I. Di Piazza

STH-CFD Codes Coupled Calculations Applied to HLM Loop and Pool Systems

This work describes the coupling methodology between a modified version of RELAP5/Mod3.3 and ANSYS Fluent CFD code developed at the University of Pisa. The described coupling procedure can be classified as “two-way,” nonoverlapping, “online” coupling. In this work, a semi-implicit numerical scheme has been implemented, giving greater stability to the simulations. A MATLAB script manages both the codes, oversees the reading and writing of the boundary conditions at the interfaces, and handles the exchange of data. A new tool was used to control the Fluent session, allowing a reduction of the time required for the exchange of data. The coupling tool was used to simulate a loop system (NACIE facility) and a pool system (CIRCE facility), both working with Lead Bismuth Eutectic and located at ENEA Brasimone Research Centre. Some modifications in the coupling procedure turned out to be necessary to apply the methodology in the pool system. In this paper, the comparison between the obtained coupled numerical results and the experimental data is presented. The good agreement between experiments and calculations evinces the capability of the coupled calculation to model correctly the involved phenomena.

Experimental activity for the investigation of mixing and thermal stratification phenomena in the circe pool facility

Since the Lead-cooled Fast Reactor (LFR) has been conceptualized in the frame of GEN IV International Forum (GIF), ENEA is strongly involved in the HLM technology development.In particular, several experimental campaign employing HLM loop and pool facilities (CIRCE [1], NACIE [2], HELENA [3], HERO [4]) are carried out in order to support HLM technologies development.In this frame, suitable experiments were carried out on the CIRCE pool facility refurbished with the Integral Circulation Experiment (ICE) test section in order to investigate the thermal hydraulics and the heat transfer in grid spaced Fuel Pin Bundle cooled by liquid metal providing, among the others aim, experimental data in support of codes validation for the European fast reactor development.The study of thermal stratification in large pool reactor is relevant in the design of HLM nuclear reactor especially for safety issue. Thermal stratification should induce thermomechanical stresses on the structures and in accidental scenario conditions, could opposes to the establishment of natural circulation which is a fundamental aspect for the achievements of safety goals required in the GEN-IV roadmap.In the present work, a Protected Loss of Heat Sink with Loss Of Flow (PLOHS+LOF) scenario is experimentally simulated and the mixed convection with thermal stratification phenomena is investigated during the simulated transient, as foreseen in the frame of Horizon 2020 SESAME project [5].A full characterization of thermal stratification inside the pool is presented, and the main results gained during the run are reported.The two tests named A (20 h) and B (6 h) here reported, essentially differs for the power supplied to the fuel bundle during the full power run (800 kW and 600 kW respectively). After the transition to natural circulation conditions, the power supplied to the bundle is decreased to about 30 kW simulating the decay heat.Finally the Nusselt number for the central subchannel of the fuel bundle simulator (FPS) is evaluated and compared with values obtained from Ushakov and Mikityuk correlations [6–7].Copyright

Coupled simulations of natural and forced circulation tests in Nacie facility using relap5 and Ansys fluent codes

In this work the activity performed at the DICI (Dipartimento di Ingegneria Civile e Industriale) of the Pisa University in collaboration with the ENEA Brasimone Research Centre is presented. In particular the document deals with the application of an in-house developed coupling methodology between a modified version of RELAP5/Mod3.3 and Fluent commercial CFD code, to the NACIE (Natural Circulation Experiment) LBE experimental loop (built and located at the ENEA Brasimone research centre).The first part of the document treats the description of the NACIE loop type facility, while in the second part, the developed coupling tool is presented and the obtained numerical results are compared to stand alone RELAP5 results and to data obtained from the NACIE experimental campaign. The experimental tests are performed varying the argon flow rate and the electric power supplied to the heater and both natural and assisted circulation tests are investigated. The numerical model set-up is based on a two-way explicit coupling scheme and 2D and 3D geometrical domain were investigated.Comparative analyses among numerical and experimental results showed good agreement, giving positive feedback on the feasibility and capability of the developed coupling methodology.Copyright

A CFD Analysis of Flow Blockage Phenomena in ALFRED LFR DEMO Fuel Assembly

A CFD study has been carried out on fluid flow and heat transfer in the HLM-cooled Fuel Pin Bundle of the ALFRED LFR DEMO.In the context of GEN-IV Heavy Liquid Metal-cooled reactors safety studies, the flow blockage in a Fuel sub-assembly is considered one of the main issues to be addressed and the most important and realistic accident for LFR Fuel Assembly. The present paper is a first step towards a detailed analysis of such phenomena, and a CFD model and approach is presented to have a detailed thermo-fluid dynamic picture in the case of blockage. The closed hexagonal, grid-spaced fuel assembly of the LFR ALFRED has been modeled and computed. At this stage, the details of the spacer grids have not been included, but a conservative analysis has been carried out based on the current main geometrical and physical features. Reactivity feedback, as well as axial power profile, have not been included in this analysis. Results indicate that critical conditions, with clad temperatures around ∼900°C, are reached with blockage larger than 30% in terms of area fraction.Two main effects can be distinguished: a local effect in the wake/recirculation region downstream the blockage and a global effect due to the lower mass flow rate in the blocked subchannels; the former effect gives rise to a temperature peak behind the blockage and it is dominant for large blockages (>20%), while the latter effect determines a temperature peak at the end of the active region and it is dominant for small blockages ( 15% could be detected by putting some thermocouples in the plenum region of the FA.© 2014 ASME

HELENA: A Heavy Liquid Metal Multi-Purpose Loop for Thermal-Hydraulics, Corrosion and Component Test

Since the Lead-cooled Fast Reactor (LFR) has been conceptualized in the frame of GEN IV International Forum (GIF), ENEA is strongly involved on the HLM technology development.Currently ENEA has implemented large competencies and capabilities in the field of HLM thermal-hydraulic, coolant technology, material for high temperature applications, corrosion and material protection. In this frame, the HELENA facility is well instrumented and it represents a loop working in pure lead for experiments in the field of corrosion for LFR structural materials, component test, and thermal-hydraulic investigations. The components and the process of the facility has been also depicted in some details. The scheduled working activities has been described and the main steps focused.A prototypical mechanical pump has been designed and manufactured to properly work in pure lead at high temperatures. The long-run test on the component will be isothermal at 400°C with low oxygen content <10−8 % in weight; the oxygen content will be monitored continuously. The pump will be gradually driven to the reference mass flow rate 35 kg/s and this mass flow rate will be maintained for 1500 h, i.e. 2 months about. After this, the pump is stopped and the loop is drained. Then, the pump impeller is disassembled by the body and it is analyzed for corrosion. Then, a test on the ball valves is carried out for a few months. At the end of 2014, the facility is upgraded with the insertion of the FPS in the heating section and with the secondary side. A 19-pin wire-spaced Fuel Pin bundle Simulator (FPS) is installed to measure clad temperature and heat transfer coefficients in different conditions in the different ranks of sub-channels of the MYRRHA bundle. Therefore a test matrix on the forced convection condition in the wire-spaced bundle will be carried out.A 7 tube/shell-and-tube Heat Exchanger couples the primary lead loop with the secondary side with water in pressure at 100 bar. The tube-in-tube technology with lead tube side, water shell side, steel powder in the gap is adopted. Bubble tubes with flowing Argon are adopted to measure pressure losses in the different branches of the loop. Several thermocouples monitor the loop in different points. An ancillary gas system ensures the cover gas.The paper reports the description of the experiments, the proposed test matrix and description the technological solution adopted for the HELENA implementation.Copyright

HLM Fuel Pin Bundle Characterization in CIRCE Pool Facility

Since the Lead-cooled Fast Reactor (LFR) has been conceptualized in the frame of GEN IV International Forum (GIF), ENEA is strongly involved on the HLM technology development.Currently ENEA has implemented large competencies and capabilities in the field of HLM thermal-hydraulic, coolant technology, material for high temperature applications, corrosion and material protection, heat transfer and removal, component development and testing, remote maintenance, procedure definition and coolant handling.In this frame the Integral Circulation Experiment (ICE) test section has been installed into the CIRCE pool facility, and suitable experiments have been carried out aiming to deeply investigate the pool thermal-hydraulic behavior of a HLM cooled pool reactor.In particular a fuel pin bundle simulator (FPS) has been installed in the CIRCE pool. It has been conceived with a thermal power of about 1 MW and a linear power up to 25kW/m, relevant values for a LMFR. It consist of 37 fuel pins (electrically simulated) placed on a hexagonal lattice with a pitch to diameter ratio of 1.8. The pins have a diameter of 8.2mm and active lengths of 1 m. Along the FPS, three spacer grid properly designed by ENEA have been installed.The FPS has been deeply instrumented by several thermocouples. In particular three sections of the FPS have been instrumented to monitor the heat transfer coefficient along the bundle as well as the cladding temperature in different rank of sub-channels.A full characterization of the FPS has been experimentally achieved both under forced and natural circulation, and the main results gained during the run are reported into the paper.Moreover the paper reports a preliminary analysis and discussion of such results, also in comparison with CFD calculations performed by CFX code.Copyright