Les futurs réacteurs à neutrons rapides refroidis au sodium (RNR-Na) doivent remplir les critères GEN-IV à savoir présenter des qualités d'économie, de sûreté améliorée, de résistance à la prolifération et de minimisation des déchets. Ce projet de thèse est dédié à l'étude de l'impact des combustibles innovants (spécialement le combustible oxyde sphere-pac chargé en actinides mineurs) sur les performances de sûreté des RNR-Na dédiés à la transmutation.Le code de calcul SIMMER-III, développé à l'origine pour les phases avancées d'un accident grave, est utilisé pour les simulations. Ce code a été étendu dans le cadre de cette thèse afin d'améliorer la simulation de la phase primaire de l'accident, en introduisant le traitement des effets en réactivité liés à la dilatation du cœur et les spécificités du combustible sphere-pac (conductivité thermique, gap). Les transitoires complets (de la phase d'initiation aux phases avancées) sont simulés avec cette version étendue du code. Dans le cadre de cette thèse, les propriétés thermiques du combustible sphere-pac ont été modélisées et adaptées à SIMMER. Une méthodologie innovante tenant compte des effets en réactivité liés à la dilation thermique du cœur dans un maillage Eulérien et dans le cadre de la cinétique spatiale a ensuite été développée. A chaque pas de temps, les dimensions et densités dilatées sont calculées pour chaque cellule suite aux variations de températures. Des facteurs correctifs sont appliqués aux densités dilatées pour obtenir une configuration équivalente (en réactivité) ayant les dimensions non-dilatées et des densités modifiées. De nouvelles sections efficaces sont calculées à partir de ces densités et l'effet en réactivité lié à la dilatation est calculé. Les résultats sont prometteurs pour des dilatations uniformes et non-uniformes. Des limitations dans le cas de dilatations non-uniformes ont été identifiées et des calculs neutroniques ont été effectués en vue de futurs développements SIMMER. Les résultats préliminaires sont encourageants. Enfin, deux cœurs RNR-Na, issus du précédent projet CP-ESFR, ont été modélisés avec des combustibles sphere-pac : le Working Horse et le CONF2 (présentant un plénum sodium élargi pour une diminution de l'effet de vide sodium). Des analyses de sûreté ont été effectuées afin de fournir une première évaluation du comportement du combustible sphere-pac comparé au combustible pastille. Les deux options sont analysées en situation nominale et accidentelle (accident de perte de débit primaire) en début de vie du cœur et après irradiation. Les analyses révèlent deux phases à considérer en début de vie pour le combustible sphere-pac. Au démarrage du réacteur, ce combustible n'est pas restructuré et sa conductivité thermique est très inférieure à celle du combustible pastille. Après quelques heures sous irradiation, il se restructure suite aux importants gradients de température, ce qui améliore sa conductivité. Il se comporte alors de façon similaire au combustible pastille. Ce travail a également permis d'évaluer le comportement accidentel du cœur CONF2 qui subit un transitoire doux, prouvant que le large plénum sodium prévient efficacement de larges insertions de réactivité positive. Cependant, avec l'ajout d'américium ou suite à l'irradiation, des excursions de puissance et de réactivité plus prononcées sont observées. Ce travail a permis de démontrer que le combustible sphere-pac ne semble pas causer de problèmes de sûreté spécifiques comparé au combustible pastille, dans les conditions de simulations actuelles. La prise en compte des effets en réactivité liés à la dilatation du cœur avec cette version étendue de SIMMER retarde et réduit le potentiel énergétique lors d'un accident. Les analyses confirment également l'action atténuante du plénum sodium sur les transitoires conduisant à la vidange du sodium du coeur. Le comportement du combustible sphere-pac dans ces conditions ouvre une perspective à son utilisation en RNR-Na. / Future sodium cooled fast reactors (SFRs) have to fulfill the GEN-IV requirements of enhanced safety, minimal waste production, increased proliferation resistance and high economical potential. This PhD project is dedicated to the evaluation of the impact of innovative fuels (especially minor actinides bearing oxide sphere-pac fuels) on the safety performance of advanced SFRs with transmutation option. The SIMMER-III code, originally tailored to mechanistically analyze later phases of core disruptive accidents, is employed for accident simulations. During the PhD project, the code has been extended for a better simulation of the early accident phase introducing the treatment of thermal expansion reactivity effects and for taking into account the specifics of sphere-pac fuels (thermal conductivity and gap conditions). The entire transients (from the initiating event to later accident phases) have been modeled with this extended SIMMER version. Within this PhD work, first the thermo-physical properties of sphere-pac fuel have been modeled and casted into SIMMER-III. Then, a new computational method to account for thermal expansion feedbacks has been developed to improve the initiation phase modeling of the code. The technique has the potential to evaluate these reactivity feedbacks for a fixed Eulerian mesh and in a spatial kinetics framework. At each time step, cell-wise expanded dimensions and densities are calculated based on temperature variations. Density factors are applied to the expanded densities to get an equivalent configuration (in reactivity) with original dimensions and modified densities. New cross sections are calculated with these densities and the reactivity of the equivalent configuration is computed. The developed methods show promising results for uniform and non-uniform expansions. For non-uniform expansions, model improvement needs have been identified and neutronics simulations have been carried out to support future SIMMER extensions. Preliminary results are encouraging. In the third part of the PhD, two core designs with conventional and sphere pac fuels are compared with respect to their transient behavior. These designs were established in the former CP-ESFR project: the working horse core and the optimized CONF2 core (with a large sodium plenum above the core for coolant void worth reduction). The two fuel design options are compared for steady state and transient conditions (Unprotected Loss of Flow accident, ULOF) either at beginning of life (BOL) or under irradiated conditions. Analyses for sphere-pac fuel reveal two main phases to consider at BOL. At start-up, the non-restructured sphere-pac fuel shows a low thermal conductivity compared to pellet fuel of same density. However, the fuel restructures quickly (in a few hours) due to the high thermal gradients and its thermal conductivity recovers. The fuel then shows a behavior close to the pellet one. The study also shows that the CONF2 core leads to a very mild transient for a ULOF accident at BOL. The large upper sodium plenum seems to effectively prevent large positive reactivity insertions. However, stronger reactivity and power peaks are observed under irradiated conditions or when americium is loaded in the core and lower axial blanket. This PhD work demonstrates, under current simulation conditions, that sphere-pac fuels do not seem to cause specific safety issues compared to standard pellet fuels, when loaded in SFRs. The accurate simulation of core thermal expansion reactivity feedbacks by means of the extended SIMMER version plays an important role in the accident timing (simulations confirm the expected delay in the first power peak) and on the energetic potential compared to the case where these feedbacks are omitted. The analyses also confirm the mitigating impact of a large sodium plenum on transients with voiding potential. The behavior of sphere-pac fuel in these conditions opens a perspective to its practical application in SFRs.

Nuclear fuel comes in many forms; oxide fuel is the most commonly used in current reactor systems while metal fuel is a promising fuel type for future reactors due to neutronic performance and increased thermal conductivity. As a key heat transfer parameter, thermal conductivity describes the heat transport properties of a material based upon the density, specific heat, and thermal diffusivity. A material’s ability to transport thermal energy through its structure is a measurable property known as thermal diffusivity; the units for thermal diffusivity are given in area per unit time (e.g., m2/s). Current measurement methods for thermal diffusivity include LASER (or light) Flash Analysis and the hot-wire method. This study examines an approach that combines these previous two methods to characterize the diffusivity of a packed bed of microspheres of depleted uranium (DU) metal, which have a nominal diameter of 250 micrometers. The new apparatus is designated as the Crucible Heater Test Assembly (CHTA), and it induces a radial transient across a packed sample of microspheres then monitors the temperature profile using an array of thermocouples located at different distances from the source of the thermal transient. From the thermocouple data and an accurate time log, the thermal diffusivity of the sample may be calculated. Results indicate that DU microspheres have very low thermal conductivity, relative to solid uranium metal, and rapidly form an oxidation layer. At 500°C, the thermal conductivity of the DU microspheres was 0.431 ± 13% W/m-K compared to approximately 32 W/m-K for solid uranium metal. Characterization of the developed apparatus revealed a method that may be useful for measuring the thermal diffusivity of powders and liquids.

El continuo af'an por reducir la cantidad de act'¿nidos minoritarios (MA) procedentes del combustible quemado en los reactores de agua ligera (Light Water Reactor, LWR) y de esa forma reducir la radiotoxicidad, ha llevado a desarrollar nuevos conceptos de combustible nuclear. El nuevo combustible por empaquetamiento de esferas (Sphere-Pac, SP) ofrece la oportunidad de reintroducir los MA en una matriz y quemarlos en un reactor r'apido de neutrones, donde se facilitan ciclos mu'ltiples por transmutaci'on de elemen- tos. Este combustible se puede utilizar tambi'en en un sistema subcr'¿tico r'apido de neutrones, es decir, un sistema nuclear accionado por un acelera- dor de part'¿culas (Accelerator Driven System, ADS), donde la subcriticidad (seguridad de parada del reactor) permite utilizar combustibles con mayor contenido de MA que en un reactor normal, reduciendo eficazmente en un solo paso la radiotoxicidad. El combustible SP se produce a partir de una soluci'on base (formada por metales y elementos qu'¿micos) mediante un proceso de gelificaci'on in- terna. Este proceso garantiza una buena homogeneidad del producto final y un riesgo de contaminaci'on mucho menor si se compara con la fabricaci'on cl'asica de pellets (combustible comprimido), puesto que se evita el uso de prensas y amoladoras. La gelificaci'on interna es una reacci'on qu'¿mica acu- osa que se produce al calentar la soluci'on hasta 80 ± 5¿ C. Cuando se realiza el proceso por calentamiento electromagn'etico, se observan algunas venta- jas con respecto al calentamiento tradicional por conducci'on (contacto de la muestra con aceite de silicio precalentado): se evita la etapa de reciclado del aceite y de los disolventes org'anicos necesarios para eliminar el aceite de la superficie de las part'¿culas producidas. En la unidad de gelificaci'on in- terna por microondas (Microwave Internal Gelation, MIG), las microondas representan una alternativa mucho m'as simple y segura: el calentamiento volum'etrico sin contacto facilita la producci'on a distancia del combustible en celdas calientes y adem'as reduce los residuos de l'¿quido contaminado. Esta tesis se enmarca dentro del proyecto Platform for Innovative Nu- clear FuEls (PINE), que tiene como objetivo fundamental la producci'on de combustible SP por MIG. En el sistema MIG, el tiempo de calentamiento es muy corto (del orden de decenas de milisegundos), por lo que se deben optimizar los par'ametros que contribuyen al calentamiento por microondas y es necesario conocer en profundidad la interacci'on entre las microondas y las muestras. En la primera parte de este trabajo se investiga un modelo t'ermico basado en diferencias finitas en el dominio del tiempo (FDTD), el cual es capaz de determinar, en cada instante durante el proceso de calentamiento, el comportamiento t'ermico de un punto definido dentro del material que se calienta. Adem'as se presenta una descripci'on detallada de los par'ametros m'as relevantes del modelo, incluyendo las condiciones de contorno (entre ellas la convecci'on). Por otra parte, se implementa anal'¿ticamente y se valida con diferentes t'ecnicas: una basada en teor'¿a de la f'¿sica, otra basada en la herramienta de ecuaciones diferenciales parciales (PDEtools) y la u'ltima basada en ejemplos encontrados en la literatura. En segundo lugar, se investigan los posibles disen¿os de cavidades de microondas para su aplicaci'on en MIG. Tanto las cavidades (selecci'on de los modos, frecuencia de resonancia, factores de calidad, etc.) como su posterior caracterizaci'on, se detallan con el objetivo de especificar el acoplamiento de energ'¿a. Los mecanismos de transferencia de energ'¿a de las cavidades se explican utilizando el m'etodo de perturbaci'on, con el que adem'as se analizan las p'erdidas de la cavidad cuando se coloca una muestra diel'ectrica en su interior. Con el modelo de transferencia de energ'¿a desar- rollado, se obtiene la tasa de generaci'on de calor por microondas, que se aplica al modelo t'ermico FDTD mencionado anteriormente. Los resultados anal'¿ticos demuestran la viabilidad de producir esferas gelificadas por MIG. Seguidamente se introducen los principales par'ametros relacionados con el calentamiento de un material por microondas, es decir, las propiedades diel'ectricas. Se desarrolla un nuevo procedimiento que permite medir estas propiedades en gotas que caen libremente a trav'es de una cavidad de mi- croondas. Se presenta el montaje experimental, cuya viabilidad se prueba a trav'es de diferentes experimentos. Las propiedades diel'ectricas medidas se incluyen en los modelos (perturbacional y t'ermico) con la intenci'on de determinar la potencia absorbida por la sustancia (en forma de gotas) y la temperatura que alcanza. En la u'ltima parte se presenta la implementaci'on del sistema MIG apli- cada al proyecto PINE, fundamental para la pr'actica de calentamiento (basado en frecuencias altas) dentro del laboratorio. Las propiedades de cada dispositivo se evaluan para realizar un estudio de potencia antes del ensamblaje del sistema MIG. De esa forma se evitan fallos al poner el sis- tema en funcionamiento. Adem'as se aportan las t'ecnicas experimentales y los resultados. La producci'on con 'exito de esferas gelificadas demuestra, sin duda, el uso favorable de las microondas en la producci'on de combustible SP por gelificaci'on interna. / In the continuous aim to reduce the amount of minor actinides (MA) from the spent fuel of Light Water Reactors (LWR) and therefore reduce its radiotoxicity (radioactive toxicity), new nuclear fuel concepts have been developed. Sphere-Pac (SP) fuel gives the opportunity to reintroduce the MA in a fuel matrix and to burn them in a fast reactor, which facilitates a multi-cycle because of its breeding feature, or in a subcritical fast system, i.e. an Accelerator Driven System (ADS) where its sub-criticality allows higher MA contents than a normal fast reactor reducing efficiently the radiotoxicity in one step. SP fuel is produced from the base solution (already containing all the elements) by internal gelation, which guarantees a good material homo- geneity and a lower contamination risk compared to the classical pellet fabrication, avoiding presses and grinding machines. The internal gelation is an aqueous chemical reaction occurring when the solution is heated up to 80 ± 5¿C. When performing the internal gelation process with electro- magnetic heating, some advantages appear with respect to the traditionally heating through conduction by contact of the sample with hot silicon oil: the recycling step of the oil and the organic solvents necessary to clean the particles from oil are avoided. In the Microwave Internal Gelation (MIG) unit, the microwaves represent a much simpler and safer alternative: the contactless volumetric heating facilitates the remote production of the fuel in hot cells and furthermore reduces the contaminated liquid waste. The fuel related project called Platform for Innovative Nuclear FuEls (PINE), in which this thesis is embedded, aims for the production of SP- fuel by MIG. In the MIG system, the heating time is very short (in the order of tens of milliseconds), therefore the microwave heating parameters have to be optimized and a good knowledge of the interaction between the microwaves and the samples must be achieved. In the first part of this dissertation a finite difference time domain (FDTD) thermal model capable to determine over each instant about the thermal behaviour of a definite point inside a material during heat process- ing is investigated. A detailed overview of the most relevant parameters on the model including the boundary conditions (e.g. convection) is pre- sented. Furthermore, the model is analytically implemented and validated with different techniques: a theoretical based physically validation, a par- tial differential equations (PDEtools) based validation and a validation with examples from the literature. Secondly, possible microwave cavity designs for MIG are researched. The cavities (selection of modes, resonant frequency, Q-factor, etc.) and its subsequent characterization for the coupling of energy are explained. Furthermore, the power transfer mechanisms of the cavities are explained using the perturbation method to analyse the losses when a dielectric sam- ple is placed inside a cavity. The developed power transfer model delivers the microwave heat generation rate which is applied to the FDTD thermal model mentioned in the previous paragraph. The analytical results provide a positive impression about the feasibility of producing gelated spheres by MIG. Next, the main parameters dealing with the heating of a material by microwaves are introduced. A new procedure that enables the measure- ment of dielectric properties of aqueous droplets freely falling through a microwave cavity is developed. The experimental setup is presented and several experiments prove its feasibility. The measured dielectric properties are afterwards included in the perturbation and thermal models with the main intention of determining the absorbed power by the material in form of drops and the reached temperature. In the last part the MIG system for the laboratory practice of the high frequency heating applied to the PINE project is implemented. Each device is characterized for a power study precedent to the MIG system assembly, avoiding then failures when putting the system into operation. In addition, the experimental techniques and the results are reported. Successful pro- duction of gelated spheres shows the favourable usage of microwave for the production of SP-fuel by internal gelation. / Cabanes Sempere, M. (2013). Innovative production of nuclear fuel by microwave internal gelation [Tesis doctoral]. Editorial Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/31641