The knowledge and understanding of the changes in nuclear fuel composition throughout the reactor lifetime is a key factor for assessing the performance of the fuel itself and, consequently, the safety related to the whole plant. Therefore, among the several issues to be considered during the operation of the fuel rods in a nuclear reactor, it is also necessary to get a thorough knowledge about the multi-component system formed at the fuel-cladding interface by fission products (which diffuse and condensate in the colder region of the fuel pin giving rise to the Joint-Oxide-Gain - JOG), and by the fuel components themselves (e.g., the Cs-Mo-Te-I-U-Pu-O system, in fast reactor MOX fuel). Indeed, the mentioned system needs to be characterized in terms of phase composition and behaviour under the conditions of a fast reactor environment. Nowadays, the state-of-the-art is not completely satisfactory and the scientific community is still investigating this subject (e.g., through the H2020 Project INSPYRE, http://www.eera-jpnm.eu/inspyre/, with the final aim of a complete characterization of the fast reactor MOX fuel behaviour. In this thesis project, I focused on two different ternary systems (namely, Cs-Te-O and Cs-Mo-O), investigating them using several experimental techniques, in order to obtain structural and thermodynamic information on the different phases composing these two systems. More specifically, X-ray powder diffraction and neutron diffraction have been used for characterizing the synthetized compounds from the compositional and structural point of view, while differential scanning calorimetry (DSC) experiments were performed to investigate phase transitions. Furthermore, for this latter technique, different experimental settings have been adopted, and in the end a specific methodology has been developed giving reliable results, and avoiding issues of volatilization of the samples and/or chemical interaction with the measuring crucibles. Concerning the Cs-Te-O system, I treated mainly the Cs2TeO4 phase, obtaining the following main results: atomic position and linear thermal expansion coefficients, using neutron diffraction and high temperature X-ray diffraction, respectively. Furthermore, a phase transition has been identified for the first time, and a preliminary estimation of transition and melting temperatures and enthalpies has been obtained by DSC. As for the second system treated (i.e., Cs-Mo-O), the experiments I performed involved the Cs2MoO4-MoO3 pseudo-binary section of the ternary phase diagram, leading to an additional validation for the non liquidus transitions for X(MoO3) = 0.55, 0.60 and 0.667 (Cs2Mo2O7). Moreover, a promising synthesis path was developed for Cs2Mo3O10. Finally, the Cs2(Tex,Mo1-x)O4 solid solutions have been considered, and several experiments were performed on the 50% solid solution, leading to a first evaluation of the phase transition temperatures, and to the determination of the atomic positions as well using neutron diffraction.
La comprensione di come cambia la composizione del combustibile nucleare durante la vita del reattore è un fattore chiave per la valutazione delle performance del combustibile stesso e, di conseguenza, della sicurezza dell’intero impianto. È quindi necessario, tra i diversi aspetti da considerare durante l’irraggiamento in reattore delle barrette di combustibile, avere una conoscenza completa del sistema multicomponente formato all’interfaccia combustibile-guaina dai prodotti di fissione (che diffondono e condensano nella parte più fredda della barretta di combustibile dando origine al cosiddetto JOG - Joint-Oxide-Gain) e dai componenti del combustibile stesso (ad esempio, il sistema Cs-Mo-Te-I-U-Pu-O nei combustibili MOX per reattori veloci). Il sistema citato necessita infatti di essere caratterizzato in termini di fasi presenti e di comportamento a cui è soggetto nelle condizioni di un reattore veloce. Ad oggi lo stato dell’arte a riguardo non è pienamente soddisfacente e la comunità scientifica sta ancora indagando questo tema (ad es. nell’ambito del Progetto H2020 INSPYRE, http://www.eera-jpnm.eu/inspyre/), con l’obiettivo finale di una completa caratterizzazione del combustibile MOX. In questo progetto di tesi, mi sono focalizzato su due differenti sistemi ternari (Cs-Te-O e Cs-Mo-O), analizzandoli tramite l’utilizzo di svariate tecniche sperimentali volte all’ottenimento di informazioni di natura strutturale, cristallografica e termodinamica sulle diverse fasi che compongono suddetti sistemi. Nello specifico, sono state utilizzate la diffrazione di polveri a raggi X e la diffrazione neutronica per la caratterizzazione dei composti sintetizzati dal punto di vita strutturale e della composizione, mentre sono stati effettuati esperimenti di calorimetria a scansione (DSC - differential scanning calorimetry) per lo studio delle transizioni di fase. Inoltre, per quest’ultima tecnica, sono state adottate diverse procedure sperimentali ed è stata sviluppata una specifica metodologia che ha dato risultati affidabili, evitando problemi di volatilizzazione del campione e/o interazione chimica con i crogiuoli utilizzati per la misura. Riguardo il sistema Cs-Te-O, ho trattato principalmente la fase Cs2TeO4, ottenendo i seguenti principali risultati: posizioni atomiche e coefficienti di espansione termica lineare, utilizzando rispettivamente la diffrazione neutronica e la diffrazione di raggi X ad alta temperatura. Inoltre, è stata identificata per la prima volta una transizione di fase e sono state ottenute tramite DSC prime stime di temperature e entalpie di transizione e fusione. Per quanto riguarda il secondo sistema trattato, ossia Cs-Mo-O, gli esperimenti effettuati hanno coinvolto solamente la sezione pseudo-binaria Cs2MoO4-MoO3 del diagramma di fase ternario, portando a una validazione addizionale per le transizioni diverse dal liquidus per X(MoO3) = 0.55, 0.60 e 0.667 (Cs2Mo2O7). Inoltre, è stata sviluppata una promettente via di sintesi per la fase Cs2Mo3O10. Infine, sono state considerate le soluzioni solide Cs2(Tex,Mo1-x)O4, con diversi esperimenti svolti sulla soluzione solida al 50%, permettendo una prima valutazione delle temperature associate alle transizioni di fase e una determinazione delle posizioni atomiche.
Experimental investigation of Cs-Te-O and Cs-Mo-O fission product systems
VOLFI, ANDREA
2018/2019
Abstract
The knowledge and understanding of the changes in nuclear fuel composition throughout the reactor lifetime is a key factor for assessing the performance of the fuel itself and, consequently, the safety related to the whole plant. Therefore, among the several issues to be considered during the operation of the fuel rods in a nuclear reactor, it is also necessary to get a thorough knowledge about the multi-component system formed at the fuel-cladding interface by fission products (which diffuse and condensate in the colder region of the fuel pin giving rise to the Joint-Oxide-Gain - JOG), and by the fuel components themselves (e.g., the Cs-Mo-Te-I-U-Pu-O system, in fast reactor MOX fuel). Indeed, the mentioned system needs to be characterized in terms of phase composition and behaviour under the conditions of a fast reactor environment. Nowadays, the state-of-the-art is not completely satisfactory and the scientific community is still investigating this subject (e.g., through the H2020 Project INSPYRE, http://www.eera-jpnm.eu/inspyre/, with the final aim of a complete characterization of the fast reactor MOX fuel behaviour. In this thesis project, I focused on two different ternary systems (namely, Cs-Te-O and Cs-Mo-O), investigating them using several experimental techniques, in order to obtain structural and thermodynamic information on the different phases composing these two systems. More specifically, X-ray powder diffraction and neutron diffraction have been used for characterizing the synthetized compounds from the compositional and structural point of view, while differential scanning calorimetry (DSC) experiments were performed to investigate phase transitions. Furthermore, for this latter technique, different experimental settings have been adopted, and in the end a specific methodology has been developed giving reliable results, and avoiding issues of volatilization of the samples and/or chemical interaction with the measuring crucibles. Concerning the Cs-Te-O system, I treated mainly the Cs2TeO4 phase, obtaining the following main results: atomic position and linear thermal expansion coefficients, using neutron diffraction and high temperature X-ray diffraction, respectively. Furthermore, a phase transition has been identified for the first time, and a preliminary estimation of transition and melting temperatures and enthalpies has been obtained by DSC. As for the second system treated (i.e., Cs-Mo-O), the experiments I performed involved the Cs2MoO4-MoO3 pseudo-binary section of the ternary phase diagram, leading to an additional validation for the non liquidus transitions for X(MoO3) = 0.55, 0.60 and 0.667 (Cs2Mo2O7). Moreover, a promising synthesis path was developed for Cs2Mo3O10. Finally, the Cs2(Tex,Mo1-x)O4 solid solutions have been considered, and several experiments were performed on the 50% solid solution, leading to a first evaluation of the phase transition temperatures, and to the determination of the atomic positions as well using neutron diffraction.| File | Dimensione | Formato | |
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2019_12_Volfi.pdf
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https://hdl.handle.net/10589/150836