Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Douglas S. McGregor / Advancements in nuclear reactor core modeling and computational capability have encouraged further development of in-core neutron sensors. Measurement of the neutron-flux distribution within the reactor core provides a more complete understanding of the operating conditions in the reactor than typical ex-core sensors. Micro-Pocket Fission Detectors (MPFDs) have been developed and tested previously but have been limited to single-node operation and have utilized highly specialized designs. The development of a widely deployable, multi-node MPFD assembly will enhance nuclear research capabilities. In-core neutron flux measurements include many challenges because of the harsh environment within the reactor core. Common methods of in-core neutron measurement are also limited by geometry and other physical constraints. MPFDs are designed to be small and robust while offering a real-time, spatial measurement of neutron flux. Improvements to the MPFD design were developed based on shortcomings of prior research in which many of the theoretical considerations for MPFDs were examined. Fabrication techniques were developed for the preparation of MPFD components and electrodeposition of fissile material. Numerous arrays of MPFDs were constructed for test deployments at the Kansas State University TRIGA Mk. II research nuclear reactor, University of Wisconsin Nuclear Reactor, Transient REActor Test facility at the Idaho National Laboratory (INL), and Advanced Test Reactor at INL. Preliminary testing of a single MPFD sensor at KSU yielded a linear response to reactor power between 10 kWth and 750 kWth and followed both positive and negative reactivity insertions in real-time. A $1.50 reactor pulse was monitored from the Intra-Reflector Irradiation System, located in reflector region of the KSU TRIGA Mk. II core with 1-ms time resolution. Improved multi-node MPFD arrays were then designed, fabricated, and deployed in flux ports between fuel rods and within an iron-wire flux port which was inserted into the central thimble of the KSU TRIGA Mk. II research nuclear reactor. Work continues to develop MPFDs for deployment at research reactors at INL and elsewhere. Results from the MPFD measurements will be useful for future validation of computational modeling and as part of advanced nuclear fuel development efforts.

Cette thèse a pour but de contribuer à la qualification du formulaire de calcul neutronique NARVAL, dédié aux coeurs de réacteurs à plaques. En particulier, l’objectif est de développer des méthodes innovantes permettant d’utiliser les données expérimentales inédites du programme HIPPOCAMPE pour évaluer la précision du profil de puissance calculé. La complexité provient de la localisation de l’instrumentation (chambres à fission placées entre les assemblages) et des hétérogénéités caractéristiques de ce type de coeurs (géométrie à plaques, poisons consommables et de contrôle solides). Pour aborder ce problème deux voies ont été mises en oeuvre : la première voie consiste à « combiner puis extrapoler » les écarts C/E observés afin de déterminer les incertitudes associées aux facteurs de puissance. Nous avons utilisé, pour ce faire, la méthode « P/A », traditionnellement employée dans les REP électrogènes mais jamais appliquée aux coeurs à plaques à ce jour. La deuxième voie passe, en revanche, par la reconstruction d’une nappe de puissance à utiliser comme référence (comparaison calcul/« expérience-reconstruite ») : nous avons focalisé notre travail sur des techniques géostatistiques. Après avoir constaté que les deux méthodes conduisent à des résultats satisfaisants (erreur comparable à l’incertitude expérimentale cible) nous avons continué notre recherche, en explorant les possibles développements et en introduisant en particulier une nouvelle méthode hybride (associant les techniques géostatistiques à la méthode P/A) qui permet d'améliorer ultérieurement la qualification du profil de puissance (écart-type des écarts C/E cohérent avec la constatation expérimentale). / The aim of this doctoral thesis work is to contribute to the experimental validation of a neutron physic code, called NARVAL, devoted to the analysis of slab cores reactors. The primary objective is to develop some innovative methods in order to validate the computed power map starting from the original experimental data, provided by the HIPPOCAMPE campaign. The particular position of the instrumentation (fission chambers located between the assemblies) and the strong heterogeneities, characterising this specific core design (slab geometry, burnable and control neutron absorbers in solid state) represent the main challenge of this work. Two different approaches are investigated : the first one consists in “combining and extrapolating” the observed calculated/experimental results in order to evaluate the uncertainty of power coefficients. Among different solutions, the “P/A” method is chosen : it is usually employed to perform conventional PWR plant analysis and has never been applied before to slab cores. The latter aims to reconstruct a power map that could be used as a direct reference for code validation : in this case the geostatistical techniques are selected. These methods provide satisfactory results as estimated errors are in good agreement with the experimental uncertainty target. Nevertheless, in this work a new hybrid method, applying the geostatistical technics to the P/A scheme, is investigated and developed. The good agreement between the experimental and the estimated validations of the computed power map attests the noteworthy performance of this new method.