<p> The relative cumulative yields of krypton and xenon isotopes from the thermal neutron fission of 237Np and Np238 and from the fast neutron fission of Np237 have been measured by means of a mass spectrometer. These are the first fission yield results for 238Np, and the first for 237Np at thermal neutron energies. The results are compared with those from other fissioning nuclides.</p> <p> Independent yields of the shielded nuclides 80Br, 82Br, 128I, and 130I from the thermal neutron fission of 235U, 233U, 239Pu, and 238Np have been determined by mass spectrometric analyses of the krypton and xenon β-decay products. The results are discussed in terms of conventional charge distribution theories, and also in terms of neutron emission effects.</p> / Thesis / Doctor of Philosophy (PhD)

La fission nucléaire, bien que connue depuis 80 ans, reste un défi théorique. La complexité et la multiplicité des interactions au sein d’un noyau fissionnant rend la modélisation du phénomène de fission délicate. Les modèles de fission nucléaire peinent à reproduire les observables. Parmi ces observables, les rendements de fissions sont pourtant des ingrédients clés pour la physique des réacteurs. S’ils ne peuvent être prédis efficacement, il est nécessaire de les mesurer.Les bibliothèques de données nucléaires évaluées, qui condensent la connaissance actuelle de ces rendements de fission, présentent pourtant des incertitudes importantes, ainsi que des divergences.Depuis quelques années, une collaboration entre le CEA, le CNRS et l’Institut Laue Langevin (ILL) vise à mesurer avec précision les rendements de la fission induite par neutron thermique pour des actinides d’importance pour les cycles Uranium-Plutonium et Thorium-Uranium, afin de compléter les connaissances actuelles et également fournir des données de référence pour le test de modèles.En particulier, au cours de cette thèse, des données expérimentales, acquises sur le spectromètre de masse LOHENGRIN à l’ILL de Grenoble (France), ont été analysées pour le 239Pu(nth,f) et le 241Pu(nth,f). Deux types de rendements ont été déterminées : des rendements en masse, à l’aide d’une chambre d’ionisation, et des rendements isotopiques, par spectrométrie γ avec des détecteurs Ge.Grâce à un protocole expérimental rigoureux testant la reproductibilité des observables et une méthode d’analyse innovante, adaptée en particulier à l’estimation des covariances et à leurs propagations, les rendements en masse obtenus présentent des incertitudes réduites et une matrice de covariance expérimentale complète. Celle-ci est essentielle pour la compréhension et l’utilisation des données mesurées.Les rendements isotopiques obtenus sont fortement dépendants des données de structure nucléaire, et présentent de facto des incertitudes importantes. L’amélioration future des données de structure permettra, grâce aux outils d’analyse développés, de réduire l’incertitude des rendements isotopiques mesurés.D’autres observables d’intérêts ont pu être extraites des mesures, en particulier la distribution de l’énergie cinétique moyenne des produits de fission en fonction de leur masse, ainsi que la polarisation de charge pour une partie du pic lourd. Ces données sont complémentaires aux rendements et présentent un intérêt pour les modèles et les futures évaluations. / Nuclear fission, although known for 80 years, remains a theoretical challenge. The complexity and multiplicity of interactions within a splitting nucleus make the modelling of the fission phenomenon delicate. Nuclear fission models struggle to replicate observables. Among these observables, however, fission yields are key ingredients for reactor physics. If they cannot be predicted effectively, it is necessary to measure them.The evaluated nuclear data libraries, which condense the current knowledge of these fission yields, nevertheless present significant uncertainties, as well as discrepancies.For several years, a collaboration between the CEA, the CNRS and the Laue Langevin Institute (ILL) has aimed at accurately measuring the yields of thermal neutron induced fission for actinides of importance for the Uranium-Plutonium and Thorium-Uranium cycles, in order to complete current knowledge and also provide reference data for model testing.In particular, during this thesis, experimental data acquired on the LOHENGRIN mass spectrometer at the ILL in Grenoble (France) were analysed for 239Pu(nth,f) and 241Pu(nth,f). Two types of yields were determined: mass yields, using an ionization chamber, and isotopic yields, by spectrometry γ with Ge.Thanks to a rigorous experimental protocol testing the reproducibility of the observables and an innovative analytical method, adapted in particular to the estimation of covariances and their propagation, the mass yields obtained present reduced uncertainties and a complete experimental covariance matrix. This is essential for understanding and using the measured data.The isotopic yields obtained are highly dependent on the nuclear structure data, and de facto present significant uncertainties. The future improvement of the nuclear structure data will allow, thanks to the analysis tools developed, to reduce the uncertainty of the isotopic yields measured.Other observables of interest have been extracted from the measurements, in particular the average kinetic energy distribution of the fission products as a function of their mass, as well as the charge polarization for part of the heavy peak. These data are complementary to fission yields and are of interest for models and future evaluations.

Uncertainties in nuclear model responses must be quantified to define safety limits, minimize costs and define operational conditions in design. Response uncertainties can also be used to provide a feedback on the quality and reliability of parameter evaluations, such as nuclear data. The uncertainties of the predictive model responses sprout from several sources, e.g. nuclear data, model approximations, numerical solvers, influence of random variables. It was proved that the largest quantifiable sources of uncertainty in nuclear models, such as neutronics and burnup calculations, are the nuclear data, which are provided as evaluated best estimates and uncertainties/covariances in data libraries. Nuclear data uncertainties and/or covariances must be propagated to the model responses with dedicated uncertainty propagation tools. However, most of the nuclear codes for neutronics and burnup models do not have these capabilities and produce best-estimate results without uncertainties. In this work, the nuclear data uncertainty propagation was concentrated on the SCK•CEN code burnup ALEPH-2 and the Monte Carlo N-Particle code MCNP.Two sensitivity analysis procedures, i.e. FSAP and ASAP, based on linear perturbation theory were implemented in ALEPH-2. These routines can propagate nuclear data uncertainties in pure decay models. ASAP and ALEPH-2 were tested and validated against the decay heat and uncertainty quantification for several fission pulses and for the MYRRHA subcritical system. The decay uncertainty is necessary to define the reliability of the decay heat removal systems and prevent overheating and mechanical failure of the reactor components. It was proved that the propagation of independent fission yield and decay data uncertainties can be carried out with ASAP also in neutron irradiation models. Because of the ASAP limitations, the Monte Carlo sampling solver NUDUNA was used to propagate cross section covariances. The applicability constraints of ASAP drove our studies towards the development of a tool that could propagate the uncertainty of any nuclear datum. In addition, the uncertainty propagation tool was supposed to operate with multiple nuclear codes and systems, including non-linear models. The Monte Carlo sampling code SANDY was developed. SANDY is independent of the predictive model, as it only interacts with the nuclear data in input. Nuclear data are sampled from multivariate probability density functions and propagated through the model according to the Monte Carlo sampling theory. Not only can SANDY propagate nuclear data uncertainties and covariances to the model responses, but it is also able to identify the impact of each uncertainty contributor by decomposing the response variance. SANDY was extensively tested against integral parameters and was used to quantify the neutron multiplication factor uncertainty of the VENUS-F reactor.Further uncertainty propagation studies were carried out for the burnup models of light water reactor benchmarks. Our studies identified fission yields as the largest source of uncertainty for the nuclide density evolution curves of several fission products. However, the current data libraries provide evaluated fission yields and uncertainties devoid of covariance matrices. The lack of fission yield covariance information does not comply with the conservation equations that apply to a fission model, and generates inconsistency in the nuclear data. In this work, we generated fission yield covariance matrices using a generalised least-square method and a set of physical constraints. The fission yield covariance matrices solve the inconsistency in the nuclear data libraries and reduce the role of the fission yields in the uncertainty quantification of burnup models responses. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

The relative fission yields of the isotopes of krypton and xenon have been determined mass spectrometrically. Abnormal fission yields, resulting in fine structure in the mass fission yield curve, have been found in both mass ranges. A shift of the fine structure to lower masses has been observed in going from U^235 + n fission to U^238 + n fission. From this shift in fine structure, it has been possible to determine the proportion of U^235 and U^236 neutron fission that have occurred in the sample. Evidence is presented to show that the observed fine structure and the shift in this fine structure is the result of a combination of two effects involved the extra stability of closed neutron shells of 50 and 82 neutrons which fall in the Kr and Xe ranges respectively. The capture of thermal neutrons by Xe^135, which modifies the fission yields in the 135 and 136 mass chains, has been studied so that the observed fission yields at these masses may be suitably corrected. Finally, the branching ratio between the isomeric states of Kr^85 and the half-life of the long-lived isomer have been re-determined and found to be 0.29 and 10.27 ± 0.18 yrs. respectively. / Thesis / Doctor of Philosophy (PhD)