collected mainly in the bottom part of the tube due to gravity and it is drawn out by its own momentum. A correlation of mixed gas heat transfer coefficient along a slightly inclined tube, in a gravity controlled flow regime, has been developed, showing a good agreement with experimental results. (C) 2012 Elsevier B.V. All rights reserved

Supercritical carbon dioxide applications: features and advantages

The Supercritical carbon dioxide (SCO2) is widely used in many industrial applications as process or heat transfer fluid. The SCO2 is characterized by good chemical stability at high temperature, supercritical conditions at limited temperature and pressure and high thermodynamic efficiency. Because of these features, SCO2 allows to operate with small sized operating machines and higher efficiency compared to conventional applications. Moreover, the SCO2 can be used in conventional thermodynamic gas cycles reaching higher efficiency at the same temperature level. The main advantage is the possibility of reaching thermodynamic efficiency values close to 50%, much higher than conventional cycles. Moreover, the critical point of CO2, low pressure and low temperature (7 MPa and 300 K), makes systems more economic and technically feasible than systems powered by other supercritical fluids. SCO2 industrial applications are analyzed in the present paper and specific advantages will be investigated

Impianti Nucleari innovativi di piccola taglia per la produzione elettrica

La tecnologia maggiormente diffusa a livello internazionale, per la produzione industriale di calore da fonte nucleare, è quella dei reattori ad acqua leggera (LWR), in particolare ad acqua pressurizzata (PWR). Con riferimento a questa tipologia di reattori, verranno analizzate soluzioni proposte a livello internazionale per impianti di piccola taglia finalizzate alla produzione elettrica e cogenerazione, in particolare quelle più innovative. Con l'obiettivo di effettuare un'analisi comparativa, saranno considerati i reattori di piccola taglia sviluppati per la propulsione navale ed i reattori specificatamente proposti per applicazioni terrestri, con una producibilità elettrica dell'ordine delle decine di MW. Nell'analisi comparativa tra soluzioni tecnologiche previste saranno analizzate le seguenti caratteristiche: circolazione del refrigerante primario, trasferimento di potenza al circuito secondario, pressurizzazione e depressurizzazione del primario, refrigerazione del nocciolo in emergenza, controllo della reattività, sistemi di spegnimento del reattore, protezioni previste per incidenti di progetto quali LOCA, LOFA e RIA, sistemi per la rimozione del calore di decadimento. Come obiettivo ultimo, lo studio si propone di individuare le soluzioni migliori tra quelle proposte per svolgere una determinata funzione e sarà valutata la possibilità di conciliare in un impianto innovativo aspetti tecnologici di particolare pregio, appartenenti a diverse alternative analizzate

Film condensation in inclined tubes with noncondensable gases: An experimental study on the local heat transfer coefficient

An experimental investigation on the role of noncondensable gases during condensation of steam inside inclined tubes is presented. In a condenser, noncondensable gases flowing with steam cause reduction of condenser performance and efficiency. Many researchers have investigated in-tube condensation for vertical heat exchangers, but very few works have been performed to study condensation in inclined tubes with noncondensable gases. In the paper, experiments dedicated to this situation are described, with reference to the following conditions: tube internal diameter: 12.6 mm, 20 mm and 26.8 mm; tube inclination: 7 degrees, 15 degrees, 30 degrees and 45 degrees; inlet noncondensable gas mass fraction omega(in) = 2%-42%; inlet mixture Reynolds number Re-m,Re-in = 5000-20000; local noncondensable gas mass fraction omega = 2%-70%; local mixture Reynolds number Re-m = 400-21000; local condensate Reynolds number Re-l = 10-290; saturated steam at atmospheric pressure; and gravity controlled flow regime. A limited influence of the inclination angle on heat transfer coefficient has been observed. Correlations to evaluate the local heat transfer coefficient along inclined tubes, in a gravity controlled flow regime, have been developed and they are in good agreement with the experimental results. (C) 2013 Elsevier Ltd. All rights reserved