The reprocessing of used nuclear fuel requires the dissolution and separation of numerous radioisotopes that are present as fission products in the fuel. The leading technology option in the U.S. for reprocessing is a sequence of processing methods known as UREX+ (Uranium Extraction ). However, an industrial scale facility implementing this separation procedure will require the establishment of safeguards and security systems to ensure the protection of the separated materials. A number of technologies have been developed for meeting the measurement demands for such a facility. This project focuses on the design of a gamma detection system for taking measurements of the flow streams of such a reprocessing facility. An experimental apparatus was constructed capable of pumping water spiked with soluble radioisotopes under various flow conditions through a stainless steel coil around a sodium iodide (NaI) detector system. Experiments were conducted to characterize the impact of flow rate, pipe air voids, geometry, and radioactivity dilution level on activity measurements and gamma energy spectra. Two coil geometries were used for these experiments, using 0.5 in stainless steel pipe wound into a coil with a 6 inch diameter; the first coil was 5.5 revolutions tall and the second coil was 9.5 revolutions tall. The isotopes dissolved in the flowing water were produced at the Texas A&M Nuclear Science Center via neutron activation of chromium, gold, cerium, and ytterbium nitrate salts. After activation, the salts were dissolved in distilled water and inserted into the radioactive flow assembly for quantitative measurements. Flow rate variations from 100 to 2000 ml/min were used and activity dilution levels for the experiments conducted were between 0.02 and 1.6 μCi/liter. Detection of system transients was observed to improve with decreasing flow rate. The detection limits observed for this system were 0.02 μCi/liter over background, 0.5% total activity change in a pre-spiked system, and a dilution change of 2% of the coil volume. MCNP (Monte Carlo N-Particle Transport) models were constructed to simulate the results and were used to extend the results to other geometries and piping materials as well as simulate actual UREX stream material in the system. The stainless steel piping for the flow around the detector was found to attenuate key identifying gamma peaks on the low end of the energy spectrum. For the proposed schedule 40 stainless steel pipe for an actual reprocessing facility, gamma rays below 100 keV in energy would be reduced to less than half their initial intensities. The exact ideal detection set up is largely activity and flow stream dependant. However, the characteristics best suited for flow stream detection are: 1) minimize volume around detector, 2) low flow rate for long count times, and 3) low attenuation piping material such as glass.

<p>Nuclear energy is currently one of the world’s main sources of electricity. Closely connected to the use of nuclear energy are important issues such as the nonproliferation of fissile material that may potentially used in nuclear weapons (safeguards), and the management of the highly radioactive nuclear waste. This thesis addresses both these issues by contributing to the development of new experimental methods for ensuring safe and secure handling of the waste, with focus on methods to be used prior to encapsulation and final storage.</p><p>The methods rely on high resolution gamma ray spectroscopy (HRGS), involving the measurement and analysis of emitted gamma radiation from the fission products ¹³⁷Cs, ¹³⁴Cs and ¹⁵⁴Eu. This technique is nondestructive, making it relatively nonintrusive with respect to the normal operation of the nuclear facilities.</p><p>For the safeguards issue, it is important to experimentally verify the presence and identity of nuclear fuel assemblies and also that the fuel has experienced normal, civilian reactor operation. It has been shown in this thesis that the HRGS method may be used for verifying operator declared fuel parameters such as burnup, cooling time and irradiation history. In the experimental part of the work, the burnup and the cooling time has been determined with an accuracy of 1.6% and 1.5%, respectively (1 σ).</p><p>A technique has also been demonstrated, utilizing the ratio ¹³⁴Cs/¹⁵⁴Eu, with which it is possible to determine whether a fuel assembly is of MOX or LEU type. This is of interest for safeguards as well as for the safe operation of a final storage facility.</p><p>As an improvement to the HRGS technique, measuring a part of the fuel assembly length in order to reduce measurement time has been suggested and investigated. A theoretical case for partial defect verification has also been studied as an extension of the HRGS technique. </p><p>Finally, HRGS has been used for determining the decay heat in spent nuclear fuel assemblies, which is of importance for the safe operation of a final storage facility. This application is based on the radiation from ¹³⁷Cs, and the accuracy demonstrated was within 3% (1 σ).</p>

Nuclear energy is currently one of the world’s main sources of electricity. Closely connected to the use of nuclear energy are important issues such as the nonproliferation of fissile material that may potentially used in nuclear weapons (safeguards), and the management of the highly radioactive nuclear waste. This thesis addresses both these issues by contributing to the development of new experimental methods for ensuring safe and secure handling of the waste, with focus on methods to be used prior to encapsulation and final storage. The methods rely on high resolution gamma ray spectroscopy (HRGS), involving the measurement and analysis of emitted gamma radiation from the fission products 137Cs, 134Cs and 154Eu. This technique is nondestructive, making it relatively nonintrusive with respect to the normal operation of the nuclear facilities. For the safeguards issue, it is important to experimentally verify the presence and identity of nuclear fuel assemblies and also that the fuel has experienced normal, civilian reactor operation. It has been shown in this thesis that the HRGS method may be used for verifying operator declared fuel parameters such as burnup, cooling time and irradiation history. In the experimental part of the work, the burnup and the cooling time has been determined with an accuracy of 1.6% and 1.5%, respectively (1 σ). A technique has also been demonstrated, utilizing the ratio 134Cs/154Eu, with which it is possible to determine whether a fuel assembly is of MOX or LEU type. This is of interest for safeguards as well as for the safe operation of a final storage facility. As an improvement to the HRGS technique, measuring a part of the fuel assembly length in order to reduce measurement time has been suggested and investigated. A theoretical case for partial defect verification has also been studied as an extension of the HRGS technique. Finally, HRGS has been used for determining the decay heat in spent nuclear fuel assemblies, which is of importance for the safe operation of a final storage facility. This application is based on the radiation from 137Cs, and the accuracy demonstrated was within 3% (1 σ).

Les légumineuses sont connues pour leurs capacités à établir une relation symbiotique avec des bactéries du sol fixatrices de l'azote atmosphérique. Cette interaction aboutit à la formation d'un nouvel organe au niveau des racines, la nodosité, au sein duquel le symbiote convertit l'azote atmosphérique (N2) en ammoniac, qui peut être directement consommé par les plantes. A l’intérieur de cette nodosité, la concentration en oxygène (O2) est maintenue à un très faible niveau car la réaction de réduction du N2 par l’enzyme bactérienne nitrogénase est inhibée par des traces d’oxygène. Un mécanisme de perception directe de l'O2 impliquant des membres de la famille des facteurs de transcription « Ethylene Responsive Factors » (ERFs) du groupe VII a récemment été découvert chez Arabidopsis thaliana. Ces facteurs de transcription (FT) possèdent une extrémité N-terminale caractéristique avec un résidu de cystéine à la seconde position. Dans des conditions normales d'O2, les FT sont conduit à la dégradation suivant une voie spécifique du protéasome. En condition de stress hypoxique, les TFs sont stabilisés et peuvent activer l’expression des gènes de réponse à l'hypoxie. Il a été démontré que la présence d’O2 et de NO était nécessaire pour déstabiliser ces protéines, et qu'une réduction de la disponibilité de l'un ou l'autre des gaz est suffisante pour protéger le résidu cystéine N-terminale de l'oxydation. L’objectif de cette thèse a été d'étudier le rôle de la famille ERF-VII dans la perception et l'adaptation au manque d'O2 chez M. truncatula. Des travaux ont aussi été menés pour déterminer l’importance du NO dans le fonctionnement en microoxie de la nodosité. Quatre gènes codant pour des facteurs de transcription de la famille ERF-VII ont été identifiés dans le génome de M. truncatula. La caractérisation de cette famille au niveau transcriptionnel a révélé que seul MtERF-B2.2 était induit par le stress hypoxique et au cours du développement des nodosités. Les trois autres, MtERF-B1.1, MtERF-B1.11 et MtERF-B2.3, sont constitutivement exprimés dans les feuilles, les racines et les nodosités. Pour étudier la stabilité de la protéine MtERF-B2.1, l’orthologue de RAP2.12 principal ERF-VII décrit dans la perception de l’O2 chez Arabidopsis, en fonction de la disponibilité de O2/NO, nous avons réalisé une protéine de fusion entre l’extrémité N-terminale de notre protéine et la protéine rapporteur luciférase. Les résultats obtenus sur des protoplastes d'Arabidopsis montrent l’implication la partie N-terminale de MtERF-B2.1 dans la régulation de la stabilité de la protéine, mais en contradiction avec les résultats obtenus en plantes composites de M. truncatula. La fonction de MtERF-B2.1 et MtERF-B2.11 a également été étudiée dans le cadre de la réponse au stress hypoxique et au cours du processus de nodulation en utilisant une stratégie d'interférence ARN. Des racines transgéniques dérégulées sur l’expression de MtERF-B2.1 et MtERF-B2.11 ont montré un défaut d’activation de plusieurs gènes de réponses à l'hypoxie tels que l’alcool déshydrogénase (ADH1) ou la pyruvate décarboxylase (PDC1). Ces racines transgéniques ARNi-MtERF-B2.1/B2.11 sont également affectées dans l'interaction symbiotique avec une réduction significative de la capacité de nodulation et de l'activité de fixation de l'azote dans les nodules matures. En conclusion, ces travaux révèlent que le mécanisme de détection d'O2 est médié par les ERF-VII dans les nodosités de M. truncatula et que ce mécanisme, associé aux cibles moléculaires régulées en aval, participe au développement de cet organe et au maintien de la capacité de fixatrice de celui-ci. De plus, les résultats indiquent que MtERF-B2.1/B2.11 sont des régulateurs positifs du métabolisme anaérobie et que les gènes associés au cycle hémoglobine-NO sont susceptibles d'activer d'autres voies de génération d'ATP. / Legume crops are known for their capacities to establish a symbiotic relationship with nitrogen fixing soil bacteria. This mutualism culminates in the formation of a new plant organ, the root nodule, in which the symbiont converts atmospheric nitrogen (N2) into ammonia, which can be directly consumed by plants. In nodules, bacterial nitrogenase enzyme is inhibited by traces of oxygen (O2) so different mechanisms maintain this organ at low O2 level. At the same time, nodules need to maintain a high ATP level to support the nitrogenase activity, which is highly energy demanding. Thus, a balance between a tight protection from O2 and an efficient energy production, referred as the “O2 paradox” of N2-fixing legume nodules, has to be reached. In Arabidopsis thaliana, a direct oxygen sensing mechanism has recently been discovered involving members of the ethylene responsive factors (ERFs) group VII. These transcription factors (TFs) possess a characteristic N-terminal amino acid with a cysteine residue at the second position that, under normal O2 conditions, leads to protein degradation following a specific pathway called the N-end rule pathway. Furthermore, it was shown that both O2 and nitric oxide (NO) are required to destabilize the ERFs VII and that a reduction in the availability of either gas is sufficient to stabilize these proteins. Therefore, the goal of this thesis was to investigated the role of ERF-VII family in O2 sensing and adaptation to hypoxia in M. truncatula, model plant for legumes, and to understand how NO interacts with O2 in hypoxic signalization in the microoxic environment that characterizes the nodule. We identified four genes belonging to the ERF-VII TF family in the M. truncatula genome, which present a strong similarity with ERF-VII of Arabidopsis. The characterization of this family at the transcriptional level revealed that only MtERF-B2.2 is up-regulated by hypoxia stress and during nodule development. The three others, MtERF-B1.1, MtERF-B1.11 and MtERF-B2.3 are found constitutively expressed in leaves, roots and nodules. To investigated the protein stability of MtERF-B2.1, the closest orthologous to AtRAP2.12 described as O2-sensors in Arabidopsis, in function of O2/NO availability, we realized a fusion protein with the luciferase reporter protein. Our results on Arabidopsis protoplasts indicated that the N-terminal part of MtERF-B2.1 drives its O2-dependent degradation by the N-end rule pathway. The function of MtERF-B2.1 and MtERF-B2.11 was also investigated both in response to hypoxia stress and during the nodulation process using an RNA interference strategy. Silencing of MtERFB2.1 and MtERF-2.11 showed a significant lower activation of several core hypoxia-responsive genes such as ADH1, PDC1, nsHb1 and AlaAT. These double knock-down transgenic roots were also affected in symbiotic interaction with a significant reduction of the nodulation capacity and nitrogen fixation activity in mature nodules. Overall, the results reveal that O2 sensing mechanism is mediated by ERF-VIIs in M. truncatula roots and nodules and that this mechanism, together with downstream targets, is involved in the organ development and ability to efficiently fix nitrogen. Furthermore, results indicated that MtERF-B2.1/B2.11 are positive regulator of the anaerobic metabolism and the Hb-NO cycle– related genes likely in order to activate alternative ATP generation pathways.