Fuel | EPFL Graph Search

A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but has since also been applied to other sources of heat energy, such as nuclear energy (via nuclear fission and nuclear fusion). The heat energy released by reactions of fuels can be converted into mechanical energy via a heat engine. Other times, the heat itself is valued for warmth, cooking, or industrial processes, as well as the illumination that accompanies combustion. Fuels are also used in the cells of organisms in a process known as cellular respiration, where organic molecules are oxidized to release usable energy. Hydrocarbons and related organic molecules are by far the most common source of fuel used by humans, but other substances, including radioactive metals, are also utilized. Fuels are contrasted with other substances or devices s

Modelling of fuel fragmentation, relocation and dispersal during Loss-of-Coolant Accident in Light Water Reactor

Vladimir Vladimirov Brankov

Recent LOCA tests with high burnup fuel at the OECD Halden Reactor Project and at Studsvik demonstrated the susceptibility of the fuel to fragment to small pieces, to relocate and possibly cause a hot-spot effect and to be dispersed in the event of cladding rupture. However, the LOCA safety criteria defined by the US NRC are still based on fuel tests with fresh and low burnup fuel and therefore require revision for high burnup fuel. In this context a PhD project with the goal of developing new models for high burnup fuel fragmentation, relocation and dispersal during Loss of Coolant Accident in Light Water Reactors was launched at Paul Scherrer Institute in June 2013 with a financial support from swissnuclear. The work continued with base irradiation simulation with a closer look at the fission gas release measurements of high burnup BWR fuel rods.The data showed significant scatter for fuel rods at the same average burnup, originating from the same fuel assembly and having almost the same enrichment. The scatter was explained with a BWR-specific mechanism for fission gas trapping. It was motivated with Scanning Electron Microscopy images at the pellet-cladding interface that showed very strong fuel-cladding bonding layer on one side of the fuel rod. Base irradiation with the EPRI's FALCON code coupled with an in-house advanced fission gas release and gaseous swelling model GRSW-A was done for selected high burnup BWR fuel rods. The calculated fission gas release was overestimated in all cases. This was expected, because at the time the modelling did not account for fission gas trapping. From the available modelling and experimental data, a model for fission gas trapping was proposed. After calibration, the agreement between calculated and measured fission gas release was significantly improved. Fuel fragmentation modelling was focused on fuel pulverization. This is a high burnup fuel-specific phenomenon, in which the fuel pellet periphery may fragment to sizes less than 100 ÎŒm. Such fragments are very mobile and can be easily relocated and dispersed in the event of cladding rupture regardless of the rupture opening size. Therefore, the fuel fragmentation model addresses the potentially most important mode of fragmentation - fuel pulverization. Fuel relocation inside fuel rod is simulated during cladding ballooning and until cladding failure. The model takes as input the time-dependent cladding deformation supplied by FALCON and the fragment size distribution either provided directly by experimental data or by the fuel fragmentation model. Fuel relocation was modelled under the specific assumption that outermost fragments (e.g. pulverized fuel) relocate first, resulting in a large packing factor and local cladding temperature increase at the balloon (i.e. hot-spot effect) and, as a consequence, in enhanced cladding oxidation at the balloon. Fuel dispersal is modelled by solving the mass, energy and momentum conservation equations for two phases - the gas inside the fuel rod and the fraction of fuel which is considered "moveable". Although the model uses simplified geometrical representation of the fuel rod and some other simplifying assumptions, the underlying reason for fuel dispersal (namely the interfacial friction between the gas outflow and the solid) is explicitly simulated. The model for fuel dispersal is calibrated using Halden and Studsvik LOCA tests.

EPFL2017

Improved methodology for analysis and design of Molten Salt Reactors

Rodrigo Gonzalez Gonzaga De Oliveira

Molten Salt Reactors is a class of advanced reactors that is typically characterised by the presence of a molten inorganic salt of a fissile material as fuel. At the moment, this class of reactors is considered one of the promising options in the long run of nuclear power.In the context of safety assessment of the MSFR, a task on analysis of selected transients was planned by the SAMOFAR project, to which this thesis should contribute. In addition, fluids with high melting temperature bring with them challenges regarding solidification of the fluid and possible flow blockage, to which a method was implemented during this thesis to evaluate the consequences. The open core cavity with curved shape adds another issue where the core is highly turbulent and requires the use of unstructured mesh codes for routine analysis, adding uncertainty and computational burden to the analytical workflow. Using ATARI, the tool developed during this thesis, we:1. Analyse the MSFR together with partners in support of a safety assessment task.2. Consider the impact and how to include solidification/melting in an analytical methodology.3. Evaluate options to manage/reduce uncertainties and the burden of routine use of high-fidelity codes.After a careful derivation of the conservation laws and assembling the code main algorithms, ATARI undergoes verification in a limited scope using the Stefan problem, manufactured solutions and energy balances.The nuclear cavity benchmark was performed between SAMOFAR partners showing equivalency of the codes within the scope of the benchmark. Then the participating institutions perform analysis of the MSFR for the safety assessment task, which ATARI is unable to followup with the transient analysis. The reason for failure is identified as the requirement of appropriate acceleration schemes by OpenFOAM-based multiphysics solvers, and the lack of one in ATARI.In the lack of accurate data regarding the heat exchangers, the impact of solidification is evaluated in a few simplified cases of a pipe with square cross-section and heat exchanger. For different salt compositions and different boundary conditions, the heat exchanger is found to recover from blockage if pump maintains its momentum and some cross-flow is allowed. The recovery of cases with abrupt solidification is questionable due to the required pressure head.The use of flow baffles is investigated as an option to structure the flow inside the reactor and reduce uncertainties originating from turbulence modelling while allowing the use of structured mesh codes for analysis. The concept is tested in a chloride based MSR, which retains its breed and burn mode while showing a quasi-1D flow appropriate for modelling with legacy codes.We recommend that the nuclear cavity benchmark should be expanded to include a more demanding transient case, representative of MSFR transient analysis requirements. A case is made for homogenised and economical models as used in ATARI to enable the practical use of coupling methods that promote the simultaneous solution of equations. A niche for ATARI is found as a special-mission code for analysis of challenging cases with deforming structure and flow field that justifies the use of unstructured meshes and the added computational burden. Due to design uncertainties and their nature, it is recommended to consider solidification implicitly when analysing the MSFR fuel circuit.

EPFL2021

Performance evolution and modeling of PEM fuel cell systems

Nicolas Roggo

In the context of climate change mitigation, different alternatives to reduce the atmospheric CO2 emissions are; increasing the efficiency, switching to renewable resource and reduce emissions from fossil resources by CO2 capture and storage (CCS) . Hydrogen is considered as potential alternative to cover the energy needs by using it as a fuel in internal combustion engines or fuel cells, or as chemical source. In proton exchange membrane (PEM) fuel cells, also known as polymer exchange membrane fuel cells, pure hydrogen fuelis combined with the oxygen from the atmosphere to produce water, heat and electricity. Many research has been done in recent years on fuel cell systems leading to higher efficiencies. The performance is influenced by the membrane properties, the catalysts, the number of cells and the operating conditions. For studying the performance of fuel cell systems process modeling is a well-known approach. Here the evolution of fuel cell efficiency, especially of PEMFC is studied within the aim of updating previously developed PEMFC process models to integrate them with different hydrogen production processes using fossil and renewable resources in order to assess the competitiveness on the energy market.

2012